The silviculture of trees used in British forestry [3rd edition] 978-1-78639-392-0, 1786393921, 9781786393937, 9781786393944

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THE SILVICULTURE OF TREES USED IN BRITISH FORESTRY, 3RD EDITION

The Silviculture of Trees Used in British Forestry, 3rd Edition Peter Savill Former Reader in Silviculture University of Oxford With illustrations by

Rosemary Wise Botanical Illustrator University of Oxford

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© Peter Savill 2019. All rights reserved. No part of this publication may be reproduced in any form or by any means, electronically, mechanically, by photocopying, recording or otherwise, without the prior permission of the copyright owners. A catalogue record for this book is available from the British Library, London, UK. Library of Congress Cataloging-in-Publication Data Names: Savill, Peter S., author. Title: The silviculture of trees used in British forestry / Peter Savill ;   with illustrations by Rosemary Wise. Description: 3rd edition. | Boston, MA : CABI, [2019] | Includes   bibliographical references and index. Identifiers: LCCN 2018059745 (print) | LCCN 2019005105 (ebook) | ISBN   9781786393937 (ePDF) | ISBN 9781786393944 (ePub) | ISBN 9781786393920   (hbk : alk. paper) Subjects: LCSH: Forests and forestry--Great Britain. | Tree farms--Great  Britain. Classification: LCC SD391 (ebook) | LCC SD391 .S285 2019 (print) | DDC  634.9--dc23 LC record available at https://lccn.loc.gov/2018059745 ISBN-13: 978 1 78639 392 0 (hardback) 978 1 78639 393 7 (ePDF) 978 1 78639 394 4 (ePub) Commissioning Editor: Ward Cooper Editorial Assistant: Emma McCann Production Editor: Marta Patiño Typeset by SPi, Pondicherry, India Printed and bound in the UK by CPI Group (UK) Ltd, Croydon, CR0 4YY

Contents

Acknowledgements Introduction

ix 1

ABIES15 ALBA15 GRANDIS20 PROCERA23 BORISII-REGIS27 CEPHALONICA27 CILICICA28 NORDMANNIANA29 PINSAPO31 ACER35 CAMPESTRE35 MACROPHYLLUM37 PLATANOIDES38 PSEUDOPLATANUS42 AESCULUS50 HIPPOCASTANUM50 ALNUS53 CORDATA56 GLUTINOSA57 INCANA62 RUBRA63 VIRIDIS64 

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Contents

ARIA67 NIVEA67 BETULA70 PENDULA70 PUBESCENS70 BUXUS78 SEMPERVIRENS78 CARPINUS82 BETULUS82 CARYA86 CASTANEA89 SATIVA89 CEDRUS95 ATLANTICA95 DEODARA99 LIBANI99 CHAMAECYPARIS102 LAWSONIANA102 CORYLUS106 AVELLANA106 CRATAEGUS110 MONOGYNA110 CRYPTOMERIA113 JAPONICA113 × CUPROCYPARIS 121 LEYLANDII121 EUCALYPTUS124 FAGUS129 SYLVATICA129 FRAXINUS137 EXCELSIOR137 ILEX147 AQUIFOLIUM147 JUGLANS151 NIGRA151 REGIA154

Contents

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LARIX161 DECIDUA161 KAEMPFERI164 × MARSCHLINSII 164 MALUS169 SYLVESTRIS169 NOTHOFAGUS172 ALPINA172 OBLIQUA172 OSTRYA177 PICEA180 ABIES180 OMORIKA183 ORIENTALIS192 SITCHENSIS196 PINUS202 CONTORTA203 MURICATA207 NIGRA208 NIGRA ssp. NIGRA 209 NIGRA ssp. SALZMANNII 209 PEUCE215 RADIATA217 STROBUS219 SYLVESTRIS219 PLATANUS223 POPULUS226 NIGRA228 TREMULA229 HYBRID POPLARS 232 × WETTSTEINII 239 PRUNUS243 AVIUM243 PSEUDOTSUGA249 MENZIESII249 PTEROCARYA253 FRAXINIFOLIA253

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Contents

QUERCUS256 CERRIS256 ILEX258 PETRAEA259 ROBUR259 RUBRA274 ROBINIA277 PSEUDOACACIA277 SALIX282 SEQUOIA289 SEMPERVIRENS289 SEQUOIADENDRON294 GIGANTEUM294 SORBUS299 AUCUPARIA299 DOMESTICA302 TAXUS304 BACCATA304 THUJA310 PLICATA310 TILIA319 CORDATA319 PLATYPHYLLOS324 TORMINARIA327 TORMINALIS327 TSUGA335 HETEROPHYLLA335 ULMUS340 GLABRA340 PROCERA342 References Key to species Index

345 375 387

Acknowledgements

I am grateful to many colleagues and generations of students at Oxford University who have contributed, in various ways, to the preparation of this book by their insights and by referring me to relevant papers in journals and elsewhere. I am especially indebted to Rosemary Wise who produced all the drawings. Drs Jo Clark, Gabriel Hemery, Andrew Leslie and Professor Jens-Peter Skovsgaard of the Swedish University of Agricultural Sciences; Professor Valeriu-Norocel Nicolescu of Brasov University in Romania; John Fennessy of COFORD in Dublin; Dr Gary Kerr from Forest Research; and Professor Jürgen Huss of Freiburg University all provided detailed information on various species. Tom Curtis, of 3Keel, helped in providing results of his work on the restoration of ancient woodlands. Miles Barn of Sotterley Estate in Suffolk knows more about oak growing than any private owner, and I have learned much from him. Drs Scott Wilson, Bill Mason and Richard Jinks collaborated with me in producing a series of papers that were published in the Quarterly Journal of Forestry (QJF) from 2015 about species that could become useful as climate change becomes more noticeable. Much of the information gathered for them has been included in the text, for which I thank the editors of the QJF, Drs Lesley Trotter and Freia Bladon. Michelle, my wife, has undertaken the tedious job of reading through and checking all the text. John Baker kindly photographed the leaves used to illustrate the key. Colleagues in five charities with which I have been associated for many years deserve particular acknowledgement: Woodland Heritage; the Royal Forestry Society; the Future Trees Trust; the Earth Trust and the Sylva Foundation. Through my connections with them, I have been able to



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Acknowledgements

visit many of the most impressive woodlands in Great Britain and Ireland, and discuss silvicultural practices with their owners or managers. The figures in Table 1 were kindly provided by Jackie Watson and Lesley Halsall of the Economics and Statistics Branch of the Forestry Commission, and are reproduced with permission. The scientific names of the trees used in this book were kindly checked by Dr Stephen Harris, Druce Curator of the Oxford University Herbaria. Several have changed since the first edition was produced in 1991. Finally, I thank the production staff at CABI. Among them are Ward Cooper for his initial encouragement, Emma McCann for seeing the manuscript through the production process with great efficiency, Elaine Monaghan who copy edited the manuscript with impressive and meticulous care and Marta Patiño, the Production Editor.

Introduction

Many books have been written about trees that grow in Britain. They range from guides for identification to books that have been lavishly illustrated by talented photographers and artists. One of the best known of the former group is Alan Mitchell’s (1974) A Field Guide to the Trees of Britain and Northern Europe and its successors, and among the latter is Thomas Pakenham’s (1996) Meetings with Remarkable Trees. Many authors who love forests have written about them, including Julian Evans (1995) A  Wood of Our Own and Richard Fortey (2016) The Wood for the Trees: The Long View of Nature from a Small Wood. Numerous publications by the Forestry Commission also provide much basic information that can be useful when selecting species for specific sites. Few books, however, cater particularly well for the person who seeks detailed information about the silvicultural characteristics and r­ equirements of individual species. Among the authors to do this were M.L. Anderson in his Selection of Tree Species, which was first published in 1950, and Macdonald et al. (1957) in their ‘Exotic forest trees in Great Britain’. The information they provided about species, though still very useful, has inevitably become somewhat dated as techniques have changed and ­ knowledge has increased. More recently, Ray (2001) and Pyatt et al. (2001) have produced a computer-based ecological site classification that has proved invaluable in assisting the selection of species, as well as indicating how species might respond to climate change. It is based upon matching a four-­ climatic variable (accumulated temperature, moisture deficit, windiness and continentality) and two soil variables (moisture and nutrient regimes) © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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2

Introduction

with the requirements of 31 possible species of tree. In addition, the shade tolerance of each species is taken into account. Yet computer programs, however sophisticated, can never substitute for educated and informed thinking, knowledge and understanding.

Matching Species to Site The climate of an area limits the range of species that can be grown successfully. Temperature, precipitation and wind are usually considered the important elements of climate for forestry. The more favourable the climate, the greater the range of species from which a choice can be made. In upland regions of Britain where the climate is harsh and soils are often poor, the number is limited to two or three; whereas on good soils, in the better climate of the lowlands, a choice from about 20 may often be possible. These latter sites are said to have greater amplitude. Forestry has traditionally taken second place to agriculture, and most forest sites are marginal for agricultural production in some way. A large proportion of British forests are at high elevations where growing seasons are much shorter than in the lowlands and exposure to gales is common. Apart from physical problems such as steepness, stoniness and exposure, many forest soils tend to be drought-prone or waterlogged, suffer from ­extremes of acidity or alkalinity, or have compacted or cemented layers. Two-thirds of the state-owned forests have soils that suffer from poor aeration due to permanent or periodic waterlogging (Toleman and Pyatt, 1974). There are numerous important interrelationships between the physical, chemical and biological influences in a soil that can affect species choice. In many places today there is considerable past experience of which species will grow and how they will perform, either from existing stands of the same tree on the site itself, or from similar nearby areas. Site conditions should guide species choice in any approach to forest management, and this is particularly important for continuous-cover forestry given its demanding requirements in terms of stand stability, diversity, the use of natural processes and multipurpose management. Exceptional climatic events can reveal inadequacies in species selection that may not be apparent in more normal periods. This is one reason why extreme caution should be observed before a relatively untried, but promising, exotic tree is planted on a wide scale. A common pattern with introductions everywhere is one of initial establishment in gardens and arboreta and then, if successful, perhaps half a rotation later, this is followed by small-scale planting on estates and by government departments. Large plantations of the most promising species are established some 20–50 years after that (Streets, 1962). Thus, Sitka spruce, which is

Introduction

3

the major plantation tree in Great Britain and Ireland, was first introduced in 1831. It was established as one of the chief exotics in the early 1920s and by the mid-1950s it became the most widely planted tree. Small-scale plantations are recommended for a number of species that appear promising at present. These include various maples, mostly from eastern North America, plane (Platanus), various hickories and wingnuts (Carya and Pterocarya) and – more tentatively – hop-hornbeams (Ostrya), sweetgum (Liquidambar styraciflua) and Tulip tree (Liriodendron tulipifera) (see Wilson et al., 2017). Premature extensive planting often leads to problems. For example, in Britain the cold winters of 1981−82 and 2010−11 killed many Eucalyptus spp. that were being tested experimentally, but at the same time they were also being promoted for widespread planting by a few people. The general considerations that must be taken into account when deciding what to plant on a site are described in many silviculture textbooks (e.g. Savill et al., 1997). The amount of information available about the climatic and site requirements of each species, and other features of their silviculture, varies considerably but is closely related to how common and important they are. For some, especially the major conifers, oak and beech, knowledge is quite comprehensive, while for others such as lime and walnut, it is scanty and sometimes almost non-existent.

Provenance When dealing with both indigenous and exotic trees, it is not sufficient simply to decide which species to plant without considering the original geographic source of the seed as well – that is, the provenance. Some of the worst mistakes in plantation forestry are the result of well-­intentioned foresters importing seed from stands that look good but which are quite unsuited to the new environment (Lines, 1967). Worse still are politicians who bully foresters to use untested species or provenances, as in the cases of Robinia (see p. 277) and Weymouth pine (p. 219). Gullible foresters, land agents and landowners need also to be more restrained on occasions, as in the case of Wildstar cherry clones (p. 247). Decisions on what to use must be based on the evidence of trials in the new environment, rather than on untested opinions. The forester has much to gain by using the best possible seed source for raising trees for woodlands. There was an unfortunate period when plants grown from unsuitable seed were widely planted, because well-intentioned but misguided bureaucrats insisted that the cheapest seed was always bought, to the serious detriment of many forests that were planted with it. Seed represents only a minute proportion of total establishment costs of woodland, and it is a false economy to buy anything but the very best that is obtainable. If genetically improved seed that has gone through a breeding programme is

4

Introduction

available it should always be preferred, especially if it has reached EU ‘tested’ status. There are many recorded cases of inappropriate seed leading to problems. For example, in 2006 Alain Valadon reported to a European Forest Genetic Resources Programme (EUFORGEN) meeting in Romania on a case in France where 400,000 hectares (ha) of red oak were planted ­between 1970 and 2000 (EUFORGEN, 2018a). By 2004 only 27,000 ha had survived. Mortality was due to unsuitable provenances being used to begin with, causing problems of poor frost or drought resistance, susceptibility to pests and diseases, and poor adaptation to site conditions.

Climate Change Apart from the fact that it is undoubtedly happening, climate change presents great uncertainties that influence the choice of species, since future conditions are difficult to predict with any precision. It is possible to make guesses based on the most likely scenarios, as Davies et al. (2008) have pointed out. In general, it will be best to favour species with greater amplitudes of site requirements and to make use of a wide range of species to spread risks. The most likely picture for climate change in Britain is that winters will become wetter and summers slightly drier which, combined with increased temperatures, will lead to increased summer soil moisture deficits. There may also be increases in the number of severe winter and summer gales (Ray et al., 2001). These changes are likely to favour species such as Douglas fir, while conditions for Sitka spruce may become less suitable in England but more so in northern and eastern Scotland (Ray et al., 2001). Most work on climate change has tended to concentrate on how average conditions will change. Just as important for forestry would be knowledge of what extremes to expect, as these tend to be very ­important for perennial organisms like trees. There are also widespread concerns that the current genetic stock of trees may not be able to cope with future climate change. Kremer (2010), who took the European oaks as a case study, has shown that rapid ­migration and adaptation, extensive gene flow and hybridization were the main processes that enabled oaks to cope with climatic warming in the past. In considering future evolutionary trends, he concluded that the potential for species to migrate via seed dispersal to more favourable locations (e.g. northwards) will be limited. On the other hand, it is likely that natural selection will act on a diverse gene pool in part because of large population sizes, perhaps allowing local adaptation even if this ultimately reduces diversity. Substantial evolutionary shifts can be expected in a limited number of generations. The high levels of genetic diversity and

Introduction

5

gene flow from other populations will favour rapid adaptation. However, he believed that many tree populations may be tested to the limits of their adaptive potential, so some intervention may be needed. Kremer (2010) believed that to enhance the adaptive potential of populations, genetic diversity should be increased by mixing local stock with non-local material. Guidelines that provide information on ­recommended directions and distances for the transfer of reproductive material need to be developed based on current scientific information, especially on data from existing provenance tests. The current guideline for England, for example, is given in a publication by the Forestry Commission (2010). It recommended, among other things, that a proportion of seed in new plantations should come from sources located 2–5° south of the planting site. Climate change means increased uncertainty for the future, and hence increased risk. We must plan ahead to help our forests adapt (Broadmeadow, 2002; Read et al., 2009; Forestry Commission, 2016). Climate change will involve increased CO2, reduced summer rainfall, increased winter rainfall, increased storm frequency and warmer temperatures. In addition to many negative effects, there are some positive ones. Beneficial effects will include: • increased CO2 levels (which will result in enhanced growth rates and reduced water loss due to closure of stomata); and • higher temperatures (which will result in longer growing seasons and hence increased potential productivity combined with a lower risk of winter cold damage). A consequence of this is that there will be a possibility of using tree species that are not hardy enough to thrive everywhere in Britain at present. Among the many negative effects will be that: • reduced summer rainfall will mean that droughts are more severe and more frequent so that some tree species are no longer suitable for commercial forestry on shallow, free-draining soils, particularly in southern and eastern England; • perhaps most importantly, stress caused by drought will make trees more susceptible to pests and diseases; • increased winter rainfall will result in more waterlogging and reduced access for forest machinery, and also increased mortality of fine roots. In turn this can worsen the effects of summer drought; • extreme rainfall is likely to cause flooding and the current forest road drainage network may be inadequate; • there will be an increased risk of infection by soil-borne diseases and also reduced stability and more windthrow due to greater storm frequency;

6

Introduction

• the warmer winters could result in leaves appearing earlier, which will make trees vulnerable to frost damage; • pests will be able to survive through winter and there will be a potential for exotic pests to spread to the UK; • species that rely on the timing of each other’s life cycles (e.g. flowers and their pollinators) could become out of synchronization with each other; • forest fires are almost certain to become an increasing factor affecting the condition and longevity of some woods and forest areas in sensitive areas; and • forests are likely to be increasingly seen as cool, shady refuges for healthy exercise; but public access to the forests may be interrupted by closures due to storm damage, and by roads and paths being washed away. Much thought is being given by foresters to possible tree species that might be used in Britain if climate change proceeds as predicted, and in the light of the threats posed by tree pests and diseases which have ­become so numerous since the turn of the century. Recently, the Wessex Silvicultural Group (Bladon and Evans, 2015) have considered the matter, as has Wilson (2010, 2011). There is an online network that promotes novel species known to have potential to grow well in the UK (SilviFuture, 2015) including nine high-priority species and 20 medium-priority species. Also, Read et al. (2009) suggested a number of emerging species as a means of adapting British forests to climate change, including 15 broadleaves and 10 conifers, as shown in Table 1. Accounts of most of the species in Table 1 have been given in a series of ­papers in the QJF since 2015 by Jinks, Mason, Savill and Scott (see http://www. rfs.org.uk/learning/forestry-knowledge-hub/species-profiles-project/). Table 1.  ‘Emerging species’ suggested by Read et al. (2009) for possible use as the climate changes. Broadleaved species

Conifers

Acer monospessulanum Acer opalus Alnus cordata Castanea sativa Celtis australis Fagus orientalis Fraxinus angustifolia Juglans regia Ostrya carpinifolia Platanus orientalis Populus alba Quercus faginea Quercus ilex Quercus pyrenacia Quercus pubescens

Abies alba Abies borisii-regis Abies cephalonica Abies cilicia Abies pinsapo Picea omorika Pinus brutia Pinus pinaster Pinus peuce Pinus pinea

Introduction

7

Effects of Climate Change John Weir of Forest Research advocates that at least a proportion of trees planted in future should be grown from seed collected from 2° to 5° south of the proposed planting site. He commented on: The opportunity to embed resilience principles within new woodland mixes, using a diverse range of species and assisting the migration of genes within tree species by planting seeds sourced from 2° to 5° south – as first suggested by Forest Research provenance trials. (Weir, 2017)

He also recommended delaying planting rather than using seed sources from Eastern Europe (John Weir, pers. comm., 2016). Hemery and Simblet (2014) suggested a different approach for ensuring resilience in the face of uncertainty in future forests. They proposed planting one-third native material (with seed collected regionally or ­locally), one-third non-native material of the same species and one-third alternative species. Read et al. (2009) stated that the warming climate may permit the wider use of species that were previously not reliably cold-hardy within the British Isles. They said that establishing fast-growing hardwood species is one of the most cost-effective means of sequestering atmospheric carbon. In 2009 a series of trials was established across England, testing the growth and survival of a range of potential species for short-­rotation forestry (McKay, 2011). This was done in support of a statement by Read et al. (2009) that ‘systematic trials of potential new species are urgently needed to provide the knowledge base to underpin future planting programmes’. The exceptionally cold winters of 2009–10 and 2010–11 caused considerable damage to eucalypts in many of these trials, particularly in inland and northern areas. Most of the areas of Eucalyptus nitens planted in 2009 were killed. There are two important parameters of climate that cannot be changed: • The southern tip of the UK is at latitude 50° N so many sites from which exotics might be introduced will have significantly more winter sunshine than UK planting sites. This is important for trees from Mediterranean climates, which are mostly evergreen and which grow in winter when rainfall is highest. • The other climatic parameter which is difficult to match is the lack of ground-penetrating frosts of long duration; these are absent in many warmer regions.

Composition of British Forests in the 2000s The relative importance of broadleaved trees and conifers can be gauged by the consumption of timbers in the UK, and by the extent of areas planted

8

Introduction

with different species. By far the greatest area of forest, 33%, is occupied by Sitka spruce and Scots pine, and the most common broadleaved species, at 8%, are the two native oaks (Table 2). It is difficult to obtain completely reliable figures for timber consumption in the UK, but estimates by the Timber Trade Federation (TTF, 2009) and conversations between the author and those in the industry indicated that about 90% was imported and 10% home-produced, and that 64% of all timber used was coniferous. In terms of the forest areas existing in Great Britain in 2011, almost exactly half was coniferous and half broadleaved (see Table 2), and amounted to 2.6 million ha in all, or 13% of the land area of Great Britain. Much of the broadleaved woodland is neglected, unmanaged and not even approaching its potential in terms of sustainable production (LINK, 2009; Forestry Commission, 2018a).

Developments in British Forestry Since 1990 The period of just over 25 years between the first and third editions of this book has been one of unprecedented changes in British and world forestry, Table 2.  Area of high forest (in thousands of hectares) by principal species based on the rolling National Inventory, from 2009 (Forestry Commission, 2018a). Species

Area (000 ha)

% of all high forest

Conifers Sitka spruce Scots pine Larches Lodgepole pine Norway spruce Corsican pine Douglas fir Other conifers

1307.5 664.6 218.2 126.0 99.8 60.6 45.6 45.6 39.6

47.6 25.1 8.2 4.8 3.8 2.3 1.7 1.7 1.5

Broadleaves Ash Birches Oak Sycamore Beech Hazel Hawthorn Willow Alder Sweet chestnut Other broadleaves

1336.6 1578.0 235.5 219.1 106.3 93.7 86.8 72.6 65.2 57.6 28.9 211.9

50.5 5.9 8.9 8.3 4.0 3.5 3.3 2.7 2.5 2.2 1.1 8.0

All species Clear felled

2646.8 96.5

100.0

Introduction

9

some of which affect species use. There have been substantial developments in the forestry profession. We have become aware of the greenhouse effect; geographic information systems (GIS) have come into everyday use; and multipurpose trees, agroforestry, social forestry and urban forestry, though not new, have all become subjects worthy of special attention. Sustainable forest management (SFM), together with ecosystem services, certification and carbon sequestration, are now familiar to everyone. Since the introduction of the government’s ‘Broadleaves Policy’ in 1985 there has been an increasing emphasis on multipurpose management of forests, and a growing pressure to diversify even-aged monocultures in terms of both species composition and structure. Sitka spruce monocultures have been the focus for most of the work into how this ‘continuous-cover’ forestry might best be achieved (e.g. Malcolm et al., 2001; Mason et al., 2004), though other species have also received attention (e.g. Kerr, 2002; Davies et al., 2008; Helliwell and Wilson, 2012). Foresters have been required ­increasingly to act as ecosystem managers rather than simply as growers of wood. This can be difficult in the many cases where the forests were established with the single objective of producing wood, because now they are expected and required to deliver wider benefits. In addition, many foresters were not trained in the type of forest management that is required today. The effects of the removal in 1988 of the generous tax allowances that had driven most of the afforestation, and which had resulted in a more than doubling of the area of forest in Great Britain during the 20th century, became evident in the following decades. Afforestation rates plummeted from almost 30,000 ha in 1988 to 16,300 ha in 1998, and 7000 ha in 2008 and 9000 ha in 2018. At the start of the period, field meetings of foresters were usually devoted to discussing silvicultural and economic issues. This changed rapidly to discussions and demonstrations of the most recent pond, glade, trail or coppiced area. Forestry became dominated by social scientists whose aim was to promote public participation. They achieved a great measure of success, and the Forestry Commission’s public image improved enormously (Leslie, 2014).

Attitudes to Non-native Species Great Britain is full of introductions. The landscape is largely a cultural one, which is a mixture of native species, ancient introductions and more recent arrivals. Modern agriculture, forestry and horticulture all largely depend on the introduced species (Kirby, 2012). Most people, of course, accept and understand this, but a few extreme conservationists dislike, and hope to eliminate, anything that that is not native (i.e. that arrived in Britain without help from humans). Lowland British woodlands suffered from this attitude in the 1990s, when attempts were made to remove exotic conifers (or, as many environmentalists refer to them, ‘alien’ conifers). The millions of pounds of

10

Introduction

public money that were spent on removing them before maturity probably ­exceeded the millions that had been spent on establishing them in the first place. The ‘restoration to broadleaves’ sometimes meant clearing the coniferous crop and then leaving the site to regenerate naturally, having collected the grant first, in the mistaken belief that ancient woodland species have long-lived seed, and will thus rise, phoenix-like, from the soil once the conifers are removed. More often than not the restored areas became covered with unproductive ‘scrub’ rather than woodland. Pryor et al. (2002) have been monitoring woods owned by the Woodland Trust since 2001 to determine the best ways of restoring ancient woodland, as a guide for managers and policy makers.

Profitability of Forestry The ‘restoration to broadleaves’ was popular with politicians and officials, who declared that SFM, under their guidance, was progressing from one triumph to the next (e.g. Forestry Commission, 2011). The reality, a­ ccording to the LINK report of 2009, was that English forests, at least, were undermanaged and deteriorating. Thinning and pest control had been ignored for years, and consequently the very wildlife values that were being so publicly lauded were becoming seriously impoverished. Woodland plants, butterflies and birds have all declined significantly in variety and number. The reason for this has been graphically illustrated by Nicholls (2006), Nicholls et al. (2013) and Hemery et al. (2018) in a series of survey reports on the profitability of forestry between 1963 and 2018. They concluded that ‘There has been a deterioration of the financial performance of many woodlands since the 1960s to the point where management has been reduced or even suspended’. In the most recent r­ eport, Hemery et al. (2018) stated that the trend of financial loss continues, though there was possibly a slight reduction in the number of people experiencing losses and a correspondingly slightly larger number (of the small minority) ­experiencing profit.

Pests and Diseases In addition, largely because of the speed and frequency of international travel (and in some cases ineffective, or virtually non-existent, controls on the movement of plant material), a number of tree diseases and pests have been introduced, most with potentially devastating effects. They i­nclude red band needle blight of pines, ramorum disease of Japanese larch, acute oak decline, oak processionary moth and the breakdown of resistance to Melampsora rusts by most clones of poplars that were in use. Most ­recently, in 2012, chestnut blight was introduced from France and the ash

Introduction

11

dieback disease, caused by the fungus Hymenoscyphus fraxineus, from The Netherlands. In fact, a large and apparently increasing number of indigenous and traditionally used species are under threat, and there is a need to find others to replace them where possible.

New Uses for Timber There have been several developments in the use of wood. These include: • Glulam, an engineered wood product composed of small pieces of wood that are bonded together with a durable adhesive into large, strong beams or columns. Curved shapes can be made from glulam, which provides huge flexibility in design. It offers a means for using small-dimensioned timber that might previously have gone to much inferior uses, or even be classed as waste. • Laminated veneer lumber (LVL) (also known as Kerto), which is similar to plywood and is used in all types of construction projects, from new buildings to renovations and repairs. It is very strong and dimensionally stable, and derives its high strength from the homogeneous bonded structure. Kerto is produced from 3 mm-thick, rotary-peeled softwood veneers that are glued together to form a continuous billet. The billet is cut to length and sawn into LVL beams, planks or panels according to requirements. • Thermally modified wood, which has been altered by a controlled pyrolysis process where the wood is heated to over 180°C in the absence of oxygen. This induces changes to the chemical structures of the cell wall components (cellulose, hemicellulose and lignin) to boost its durability. • Its antimicrobial value. Research, mostly in the USA, has shown that wood in general has good antimicrobial properties, usually much better than plastics or stainless steel. This makes it particularly suitable for contact with food and for food preparation, such as chopping boards (Park and Cliver, 1997). • ‘Densified’ wood. Synthetic structural materials with exceptional mechanical performance suffer either from great weight and adverse ­environmental impact (for example, steels and alloys) or complex manufacturing processes and thus high cost (for example, polymer-­ based and biomimetic composites). Natural wood is a low-cost and abundant material and has been used for millennia as a ­structural material for building and furniture construction. However, the mechanical performance of natural wood (its strength and toughness) is unsatisfactory for many advanced engineering structures and ­applications. According to an article in the Scientific American based

12

Introduction

on work at the University of Maryland (Perkins, 2018), densified wood is three times as dense as the untreated substance. It can become about 50 times more resistant to compression and almost 20 times as stiff. It is also substantially harder, more scratch resistant and impact ­resistant. It  can be moulded into almost any shape. Perhaps most ­importantly, the densified wood is also moisture resistant. A fivelayer, plywood-like sandwich of densified wood can stop simulated bullets fired into the material, which could lead to the development of low-cost armour. It can also be made transparent. It seems very likely that there will be considerable advances in the use of densified wood in the near future.

Biomass Growing biomass specifically for the production of renewable energy is a development of a traditional technology that is now receiving the backing of many governments. The EU’s 2009 Renewable Energy Directive set a target for the UK to achieve 15% of its energy consumption from renewable sources by 2020. This compares to 3% in 2009. Woody biomass, often converted into wood chips, can be used in power stations or for institutional or domestic heating such as schools and hospitals. Because of its ­dimensions or shape, a relatively high proportion – often 60% or even more – of all timber from woodlands is suitable only for fuel or other low-value uses, however valuable the stemwood may have been. This can provide part of the supply. Quite commonly, special biomass plantations are also created, mostly with poplars and willows grown on short coppice ­rotations. Once established, these can produce around 10 dry tonnes (t) ha−1 year−1 compared with about 4 dry t ha−1 year−1 from conventional coppice such as hazel and oak. Various pharmaceutical and other products derived from wood are being developed (MacKay et al., 2009), most notably from lignin removed from wood in papermaking. Formerly, this was a major pollutant to waterways around pulp mills.

Other Developments Paradoxically, although the period from 1991 was one when the value of forests globally became more widely appreciated than in any other period of history, it was also one of rapidly diminishing support for British ­forestry. Several universities that had taught forestry since the early 1900s closed their undergraduate and graduate courses because of declining ­applications and funding for courses. Only Bangor still runs comprehensive Forestry degree courses.

Introduction

13

Devolution occurred in Scotland and Wales, and with this the Forestry Commission in each country went its own way. In Wales it vanished in 2013 after incorporation into Natural Resources Wales. In England more than one attempt was made by politicians to dispose completely of the publicly owned forest estate. The plans were thwarted by the strength of public opinion against the proposals (Leslie, 2014). More recently, timber prices have increased very appreciably with China’s entry into the market in a major way, and the promotion of wood fuel has meant that demand for firewood and its price have also increased significantly, allowing woodland management and thinning of young broadleaved plantations to show some financial return for the first time in many years. There appears to be a revival of forestry teaching, at least at Oxford University, with the endowment (by Sir Martin and Lady Wood) and appointment of a new Professor of Forest Science. With the shortage of funds since 2008, state forestry in the UK has suffered quite seriously as financial cuts have been made. The Forestry Commission has lost many staff and, as a result, is very limited in what it can do. At the time of writing, in 2018, charities such as the Woodland Trust provide most of the impetus and much of the funding for new schemes. They certainly get most of the credit for new initiatives. Today there is a recognition that, in forestry, timber production must be balanced with care for the environment, wildlife, landscape, heritage, local employment and recreation. Where the population is largely urban, rural areas lose out. Urban people see the countryside simply as a place for entertainment and leisure, and do not recognize the wider social and economic diversity. In England today there is a new and powerful alliance between the Forestry Commission and private sector. Such things as concerts, mountain biking and ‘Go Ape!’ have proved immensely popular with teenagers and young adults. The latter two activities also provide much appreciated adrenalin fixes. Leslie (2014) believes that there are two obvious, big, directions for forestry in England: the first is around towns and cities, and the second is related to the low-carbon economy and climate change. These will be the challenges for the future.

This Book This book aims to provide a guide that can be used when selecting species and managing trees. The requirements of individual species are described for areas in which they are likely to do well. No particular consideration has been given to the relative economics of the different species: the main emphasis is upon the biological suitability of species to sites. It is assumed

14

Introduction

that the reader is reasonably well informed about the principles of forestry practice. About 35 species of trees are native to Britain but, according to Mitchell (1974), over 500 can easily be encountered by anyone looking in parks and gardens. If special collections in botanic gardens are included, the number rises to about 1700. The choice of the 42 genera and some 73 species i­ ncluded here has, therefore, (inevitably) been arbitrary. Because of the current interest in and emphasis on conservation, almost all native species that grow to reasonable-sized trees have been included, even though some – such as box, rowan and the wild service tree – will never be anything but minor species. Among exotics, all the commonly planted trees are ­included and, because climate change might make others desirable, many of those suggested in the Read Report on combating climate change (Read et al., 2009) are included. Among those deserving more serious consideration than they have received in the past are Cryptomeria japonica, Sequoia sempervirens, Pinus peuce, Thuja plicata, walnuts and red oak. A few minor native species have been added since the second edition of the book, ­including hawthorn, the true service tree and rowan. This book does not provide detailed botanical descriptions of trees, nor has there been any attempt to cover the means by which pests and diseases are controlled. The more serious ones are simply noted as potential risks. The remainder of the text provides information about the origin and introduction (where applicable) of each species, climatic and soil requirements, other silvicultural characteristics, diseases, natural regeneration, provenance, seed production, nursery treatment, yield and timber characteristics. Information about the ages at which trees first produce seeds should be treated with prudence. It is reasonably accurate for trees in even-aged plantations or groups at fairly close spacings. Widely spaced trees often produce seeds earlier than the times quoted. The book concludes with a simple key for identifying the trees most likely to be encountered in woods. It has been tested on generations of undergraduate and graduate students and has proved successful and easy to use. Species are arranged alphabetically through the text, by scientific names. In the line drawings a vertical line indicates a scale of 3 cm. It should be noted that it was not always possible to obtain mature specimens of fruits or cones to draw.

ABIES Mill.

Silver firs

Firs (Abies) are a genus of evergreen coniferous trees in the family Pinaceae (Farjon, 2017). Most of the well-known conifers of commercial importance – including cedars, firs, hemlocks, larches, pines and spruces – are in this family. There are about 49 species of Abies, most of which occur in temperate regions. The main areas of occurrence are western North America (including the Pacific Northwest), eastern North America (e.g. the Appalachians), Central America, western Eurasia (including the Mediterranean Basin), the Himalaya and allied montane regions and the Pacific Northeast of Asia. Common features of many, but not all Abies species, are that: • they are strong shade bearers and so have a potential role in continuous-cover silviculture; • most grow naturally in mixed-species stands; • most are remarkably winter cold hardy; and • several species grow well on calcareous soils, an unusual attribute among conifers. The form of Abies trees differs from most genera of Pinaceae in its exceptional uniformity. The trees typically possess a single straight stem with regularly spaced branch whorls produced at the rate of one whorl per year. The bark is generally smooth and thin on young trees, frequently bearing resin blisters. In older trees it is often thicker and furrowed or flaking in plates. In most Abies species the bark provides little protection against fire, although a few species are partially fire-tolerant. Because of their shade tolerance many silver firs feature prominently in the later stages of ecological successions in their natural habitats. Cones are borne on 1-year-old twigs, and mature in one season. They are erect, ovoid to cylindrical, generally resinous and dehiscent (i.e. they fall apart on maturity); the cone axis persists as an erect ‘spike’ on the branch. The wood lacks resin ducts. Only three silver firs are grown as forest trees in Britain.

ABIES ALBA Mill.

European silver fir

Origin and introduction The European silver fir occurs naturally, scattered through the mountains of central, southern and eastern Europe, from the Pyrenees, through the Alps and Apennines to the Carpathians and Balkans, with outlying © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

15

16

ABIES ALBA Mill.

lowland stands in France (Normandy) and eastern Poland. It is especially prominent in the Vosges, Jura, Black Forest and northern Bavaria. It was introduced to Britain in about 1603. Climatic requirements This species is said to be winter cold hardy to around −30°C (Gymnosperm Database, 2018). High humidity and not too high a maximum summer temperature are required for successful growth. European silver fir is sensitive to late spring frosts and, like most shade-bearers, ideally needs overhead cover when young. Frost-prone flat areas and hollows should be avoided. It does not tolerate exposure well, or atmospheric pollution. Site requirements The European silver fir grows well on most soils that do not dry out including heavy clays, but waterlogged soils, including peats and dry, infertile sands and gravels are best avoided. It does particularly well on fertile calcareous soils, in the west and south-west of Britain at moderate elevations and on north-facing aspects. Other silvicultural characteristics The European silver fir can become a large tree, up to 50 m tall (see Table 3, p. 26), and is recorded by Mabberley (2008) as being the tallest European tree. It self-prunes slowly and is inclined to produce heavy branches unless it is discouraged by close planting or pruning. The species is a very important constituent of many continental European selection forests, where spruce regeneration tends to occur under the European silver fir and vice versa. On good sites, advance regeneration can be both prolific and persistent. Large ‘seedling banks’ are formed, awaiting an overhead gap in the canopy before growing. Young seedlings are very palatable to deer. Early growth is slow, like that of many silver firs, but it is a considerable volume producer once established, after 8–10 years. It is said to be a much deeper-rooting tree than Norway spruce and therefore more stable, especially on heavy soils. Evidence of this was recently seen by the Wessex Silvicultural Group at Stavordale Estate (Wiltshire) where individual fir stems had withstood the early 2014 gales and the surrounding Norway spruce had been significantly damaged. It was introduced to Britain around 1600 and over the following three centuries was deployed as a component of mixed woodlands on private estates throughout the UK. Objectives of establishment and ­silviculture

ABIES ALBA Mill.

17

combined botanical curiosity, landscape amenity and estate timber production. During the period 1880–1910, some more extensive industrial timber plantations were established, for example in west Argyll. However, increasing incidence of the woolly aphid, Adelges nordmanniana, led to its progressive abandonment, such that by the mid-1950s it was no longer recommended for planting (Varty, 1956; Macdonald et al., 1957). Much of the existing stock is in Scotland, but large individual specimens can be found almost everywhere dating from the period prior to 1910, where these escaped subsequent fellings during the Second World War. The current growing stock of younger material is very limited, largely comprising trial plots from the period 1930–1970 in Wales and western Scotland. Recent years have seen renewed interest. There are no modern British yield class (YC) tables specifically for Abies alba due to lack of suitable mensuration plots, but based on recommendations by Hamilton and Christie (1971) that the tables for Abies procera be used, the potential productivity ranges from 12 to 22 m3 ha−1 year−1. Ages of maximum mean annual increment would occur between 64 years (YC22) and 73 years (YC12), and mean diameters at these ages are between 28 and 40 cm. Kerr et al. (2015) reported productivity varying between 10 and 22 m3 ha−1 year−1 after 46 years for a range of provenances planted in experiments at Thetford, Radnor and Benmore. Pests and diseases Since 1910 the European silver fir has suffered badly from attacks by an aphid, Adelges nordmanniana (Wilson, 2010), which causes defoliation, dieback and ultimately death. This is the main reason that it is not planted very much today. The relationship between site and severity of attack by A. nordmanniana is not fully understood, but stands on suitable soils in cool, moist climates recover better from attack than do those on poorer soils and in warm, dry areas (Varty, 1956). The severity of outbreaks in mixed forestry is known to be much less serious than in extensive monospecific plantations (Kerr, 1999). European silver fir is less susceptible to attack by fomes (Heterobasidion annosum) than many other conifers. Neonectria canker of Abies is an emerging disease caused by the fungus Neonectria neomacrospora. It causes severe cankers leading to crown dieback. Although the disease was first noticed in Germany more than 100 years ago, it was only in 2008 when the high damage levels were first reported in Norway and in Denmark in 2011. There were also sporadic reports of it in Britain in the 1960s and the 1990s. It appeared on different Abies species in 2015 when several cases were reported to Forest Research. There are no effective control measures against N. neomacrospora in forest stands (news from Forest Research, July 2017).

18

ABIES ALBA Mill.

Natural regeneration On good sites natural regeneration can be both prolific and persistent. Large ‘seedling banks’ are formed, if not eaten by deer. They await an overhead gap in the canopy before growing. However, European silver fir is the most heavily browsed of the commercially important tree species in montane forests of central and south-eastern Europe (Senn and Suter, 2003). Flowering, seed production and nursery conditions The tree flowers from May to mid-June; seeds ripen between mid-­ September and mid-October. The earliest age at which the tree bears seeds is between 25 and 30 years; maximum production is between 40 and 60 years. There are commonly 2 or 3 years between good seed crops. There are about 22,500 seeds kg−1 (range 17,400–41,000). If seed is required for sowing in a nursery, it should be collected as soon as it is ripe, before the cones disintegrate. For the production of bare-rooted seedlings, it is normal to stratify (or pre-chill) the seed for 6–12 weeks, prior to sowing in March. Unstratified seed should be sown in January or February (Aldhous, 1972). Gosling (2007) stated that Abies seeds exhibit shallow or intermediate dormancy. They can be stored at 9–12% moisture content at < 5ºC. On current evidence (Gosling, 2007), Abies seeds are less tolerant of drying and are shorter-lived than the majority of seeds with orthodox storage characteristics. They are sensitive to desiccation, but not as much as recalcitrant seeds, or as highly perishable. A difficult problem for nurseries is that the seeds are quite often mixed with scales and other material, because of the way the cones break up on the trees. The seeds exhibit the usual features of shallow dormancy in that some untreated seeds will germinate, but pre-treatment will usually stimulate quicker germination of more seeds, over a wider range of conditions. When grown in bare-root nurseries, European silver fir can be slow to develop in the seedbed, requiring 1–2 years before ‘taking off’. A longer nursery residence will therefore be required than in more familiar pines and spruces. It is possible that usable 2-year-old seedlings could be obtained from containerized production. Provenance According to Lines (1978b), the tallest sources after 10 years’ growth in a comprehensive series of provenance trials came from such divergent places as Calabria (Italy), Czechoslovakia and the Swiss Jura. The current Forest Research website (2018c) says much the same: seed sources from the Czech Republic and nearby areas in central Europe should be favoured. More recent analysis of the same series of provenance trials

ABIES ALBA Mill.

19

s­ uggests that Calabrian provenances from > 100 m in southern Italy perform best in British conditions (Kerr et al., 2015). However, these authors note that there was relatively little provenance variation, with origins grouping within 80–120% of trial mean height, so that good seed sources within the natural range should be an acceptable alternative. Timber and uses A. alba is highly regarded in continental Europe as a general construction timber. The white wood is similar to that of Norway spruce, though less resinous, and in continental Europe it is used for the same purposes (general construction and paper). The average density is about 400 kg m−3 at 15% moisture content though it can be up to 480 kg m−3 (Ramsay and Macdonald, 2013). It is not durable outdoors. A. alba is grown for timber in central and western Europe. It is grouped with Norway spruce by the timber trade, within the ‘European whitewood’ category. It often forms a minor component of mixed ­imported timber packs otherwise comprising Norway spruce. Traditionally, large-section European silver fir (e.g. 30 cm × 30 cm balks) were used for heavy construction framing in alpine districts of Germany, Austria and Switzerland. In classical Greece and Italy (Rome and, later, the Genoese and Venetian maritime territories), montane fir supplies from the Apennines, Alps and Macedonia were sought for naval construction because of their high strength-to-weight ratio (Thirgood, 1981; Meiggs, 1982). This mirrors the modern values attributed to Sitka spruce (e.g. for aircraft framing). More recently, silver fir timber has been used as an internal component of the massive and cross-laminated timber elements found in novel engineered-timber construction systems, notably in Austria (Wilson, 2011). There is a limited record of processing European silver fir timber in Britain, though large-section material from the Victorian and Edwardian plantations of Argyll was used by shipbuilders on Clydeside as keel blocks (Elwes and Henry, 1906–1913), with similar applications occasionally arising in the North Sea oil sector. Traditionally, home-grown fir timber was treated and sawn for non-market ‘estate uses’ and was seen as better than grand and noble firs for these applications. The European silver fir makes a good Christmas tree, though it has now largely been superseded for this purpose by other species of silver fir, especially A. nordmanniana (see below). Its resin is the source of Strasburg turpentine, which is used in many bath products to give them a ‘pine’ scent. Place of European silver fir in British forestry The European silver fir was widely planted in Victorian and Edwardian times but is used very little in Britain today because, at least in pure

20

ABIES GRANDIS (D. Don) Lindl.

even-aged plantations, it is badly attacked by Adelges nordmanniana. Even if genotypes resistant to it could be found, it is still unlikely to be widely used because it is a species that will become less suited to Britain, especially the south, if climate changes proceed as predicted. In the north it might fulfil a valuable role as a shade-bearer, as a minor component of mixed-age and mixed-species woodlands.

ABIES GRANDIS (D. Don) Lindl.

Grand fir

Origin and introduction Grand fir is native to the Pacific coast: Washington, Oregon and north-­ western California, and to the coast of southern British Columbia, including Vancouver Island, Canada. It will grow to heights of 40–70 m in its native range. Brown and Nisbet (1894) referred to it as the ‘Great Californian Silver Fir ’ and said it was ‘really worthy of its botanical name, for it is a tree of magnificent growth’. Separate populations exist further east, in western Montana and northern and western Idaho, USA (Steinhoff, 1978). The species achieves its best development in the Olympic Peninsula of Washington State. It is one of several north-western American species discovered by David Douglas, who sent seeds to Britain in 1831. Climatic requirements In its natural range the tree does best where conditions are cool and moist. It grows well where rainfall is as low as 750 mm but responds to higher levels. The best stands in Britain occur in the wetter parts of the country where precipitation is 1000 mm or more in the north and 1150 mm in the south (Macdonald et al., 1957). It is not as demanding as Sitka spruce. Temperatures in Britain are not limiting: it performs as well in the north as in the south, but the tree is susceptible to damage by late spring frosts, so hollows should be avoided. Grand fir is sensitive to exposure, unlike noble fir, and should only be planted at low elevations; it will not thrive in areas of high atmospheric pollution or exposure to sea winds. Site requirements Though requirements are not excessive, grand fir does best on well-drained, moist, light and deep soils of at least moderate fertility. It should be used cautiously on heavy and infertile soils and will not thrive on calcareous soils. On drought-prone soils it may suffer from drought crack. It appears to root much more deeply than many other conifers (Carey and Barry, 1975).

ABIES GRANDIS (D. Don) Lindl.

(A)

21

(B)

(C)

Fig. 1.  (A) European silver fir, Abies alba; (B) Grand fir, Abies grandis; (C) Noble fir, Abies procera.

Other silvicultural characteristics The most important silvicultural characteristic of grand fir is its strong shade tolerance, especially when young, and consequently it is valued for underplanting and in continuous-cover systems. It usually has a good, straight stem, though it often tapers markedly. Older trees suffer crown damage on exposed sites and the species is moderately susceptible to windthrow and windbreak. It will grow to over 50 m tall in Britain. Grand fir is more resistant to Heterobasidion annosum than most conifers but prone to damage and associated wood deterioration due to a syndrome linked with drought, exposure and attack by Adelges piceae (Aldhous and Low, 1974), so it is probably safest in the western half of

22

ABIES GRANDIS (D. Don) Lindl.

Britain where there is less risk of drought and frost. The litter, like that of Douglas fir, breaks down quickly. In common with all silver firs, early growth is slow though later it can become very fast, averaging 1 m in height a year over the first 40 years. The treatment of the seed and sowing times in nurseries are the same as for the European silver fir. Flowering, seed production and nursery conditions The tree flowers in May, and seeds ripen in August and September; they should be collected immediately, before the cones disintegrate. Grand fir is a rather poor seed producer in Britain. The earliest age at which it might bear seed plentifully is between 40 and 45 years. There are commonly 3–5 years between good seed crops. There are about 40,600 seeds kg−1 (range 26,200−63,500), of which 20,000 are normally viable. Provenance In Britain there appears to be a latitudinal cline of increasing vigour in a southerly direction among provenances from the coasts of Canada and the USA. Inland origins grow too slowly to be of much use (Lines, 1978c). Limited Forestry Commission experiments have shown differences of ten YCs between the most and least vigorous provenances. Many existing stands are thought to originate from sources at the poorer end of the range (Lines, 1978a). The best growth after 6 years has been found in provenances from the Olympic Peninsula in Washington (Lines, 1986, 1987). Yield and timber On suitable sites grand fir can be one of the most productive trees in Britain. YCs in excess of 30 m3 ha−1 year−1 have been recorded, and on many sites it will out-produce most other conifers. In fact, its rapid growth in even-aged systems of silviculture and drought cracking has led, unjustly, to its timber having a particularly poor reputation among British sawmillers and there are few existing markets for it. The timber is perceived by most as being much inferior to that of the European silver fir. It is, therefore, not widely planted, though if it is used more in uneven-aged systems where growth would be slower, its reputation could improve. The average density of the wood is about 450 kg m−3 at 15% moisture content. In north-western America it is used as a general construction timber and is commercially valuable even though it is weaker and more prone to decay than many other species.

ABIES PROCERA Rehder

23

Place of grand fir in British forestry The most important attribute of grand fir is its shade-bearing qualities, which give it an important potential place in continuous-cover silvicultural systems. Though it can be productive on suitable sites, other species are usually favoured, and it is therefore likely to remain a tree of minor importance.

ABIES PROCERA Rehder

Noble fir

Origin and introduction The natural distribution is more restricted than those of many other north-western American species, being confined to the Pacific coast from northern Washington to northern California, mostly at elevations between 1000 and 1500 m (Franklin et al., 1978). Like grand fir, it was first found by the Scottish botanist and explorer David Douglas, and was introduced by him to Britain in 1831. Climatic requirements In its natural range noble fir does best where there is a cool, short growing season with abundant precipitation, mostly as snow. There are no well-­ defined climatic limits in Britain, but it grows best in the north and west, where temperatures and evaporation rates are rather low and precipitation high. The tree has a high tolerance of exposure and does not become deformed. This valuable characteristic gives it a potentially useful place in the uplands. Noble fir is less moisture-demanding than Sitka spruce, which makes it suitable for well-drained, occasionally dry, exposed sites where Sitka spruce will not grow. It is less frost-tender than other silver firs but can be damaged by late spring frosts. Like most species of Abies, it will not tolerate atmospheric pollution or salty sea winds. Site requirements In its native habitat noble fir is a pioneer species that regenerates after major disturbances such as fires. It does not need high fertility and will thrive on most neutral or acidic soils, provided adequate moisture is available, though it will not withstand waterlogging. It does best on welldrained deep, moist soils, but grows quite well on boulder clays and some peats, but not in the presence of heather. Very dry sites should be avoided as it is susceptible to drought crack.

24

ABIES PROCERA Rehder

Other silvicultural characteristics In north-western America noble fir will grow up to 70 m tall or more. Early growth is always slow, necessitating prolonged weeding and making the species vulnerable to damage from browsing animals. It is normally wind-firm. The tree is relatively resistant to H. annosum. It can be used for underplanting and in continuous-cover silvicultural systems, but it is not as tolerant of shade as grand fir or hemlock, and needs freeing quite early. Flowering, seed production and nursery conditions The tree normally flowers in June, and seed production is better than the other species of Abies in Britain. Seeds ripen from mid- to late September and are dispersed early in October. Cones should be collected in late August or early September, because after this the scales loosen prior to the disintegration of the cone and all the seed can be lost. The earliest age at which the tree bears a good seed crop is between 30 and 35 years, but the best ones are usually between the ages of 40 and 60 years. There are commonly 2–4 years between good seed crops. There are about 29,800 seeds kg−1 (range 20,300−42,100), of which only 12,000 are normally viable. The treatment of the seed and sowing times in nurseries are the same as for the European silver fir. Provenance The most promising source appears to be Larch Mountain, Oregon, USA, at 75 m, which lies east of Portland (Lines, 1987), and provenances from the Washington or northern Oregon Cascade mountains or from good-quality British stands are also suitable. Timber and uses The timber of noble fir is regarded in the USA as one of the best of the true firs and it is used for general structural purposes and joinery. It was once used for aeroplanes because its wood is light, strong and can be bent before it breaks (ISI, 2011). In the USA noble fir is regarded as suitable for general structural purposes and joinery. Its wood has always been valued over that of other true firs because of its greater strength. It has been used in the USA for specialty products, such as stock for ladder rails (Burns and Honkala, 1990). In Britain, by contrast, it has a reputation for having poor strength properties, poor durability and a low density (about 420 kg m−3 at 15% moisture content). It is said to be suitable for general interior joinery and light structural work (Patterson, 1988).

Other European and Near-eastern Species of Silver Fir

25

It is a species of great ornamental value and is popular as a Christmas tree. It is grown profitably in parts of Europe specifically for its foliage, and varieties with blue needles are preferred for this. Place of noble fir in British forestry Noble fir will thrive, without becoming deformed, on exposed dry sites at high elevations that are unsuitable for Sitka spruce. This is probably its main place in British forestry. There is little case for large-scale planting because of the slow early growth. Many people believe it could become a much more useful tree if the density of its wood could be increased, by selection and breeding.

Other European and Near-eastern Species of Silver Fir that might be Useful in a Changing Climate As climate change proceeds, various other silver firs might come to have a place in British forestry. Prominent among the candidates of potential interest are a number of European species. There seems to have been a certain amount of ‘botanical nationalism’ (Farjon, 1998) with some of the Abies species, in that many phenotypic varieties, subspecies and sometimes even species have been described relating to particular geographical regions (notably within A. nordmanniana) (see Coode and Cullen, 1965; Liu, 1971). A more recent genetic investigation by Knees (2011) has indicated that, while there are identifiable morphological differences between the western Eurasian Abies populations, these may in fact represent a single super-species with persistent regional variants responding to climatic and soil conditions. At least eight, and arguably up to 12 Abies spp. are found west of the Urals, including the far north-west of Africa (Liu, 1971; Knees, 2011). Two occur in inland montane regions of central Europe (A. alba) and the Caucasus (A. nordmanniana) which enjoy a more temperate or alpine climate. All of the other species are restricted to montane areas surrounding the Mediterranean Basin, with a warm, dry regional climate tempered by altitude. The coniferous forests of the Mediterranean Basin in particular have been subjected to severe over-exploitation by human societies since ancient times (Thirgood, 1981; Meiggs, 1982; Andersson, 2005). Some species have survived only in inaccessible montane refugia, but ranges of most species have been greatly reduced by the effects of clearance for agriculture, livestock pasturage, illegal felling and, in some cases, wildfire damage. The Abies species are no exception and some now exist only as relict species: two extreme cases being Sicilian fir (A. nebrodensis) and Algerian fir (A. numidica).

26

Other European and Near-eastern Species of Silver Fir

The five silver firs mentioned by Read et al. (2009) are covered here, these being A. alba (European silver fir – see above), A. borisii-regis (Bulgarian fir), A. cephalonica (Grecian fir), A. cilicica (Cilician fir, Syrian fir) and A. pinsapo (Spanish fir). Another very promising species is A. nordmanniana (Caucasian fir), which is also described below. A. equi-trojani (Trojan fir) is included here as a subspecies of A. nordmanniana. It incorporates A.  bornmuelleriana which is no longer regarded as a separate species. Wilson (2014) and Savill et al. (2016) reviewed the potential relevance of certain of the Mediterranean Abies spp. for forestry in the drier British lowland areas. Some details about them are shown in Table 3. Table 3.  Abies. Heights, diameters and locations of the ‘champion trees’ (the tallest of the Abies species) covered in this book in Britain and Ireland (from Johnson, 2003). Maximum recorded height (m)

Dbh (cm) Place

Species

English name

Abies alba

European silver fir

50 50

47 52

A. nordmanniana

Caucasian fir

32 48

52 36

A. nordmanniana subsp. equi-trojani (bornmuelleriana) A. cephalonica

Turkish fir

37

42

Grecian fir

35

73

41 41

A. pinsapo borisii-regis A. cilicica

Bulgarian fir

36

Cilician fir

33 26

A. pinsapo

Spanish fir

33

A. grandis A. procera

Grand fir Noble fir

19 61 50

Dbh, diameter at breast height; hyphen, no dbh given.

Thirlmere, Cumbria Armadale Castle, Highlands Endsleigh, Devon Cragside, Northumberland Fulmodeston Severals, Norfolk

Culcreuch Castle Hotel, Stirling 41 Bicton Park, Devon 39 Tendring Hall, Suffolk (Macdonald et al., 1957) 49.3 Stonefield House, Argyll 24 Glenapp, Ayr 31 Speech House, Gloucester – Balmacaan, Highlands 45 Harestanes, Borders 62 Ardkinglas, Argyll 37 Ardkinglas, Argyll

ABIES CEPHALONICA Loudon

ABIES BORISII-REGIS Mattf.

27

Bulgarian fir

A. borisii-regis is native to Bulgaria, northern Greece, Albania and the former Yugoslavia. It was named in honour of Tsar Boris III of Bulgaria (1894–1943, King – later dictator – of Bulgaria 1918–1943). This species is generally regarded as of stable hybrid origin, originating from ancient introgression between A. alba and A. cephalonica (Delcheva, 2010; Bella et al., 2015). It is found at elevations of 700–1700 m in the Pirin Mountains of south-western Bulgaria and to 1800 m in Greece, areas with typical rainfall exceeding 1000 mm. It is cold hardy to between −17.8 and −23.2°C (Gymnosperm Database, 2018), hence rather less so than A. alba or A. nordmanniana. In most other respects it appears to be very similar to A. alba. Matis (1988) produced a volume table for this species based on measurements from 3011 trees in Pertouli Forest, Greece. Within its native range it is generally regarded as superior to A. cephalonica as a timber tree, more similar to the valued Macedonian provenances of A. alba. It is not common in Britain and the few forest plots that exist have only been planted since 2010. In south-western Bulgaria, it grows mainly in mixture with Picea abies and Macedonian pine (Pinus peuce), whereas in northern Greece it can be found in mixture with Scots pine and beech. No specific details about nursery practices for this species are known, but those given on page 18 for A. alba are likely to apply.

ABIES CEPHALONICA Loudon

Grecian fir

A. cephalonica is native to Greece: Cephalonia, Euboea, Sterea Hellas and the Peloponnese, at 600–2100 m elevation, primarily over calcareous soils. The climate experienced is montane Mediterranean, with 750–1500 mm annual precipitation. The cold hardiness limit is between −23.3 and −28.8°C (Gymnosperm Database, 2018). In its natural habitat it grows best on moist, but not waterlogged soils in the cooler and moister parts of the country. It can form pure stands at higher elevations, but at lower elevations occurs together with Fagus orientalis, Quercus spp., Castanea sativa and P. nigra. Northern portions of its range show evidence of past (and perhaps ongoing) natural hybridization with A. alba (hybrids are known as A. borisii-regis – see above), which complicates the genetic conservation of this species (Farjon, 2010). It is described in its natural habitat as a broadly pyramidal tree reaching 30 m tall, 2.7–4.5 m in girth (86–143 cm diameter at breast height, dbh), with long horizontal branches. The species was exploited for timber (especially naval construction) in Greece in classical antiquity, but was always regarded as notably inferior to A. alba of Macedonian origins owing to its slower growth and heavier branching. Natural populations in Greece have been very heavily depleted by historical felling, clearance for agriculture, pastoral grazing pressure and many

28

ABIES CILICICA (Antoine & Kotschy) Carrière

recent incidences of deliberate and accidental wildfires. Dallimore and Jackson (1948) state that on Mount Enos in Cephalonia, there was once a forest of this fir 19–24 km in length and 58 km round but its area has since been much reduced by fires. It is extremely sensitive to damage by fires and regenerates very slowly, if at all, after one (Ganatsas et al., 2012). Outside its native range, A. cephalonica has found a place as a Mediterranean plantation tree, for example in southern France and Italy, but early budburst when young can cause frost damage and problems with establishment in more frost-prone regions (Aussenac, 2002; Carey, 2004). It was introduced to Britain as seed in 1824 by General Napier. There are a few trial plantations in Britain dating from the period 1930 to 1960, for example at Crarae and Kilmun (Argyll) and at Weston Common, near Alice Holt (Surrey). It has also been tried in mixed-species forestry on private estates, for example at Weasenham Woods in Norfolk. It is considered winter hardy over most of Britain but starts annual growth early, so is liable to late spring frost damage (Kent, 1900; McDonald et al., 1957). In milder British climates, such as the west coast of Scotland, the tree becomes established quickly and grows reasonably vigorously. In Britain, it is said to thrive in the driest regions (Johnson, 2003), though the biggest trees are mainly in the west. George et al. (2015) indicate that this species shows better recovery from drought than other Eurasian Abies spp. The timber form of the tree is usually significantly inferior to the more familiar Abies spp. in Britain, according to Macdonald et al. (1957). The species has a reputation for a coarse branching habit. No specific details about nursery practices for this species are known, but those given on page 18 for A. alba are likely to apply.

ABIES CILICICA (Antoine & Kotschy) Carrière Cilician fir or  Syrian fir A. cilicica is a tree native to montane areas of southern Turkey (Antitaurus), western Syria and northern Lebanon at 1000–2100 m elevation. It is winter cold hardy to between −23.3 and −28.8°C (Gymnosperm Database, 2018), but because bud burst occurs early, as in A. cephalonica, it is liable to late spring frost damage. It is generally a narrowly pyramidal, small to medium-sized tree reaching 25–35 m in height, to 75 cm dbh, with ascending branches. In natural conditions, it grows in mixed montane forests in association with the Cedar of Lebanon (Cedrus libani) and Italian cypress (Cupressus sempervirens). These forests generally experience a rather hotter, drier climate than those of northern Turkey, the Caucasus and the Balkans. According to Dallimore and Jackson (1948), A. cilicica has timber similar to that of A. nordmanniana and grows under similar conditions, though it is said not to be so susceptible to insect attacks, for example by Adelges nordmanniana. The timber was extensively harvested in antiquity for building

ABIES NORDMANNIANA (Steven) Spach

29

construction and naval purposes, where its long, straight lengths gave it a distinct role as compared to the true cedars. This exploitation, together with later heavy pastoral grazing pressure and harvesting for railway construction during the Ottoman period, led to severe depletion of the natural stands. Botanically described in 1853, according to Streets (1962), Abies cilicica has been planted successfully in the Troodos mountains of Cyprus on north-facing aspects between 1050 and 1675 m, starting growth slowly, but accelerating to 30 cm year−1 on better sites. No specific details about nursery practices for this species are known, but those given on page 18 for A. alba are likely to apply.

ABIES NORDMANNIANA (Steven) Spach

Caucasian fir

[including A. nordmanniana subsp. equi-trojani (Asch. & Sint. ex Boiss.) Coode & Cullen – Trojan fir, which includes A. nordmanniana subsp. bornmuelleriana Mattfeld – Turkish fir, which is now regarded as the same as A. nordmanniana subsp. equi-trojani.] A. nordmanniana is described in the Gymnosperm Database (2018) as an evergreen tree reaching 61 m tall, densely branched from the ground up, with branches regularly arranged. It was first botanically described in 1836 and introduced to Britain about 10 years later (Bean, 1929). According to the Gymnosperm Database (2018), the typical subspecies A. nordmanniana subsp. nordmanniana occurs in the western Caucasus (Abkhazia and Georgia) and in the Pontic mountains of north-eastern Turkey (e.g. Paphlagonia). It typically occurs in natural montane and sub-alpine forests, locally in pure stands, but more often in mixture with Picea orientalis, Pinus sylvestris, Fagus orientalis, Acer trautvetteri, Carpinus caucasica, Ulmus elliptica, Acer pseudoplatanus, Tilia caucasica, Taxus baccata and Rhododendron ponticum. These rich forests contain a number of tree species of potential relevance to silviculture in Britain under a warmer climate and their silvicultural dynamics are also of notable interest. Geographically separated and phenotypically distinct populations in western and northern Turkey have given rise to taxonomic disputes about this species (Gymnosperm Database, 2018). In central northern Turkey a clinal type, or subspecies, referred to as Abies nordmanniana subsp. bornmuelleriana, has been described, though not officially recognized as a distinct subspecies. It possibly exhibits greater drought tolerance at lower elevations. At the far western end of the range, in north-western Turkey, a second subspecies – A. nordmanniana subsp. equi-trojani – has been described in the Gymnosperm Database (2018) as a narrowly conical tree, 20–30 m tall, 1.8–4 m in girth, with a somewhat rounded crown, occurring at 700–2000 m asl on the northern slopes of high mountains (e.g. behind Troy) and on Ulu-Dagh in Bithynia. Subsp. equi-trojani grows in pure stands as isolated relict populations and is also reported to grow well

30

ABIES NORDMANNIANA (Steven) Spach

in silviculturally valuable natural mixtures with P. nigra. It is said to do best on calcareous soils (Farjon, 1990). Sometimes A. nordmanniana subsp. equi-trojani has been treated as a distinct species, especially by Turkish authors (e.g. Ata, 1989). There is some evidence that it represents a stable hybrid, resulting from ancient introgression with A. cephalonica, and hence it may be an eastern analogue of A. borisii-regis (A. alba × A. cephalonica) (Liu, 1971). Hybrid vigour has been reported, producing faster growth and better timber properties, leading to silvicultural interest in this species in Turkey (Ata, 1982). Climate and site requirements A. nordmanniana is native to high mountains around the Black Sea at 900–2100 m asl, typically on cool, steep, north-facing slopes. The climate is temperate and moist, with annual precipitation of 1000–3000 mm. The species has a cold hardiness limit between −28.9 and −34.3°C (Gymnosperm Database, 2018). A. nordmanniana subsp. equi-trojani is regarded as potentially more drought tolerant than the type species, growing to appreciably lower elevations (Aussenac, 2002). The species grows on a great variety of soils and situations, though does best on strong, deep siliceous loams rich in organic matter and unlikely to dry out in summer, nor retain too much moisture in winter. Cold, stiff clays should be avoided. A. nordmanniana subsp. equi-trojani is said to grow well on calcareous soils, reflecting the genetic influence of A. cephalonica. In Scotland there was a belief that A. nordmanniana could potentially tolerate dry, infertile heathland sites in the east of the country to a greater extent than could A. alba (Elwes and Henry, 1906–1913). Interestingly, George et al. (2015) commented that this species showed greater resistance to drought than did other Eurasian Abies spp., due to later flushing, which might also confer enhanced frost resistance. Silvicultural record The timber of A. nordmanniana is considered similar to that of A. alba and has been used for comparable applications in naval and building construction since classical times. Tutin et al. (1964) reported that it was planted for timber in central Europe, and occasionally elsewhere. In Britain, Kent (1900) stated that ‘as an ornamental tree for landscape gardening, few can compare with it for beauty of outline, symmetry and the rich contrast produced in summer by the dark glossy green of the old and the light lively tints of the young foliage’. This is its main current attribute in Britain, where it has become a much sought-after Christmas tree. In 2017, some 80% of all Christmas trees sold in England were A. nordmanniana. Some

ABIES PINSAPO Boiss.

31

Victorian and Edwardian trial plantings on Scottish estates have given rise to large specimen trees today, and there is a plot of trees dating from the 1950s at the Kilmun forest garden. A. nordmanniana subsp. equi-trojani is ­regarded as a valuable timber tree within its native range, and is preferred to admixed species such as P. nigra and Picea orientalis. Timber from A.  nordmanniana subsp. equi-trojani was reputedly used to construct the Trojan horse in 1178 BC. A stand of A. nordmanniana subsp. equi-­trojani (bornmuelleriana) at the Bedgebury Pinetum in Kent has performed well, showing potential promise for the drier lowland areas of Britain. Susceptibility to Adelges nordmanniana is comparable to that of Abies alba, or worse. No specific details about nursery practices for this species are known, but those given on page 18 for A. alba are likely to apply.

ABIES PINSAPO Boiss.

Spanish fir

As stated above, the conifer forests of the Mediterranean Basin have been subjected to overuse by humans since ancient times. Some species now survive only as relict populations. A. pinsapo is a good example, with the species surviving in only three enclaves in southern Spain and two in northern Morocco. Until the mid-20th century A. pinsapo forests were subject to major anthropogenic pressures, and in Spain they were under constant threat from fire, grazing and overcutting until they were acquired by the state. Conservation efforts, however, have now been undertaken in both Spain and Morocco, and the fact that all the A. pinsapo forests are covered by some form of protection preserves them from further inappropriate use or exploitation. These forests are now recovering after years of intensive grazing and use of their timber for construction, firewood and charcoal making. However, these relict forests face new threats from climate change, arson and the appearance of pests. The limited area occupied by the forests makes them highly vulnerable to disturbance (Esteban et al., 2010). Two varieties of A. pinsapo are recognized that have disjunct (i.e. geographically separated) ranges. According to Farjon (1990): • var. pinsapo occurs in Spain: in the provinces of Malaga and Granada, in the Sierrania de Ronda (Sierra de las Nieves, S. de Bermeja and S. de Yunquera). It grows at 1000–2000 m elevation. • subsp. marocana (Trab.) Emb. & Maire occurs in Morocco: the western Rif Mountains (Mts. Tissouka, Mago, Kraa, Mt Tazaot and Bab Rouida), at elevations of 1400–2100 m. In its native habit, the climate is montane Mediterranean, at between 1000 and 2000 m elevation: dry, warm summers alternate with cool, moist winters and annual precipitation is around 1000 mm.

32 Likely Silvicultural and Utilization Potential of Southern European Silver Firs in Britain

Both varieties occupy similar habitats: north-facing slopes on rocky soils derived from dolomitic limestone or serpentine, with deep drainage (Tutin et al., 1964). A. pinsapo was introduced to Britain in 1839. It is not a fast-growing tree and never achieves very large dimensions: 27 m is about its limit. A species plot at Crarae, Argyll, averaged just under 4 m at age 20 (Macdonald et al., 1957). It has subsequently lacked vigour and been subject to some windthrow, while the plot at Kilmun (Mason et al., 1999) has also been severely damaged by windthrow. Like A. cephalonica it is probably better suited to drier and warmer parts, particularly in the south of England, including on chalk (Johnson, 2003), though the existing trees are mainly in the west. Dallimore and Jackson (1948) stated that ‘the wood does not appear to possess any commercial value outside its native country’. The tree frequently displays a coarse stem form, often with multiple leaders and forks (Macdonald et al., 1957) as at Crarae, Argyll. It is, however, quite frequently planted for timber outside its present natural range (Tutin et al., 1964), notably in southern France. Like Cedrus atlantica, which grows in the same areas (sometimes in intimate mixture, as at Mt Ventoux), it is one of a small group of conifers adapted to calcareous soils. No specific details about nursery practices for this species are known, but those given on p. 18 for A. alba are likely to apply.

Likely Silvicultural and Utilization Potential of Southern European Silver Firs in Britain The limited growth information that is available from forest plots in Britain (Table 4, p. 33) suggests that, in the more oceanic parts of Britain, none of these southern European and Asian species is likely to be as productive as Abies species from north-western America such as noble fir, grand fir and Pacific silver fir. British forests already contain some 8000 ha of stands comprising Abies species, but these are dominated by the North American grand fir (A. grandis) and noble fir (A. procera), in a roughly 2:1 ratio (Mason, 2012). Most existing plantations of these species were ­established during the period 1920–1980, with very limited deployment in woodland creation or restocking since 1980. Noble fir is primarily used in upland areas of Scotland and Wales where the dominant plantation crop is currently either Sitka spruce (Picea sitchensis) or Scots pine (Pinus sylvestris). Grand fir is used in a wider range of situations across Britain, weighted towards better sites at lower elevations, where a range of conifer and hardwood species are potentially suitable. Top heights converted to YC using tables for noble fir as advised by Hamilton and Christie (1971). No basal areas are provided for the A. cilicica plot at Kilmun because its small size means that such a measure would be unreliable. Quantities of any other Abies species are very much

Likely Silvicultural and Utilization Potential of Southern European Silver Firs in Britain 33

Table 4.  Growth of Abies species from southern Europe and the Near East in trial plots in forest gardens in Britain. Age (years)

Top height (m)

Cumulative Estimated basal area yield class (m2 ha−1) (m3 ha−1 yr−1)

47 53

22.5 22.4

75.6 74.4

18 14

Species

Location

Abies alba Abies bornmuelleriana Abies cephalonica

Kilmun, Argyll Bedgebury, Kent Bedgebury, Kent Kilmun, Argyll Kilmun, Argyll Brechfa, Wales

50

19.7

47.0

8

48 40 41

21.5 16.8 22.7

79.2 N/A 61.4

16 12 16

Kilmun, Argyll

50

19.8

84.4

14

Kilmun, Argyll

41

18.0

49.4

14

Abies cephalonica Abies cilicica Abies nordmanniana Abies nordmanniana Abies pinsapo

Notes: For details of Brechfa and Kilmun, see Danby and Mason (1998) and Mason et al. (1999). Information on Bedgebury was kindly provided by Ian Craig, Forest Research, Alice Holt.

smaller and are dominated by older material in mixed woodlands established prior to 1920. Few stands of other Abies species can be found outside arboreta and forest gardens. Perceptions of Abies (including the north-western American species) among foresters and timber processors in Britain are currently adverse, because of the established risk of drought crack (Day, 1954) and the lower density and market prices for the timber (Wilson, 2010, 2011). Markets are typically for lower grade, non-load-­ bearing applications. Commentary in recent years has suggested an expanded role for European silver fir (A. alba) and Pacific silver fir (A. amabilis) in forests managed under selection systems in Britain (Kerr, 1999; Wilson, 2010, 2011, 2014; Mason, 2012; Savill, 2013; Kerr et al., 2015, 2016). Such applications would take advantage of the advance regeneration and shade tolerance of these species, while use in mixed-species stands would be expected to significantly mitigate the risks from Adelges nordmanniana. Pacific silver fir would primarily be a species for the wetter western upland areas on freely draining soils, potentially operated in mixture with Sitka spruce and western hemlock. European silver fir would be expected to grow best on better, mid-slope sites across the country, in mixture with a wide range of other conifers (e.g. Douglas fir, Norway spruce) and hardwoods (oak, beech, sycamore). Field reports also suggest that the timber quality of European silver fir, in particular, exceeds that of the North American Abies spp., as used in British forestry, although there is little direct evidence to date. This leaves the question as to whether there is a class of sites

34 Likely Silvicultural and Utilization Potential of Southern European Silver Firs in Britain

in eastern Britain that are too dry for European silver fir but are capable of carrying productive crops of the more drought-tolerant Abies species. Kerr et al. (2015) have reported that suitable provenances of A. alba performed satisfactorily within a provenance trial at Thetford. There have also been promising trial plantings of Turkish fir (A. nordmanniana subsp. equi-trojani (bornmuelleriana) at Bedgebury Pinetum in Kent and of Grecian fir (A. cephalonica) at a number of southern localities. However, as Mason (2012) pointed out, we do not yet have a sufficient number of trial plots to evaluate reliably and compare the performance of these lesser known species under dry lowland climates. However, George et al. (2015) have reported relevant findings from Austria. Wilson (2014) suggested that there may well be a role for Mediterranean Abies spp. for use in mixture with Scots or Corsican pine (Pinus nigra subsp. laricio) and/or with Atlas cedar (Cedrus atlantica) in developing more resilient planted forests on the lowland heaths (Thetford, Wareham, etc.). This was based on observations from parts of southern France where such mixtures are employed. The severity of Dothistroma might be reduced in such mixed-species stands, potentially allowing the current moratorium on use of Corsican pine to be selectively relaxed. The information presented here suggests that the prime candidates for such comparative trials are A. nordmanniana (all variants), A. borisii-regis, A. cephalonica and A. cilicica. By contrast, A. pinsapo does not appear to be as drought tolerant as some of the other species considered (Aussenac, 2002) and is probably not a prime candidate for further trials.

ACER L. The genus Acer is in the family Sapindaceae, which contains approximately 140–150 genera and 1400–2000 species, including maples and horse chestnut. The Sapindaceae occur in temperate to tropical regions, many in laurel forest habitat, throughout the world. Many are laticiferous: that is, they contain latex (a milky sap), and many contain mildly toxic saponins with soap-like qualities in either the foliage and/or the seeds, or in the roots. The largest genera are Serjania, Paullinia, Acer and Allophylus. The largely temperate genera formerly separated in the families Aceraceae (Acer, Dipteronia) and Hippocastanaceae (Aesculus, Billia, Handeliodendron) were included within a more broadly circumscribed Sapindaceae by the Angiosperm Phylogeny Group. Recent research has confirmed the inclusion of these genera in Sapindaceae. There are 114 north-temperate species of Acer and one tropical mountain species in western Malaysia. Most occur in East Asia. There are 14 European species, but only one of these, A. campestre, is native to Britain. Many produce important timbers; one is the source of maple syrup; and there are numerous cultivated ornamental varieties, which are mainly known for their spectacular autumn colours (Mabberley, 1990).

ACER CAMPESTRE L.

Field maple

Origin Field maple is the only indigenous maple, found most commonly at low elevations in the south of England, in woodlands, hedgerows and scrub. It is absent from Scotland and Ireland, and uncommon in Wales. Its range extends over most of Europe south of Norway, from 55° to 38° N (EUFORGEN, 2018b); it extends eastwards to Poland, Belarus and southwestern Asia and is also found in the Atlas Mountains. Climatic requirements The trees grow best in warmer, drier climates and are consequently more common at lower elevations and in the south of England. They are hardy to low winter temperatures and not damaged by frost.

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

35

36

ACER CAMPESTRE L.

Site requirements Field maple is most common on neutral to calcareous soils, especially when heavy, but it will not grow on waterlogged soils. Though it grows on a range of other soil types as well, it is seldom found when the pH is below 6 or above 8. It is frequently associated with ash, together with a ground flora of dog’s mercury, bluebell, sanicle, sweet woodruff and similar woodland herbs. Other silvicultural characteristics Field maple is a small tree or shrub that seldom exceeds about 15 m tall but occasionally grows to 25 m. It is a later seral species that typically occurs in the understorey of deciduous woodland and is never found i­nvading grassland or the early stages of plantations. As such, it is shade tolerant when young when it can grow fast, but it is soon overtaken by other species as the forest matures. It coppices well. Flowering and seed production The tree flowers in the spring and is unusual among British trees in being insect pollinated. It is regarded as a good bee plant. Seed is produced when the tree is aged around 10 years. There are about 15,000 seeds kg−1. The seeds need 16−18 months’ stratification if they are to be grown in the nursery; otherwise they should be treated in a similar way to sycamore. Timber and uses The wood of field maple is white, hard and strong. It is slightly heavier than sycamore, at about 690 kg m−3, and is used for similar purposes: furniture, joinery and flooring. Place of field maple in British forestry Though only a small tree, field maple’s bright amber-yellow autumn colour makes it one of the most striking native species. However, as a timber producer, its size and slow growth mean that it is an unimportant forest tree.

ACER MACROPHYLLUM (Pursh)

37

ACER MACROPHYLLUM (Pursh) Bigleaf maple or Oregon maple Origin and introduction The native range of the species is mainly a coastal distribution from Sullivan Bay (British Columbia) to 16 km north of San Francisco Bay (Burns and Honkala, 1990). It was introduced to Britain in 1827. Climate and site It grows over a wide range of temperature and moisture conditions, from the cool, moist, maritime climate of coastal British Columbia to the warm, dry growing seasons of California. Evidence from a plot at the Kilmun Forest Garden, Argyll, does suggest that bigleaf maple can perform well under sheltered upland conditions on less fertile soils; so it, with sycamore, offers opportunities for better upland sites. Bigleaf maple grows best on well-drained alluvial and colluvial soils. Abundant moisture and a deep, gravelly profile produce the best growth, usually found on river terraces, flood plains and seepage sites. It does not require high concentrations of soil nutrients. It is said to be a soil improver on sites where it grows. Other silvicultural characteristics The tree coppices profusely after cutting. Height growth is rapid through the sapling stage but slows thereafter. The root system is shallow and spreads widely, making the tree well suited to the shallow or saturated soils on which it often grows. Bigleaf maple is often present in undisturbed stands and is able to respond with vigorous coppice growth after disturbance. Maple seedlings often appear in intermediate or late seral communities. According to Burns and Honkala (1990), bigleaf maple frequently follows willow (Salix spp.) or red alder in riparian seres. The silviculture of bigleaf maple usually involves control rather than culture. It does not invade clearcut areas aggressively, but vigorous stump sprouting is a problem when it occurs in harvested stands. Sprouting can be controlled by applying water-soluble amines or potassium salts of phenoxy herbicides around the sapwood perimeter on freshly cut stumps. Girdling the uncut trees is ineffective, because girdled bigleaf maples survive for several years, and sprout.

38

ACER PLATANOIDES L.

Bigleaf maple is moderately shade tolerant and can be managed in mixtures with oceanic conifers, making it attractive for diversifying upland conifer forests on better soils. It was regarded by Case and Lawler (2016) as being the least vulnerable to climate change of 11 north-western American species studied, largely because of its adaptive capacity. Bigleaf maple can reproduce quickly after disturbances, and its seeds can disperse long distances, potentially allowing it to move in response to a changing climate. Thinning to final spacing by 15 years is recommended, with a spruce-­ compatible rotation of 40–50 years. The productive bigleaf maple stand at Kilmun Forest Garden reached 11 m after 22 years and 28 m after 63 years, and is recorded as achieving general yield class (GYC) 14 (Mason et al., 1999). Timber properties and utilization Interest in these species centres on their attractive, light-coloured hardwood timbers suitable for furniture making, decorative carpentry (e.g. kitchen worktops) and musical instruments. Figured material can attract especially high values for such applications, as in the case of sycamore. The flow of sap is adequate for syrup production in January and February, but the syrup is of a lower quality than that made from sugar maple.

ACER PLATANOIDES L.

Norway maple

Origin and introduction Norway maple is native to central and northern Europe, Asia Minor, the Caucasus and northern Iran. It was introduced to Britain before 1683. Climatic requirements Norway maple is much less tolerant of exposure than sycamore and should never be planted on windy sites. A consequence is that it is far more common in the lowlands of southern England than elsewhere. In common with sycamore, it can withstand moderate pollution by smoke but not salt spray. Site requirements Its site requirements are similar to those of sycamore, though it is less demanding of soil fertility and will tolerate drought better. It will grow

ACER PLATANOIDES L.

39

on most types of soil, except peats, provided they are moist; and it is ­successful in a wide range of pH conditions, though does not thrive on ­extremely acid or alkaline soils. According to Kerr and Niles (1998), it grows best on deep, fertile, moist soils that are adequately drained and have a pH in the range 5.5–6.5. It also does well on heavy soils. The species is regarded as one of the best broadleaves for planting over chalk and is said to grow better than sycamore on more acid soils. Norway maple is a tree best suited to lowland sites and lower hills. It will not thrive at such high elevations as sycamore.

Other silvicultural characteristics Norway maple is a medium-sized tree that grows to about 25 m tall. It will not thrive in large pure stands, but rather singly or ideally in small groups, because its rapid early growth may cause suppression of companion species. It does well in mixture with beech, Norway spruce and western red cedar. Norway maple is sensitive to competition from grass when young, and therefore growth benefits considerably from careful weeding. It grows fast when young, usually faster than sycamore for the first 30 years or so, and needs to be heavily thinned to maintain growth but does not reach such large dimensions as sycamore, or live for as long. It is usually described as moderately light-demanding. On suitable sites, and with proper thinning and care, trees of 40 cm diameter at breast height (dbh; 1.3 m) may be achieved in 40 years. The tree is usually r­ egarded as being more attractive than sycamore: it produces spectacular yellow or orange-red autumn colours. It makes a good street tree, and many cultivars have been selected for this purpose. Like sycamore, it is very susceptible to bark stripping by grey squirrels throughout the pole stage and this makes growing it for high-quality timber production extremely difficult in most of Britain. Otherwise it is generally much more free from most species of pests or diseases. In common with other maples though, it is one of the trees that are potentially at risk from the larvae of the Asian longhorn beetle (Anoplophora glabripennis). Street trees will apparently be particularly at risk if the beetle gets a foothold in Britain.

Natural regeneration Norway maple has become naturalized on many sites but especially on sandy soils.

40

ACER PLATANOIDES L.

(A)

(B)

(C)

Fig. 2.  (A) Sycamore, Acer pseudoplatanus; (B) Field maple, Acer campestre; (C) Norway maple, Acer platanoides.

ACER PLATANOIDES L.

41

Flowering, seed production and nursery conditions In common with sycamore and field maple, Norway maple is insect pollinated. It flowers in the spring and is regarded as a good pollen- and nectar-­ producing tree for bees. It produces its first fertile seed between 25 and 30 years, with the best crops every 2 or 3 years between 40 and 60 years, though some seed is produced every year. It is usually ready for collection in September or October. There are about 7500 seeds kg−1, of which about 80% normally germinate. For nursery production the seed should be treated in the same way as sycamore. Provenance Work by Kerr and Niles (1998) has shown that good growth and form of Norway maple can be achieved from a wide range of sources in Europe. These include Germany, the Netherlands, Denmark and the former Yugoslavia. A provenance from Russia performed poorly, as expected. Growth and yield Field trials have indicated that height growth can be fast; up to 18–22 m tall in 40 years is possible (Evans, 1984; Kerr and Niles, 1998). Timber The wood is similar to that of sycamore, being white and fine-textured, though harder. It is used for the same purposes. Wavy- (or ripple-) grained maple is in great demand and is sometimes used for making m ­ usical instruments. The average density at 15% moisture content is slightly ­ greater than that of sycamore, at about 660 kg m−3. Place of Norway maple in British forestry It is considered by Kerr and Niles (1998) to be a useful tree for high pH and heavy soils, and as an alternative species to sycamore (which does better in the north) on sheltered southern sites. It has become a popular street tree in British towns because: • • • •

early growth is vigorous; leaf emergence is early and autumn colour is attractive; form and size are ideal; it tolerates urban influences well;

42

ACER PSEUDOPLATANUS L.

• it transplants well; • it grows on a variety of soils; and • it withstands ice and snow damage better than other maples.

ACER PSEUDOPLATANUS L.

Sycamore

Origin and introduction Sycamore (or ‘plane’ in Scotland) is native to high elevations in southern and central Europe; it extends northwards to Paris and east as far as the Caucasus. Its time of introduction to Britain and Ireland is uncertain, but it was possibly brought by the Romans, or as late as 1550. It is certainly reported to have been rare in the 16th century and has only become properly established within the last 200 years. Writing in 1794, Samuel Hayes mentioned the fine sycamore trees present in Ireland at the time. The ecology of sycamore has been described in detail by Jones (1945) and more recently by Taylor (1985). Sycamore is most common in the north and west of Britain. Climatic requirements Sycamore is much less sensitive to late spring frosts than most important deciduous trees and is tolerant of exposure to wind on suitably moist and fertile sites. It makes good windbreaks in exposed and maritime areas and can recover from the effects of salt spray by producing new leaves, but the foliage itself is not resistant to the effects of salt, as in truly maritime species. Sycamore withstands smoke pollution well. It grows better at higher elevations than any other broadleaved trees ­except rowan and the birches, and its potential upper limit is determined more by the presence of suitable soil than by climate. According to Leslie (2005), the most northerly woodland in Britain, on the exposed Mey ­estate in Caithness, is composed of sycamore. Sycamore thrives best in the damper western parts of Britain. Predictions for climate change ­indicate that it will become less suited than at present to south-eastern England and the Midlands if summer droughts become more frequent (Morecroft et al., 2008), but parts of south-eastern Scotland will become more favourable. A study by Willoughby et al. (2009) showed that providing shelter to both ash and sycamore, even in the relatively sheltered English lowlands, resulted in between two and four times faster growth. Side shelter from nurse species is likely to be most effective.

ACER PSEUDOPLATANUS L.

43

Site requirements Though it will grow on a wider range of soils than many species of trees, sycamore does not grow in soils that are either too dry or too wet. Most writers agree that a deep, moist soil, freely drained and of a reasonably high pH and fertility is ideal, but this does not imply that it is particularly site-­demanding in Britain. Sycamore grows well and regenerates on neutral shaley soils, on many of the heavier calcareous loams, clay with flints over chalk and even on acid sandy soils, provided they are deep and welldrained but retain some water. Sites to avoid are podzols, heavy clays that are gleyed close to the surface, and where the winter water table comes to within 30 cm of the surface. Sycamore grows particularly well in hilly country. Other silvicultural characteristics Sycamore is a hardy tree that will grow to 30 m tall. It is rarely found growing in pure stands and is usually a component of mixed woodland in small groups or in intimate mixtures. It is moderately shade bearing when young, making it a useful addition to the native tree flora, which is dominated by light-demanding species; however, more mature trees will die rapidly of suppression if their crowns are not kept free. The tree has poor powers of recovery if suppressed after the age at which height growth slows considerably (Hein et al., 2009). Seed production and natural regeneration in moderately shaded woodlands are often prolific, and the species has potential in continuous-cover systems, at least when young, forming large seedling banks under canopies. Regeneration is frequently associated with sites where dog’s mercury thrives and where elder also ­regenerates. These tend to be somewhat drier than sites where ash does best. Young plants are intolerant of competition from grasses, so if planted in the open, weeding is important. Natural regeneration in open grassland communities is consequently rare. In mixtures, Stevenson (1985) ­considered that sycamore and European larch are compatible with each other in terms of growth rates. In windy parts of the country, such as parts of Yorkshire, sycamore tends to produce buttresses at the bases of stems. Growing it in mixture with larch nurses can prevent their development, by preventing excessive swaying in the wind. Sycamore trees up to 80–100 years old coppice and normally self-prune well, but if pruning should be necessary it should be done in summer. Winter or spring pruning is followed by profuse ‘bleeding’. Grey squirrels prefer sycamore to most other species and can be ­exceedingly damaging to trees of up to 25 years old by stripping bark

44

ACER PSEUDOPLATANUS L.

to eat the phloem tissue beneath and killing the stems. This has become so serious that the amount of sycamore being planted is likely to reduce ­considerably unless a solution can be found. The species is less attacked by rabbits than many others, nor is it damaged seriously by deer, though seedlings, buds and young shoots are palatable to them and this can result in forked stems. Pests and diseases The sycamore aphid, Drepanosiphum platanoidis, is often present on the leaves, where it can produce large quantities of honeydew. Sycamore also suffers from ‘sooty bark disease’ caused by the fungus Cryptostroma corticale, which has been described by Young (1978). It can cause dieback and occasionally death, but serious outbreaks are, apparently, only likely in the Greater London area. The tar spot fungus Rhytisma acerinum is often seen on sycamore leaves in unpolluted areas. It does no damage to the tree, apart from reducing the photosynthetic area a little. Natural regeneration A feature of the tree is that it is one of the few introduced species that has not only become naturalized but is also spreading, especially in the lowlands. It is continuing to invade many different habitats, particularly valley floor ash woods and calcareous beechwoods, though its requirements do not entirely coincide with those of either ash or beech. Because of its invasive tendencies and the fact that it is an exotic, it has become a controversial species. Mabberley (1990), for example, described it as ‘an aggressive weed of British woodland’. A characteristic of the sites where it grows best and regenerates naturally is that the decay of organic matter is rapid. Flowering, seed production and nursery conditions The tree flowers in mid-April. Sycamore is the only common tree that produces insect-pollinated flowers (though the less common field and Norway maples, cherry and the limes do, too). It is a good early source of pollen and nectar for bees; seeds ripen in September and October, and are dispersed between mid-September and mid-October. The earliest age at which the tree bears good seed crops is between 25 and 30 years, and the best ones are usually between the ages of 40 and 60 years. Some seed is produced every year, but it is produced plentifully every 2nd or 3rd year. There are about 9400 seeds kg−1 (range 5400–15,800), of which 70% to

ACER PSEUDOPLATANUS L.

45

80% will normally germinate after 4 weeks’ stratification at 2°C if sown no later than February. About 3500 seedlings are normally produced per kg of seed, of which 1300 are likely to be saleable. The seed is recalcitrant and is best sown immediately after collection or, if stored (for a maximum of 12−16 weeks), it must not be dried below 35% moisture content. It should then be stratified for 8 weeks. Aldhous (1972) stated that if it is stratified too soon the seed may germinate early, before the nursery beds are fit to be sown. Conservation Sycamore has two characteristics that are believed to have detrimental ­effects on the ground flora of deciduous woodland: (1) on some sites its litter decays slowly; and (2) the heavy shade it casts. In a detailed discussion of these characteristics, Taylor (1985) concluded that, in long-established woodland in which sycamore is not totally dominant, it does not cause the extinction of species in the ground vegetation. It may even increase diversity by leading to an increase of those species that are shade tolerant in ash woods. It is in no way the ecological menace that some suggest. Sycamore supports a relatively poor invertebrate fauna compared with most native species. According to Kennedy and Southwood (1984), it has 43 species associated with it compared with 423 on oak. This number is similar to lime, with 57 species, rowan with 58, ash with 68, and hornbeam and field maple with 51 each. All the latter are native species. However, the aphids that thrive on sycamore trees are an important source of food for blue tits and great tits in the early spring (Elton, 1979). It also supports more lichens than most native trees. The meagre evidence that exists suggests that sycamore will eventually be replaced by another species rather than by itself. In ash–sycamore woodland, for example, the regeneration of either species is less likely under their own canopies than beneath each other’s (Waters and Savill, 1991). The proportion of sycamore regeneration therefore declines as the proportion in the canopy increases; consequently, the two species will probably alternate with each other, and it is unlikely that either will dominate. Woodland that has been undisturbed for some time contains little sycamore. It tends to colonize disturbed woodland areas (Taylor, 1985; Leslie, 2005). Invasion is a slow process up to a point of equilibrium ­determined by the woodland type. In disturbed woodland invasion is often rapid and apparently aggressive, with sycamore initially becoming a dominant species, but then it declines again to the equilibrium point. Attempts to eradicate sycamore, a widely practised policy in nature reserves, usually provide ideal conditions for its expansion. This is clearly a waste of effort unless it can be shown that sycamore is undesirable for a reason other than that it is a threat to native trees and shrub species.

46

ACER PSEUDOPLATANUS L.

Provenance Continental European botanists have recognized several ecotypes and varieties but none is distinguished in Britain. Cundall et al. (1998) have ­reported on the early growth of British and continental provenances of sycamore in a series of five trials. After four seasons in the field, the ranking of provenances for height was not significantly different, though they had been significant in the nursery, nor was there any provenance × site interaction. There were, however, indications that some continental provenances may be superior to British ones, and also that others are inferior and should probably be avoided. The Future Trees Trust is in the process of selecting and grafting material from exceptionally good ‘plus’ trees with the aim of developing clonal seed orchards, prior to a breeding programme. Because sycamore is insect pollinated and usually occurs as scattered individual trees and is also self-compatible, it can be deduced that it is more differentiated than wind-pollinated species with a continuous distribution, such as birch and spruce (EUFORGEN, 2018b). Area and yield An analysis of European height growth curves by Hein et al. (2009) indicated that the species grows fast up to 20–25 years and then slows considerably. The British tables (Hamilton and Christie, 1971) show the same pattern. Yields of 750–1000 m3 ha−1 of high-quality timber are achievable on the best sites over rotations of 70–75 years (i.e. about 12 m3 ha−1 year−1). This is higher than can be expected from ash, where the maximum is 10 m3 ha−1 year−1. Because it grows mostly in mixed stands, not much work has been done on refining growth models. It is estimated that sycamore is the dominant species over 106,300 ha of British woodlands (Forestry Commission, 2018). Timber The diffuse-porous wood of sycamore normally has no well-marked figure or grain, or visible distinction between sapwood and heartwood. Logs do not store well and should be sawn and dried soon after felling. The wood is hard, strong, stable and can be worked to a very smooth finish. Sycamore dries quickly and is described as a ‘friendly’ wood to use. Its average density at 15% moisture content is about 610 kg m−3. It is widely used for furniture making and joinery, and is also suitable for flooring, but is not durable outside without preservative treatment. Because it is free from smells or tastes, it is also ideal for uses associated with food. Sycamore wood has the added advantage in that it is

Other Species of Acer that might be Useful in a Changing Climate

47

one of several species that have particularly good antibacterial and antiviral properties, and so is very suitable for using as chopping boards for food preparation – much better than plastics or stainless steel (Park and Cliver, 1997). The white or creamy coloured timbers of sycamore and ash are among the most easily saleable of all British hardwoods as there is a market for all grades. Venables (1985) considered more of these two species should be grown, and more recently Leslie (2005) has stated that sycamore can provide one of the best financial returns of any hardwood tree. ‘Wavy-grained’ sycamore, which arises occasionally, can fetch extraordinarily high prices and is used for making good violins and other musical instruments, and for veneers: the intensity of waviness increases with age, so the figure is difficult to detect in young trees. Discoloured heartwood tends to occur in larger trees over 45 cm diameter. Place of sycamore in British forestry Sycamore is one of the fastest-growing broadleaves and produces a ­potentially valuable white timber. Silviculturally it is similar to ash in many respects, though it is much more frost hardy and less site-demanding. Both ash and sycamore occur on similar soils and can be relatively fast growing. Both are opportunist species that spread into gaps in the canopy. Unfortunately, sycamore is often badly attacked by grey squirrels and this limits its potential considerably. There is also much prejudice against the species from naturalists’ groups.

Other Species of Acer that might be Useful in a Changing Climate Maples are likely to provide the most suitable substitutes for ash if much of it succumbs to the dieback disease caused by Chalara fraxinea on moist fertile soils in warmer, lowland parts of Britain, including valley woodlands. These are the conditions of the lowland mixed ashwood (National Vegetation Classification, NVC W8) ecosystem (as described in Rodwell, 1991) and analogous plantations. These conditions most closely resemble the native temperate mixed woodland ecosystems of many species of Acer. Cold hardiness for British conditions is good across this group. With the exception of sycamore, maples are unsuited to wind-exposed conditions and are less likely to be useful substitutes for ash in upland ashwoods (NVC W9). Maples do better with a soil moisture regime (SMR) of Fresh to Moist and a soil nutrient regime (SNR) of Poor to Rich in terms of the Forestry Commission Ecological Site Classification (ESC) (Pyatt et  al., 2001). None tolerates very infertile, sandy, waterlogged or peaty soils. Where soil conditions are droughty and/or calcareous, selection of ­species

48

Other Species of Acer that might be Useful in a Changing Climate

from this group is restricted to field maple and Norway maple. Such sites occur in limestone ash–beechwoods (NVC W12). Provenance, seed production and nursery practice Maple species produce winged seeds or samaras, carried some distance from the parent tree by the wind. Field maple and sycamore reproduce well by natural regeneration in Britain and Ireland. Norway maple produces seed early, and copiously by 25 years, typically with 11,000 seeds per kg with a 70% germination rate. Seed can be over-wintered in moist sand. No nursery problems have been reported. Forest Research (2018b) recommends seed is taken from good British stands or selected stands in western Europe. This is supported by provenance trials, reported by Kerr and Niles (1998), comparing European provenances. The flowers of bigleaf maple (A. macrophyllum Pursh) are described as polygamous, with both staminate and perfect flowers mixed in the same dense, cylindrical racemes. It will produce seed from age 10 (Iddrisu and Ritland, 2004). There are 5000–8000 winged samaras per kilogram, which can be stored for 1 year if collected dry (10–20% moisture content, MC). In nature, regeneration from seed is good under a partial canopy but poor in open areas and under denser canopy shade. It grows rapidly in the nursery, reaching 1–2 m within 1 year, but limited planting is undertaken within the native range. There are no provenance trials in Britain, but Forest Research (2018b) recommends the use of Washington provenances. Silver maple (A. saccharinum) produces seed from age 11. There are 2000–7000 samaras per kilogram. This species regenerates well from seed under suitable conditions in the native range. Seed is rarely set in British conditions but natural regeneration has been found locally in Surrey. Silver maple seed germinates in the nursery without pre-stratification (Leopold et al., 1998). There are no British provenance trials, but Forest Research (2016) recommends silver maple from the northern part of its native range, to ensure cold hardiness for Britain. Silviculture The maple species naturally form a component of mixed-hardwood stands on productive sites. Silvicultural regimes and yield predictions for these species in Britain currently rely on experience with even-aged sycamore. Table 5 presents growth and production data from older Norway maple trial plots. Kerr and Niles (1998) reported early height growth data from provenance trials in southern England. Norway maple grows rapidly for its first 40–50 years and is difficult to manage in mixtures (Forest Research,

Other Species of Acer that might be Useful in a Changing Climate

49

Table 5.  Growth and production data for early Norway maple (Acer platanoides) trial plots. Metric conversions from Table 54 in Macdonald et al. (1957). Site Gravetye, Sussex Manby, Lincs. Dunkirk, Kent Cirencester, Gloucs. Betteshanger, Kent

Age (yrs)

Height (m)

Dbh (cm)

Volume (m3/ha)

Stocking (stems/ha)

42 38 40 46 40

19.2 17.9 14.2 18.2 15.1

21.8 24.3 22.6 18.6 20.6

199.8 96.1 84.8 112.8 127.2

642 435 351 874 667

Dbh, diameter at breast height.

2016). Macdonald et al. (1957) suggested that Norway maple can outperform beech in mixture if it is carefully thinned. Pests and diseases The major challenges with the maples are their vulnerability to bark stripping by grey squirrels, as with sycamore, and susceptibility to leaf spot diseases and Verticillium wilts (Forest Research, 2016). Root-rot fungi such as Phytophthora and Armillaria (honey fungus) can be a problem, particularly in mature specimens. The specific butt rot fungus Ganoderma applanatum affects bigleaf maple. Susceptibility to P. ramorum has been demonstrated in the Pacific Northwest and might constrain deployment in Britain on infected former larch sites. The sapstreak disease (Ceratocystis virescens) is a serious threat to silver maple in North America, but it is not thought to be present in Europe.

AESCULUS L. AESCULUS HIPPOCASTANUM L.

Horse chestnut

The genus Aesculus is in the family Sapindaceae (see p. 35). There are about 25 species of Aesculus, mostly in North America where they are often known as ‘buckeyes’, and a few in Europe and Asia. Horse chestnut, the only European species, is recognized in late winter/early spring by its very large reddish-brown sticky buds; then by its opposite and palmately compound leaves, with 5–7 leaflets, the whole being up to 30 cm long and 60 cm across; and by its seed. The scars left by fallen leaves leave an obvious horseshoe-shaped scar with ‘nail’ marks (More and White, 2003). The flowers are usually white with a yellow to pink blotch at the base of the petals; they are produced in spring in spectacularly attractive erect panicles 10–30 cm tall with about 20–50 flowers on each panicle. Usually only 1–5 fruits develop on each panicle; the shell is a green, spiky capsule containing one (rarely two or three) nut-like seeds, known as ‘conkers’ or horse-chestnuts. Each conker is 2–4 cm diameter, glossy nut-brown with a whitish scar at the base. They are liked by deer (Bean, 1929). Several other species of Aesculus are grown in Britain, but A. hippocastanum is by far the most common. Origin Horse chestnut is native and endemic to Albania, southern Macedonia and Greece, a small area in the Pindus Mountains mixed forests, and in the Balkan mixed forests of south-eastern Europe. Röhrig and Ulrich (1991) stated that the Balkan peninsula is particularly rich in endemics. According to Mitchell (1974) it was introduced to Britain in 1616. A. hippocastanum is very common as a parkland, avenue and garden tree; in town parks, squares and churchyards; and particularly on village greens in England. It is probably the most recognizable species of tree in Britain, despite not being a native. It will grow up to almost 40 m tall. It is not a traditionally planted forest tree, but rather an urban one. Climatic and site requirements Remarkably little has been written about the silviculture of the horse chestnut, perhaps because it is not used as a forest tree. It is hardy but, where late frosts are frequent, the trees do not produce fruits so well. Trees

50

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

AESCULUS HIPPOCASTANUM L.

51

do not withstand exposure but will tolerate atmospheric pollution well (Brown and Nisbet, 1894). They are vulnerable to drought. Horse chestnut grows naturally in the sub-alpine zone of ‘ravine forests’, in narrow, humid, warm valleys and gorges (Röhrig and Ulrich, 1991). It needs a ‘good, rich, loamy and dry soil, and a rather sheltered situation’ (Brown and Nisbet, 1894), though More and White (2003) considered that it needs good soil moisture and full sun to grow best. Other silvicultural characteristics Horse chestnut trees up to 27 m are reasonably common. Johnson (2003) recorded one of 35 m at Hurstbourne Priors in Hampshire, and one of 39 m in Screens Wood, west Sussex. It is a wide-spreading, relatively short-lived tree (usually about 150 years) which, when grown in the open, produces massive branches. It grows rapidly, by 60–90 cm a year. In natural conditions, it grows with beech, walnut, large-leaved lime and oriental plane. Trees are shade tolerant at all ages, though Edlin (1945) says that it is light demanding. Trees tend to shed branches after 80 years. Various species of Aesculus hybridize with each other and hybrids are very difficult to identify (International Dendrology Society, 2018). Pests and diseases Pseudomonas syringae, which causes ‘bleeding canker’, is a pathogenic bacterial strain carried by the leaf miner beetle. It is one of the most important plant pathogens of horse chestnut trees, having a very wide host range and causing spots; necrosis of leaves, twigs and fruits; and stem cankers which can kill the tree. Bleeding canker was first reported in Britain in the 1970s, although it was recognized in the USA much earlier, in the 1930s. Pre-2000, symptoms of the disease were associated with two Phytophthora pathogens. Today, the incidence of the disease within the UK has increased dramatically. In 2000 only four cases were reported but this rose to more than 110 in 2006, and survey results show that in 2007 around half the horse chestnut trees in Britain showed some degree of symptoms. Anthracnose caused by the fungus Glomerella cingulata is a common leaf disease on horse chestnut. The fungus is particularly aggressive ­toward plant parts already weakened by environmental stress, inadequate nutrition or natural senescence. On affected leaves, the petioles, midribs and veins turn brown. Horse chestnut leaf miner (Cameraria ohridella) is an exotic insect pest which lives in horse chestnut trees. It was first reported in the UK in 2002

52

AESCULUS HIPPOCASTANUM L.

(in the London Borough of Wimbledon) and has since spread north, south and west to most of England and parts of Wales, and there has been one confirmed sighting in Scotland. Its larvae (caterpillars) mine within the leaves, and at high population densities they can destroy most of the leaf tissues, making the trees look very unsightly in late summer. Although it can cause severe damage to horse chestnut leaves on an annual basis, and discolouration and ­defoliation before normal autumn leaf-fall, on its own the pest does not ­significantly impair tree health, and the trees will usually flush normally the following spring. Timber and uses Horse chestnut has never been planted as a timber tree in Britain, but solely as an ornamental, mainly in urban situations. The wood of horse chestnut is soft, and creamy-white or yellowish. The sapwood is not easily distinguishable from the heartwood and the texture is very fine and uniform. The density is about 510 kg m3 when seasoned. It is stable in that it works relatively easily and does not move or shrink much; it bends well but is not durable. The wood is described as brittle and perishable, but permeable to preservatives (Farmer, 1972). It is reputed to be of special value to turners, cabinet makers and wood carvers.

ALNUS Mill.

Alder

Alders are in the birch family (Betulaceae), which includes six genera including the birches, alders, hazels, hornbeams, hazel-hornbeam and hop-hornbeams, numbering a total of 167 species. They are mostly natives of the temperate northern hemisphere, with a few species reaching the southern hemisphere in the Andes. Their typical flowers are catkins and often appear before leaves. An unusual feature of alders is that they fix atmospheric nitrogen through a symbiotic association in root nodules with a nitrogen-fixing, filamentous bacterium, Frankia alni. The nodules are perennial structures and can achieve the size of a cricket ball or larger as a result of s­ easonal growth. Nodule masses may contain one or several Frankia strains that can vary widely in effectiveness in nitrogen fixation (Aldhous and Mason, 1994). Frankia is similar to the Rhizobium bacterium that is found in the root nodules of legumes in the Fabaceae family. The main interest in alders in British forestry, rather than as timber-producers in their own right, derives from this nitrogen-fixing ability (see, for example, the section on walnut). Through it, they have a potential as ‘nurses’, or as soil improvers on sites with undeveloped soil such as reclamation areas and roadside cuttings. According to Mabberley (2008), there are 35 species of alder, of which five are shrubs or small trees. There are also many hybrids (see text for some of them and also Tables 6 and 7, pages 54 and 55). The genus is distributed throughout the cooler regions of the northern hemisphere, and it also extends over the mountains of Central America to the highlands of Bolivia, Colombia, Peru, Ecuador, Venezuela and Argentina. Alders seldom grow to large trees. Most are pioneer species that invade gaps and clearings in forests. They are capable of direct colonization even on the most undeveloped soils. They are usually relatively small and short-lived and give way in most instances to later successional species. Alders are intolerant to shade and competition and will not grow under a canopy. Only on sites suited exclusively to alder will seedlings regenerate under mature alder trees. They have value for wildlife conservation, ­especially along the edges of water. In Britain the alders are largely neglected as potentially useful trees. Alders in general are prone to a number of diseases that are probably brought on by stress resulting from the difficult sites where they usually grow. All alders that are grown in Britain are susceptible to a soil-borne fungal disease discovered in the early 1990s, Phytophthora alni. It occurs in young woodland plantations and orchard shelterbelts, but its greatest impact is on the riparian alders of southern Britain. The disease has © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

53

Mean height (m)

Mean dbh (cm)

Basal area (m2 ha−1)

Cumulative volume (m3 ha−1)

Yield class

Source authors

Perthshire

23–24

17.5

23

n.d.

283.4

n.d.

1

Suffolk Dumfriesshire Argyll Hampshire Hampshire Northamptonshire Argyll Argyll Hampshire Northamptonshire Dunbartonshire Midlothian

23–24 38 28 22 21 21 31 63 26 21 17 5

13.1 22.8 16.2* 13.8 17* 7.4 16.3 24.8 14.5 9.5 n.d. n.d.

16 30 n.d. 13.7 22 n.d. n.d. n.d. 22.6 n.d. 10.3 9.6

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 34.9 19.2

131.9 n.d. n.d. 171.1 n.d. n.d. n.d. n.d. n.d. n.d. 41 47

n.d. n.d. 4 11 12 n.d. n.d. 12 n.d. n.d. 12 14

1 1 4 1,2 2 1 1 4 1 1 3 3

Midlothian

15

n.d.

n.d.

55

n.d.

n.d.

3

Sutherland Dumfriesshire Northumberland Yorkshire

11 10 11 10

2.5 4.7 6.2 10

n.d. n.d. 7.2 12.4

n.d. n.d. 15.3 25–26

n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d.

5 5 5 5

Yorkshire

10

6.8

n.d.

n.d.

n.d.

n.d.

5

Location

County

Alnus incana

Drummond Hill Thetford Nith Bridge Kilmun Buriton Micheldever Rockingham Benmore Kilmun Buriton Rockingham Lennox Glencorse, Bush Glencorse, Bush Shin 112 Solway 12 Falstone 14 North York Moors 79 York 13

Alnus rubra Alnus rubra Alnus rubra Alnus rubra Alnus rubra Alnus rubra

*indicates that reported measure is of top height rather than mean height in this particular instance. Published data sources above: 1, Macdonald et al. (1957); 2, Evans (1984); 3, McIver (1991); 4, Mason et al. (1999); 5, Mason (2011) in McKay (2011). dbh, diameter at breast height; n.d., no data for that parameter.

ALNUS Mill.

Age (years)

Species

Alnus incana Alnus incana Alnus incana Alnus cordata Alnus cordata Alnus cordata Alnus cordata Alnus cordata Alnus rubra Alnus rubra Alnus rubra Alnus rubra

54

­Table 6.  Growth and yield of non-native alder species in British recorded sample plots.

Table 7.  Summary data from Forest Research sample plots for three species of alder.

Location

County

Alnus glutinosa Alnus cordata Alnus incana Alnus incana

Mildenhall Holt Down Drummond Hill Croxton Mousehall

Suffolk Hampshire Perthshire Suffolk

56 52 40 40

Top height (m)

Mean dbh (cm)

Cumulative basal area (m2 ha−1)

Cumulative volume (m3 ha−1)

General (local) yield class (m3 ha−1 yr−1)

20.7 19.7 18.8 16.1

32.1 29.9 35.8 25.7

57.6 76.3 – 40.1

476 559 417 253

8 (9) 8 (11) 8 (10) 6 (6)

ALNUS Mill.

Species

Age (yrs)

Data courtesy of Ian Craig, Forest Research, Alice Holt. dbh, diameter at breast height; hyphen, records enabling this calculation were not kept.

55

56

ALNUS CORDATA (Loisel.) Loisel.

i­ncreased steadily over the years; by 2003 more than 15% of the surveyed trees had been affected or killed by it (Webber et al., 2004). The disease tends to be less frequent in trees that are 1–10 m or more away from the water’s edge and is especially infrequent in areas that are not exposed to periodic flooding.

ALNUS CORDATA (Loisel.) Loisel.

Italian alder

Origin and introduction Italian alder is native to southern Italy and Corsica and was introduced to Britain in 1820. Climatic requirements It appears to have no major climatic limitations in Britain. Site requirements It will tolerate both dry and calcareous sites, such as thin soils over chalk and reclaimed sites, but it does best on deep chalky soils and least well on acid soils. Unlike most species of alder, it is less confined to riversides. Other silvicultural characteristics It is a medium-sized tree, seldom taller than 20 m, and potentially a valuable pioneer species for sites that are being planted for the first time. Its most useful feature, which is uncharacteristic of most other alders, is its tolerance of comparatively dry sites. Like all alders, it is a strong light-­ demander and withstands exposure and pollution well. The coppicing ability of this species is so variable that some authors have claimed that it does not coppice at all. Others say that it coppices best if the stem is cut at a height of 30 cm, rather than at ground level. Diseases In common with all alders, this species is susceptible to attack by P. alni (see above) but, unusually, it is said to be immune to damage by grey squirrels (Dutton, 2018).

ALNUS GLUTINOSA (L.) Gaertn.

57

Flowering and seed production Italian alder flowers and fruits at an early age. There are 363,000 seeds kg−1, of which half will normally germinate. The seed does not store well. Timber and uses The species is suitable for planting for firewood or pulpwood production on coppice rotations, though it is not a high-volume producer per unit area planted. The timber is reddish-orange. Its quality is said to be similar to that of hybrid poplar, though it is heavier, shrinks much more and has a high modulus of rupture. It is durable when immersed in water. It is used for turnery and carving, and for the production of mouldings, furniture, panelling and plywood (EUFORGEN, 2018b). Italian alder has been described as a superb landscape tree, with an upright conical shape, glossy green leaves and distinctive bark, foliage, flowers and fruit. Place of Italian alder in British forestry Italian alder has a potentially useful place on dry calcareous soils, either as a nurse for beech, ash or maple, or as a cover tree (Wood and Nimmo, 1962). Its value for these purposes has probably been overlooked in the past, and as a consequence it has been planted less than it might otherwise have been.

ALNUS GLUTINOSA (L.) Gaertn.

Alder, Black alder

Origin Black alder is an indigenous species found in all parts of Britain and Ireland. Its natural range extends as far east as Siberia and southwards to North Africa. It is the only native British nitrogen-fixing tree. Climatic requirements Black alder is very hardy indeed and has no serious climatic limitations in Britain, though it does not grow well at such high elevations as birch and rowan. The tree is found up to 500 m in the Cairngorms but is said not to thrive if precipitation is less than 1500 mm in places where access to groundwater is not possible (Claessens, 2005; Claessens et al., 2010). The frequency of natural populations increases towards higher rainfall areas

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of the west and north: soil moisture probably exercises more control over local and regional distributions than atmospheric humidity. Even though flushing is early, trees do not suffer from late spring frost damage and are moderately resistant to salt spray. They tolerate exposure reasonably well, and better than grey alder. However, straight-stemmed trees will only grow in relatively sheltered situations. Site requirements Black alder is relatively undemanding except that it will only grow in moist soils, and not at all on the most infertile soils. It is truly at home on sites not subject to drought (such as close to lakes and streams), and with restricted internal drainage. It also does well on coarse sands and gravels if moisture is adequate and the pH is above 6. However, it tends not to grow as well as grey alder on calcareous soils. Black alder can withstand short periods of flooding outside the growing season, but ideally soil water should be aerated. It does not grow on acid peats and grows badly on dry, sandy soils. Other silvicultural characteristics Black alder grows up to a maximum of about 25 m tall, though it is usually no more than half that height in natural conditions. The taproot is able to penetrate anaerobic water sources that other species cannot use (McVean, 1953b), so it sometimes appears to be drought resistant. It is difficult to establish black alder on grassy sites unless weed control is good. Black alder can contribute significantly to the nitrogen content of litter and the soil, and consequently benefit the growth of companion tree species. Nodule formation proceeds best in the pH range 5.4−7.0 (Ferguson and Bond, 1953) but, after nodules have formed, growth is best at a pH of about 5.4. There is also evidence that the fixed nitrogen, subsequently translocated to the leaves of alders, is returned to the litter in nitrate form and this inhibits the development of certain potentially pathogenic fungi of many trees such as Poria and Armillaria (Bollen et al., 1967; Li et al., 1967). Black alder coppices well when young and is notable in that it is among few hardwoods that are not seriously attacked by hares and rabbits. Moderate grazing levels favour the spread of the trees by reducing the shading and smothering effects of tall herbaceous vegetation on seedlings. It can sometimes become invasive. The early growth of black alders is fast, which makes their use as nurses difficult because the trees being nursed can easily be suppressed and killed if the alders are not removed. The rapid early growth is due to the speedy development of a large area of leaves and the long period in

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leaf. Black alder is not long-lived, especially on poor sites where its life may be only 20–25 years. Black alder is sensitive to shading, so that internal regeneration in woods is unknown except on sites unsuited to the growth of any other trees. Thinning has to be started early, and has to be heavy and frequent around selected final crop trees, to achieve marketable timber before heart rot sets in. Trees do not respond to delayed thinning. Black alder is the principal or only food of 141 insects and mites (Kennedy and Southwood, 1984) and, if planted along the banks of rivers and lakes, these insects can provide an important source of food for fish. A policy of planting for this purpose is followed in parts of Canada and the USA. Matthews (1987) considered that black alders might have a place in the British uplands for diversifying the large areas of coniferous forests. Natural regeneration As mentioned above, black alders are natural pioneers in successions. According to Claessens (2005), for natural regeneration to be successful, the seeds must be in contact with the mineral soil. Seedlings will only survive and become established on soil surfaces that come within the capillary fringe of the water table in drier regions, where surface layers remain continuously moist for 20–30 days in the period April–June. Seedlings do not survive if there is competition for light. These conditions do not arise every year in most places. To regenerate naturally black alder requires high levels of both light and moisture, and this is usually achievable only on disturbed sites. Flowering, seed production and nursery conditions Black alder flowers between early March and late April, before the leaves are fully out in the spring, the catkins having formed the previous autumn. The seeds ripen between September and November, and are best collected for nursery purposes in October. They are dispersed from the time they ripen to early spring. The earliest age at which the trees bear reasonable amounts of seed is about 15–20 years and the best seed crops are usually after the age of 30, at 2–3-yearly intervals. There are about 767,000 seeds kg−1 (range 582,000−1,406,000) of which 35% are normally viable. Nodule formation on the roots of black alder transplants may not occur satisfactorily if they are planted on sites that are being reclaimed, resulting in comparatively poor growth, unless the young plants are inoculated with Frankia in the seedbeds of nurseries. An application of extract of crushed nodules, collected from healthy, well-grown tree roots, is now recommended to nursery managers as a standard treatment for alder seedbeds (McNeill et al., 1989).

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Provenance McVean (1953a) has described some of the variation that occurs in black alder over Britain and Ireland, and it is clear that it is considerable. Comprehensive provenance trials with black alder are now being carried out in many European countries (though not in Britain) and are revealing the variation associated with geographical origin. Worrell (1992) has found that survival in black alders of British origin is better than in provenances from continental Europe, while Matthews (1987) stated that the black alders are promising subjects for improvement by selection and breeding. Several varieties exist in cultivation. Area and yield The fixation of atmospheric nitrogen requires much energy, which might otherwise be diverted into vegetative growth: for this reason, black alders are not productive trees, especially on sites where soil nitrogen is low. As they are such relatively short-lived trees, they seldom grow to large sizes. Growth rates up to age 7–10 are fast but they then slow rapidly. The maximum rotation for growing timber is 60–70 years if heart rot is to be avoided. Maximum mean annual increments range from 4 to 14 m3 ha−1 year−1 (Claessens et al., 2010). No recent figures exist for the area of woodland where alder predominates, though in 1978 it was estimated that there were perhaps 10,000 ha of alder of all species in Britain. Timber and uses The timber of black alder is light (about 530 kg m−3 at 15% moisture content), not very strong, and is soft and resilient. A blow causes a temporary depression unaccompanied by a permanent indentation. This is in complete contrast to redwood, for example, which can be dented easily. The boundary between the heartwood and sapwood is very clear. The heartwood is typically orange in colour, but it can be dark brown. One of its main attributes is its resistance to decay when submerged in water, but otherwise it rots quickly. It is sometimes used for making sluice gates and other structures along streams and rivers. Black alder is a traditional timber for general turnery work, is acceptable for hardwood pulp and is used for making medium-priced furniture. Where alder timber is available in large quantities, as in the western USA, it tends to be much better accepted and widely used. At one time charcoal made from alder was used in the manufacture of gunpowder, and the wood was in demand for sawing to make herring-barrel staves (Brown and Nisbet, 1894). Alder wood has been used by many electric guitar manufacturers since the 1950s. It is apparently appreciated for its

ALNUS GLUTINOSA (L.) Gaertn.

(A)

(B)

(D)

(C)

Fig. 3.  (A) Italian alder, Alnus cordata; (B) Black alder, Alnus glutinosa; (C) Grey alder, Alnus incana; (D) Red, or Oregon alder, Alnus rubra.

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ALNUS INCANA (L.) Moench

‘bright’ tone. It is similar to poplar in grain and texture and is one of the weakest of hardwoods. In some parts of the world there is considerable interest in growing alders on short rotations for fuel, but on good sites they are not normally as productive as alternative species. Place of black alder in British forestry Because of its nitrogen-fixing ability, black alder is often planted on reclamation sites where soil nitrogen is in short supply. Apart from this specialist use it is likely to remain a minor species in British forestry.

ALNUS INCANA (L.) Moench

Grey alder

Origin and introduction Grey alder is found over most of central Europe and extends westwards to France. It is also native to Scandinavia and has been planted in Britain since it was introduced in 1780. Silviculture In most respects, the silviculture of grey alder is the same as that for black alder. It is slightly less hardy than black alder and, while having no serious climatic limitations in Britain, it will not withstand excessive exposure. It will grow on all but the most infertile soils, but requires sites not subject to drought. Grey alder grows better on slightly drier and more calcareous sites than black alder. It is a light-demanding pioneer species. It has a rather wider site tolerance than either black or red alder, being suited both to moderately dry and to wet soils of poor to medium nutrient status. However, it does not tolerate alkaline soils and will not grow on peats or nutritionally poor soils (Forest Research, 2018b). Grey alder coppices well when young and often spreads by root suckers, especially after mature trees have been felled. They can become invasive. It is more resistant to Phytophthora alni than black alder. Flowering and seed production Grey alder flowers from late February to May. Seeds ripen between September and November. It produces an average of 1,460,000 seeds kg−1 (range 961,000−1,980,000), of which only 25% are normally viable.

ALNUS RUBRA Bong.

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Provenance No provenance studies have been undertaken in Britain so, according to Forest Research (2011), seed should be obtained from good British stands or possibly from western Sweden or Norway. Grey alder can hybridize with native black alder (Hora, 1981) to form A. × pubescens Tausch, frequent on sites where both parents occur (Tutin et al., 1964). Timber The timber is essentially the same as that of black alder. Place of grey alder in British forestry Similar to black alder, except that it should not be used near native black alder because of probable hybridization.

ALNUS RUBRA Bong.

Red or Oregon alder

Origin Red alder has an extensive natural range along the Pacific coast of North America, where it is closely associated with Sitka spruce. It was introduced to Britain in the second half of the 19th century. Site requirements Site requirements for the spectacular, sustained and rapid growth that occurs so rarely are not really understood. Red alder is reported to have grown particularly well on a clay-with-flints soil over exposed chalk in Hampshire and on calcareous boulder clays in Northamptonshire (Macdonald et al., 1957). In common with black alder and grey alder, it has the reputation of being drought resistant once taproots have developed. Fraser (1966), for example, has shown one sample of red alder rooting to 89 cm on a surface water gley soil, where Sitka spruce only rooted to 45 cm. Silviculture Red alder has proved to be a most disappointing species as a timber-­ producing tree in Britain, though growth for the first 10–15 years can be

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spectacular. It can reach 15 m tall in 15 years, after which growth usually declines rapidly and the trees begin to die back. They seldom fulfil the signs of their early promise. There is little indication that the poor performance is related to unsuitable provenances as several, at least from British Columbia, have been tried. Red alder is more susceptible to spring frosts than either grey or black alder; this may be a contributory factor to the dieback from which it suffers. Timber The wood of red alder is similar to that of black and grey alders. It is an important pulpwood in north-western America. Place of red alder in British forestry Red alder is a species that causes intense interest at intervals due to its extremely rapid early growth which is, unfortunately, seldom sustained. Its most likely value in Britain is as a parent of hybrids in any future breeding programme.

ALNUS VIRIDIS (Chaix) DC. including Sitka alder (formerly Alnus sinuata)

Green alder

Taxonomy and distribution Green alder occurs widely across the northern hemisphere as several distinct subspecies. The European subspecies (A. viridis ssp. viridis) is found in the Alps and Carpathian mountains. The western North American species (A. viridis ssp. sinuata), known as the Sitka alder (formerly A. sinuata), is also of significance; it extends into Kamchatka. The mountain alder (A. viridis ssp. crispa) is found in eastern Canada and Greenland, while A. viridis ssp. fruticosa is found widely across Siberia and Alaska in boreal forests. At high latitudes the species functions as a pioneer colonist of fluvioglacial gravels, with willows. At lower latitudes, green alder becomes strongly associated with mountain environments, acting as a pioneer colonist of disturbed sites (e.g. after avalanches). It occurs above red alder in the oceanic mountains of the Pacific Northwest and above grey alder in the mountains of central and southern Europe. The habit of green alder is described by Mitchell (1974) as ‘an erect-branched shrubby plant, rarely a small tree’, seldom exceeding 3 m in height in Europe, but reaching 12–13 m in the case of western American Sitka alder (Hora, 1981).

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It has shiny green ovate leaves (3–8 cm long; 2–6 cm broad). Green alder was introduced into Britain over a century ago (e.g. Sitka alder in 1903 according to White and Gibbs, 2000) and has been used to some extent for reclamation forestry (e.g. Moffat and Roberts, 1989). Climate and soil requirements This species is winter cold hardy in Britain and is believed to be more frost resistant than red alder (Forest Research, 2018b), but is intolerant of wind exposure. It grows best on a free-draining soil of poor to moderate fertility. Droughty, poorly drained and alkaline soils are unsuitable. Its nitrogen-fixing capacity (due to Frankia in root nodules) helps to ameliorate infertile sites. Provenance selection There is currently no information available to guide provenance selection in Britain. Seed production and nursery practice As with other alders, the seed is light and is wind and water dispersed. This species also reproduces by profuse stump and root suckering and can, therefore, become invasive. Nursery practice is believed similar to other alder species (Gordon and Rowe, 1982; Gordon, 1992; Aldhous and Mason, 1994; Gosling, 2007). This includes cold storage below 4°C, after drying to less than 10% moisture content. Silviculture and yield Green alder may find a role as a soil-improving or diversification species within upland conifer forests on certain site types. It is less likely to become over-competitive than red alder (Evans, 1984). Vann et al. (1988), reviewing the establishment success of alder species on former industrial sites in the Pennines, suggested that the European subspecies of green alder had performed as well as native black alder, while red and Sitka alders had performed poorly. The use of hybrids with red alder to enhance frost resistance has been proposed (Evans, 1984). Green alder has become an invasive weed in New Zealand, and this should promote caution as to its potential deployment in Britain, especially near native woodland sites.

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Pests and diseases Forest Research (2018b) indicates that susceptibilities are likely to be similar to those of Italian alder. Timber properties and utilization There is little record of domestic use of the timber due to its poor stem form: any material harvested from British deployments would be likely to supply woodfuel biomass markets.

ARIA (Pers.) Host ARIA NIVEA Host

Whitebeam

The family Rosaceae to which Aria belongs is large and consists of trees, shrubs and herbs. Among the trees are most of the temperate regions’ fruit trees, including plums, cherries, peaches, apricots and almonds, as well as apples, pears and quinces. There are approximately 100–160 genera. The scientific name of A. nivea was, until recently, Sorbus aria (see p. 299). It is distinguished from Sorbus by the simple (not compound) leaves which have a persistent white or greyish tomentum (felty hairs) beneath. The ripe fruits taste sweet, without the bitterness found in the fruits of almost all species of Sorbus (International Dendrology Society, 2018). Origin The tree occurs throughout temperate Eurasia from western Europe to Japan. It is usually found on chalk and limestone, and also on sandstone hills from Kent and Hertfordshire to Dorset and the Wye valley. It has been widely planted elsewhere. It is a widespread species, occurring from Norway to central China. Climate and site Apart from requiring a well-drained soil, the species is not demanding. It will grow in both acid and alkaline conditions and will tolerate drought, unseasonable frosts, exposure, salty sea winds and atmospheric pollution, but not wet soils. It is very hardy and particularly thrives on dry soils derived from limestone and chalk.

Silviculture Whitebeam is a small, relatively short-lived, rather uncommon, native tree, which occasionally grows to 20 m tall on good soils, but more commonly does not exceed 12 m. It is a colonizing and strongly light-­demanding species, though it becomes reasonably shade bearing with age. It is normally found in open situations where it can be a useful pioneer, especially in windy places and on chalky soils. Whitebeam is said to make a valuable nurse for beech (Anderson, 1950). It is rather susceptible © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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to fireblight (Erwinia amylovora). It coppices well and also produces root suckers o ­ ccasionally. Huss et al. (2016) stated that it is ‘poorly competitive’ and therefore only able to thrive in mixture with other broadleaves in open, slow-growing woods or on rocky ground where it cannot be overtopped by neighbours. Natural regeneration The seeds of whitebeam are distributed by birds, and regeneration will appear on suitable sites in open places, though never prolifically. Flowering, seed production and nursery conditions Whitebeam produces showy white, insect-pollinated flowers in May and June. The seeds, in attractive red berries, mature in September and October. Propagation in the nursery normally requires the berries to be macerated in order to separate out the seeds, which should then be stratified and sown the following spring. If the seed is not separated it may remain dormant for an additional year (Aldhous, 1972). There are about 1500 seeds kg−1 of fruit.

Fig. 4.  Whitebeam, Aria nivea.

ARIA NIVEA Host

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Genetics There are many subspecies, varieties and hybrids of whitebeam. Timber The wood is hard, heavy (at about 860 kg m−3), fine grained and white, but is never found in large dimensions or in great quantities. In the past it was widely used for turnery; for joinery and the handles of cutlery; and, because of its hardness, as cogs in early machinery. It is said to be durable and will take a high polish. It warps badly on drying and is liable to split (Elwes and Henry, 1906–1913). Place of whitebeam in British forestry In common with a related species, rowan, whitebeam has considerable decorative value and is often planted for this purpose in towns. This is its main use today, though the closely related Swedish whitebeam is probably used more frequently. It is also a useful species for planting on landfill sites that are being reclaimed (together with ash, white poplar and wild cherry) (Moffat et al., 2008). Otherwise, it is an occasional and attractive constituent of woodlands where it is also a source of winter food for animals and birds.

BETULA L. The birch family (Betulaceae) has six genera: the birches, alders, hazels, hornbeams, hazel-hornbeam and hop-hornbeams, numbering a total of 167 species. They are mostly natives of the temperate northern hemisphere, with a few species reaching the southern hemisphere in the Andes in South America. Their typical flowers are catkins and often appear before leaves. Currently, 102 species of Betula are recognized, distributed across the northern hemisphere. The number of species is, however, widely disputed: species delimitation is problematic and hybridization is rampant (International Dendrology Society, 2018). There are 35 species of birches in the northern hemisphere, four of which occur in Europe, with three native to Britain. One of them, B. nana (dwarf birch), is a localized shrub of mountains and moorlands. Most birches are attractive, rather short-lived (ca 70 years) pioneer species of small trees. They are hardy and their main values are as soil improvers and nurses to protect more tender species from frost damage. They seldom grow to large dimensions. In recent years there has been a resurgence of interest in birch as a ­potentially productive native species for use in upland and more northern parts of Britain.

BETULA PENDULA Roth BETULA PUBESCENS Ehrh.

Silver birch Downy birch

Origin Silver birch and downy birch may only be distinct at a subspecific level, according to Mabberley (2008), and are native to the whole of Europe (­including the British Isles) and to parts of Asia. They extend to the northern limits of tree growth. In northern Europe, birches are commercially the most important broadleaved tree species. Though not often planted, natural regeneration is plentiful and they are among the most common of all British forest trees (see p. 8). Silver birch is more common in the south and east of Britain, while downy birch occurs more in the north and west. It can be difficult to distinguish between the two species, but silver birch characteristically has hairless leaves. They are triangular in shape with a long tip and have margins that are doubly serrated with the primary serrations curved up towards the apex. Downy birch has hairs, at least on the lower veins. The underside of the leaves has white hairs in the axils of the veins and on the leaf stalk. The leaf is rounded in shape

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© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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and normally has single teeth which do not curve up to the apex. In difficult cases, a chemical test developed by Lundgren et al. (1995) using high-performance liquid chromatography (HPLC) may be used on inner bark extractives. Diarylheptanoid glucoside platyphylloside is present at high levels in B. pendula (20–60 mg/g dry bark) and at very low levels in B. pubescens (≤ 0.5 mg/g dry bark). Based on these observations, a new simple and accurate precipitation method to distinguish between B. pendula and its hybrids, and B. pubescens has been developed by the authors. Climatic requirements Both species are extremely hardy and have no major climatic limitations, although silver birch does better in slightly drier conditions than downy birch. They both thrive at elevations at which no other broadleaved species except rowan will grow, but the form of the trees can be adversely affected by even quite low levels of exposure. The birches (particularly downy birch) are more sensitive to atmospheric pollution caused by sulfur dioxide and nitrogen dioxide mixtures than many other trees (Freer-Smith, 1984). With climate change the species may be increasingly vulnerable to drought on drier sites (Forest Research, 2018b). Site requirements The ranges of the two species overlap, but silver birch occurs most commonly on lighter acid soils, heaths, gravels and shallow peats in the drier south and east of the country at low elevations. It will also grow on heavy clays, and on chalk and limestone soils, but on these it is much less common. Downy birch replaces it on badly drained heathlands and on damper soils generally, especially in waterlogged and peaty conditions in the north and west at higher elevations. On sites where both species flourish, silver birch is considered the faster-growing and higher-yielding tree. The absence of birch from a site is often said to indicate a deficiency of phosphate in the soil. Both species are reported to grow over a wide range of soil pH conditions, but to grow good birch trees of 30−35 cm diameter at breast height (dbh) over a rotation of about 30 years, oak or ash sites are required. In the northern part of their range birches occur in both lowlands and uplands; but, in the south, they are almost exclusively mountain species. Other silvicultural characteristics Birches seldom grow taller than 20 m, though may occasionally reach 25−30 m. The biggest birches are found in parts of central Europe where

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they develop into trees of ‘very considerable dimensions’ (Brown and Nisbet, 1894). Their main features are their ability to colonize bare areas quickly, their intolerance of shade, their lack of affinity for any particular soil type and their ability to grow on nutrient-poor soils (Atkinson, 1992). In Britain the birches are pioneers in forest or heathland recently cleared by felling or fire, in gaps left by canopy trees, as primary colonizers of gravel and scree, and in habitats climatically unsuitable for most other tree species. They become established most effectively on bare soils but establish poorly in even the lowest vegetation (Atkinson, 1992). Both species are gregarious and often occur in even-aged stands. These may be pure birch stands (often a mixture of silver birch and downy birch) or they may be mixed with other species. Apparently, early mycorrhizal infection is an important factor in the successful establishment of birch. Birches are also commonly associated with oak and pine woodland on better-drained soils. The birches grow fast when young but very seldom achieve large dimensions in Britain. Except on the most favourable sites, they rarely attain 30 cm dbh. Though light-demanding, the birches will grow as an understorey in open forest. Vegetative reproduction by sprouting from buds at the base of the stem occurs as a response to damage such as burning, felling or grazing. Clusters of buds are more common in downy birch than in silver birch, which possibly explains the slightly poorer sprouting ability of the former. Nevertheless, both species are ­regarded as weak at coppicing in comparison with many other broadleaved trees (e.g. Leonardsson and Gotmark, 2015). The main silvicultural values of birches are: • their improving qualities on soils such as podzols (Miles, 1981). These arise because birch has access to sources of calcium that are unavailable to many other species because they can root more deeply and also penetrate iron pans. Calcium and other plant nutrients are deposited on the surface in fallen leaves, and gradually the surface pH of the soil increases, eventually allowing earthworms to colonize the site. These then mix the surface organic layers with the mineral soil; • their hardiness, which allows them to be valuable nurses for oak, beech and frost-tender conifers on sites liable to early frosts; and • their considerable amenity value, especially of silver birch. Birches have uses in the afforestation of industrial waste sites and for the shelter they can provide for wild and domestic animals, especially in hilly areas. They are of most interest to foresters in the more exposed districts of northern England and Scotland than elsewhere. Where there is a market for birch, encouraging natural regeneration can be quite a profitable silvicultural option (using the conventional net discounted revenue criterion) because regeneration is free, rotations short (often 30−40 years)

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and yields reasonable for broadleaved species. Willoughby et al. (2007a) have also described recent interest in using direct sowing in some upland situations where natural regeneration fails to occur. For direct seeding they recommend sowing a minimum of 1 million viable seeds per hectare (i.e. about 0.5 kg) into weed-free and brash-free seedbeds no later than the middle of March, and that browsing mammals and insects are controlled or excluded. Grazing by deer and sheep inside and immediately outside woodland prevents regeneration of upland birch woodland. Because of their tolerance to a wide range of soils, relatively small sizes and attractiveness, the birches are favourites for planting in small gardens and in unpolluted urban areas. They have few pests. They are sometimes browsed by deer, and when a choice of both species is available, downy birch seems to be preferred. ‘Witches’ brooms’ are quite common on birch trees and are caused by the fungus Taphrina betulina. The species are among the most susceptible of broadleaves to honey fungus, Armillaria spp. Natural regeneration Natural regeneration is plentiful and this causes birches to be among the most common of all British forest trees. It can be so prolific that it can be a nuisance in plantations of other species. Flowering, seed production and nursery conditions The birches flower before the leaves are fully out: silver birch may begin in late March, but the main flowering period is April. Birches are wind pollinated. In the south, seeds ripen in July/August in silver birch and in August/ September in downy birch. They ripen a few weeks later in the north. They begin dispersal at once, and seed continues to be released throughout most of the winter. The first good seed crops appear at about 15 years, and the best ones between 20 and 30 years. Seed is commonly produced in profusion every year or every other year. There are about 1,900,000 seeds kg−1 of silver birch (range 1,200,000−2,900,000) and 3,570,000 kg−1 of downy birch (range 1,650,000−9,900,000), of which 40% normally germinates. For nursery purposes the seed should be collected immediately before natural dispersal and stratified for 2 or 3 months before sowing in March or early April. Aldhous (1972) stated that birches are very sensitive to seedbed surface conditions; these should be smooth and well-firmed, with an even, light covering and kept moist. Birch seeds lose their viability quickly. Less than 10% remain viable after 1 year.

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BETULA PUBESCENS Ehrh.

(A)

(B)

Fig. 5.  (A) Downy birch, Betula pubescens; (B) Silver birch, Betula pendula.

Provenance and breeding Hybridization between the two species is very rare; a possible 0.02% of hybrids have been found in Finland (O’Dowd 2004; Huss et al., 2016), and when hybrids occur they are sterile. Atkinson (1992) and Worrell (1992) have summarized the results of several studies carried out in Scandinavia and Scotland on the effect of growing birch seedlings at different latitudes. The primary outcome of these trials was that when seed lots are transferred over ranges of latitude greater than 3° (about 300 km) their growth is reduced, but when transferred over shorter distances growth is essentially the same. For provenance selection Lines (1987) recommended that seed should be collected from British stands of good appearance taken from similar or slightly more southerly latitudes than that of the planting site. Forest Research (2018b) currently recommends that British seed sources of good form or ­material

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from breeding programmes should be used. Seed sources from more continental European climates should be avoided. Hubert and Cundall (2006) evaluated up to 40 Scottish and northern English sources planted at four contrasting Scottish sites. Their results showed that seed sources from the south and east of the trial sites were generally more vigorous than those from the north and west. This could possibly be e­ xplained by the fact that more southerly sources come into leaf earlier in the spring and lose their leaves later in the autumn than those from further north, so they have a longer growing season. Thus, transfer of northern English silver birch to Scottish sites gives an increase in growth rate compared with most local material, at least in the early years, but the long-term r­eaction of such provenances to spring and autumn frosts and exceptionally low winter temperatures, ­especially at more ­exposed sites, is not known. Care should be taken to avoid moving birch provenances over distances greater than about 2−3° north or south. According to the EUFORGEN (2018b) website, in the northern parts of the distribution area (Finland and Sweden), controlling the distance of provenance shifts is crucial for the successful use of planting material. To avoid the risk of late spring and early autumn frosts, the recommended maximum transfer distance in Finland is 150 km either north or south. The transfer distance could be larger at low elevations in central European countries. In view of their widespread occurrence, there is scope for breeding to improve the qualities of these species, and this work is in progress (Hubert et al., 2010). Birch is a rewarding tree for breeders because it flowers early and produces plentiful seed: potted grafted cuttings of superior trees grown in a polytunnel will produce seed in only 3 years. Considerable progress has been made in recent years jointly by the Forest Research Agency and Future Trees Trust, and ‘Qualified’ (see note on pp. 76–77) seed for some areas of Scotland became available for the first time in 2012. Two types of birch trees can commonly be found: smooth- and roughbarked trees. The latter give an attractive ‘flaming’ grain to the wood, but unfortunately the smooth-barked specimens overwhelmingly provide the trees included in breeding programmes. There are a number of ornamental forms of silver birch. Area and yield Birches occupy some 235,500 ha, or almost 9% of the total forest area of the country (Forestry Commission, 2018a). Much of it was classed as ‘scrub’ in the Forestry Commission census of 1979−1982 (Locke, 1987). Though this term is no longer politically acceptable, it implied that much was unmanaged, and this is almost certainly still the case. The birches are the fourth most common trees in Britain (after Sitka spruce, Scots pine and the oaks). Maximum yields of about 7 m3 ha−1 year−1 may be obtained on the best sites.

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Timber and uses The wood is diffuse-porous, fine-textured and of uniform, sometimes decorative, appearance. Fast growth does not reduce its quality. By most measures it is one of the strongest timbers commonly grown in Britain. It can be worked easily but is not naturally durable (Lorrain-Smith and Worrell, 1991). Its average density at 15% moisture content is about 670 kg m−3. The wavy markings on some boards give a spectacular ‘flaming’ sheen that is sought after by cabinet makers. There is a limited market for birch wood suitable for turnery, but trees of good form have a potentially much wider range of uses, including for plywood, particle board, furniture and high-class joinery as well as pulp. If treated with preservatives it can be used for pallets and fence posts. A highly decorative and much sought-after form of silver birch timber is known as Karelian birch (from its place of origin in Finland and neighbouring parts of Russia). It is believed to be caused by a virus infection. Birches produce an abundance of slightly sugary sap in early spring and it is sometimes collected for making wine. Birch twigs are widely used on horse race courses, to fill out steeplechase fences. The form and autumn colour of the birches, particularly silver birch, are very appealing. Their general elegance of habit leads to their being widely planted, both appropriately where a smaller tree is genuinely ­required, and inappropriately where a larger, more permanent tree would be more suitable. Place of birches in British forestry There is little tradition of using birches as anything much beyond nurses and soil improvers in Britain, but an interest has developed in recent years in their potential as productive native species for use in upland areas. If efforts at breeding them for improved form are successful they could become economically important trees, as they are in Finland. Note Under EU regulations reproductive material has to be categorized in one of the following groups: 1. Tested material comes from selected individual trees or stands that have been evaluated for genetic quality or, in comparison to accepted standards, have been shown to be superior.

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2. Qualified material originates from selected superior individual trees that have not undergone any form of testing. 3. Selected material is collected from stands showing superior characteristics such as better form, growth rate, health or wood properties. 4. Source-identified material comes from general or specific locations within a single region of provenance or native seed zone with a specified altitudinal band but with no specific superior qualities recognized (Forestry Commission, 2007).

BUXUS L. BUXUS SEMPERVIRENS L.

Box

The Buxaceae is a small family with five genera (Plant List, 2018). Origin and distribution The evergreen box is one of about 30 species of box (Buxus), two of which are western European. Although it is thought to be native at some sites, including Box Hill (Surrey), Boxley (east Kent), Ellesborough and Kimble Warrens (Buckinghamshire) and possibly elsewhere, it has been widely cultivated since Roman times and the limits of its native range are uncertain. It was extensively planted in Victorian times for pheasant cover, and its recorded introduced range has increased dramatically since 1960 because of planting and more efficient recording of introduced trees. The species is believed to be native to western and southern Europe, extending from southern England southwards to northern Morocco and eastwards through the northern Mediterranean ­region to Turkey. It is widely naturalized outside its native range. Climatic requirements Box can withstand temperatures down to −25°C: it is thus hardy, though it does best in regions where winters are relatively mild. Site requirements In natural conditions the species typically grows on soils derived from chalk and limestone, often as an understorey in forests of larger trees, and is most commonly associated with beech. It also sometimes occurs in open dry montane scrub, particularly in the Mediterranean region. Box Hill, Surrey, is named after its notable box population, which comprises the largest area of native box woodland in England. Virtually pure box woodland can be found on loose, dry, crumbly chalk on steep slopes. On such sites bigger trees, including beech and yew, become unstable and soon fall. Chard (1949) noted that box favours the lower and outer edges of ‘hanging’ woods, on south-facing slopes above the level at which valley frosts occur, in the few places in Britain in which it is indigenous. These are the same sites where walnut will grow particularly well.

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When planted, box will grow on most soils that are well drained, r­ anging from light sands to heavy clay soils, and at a wide pH range from 5.5 to 7.4, so it will thrive on acid, neutral and basic soils. However, it does best on chalky soils and is unusual in this respect. It will grow well on both dry and moist soil, but soils prone even to short periods of waterlogging should be avoided. Other silvicultural characteristics Box is normally a shrub, sometimes a small tree. It is evergreen, growing to 1–9 m tall, with a trunk up to a maximum of 20 cm diameter. Its flowers are a good source of nectar for bees. Box is a slow-growing late-successional species and as such is a strong shade bearer but grows best under a light woodland canopy and will even grow in dry shade. Little is known about appropriate silviculture for the species. The CABI Forest Science database (www.cabi.org/forestscience) yields few abstracts on the species and none provides information that is helpful for its management as a potential producer of valuable wood. In spite of this, there is no reason to suppose that box is fundamentally different from other trees and shrubs. On the right sites it is likely to respond well to weed control when young and reductions in competition from neighbours when older. It is known to be normally a rather contorted shrub, and if the production of straight wood is an objective, it will certainly respond to pruning, which it withstands well. The foliage of box is poisonous, which causes the species to be viewed as a problem in places where grazing is practised. In parts of north-western Anatolia (Turkey) this has resulted in attempts to eradicate it and caused it to be classed as a protected Red Data Book species. Box is a popular plant in gardens, being particularly valued for ornamental hedging and topiary because of its tolerance of close shearing, its small leaves and pungently scented foliage. The scent is not to everyone’s liking: the herbalist John Gerard found it ‘evill and lothsome’ and at Hampton Court Palace Queen Anne (1702−1714) had box hedging grubbed up because the smell was offensive to her. Plants are tolerant of being trimmed, and can be cut right back to the base if required and will usually coppice freely. Diseases and pests This species is notably resistant to honey fungus but suffers from a number of dieback diseases, including Cylindrocladium buxicola. This was reported as a pathogen of the genus in the 1990s in Britain and since then, it has spread throughout northern Europe. Where waterlogging occurs it can

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Fig. 6.  Box, Buxus sempervirens.

also be killed by Phytophthora parasitica. Box rust (Puccinia buxi) is an occasional problem, especially on old plants. Box is not palatable to either rabbits or deer. Natural regeneration and nursery propagation On sites where the species grows well, natural regeneration can be both prolific and vigorous. In nurseries, seed stratification is not necessary but can lead to more regular germination. Cuttings from short side shoots with a heel will root easily if taken in September. Provenance Nothing has been written about provenance, but many named varieties of box have been developed for their ornamental values. Area and yield The area covered by box in Britain probably does not amount to more than a few hundred hectares. No information exists about growth rates, except that they are slow. Timber and uses Due partly to its slow growth, the wood is the heaviest (at 950−1200 kg m−3) and hardest of any that will grow in Britain. It is also free of grain produced by growth rings and is in limited demand for making musical instruments and for inlaid work, for wood turning and tool handles, and

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formerly for mathematical instruments such as rulers. Most now comes from Turkey. It was also used for making engraving blocks for printing illustrated books and newspapers but has been largely replaced by Venezuelan box (Casearia praecox) and Japanese box (Betula microphylla). For musical instruments such as the classical oboe, dense, slow-grown wood is required that provides good sound projection. Ideal logs are 20−23 mm diameter, grown on rotations of 120−150 years. Box was once used for making these instruments but was replaced for a long period by African blackwood (Dalbergia melanoxylon). The latter is now becoming scarce, so demand for box is increasing again. Place of box in British forestry Box is a very minor species in British forestry, but there is a limited ­demand, for making high-quality musical instruments.

CARPINUS L. CARPINUS BETULUS L.

Hornbeam, Musclewood (USA)

The Betulaceae, of which Carpinus is a member, has six genera: the birches, alders, hazels, hornbeams, hazel-hornbeam and hop-hornbeams, numbering a total of 167 species. They are mostly natives of the temperate northern hemisphere, with a few species reaching the southern hemisphere, in the Andes in South America. Origin There are some 41 species of Carpinus, and C. betulus is one of two species native to Europe. Its natural range extends from the Pyrenees to southern Sweden and eastwards to Iran. In Britain it mainly occurs in the south-east of England at elevations below 600 m. It resembles beech in many superficial respects and is often confused with it (Brown and Nisbet, 1894). The trunk is usually irregular and ‘lumpy’, the suggestion of bulging muscles under the bark skin giving rise to the American name ‘musclewood’ (International Dendrology Society, 2018). Climatic requirements These are not clearly known, but it appears to require warm summers and so is normally a tree of lowlands. Hornbeam is, however, very hardy, even in frost hollows. According to Schlich (1910), it will thrive ‘even in cold moist localities unsuited for beech’. The cold hardiness does not seem to extend to hardiness to wind, or to climatic exposure (Hemery and Simblet, 2014). Site requirements The tree is adapted to a wide range of soils, from wet (even waterlogged) heavy clays to light, dry sands and mildly acid to alkaline conditions, but not to chalky soils. It does best on moderately fertile damp sites: Evelyn (1678) suggested it was suited to cold hills, stiff ground and the most ­exposed parts of woods. It requires more fertile sites than beech and achieves its largest dimensions on clayey soils. Shallow soils should be avoided. Brown and Nisbet (1894) said that it is ‘essentially a tree of the plains and low-lying localities and can there often yield good service as an underwood on soils that are somewhat too moist for beech’.

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Other silvicultural characteristics Hornbeam is usually a small- to medium-sized tree, seldom exceeding 25 m tall, frequently much smaller. Stems are often fluted and crooked, and the tree tends to branch low down so the stem is short. In fact, it is seldom seen growing to the dimensions of a timber tree though this tendency can be minimized by growing it in close proximity with neighbours. The species is one of few strongly shade-bearing native trees, though slightly less so than beech. Its main silvicultural value is as an understorey tree on soils that are too moist for beech, including those prone to waterlogging. In continental Europe, where it is believed to be a soil-improving species, it is commonly encouraged on wet soils as an understorey to oak, to help in the suppression of epicormic branches. Hornbeam coppices very well and was once important as a coppice or pollarded species, when it was grown for fuel and basketmaking. Hornbeam, like beech, is less browsed by deer than oak, but it can be severely attacked by grey squirrels. In natural conditions, hornbeam is a successional species, succeeding pedunculate oak, together with elm (Ulmus minor) and field maple (Zlatanov, 2006).

Fig. 7.  Hornbeam, Carpinus betulus.

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Natural regeneration On suitable soils natural regeneration can be plentiful and vigorous. Flowering, seed production and nursery conditions The tree is monoecious and flowers in March as the leaves come out. Seeds, which are produced prolifically, ripen between August and November and are dispersed from then until the spring. They are best collected for nursery purposes in November. The earliest age at which hornbeam bears seeds is about 20–30 years, but the best seed crops are usually at intervals of 2–4 years between the ages of 40 and 80. There are about 24,200 seeds kg−1 (range 16,000−30,800), of which 45% normally germinates. Seeds need stratifying for about 1 year before being sown in the nursery unless they are collected and sown ‘green’ (i.e. as soon as they are ripe but before they have dried fully on the tree). Provenance No specific work has been done, but Worrell (1992) speculated that, as a relatively recent colonizer in Britain after the Ice Age, hornbeam is ­unlikely to be affected by the use of continental European seed, particularly when material from the near continent is planted in England. Continental European authors usually state that big differences in stem quality can be found according to provenance. Area and yield No recent figures are available about the area of hornbeam in Britain, though Locke (1987) estimated that there were 3400 ha of coppice being worked in England. Timber and uses The wood is hard, heavy, strong, tough and white, and finishes very smoothly. It was once the main source of very hard wood in Britain that was available in larger sizes than the equally prized boxwood. It is used for anything that requires a high resistance to wear such as piano mechanisms, drumsticks, billiard cues, chopping blocks and for flooring as a satisfactory alternative to maple. Its utilization is not helped by the fact that the tree is frequently fluted and of poor form. Because the wood is very

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dense (about 770 kg m−3 at 15% moisture content), and consequently has a high calorific content, it makes an excellent fuel. It is difficult to work. Its modest size, dense crown, ability to withstand cutting and dead leaf retention during winter makes it a popular hedging plant and a common tree for planting in streets, parks and gardens. As a street tree, a fastigiate variety is normally used. Place of hornbeam in British forestry Hornbeam is of limited importance in Britain. As a shade-tolerant tree its main silvicultural value is as an understorey species on soils prone to waterlogging and as a hedging plant. Its wood is valued for a number of specialized purposes, though never in large quantities. Its stem form is often very bad, but it seems that this could be improved by selection of suitable provenances (e.g. Huss et al., 2016) as well as by breeding. Hemery and Simblet (2014) stated that ‘It is mystifying that the species is not planted more widely, especially on “difficult” sites’. They regarded it as an overlooked tree that could play a much larger role in forests of the future, especially if its form could be improved by breeding.

CARYA Nutt.

Hickory, Pecan

Carya species produce the pecan nuts and hickory timber of commerce. They are in the Juglandaceae, a family of 12 genera and some 89 species, ­including walnuts (Juglans) which are also important nut-producing trees, and wingnuts (Pterocarya). They have large, aromatic compound leaves.

Origin, taxonomy and introduction The hickories are a large group of hardwoods with a few species native to China, Indochina and Assam (India). Most are in North America. Of the total of about 19 species, those most likely to be sufficiently cold hardy for British conditions are the ones native to the northern Appalachians and New England. Those of primary interest are shagbark or little shellbark hickory (C.  ovata (Mill.) K. Koch); the mockernut, big bud or white heart hickory (C. tomentosa (Poir.) Nutt.); pignut hickory (C. glabra (Mill.) Sweet); bitternut or swamp hickory (C. cordiformis (Wangenh.) K. Koch); and big shellbark, bottom shellbark or king-nut hickory (C. laciniosa (Michx. f.) Loud.). These are medium to large deciduous trees capable of reaching 25–30 m tall, and exceptionally 35–40 m. All have paired compound pinnate leaves, some visually resembling those of ash. Hickories typically produce long, clean boles with the crown profile varying by species. The bark is smooth and grey when young, becoming fissured or shaggy with age and species. The record of the hickories in Britain is not distinguished, with Macdonald et al. (1957) reporting slow early growth in arboretum collections where mature specimens have exceeded 20 m tall. Performance at Kilmun, Argyll, has been poorer, with C. tomentosa failing completely and C. ovata leaving few survivors. According to the International Dendrology Society (2018), bark is often of taxonomic significance in Carya, though is usually absent from herbarium specimens. Climatic and site requirements Hickories are climatically sensitive, requiring a warm temperate regime without severe frosts. The International Dendrology Society (2018) stated that, like many other North American trees, the hickories need a very hot, humid summer to flourish to their best potential. These tend not to occur very often in Britain. Species suitable for British conditions are those hardy down to −15°C or below. Most successful trial introductions of hickories to date have been within southern England. Hickories tolerate a wide range

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of soils, but not very infertile sandy soils. Within the native range in the eastern USA, the more northerly species occupy freely draining montane upland soils of moderate fertility. There is considerable edaphic variation between hickory species and native ecotypes; this is explored in more ­detail by Burns and Honkala (1990). Provenance, seed production and nursery practice All hickories are wind-pollinated monoecious species, with separate male and female flowers forming on the same plant. They produce heavy, nut-like ‘drupe’ fruits, some of which are commercially important for human or animal foods. This applies particularly to shagbark and shellbark hickories and pecan (C. illinoiensis (Wangenh.) K. Koch). In C. ovata there can be as few as 220 such seeds per kilogram. Seed production begins after 25–40 years and displays a strong ‘masting’ cycle, as in oak and beech (2–3 years is typical for C. tomentosa and C. ovata in their native ranges). Hickory seeds display pronounced dormancy which must be broken by pre-stratification (e.g. 60–150 days at 1–5°C in the case of C. ovata). There is a particular difficulty with propagating Carya spp. in the nursery owing to their intolerance of root disturbance at transplanting. This may imply that containerized nursery systems or direct sowing of germinated seed would be more effective. Planting seeds directly into tree shelters placed where they are wanted as trees, and covering them with a little soil as in the case of walnut (see pp. 158–159), might give good results and save several years of establishment time as well as the cost of having to buy plants. Many hickory species hybridize with each other. It is difficult to source plants in Britain, but seed is available on the international market. There are no domestic provenance trials and, consequently, no provenance advice is available. Those interested in trying hickories should favour northern species and provenances and perform ecological site comparisons. Silviculture In America, hickories are slow-grown, medium-tolerant, late-successional components of temperate hardwood forests, silviculturally comparable to species such as hornbeam. Hickories are not particularly suitable for even-aged plantation silviculture (Burns and Honkala, 1990) and selection silviculture is preferred (Cowden et al., 2014). Most prescriptions in the American literature are for natural oak–hickory stands, with a variety of red and white oak species, where hickory is a minority component. This stand type extended northwards during the 1900s, replacing the former oak–chestnut community, which had been devastated by chestnut blight.

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Hickory is generally slower growing than its oak companions and can be suppressed by mid-rotation unless released by thinning. However, if preserved, it is a long-lived component and can be managed on rotations of up to 200 years. Consideration might be given to testing oak–hickory plantations in lowland Britain, especially if sweet chestnut is seriously ­affected in future by blight (Cryphonectria). Pest and disease issues Little is known of the susceptibility of hickories to pests and diseases present within Britain and Ireland, owing to the lack of trials. Their attractiveness to deer and grey squirrels in the earlier, smooth-barked, phase may be significant. Timber properties and utilization An advantage of many hickory species is their hard, heavy (to 800 kg m-3), ring-porous, shock-resistant, tough timbers. These are suitable for furniture, interior carpentry and fencing, and particularly for tool handles and sports goods (Sachsse, 1980; Seeling, 1998). Some are therefore potential substitutes for ash, particularly in the sports grades (e.g. for hockey/ hurley sticks), although the time required to reach the desired dimensions might be considerably longer than for ash. The firewood of many hickory species is favoured in the North American range and is suitable for smoking and barbecuing meat and fish. Edible nuts are produced from C. ovata, although less favoured than pecans (C. illinoiensis). In Europe, it is mostly used as an ornamental tree (EUFORGEN, 2018b). Place of hickories in British forestry There is probably little case for planting hickories in Britain for the foreseeable future but, as climate warming proceeds, conditions might become more suitable.

CASTANEA Mill. CASTANEA SATIVA Mill.

Sweet chestnut, Spanish chestnut

Castanea is a genus in the family Fagaceae which is made up of eight genera and almost 1000 species. Species in two other genera will be ­familiar to British foresters: Fagus and Quercus. The family is widespread in the northern hemisphere and members of it are often ecologically dominant in northern temperate forests. Origin and introduction There are 12 north-temperate species of Castanea. Sweet chestnut is the only European species, being native to southern Europe (hence the alternative English name, Spanish chestnut), western Asia and parts of North Africa: essentially across the Mediterranean region, from the Caspian Sea to the Atlantic Ocean, and it is widely cultivated elsewhere. It was probably introduced into Britain by the Romans, and is now extensively naturalized in south-eastern England. It was said by Elwes and Henry (1906–1913) to reach its best development in Spain, though the most famous tree – the ‘Castagno dei Cento Cavalli’ – is in Italy. It is located on the eastern slope of Mount Etna in Sicily and is believed to be 2000–4000 years old. Guinness World Records has listed it for the record of ‘Greatest Tree Girth Ever’, noting that it had a circumference of 57.9 m when it was measured in 1780. Above ground the tree has since split into multiple large trunks. Climatic requirements The tree needs a warm summer to do well. Northern Britain is too cold and parts of the east are too dry for it to thrive. It does best in the south of England, on the right soils. Generally, it needs reasonable levels of precipitation with no dry season, or only a very short one (EUFORGEN, 2018b). The tree is rather frost tender and will not tolerate exposure from the wind. It is shade tolerant when young, but becomes increasingly less so. Site requirements Sweet chestnut grows best on deep, reasonably fertile, light soils (especially greensands), with ample but not excessive moisture. It does ­moderately

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well on clays and other stiff soils if the subsoil drainage is good. It does not require high fertility but grows badly and may die back on infertile soils, calcareous soils, badly drained sites and deep heavy clays. Johns (1912) said that ‘chestnut will thrive in most situations except where the soil is stiff and tenacious’. Brown and Nisbet (1894) reiterated this by saying that it will not compete well with other species on indifferent soils. It requires a sheltered site and a dry-bottomed soil. Huss et al. (2016) stated that it is a tree of the lower hills and mountains and is rarely found in the lowlands. According to Rollinson and Evans (1987) the ideal soil pH is 4–4.5. Other silvicultural characteristics Chestnut will grow to large dimensions in Britain: up to 35 m tall and 2 m diameter. The species is slightly more shade tolerant than oak and is sometimes planted as a soil improver on lighter soils. It grows fast when young, coppices well and will also reproduce by suckering. It is said to be unpalatable to fallow deer. It will only grow in diameter if it is well thinned. Where it is not liable to shake (see sections on shake (below) and timber and uses (p. 93)), coppice is commonly ‘stored’ so that it will grow to timber sizes, receiving its first thinning at age 20–22. It never dominates as a forest tree but can be found scattered among other hardwoods (Hemery and Simblet, 2014). Shake Ring shake is a frequent defect in trees even as small as 40 cm diameter, especially on drought-prone sites. Practically all trees over 60 cm diameter are shaken. Sweet chestnut is not extensively planted in Britain mainly because of the widely held belief that it is difficult or impossible to produce logs that are not shaken. Shake is a problem of over-mature trees (Everard and Christie, 1995), though in the Forest of Dean, trees of 40 cm diameter at breast height (dbh) can be badly shaken on light soils. In a very detailed study by Mutabaruka et al. (2005) it has been shown that the majority of trees exhibiting ring and star shake flush later in the spring than sound trees and are normally 60 years old or more. Others have found that trees of more than 60 cm diameter are very likely to be shaken. Shaken trees also tend to have narrower annual rings and larger diameter earlywood vessels compared with their sound counterparts. Most of these findings are similar to those of Mather (1992) for oak, the other species in which shake is common. The incidence of shake can be minimized in the same ways as for oak: by having short rotations and by removing trees in thinnings that exhibit late flushing. In addition, in the case of sweet chestnut,

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removal of those with abundant epicormic shoots, bole curvature or spiral bark is also likely to help. Diseases Chestnut blight, a serious and often fatal disease caused by the fungal pathogen Cryphonectria parasitica, was discovered for the first time in a nut orchard in Warwickshire in November 2011. Another outbreak was found from the same origin in East Sussex, having been introduced on plants imported from a French nursery. Further areas of infection were discovered in 2017 in Devon and south-east London. Plant health ­authorities imposed restrictions on the movement of plants, logs, branches, ­foliage and firewood out of or inside zones within a 5-km radius of three sites in Devon. The same restrictions applied to oak within 1 km of the three sites. The fungus can occasionally affect oak trees, usually when they are standing very close to heavily infected sweet chestnuts, but it does little damage to them. The disease is thought to have originated in Japan and is now also present across much of continental Europe. In the eastern USA this disease has virtually wiped out a closely related species of chestnut, Castanea dentata, since it was first noticed in 1904. An estimated 3.5 billion trees were killed. It has also caused serious damage in orchards and forests in continental Europe. However, there is evidence that the pathogen can weaken in virulence in Europe, allowing infected sweet chestnut plants to recover. This weakening or attenuation in virulence is due to a phenomenon known as hypovirulence (Forestry Commission, 2012). In addition, researchers at the College of Environmental Science and Forestry (ESF), part of the State University of New York, have planted 100 transgenic, young, native chestnuts in a ‘seed orchard’. They carry a wheat gene enabling them to withstand the blight. If that project goes ahead, it could it offer a solution to breeding sweet chestnut that is tolerant to the disease. Chestnut roots are prone to ‘ink’ disease, caused by several species of Phytophthora, mainly P. cinnamomii and P. cambivora. The disease can cause significant damage in mild, humid climates on soils that are wet. Natural regeneration Though naturalized in south-east England, natural regeneration is ­unusual and the spread of the tree limited because the seed crop is never good. Kirby (2009), however, believed that with predicted increased summer temperatures, seed set and natural regeneration may improve, though any tendency towards the tree becoming invasive is likely to be countered by the probable spread of chestnut blight (see above).

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Flowering, seed production and nursery conditions Sweet chestnut flowers in late June and July, later than practically any other tree except the limes. Flowers are pollinated by both wind and ­insects. The large seeds ripen in the autumn if the summer is warm and late enough, but good crops are rare, even in southern England. The earliest age at which the tree bears seeds is 30–40 years; good production begins after the age of 50. The seeds are large, at about 239 kg−1 (range 150−330). According to Gosling (2007), they begin to die if their moisture content falls below 40%, which makes them virtually impossible to store beyond the first spring: viability typically declines from 90% to 50% over the 10–24 weeks between collection in October/November to spring sowing in March/April. There are several horticultural varieties of chestnut and many nut cultivars. Choice varieties are propagated by grafting. Provenance Studies of provenance variation in Britain started very recently, so provenance researchers (Hubert and Cundall, 2006) currently recommend that material should be sourced from good British stands or from selected stands in western Europe. French seed orchards, where trees have been selected for timber production rather than edible nuts, are particularly ­appropriate (Huss et al., 2016). There are a large number of old, grafted cultivars of chestnut grown for nut production. For these varieties, at least, resistant genes have been bred into the gene pool to counter major fungal diseases. This has been achieved by hybridization with the more resistant Asiatic species, C. crenata and C. mollissima. Despite this benefit of resistance and growing ­vigorously in humid climates, the earlier bud burst of the hybrids makes them more vulnerable to frost damage than C. sativa, and they are also less tolerant to drought (EUFORGEN, 2018b). A programme of selection and breeding sweet chestnut for timber in Britain and Ireland was started in 2000 by the Future Trees Trust. It will be many years before seed from this improvement programme can be produced in adequate quantities for planting, partly because of the ­irregularity of seed years and partly because of the small quantities produced per unit area. Area and yield Sweet chestnut is one of the most productive broadleaved species, with mean yield classes (YC) of up to 8 m3 ha−1 year−1 in some parts of England

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(Locke, 1978) and possibly maximum yields of 11 m3 ha−1 year−1. In 1987, Locke estimated that there were 29,000 ha of sweet chestnut in Britain, of which 19,000 ha were coppice. More recently the area has been estimated at 10,800 ha of woodland in which sweet chestnut is the predominant tree species (Forestry Commission, 2003). Yields of coppice are similar to those for high forest (Rollinson and Evans, 1987). Timber and uses The main features of the timber of sweet chestnut are its natural durability, even at small dimensions, the ease with which the wood can be split and its stability after processing, which is superior to that of oak. The strongly ring-porous timber resembles oak, but it is lighter (about 560 kg m−3 at 15% moisture content), less strong and more easily worked. It also lacks the very distinctive medullary rays that are such an attractive characteristic of the timber of native oaks. The heartwood is extremely durable, and an important feature is that, unlike oak, the sapwood (which rots fast when in contact with the soil) is very narrow, rarely exceeding about 1 cm, or about three annual rings. It is a good carpentry and joinery timber. Sound timber works and finishes well and is used for furniture and other similar purposes to oak. In common with oak, the wood also has a slightly corrosive effect on metals and it becomes stained when in contact with iron. As mentioned above, ring shake is a frequent defect in the timber, and spiral grain is also a serious problem in large stems. It was used, before the Great

Fig. 8.  Sweet chestnut, Castanea sativa.

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Fire of London (in 1666), in the construction of many houses and other buildings there, including the hammerbeam roof of Westminster Hall. The tree has been cultivated for over 2000 years in parts of continental Europe, especially Spain, southern France and northern Italy, for its ­edible starchy nuts. They were once a staple food after grinding into flour or coarse meal. They are nutritionally similar to wheat, except that they lack the protein gluten, which is a binding agent, so that anything baked with chestnut flour tends to have holes and is crumbly. In a few southern parts of Europe, chestnuts are still an important source of starch and were described by Evelyn (1678) as a ‘lusty and masculine food for rusticks’. Today they are mostly regarded more as a delicacy, and roast chestnuts are a well-known Christmas treat. Elsewhere they are used in confectionery (e.g. marrons glacés from France), puddings and cakes, and in stuffings. The flour is still used to make polenta which, according to Drori (2018) is tastier than cornmeal polenta. Along with oak, chestnut also provides the staves used for making casks in continental Europe. At one time extensive areas were grown on coppice rotations for posts, hop poles, fuel and split fencing (Begley, 1955; Evans, 1982). Quite large areas are still actively coppiced, especially in Kent and Sussex where it is the last remaining important coppice ­species in the country, though the extent of coppicing is declining. Most of it is worked on a rotation of 12–16 years, which is well short of the age of maximum mean annual increment, but provides for the technical requirements of the split fencing markets. In the Forest of Dean, sweet chestnut is grown on a small scale for pulpwood, on 25–30-year rotations. Place of sweet chestnut in British forestry At present sweet chestnut has a restricted distribution, mainly in the south of England, but – if it escapes chestnut blight – it is one of few species that is likely to become much more widely used if the climate changes in the predicted way. On suitable sites, it produces a valuable and sought-after timber that is similar in some respects to oak. Among its advantages over oak are that it grows faster so rotations are much shorter and it has very little sapwood compared with oak.

CEDRUS Trew

Cedars

Cedrus is a genus of trees in the Pinaceae that grows in the mountains of the Mediterranean region and in the Himalaya. Most of the well-known conifers of commercial importance including firs, hemlocks, larches, pines and spruces are in this family. Most taxonomists (e.g. Farjon, 2017) describe cedars as a genus of two or four species, of which the Indian C. deodara (deodar) is generally agreed to be sister to a clade containing the other three taxa: C. atlantica (Atlantic cedar), C. brevifolia (Cyprus cedar) and C. libani (cedar of Lebanon). No distinguishing gene marker can be found that separates the three (Pijut, 2000). These cedars are often referred to as ‘true cedars’, to distinguish them from numerous other species, both broadleaved and coniferous, that are also called cedars. The name is often used to indicate quality among both broadleaved trees and other conifers (Evans, 2013), including western red cedar (Thuja ­plicata), incense cedar (Calocedrus decurrens), yellow cedar (Chameacyparis nootkatensis) and Japanese red cedar (Cryptomeria japonica). Wikipedia lists 27 such species, including several species in the mahogany family (Meliaceae). True cedars are quite often cultivated in Britain as ornamental specimen trees in landscaped parks and large gardens, most notably at Highclere Castle in Berkshire (as seen frequently on ITV’s Downton Abbey). Mabberley (1990) stated that part of their attraction is that they rapidly develop ‘an air of ­antiquity’ as compared with native trees such as oaks. There is rather little ­experience of them as plantation or forest trees in Britain but, as climate change proceeds, they may come to have a place as productive species on drier sites. It can be difficult to distinguish visually between the Cedrus species, but the following may help: the branch tips and leader of C. deodara droop noticeably, in a similar way to western hemlock. Cyprus cedar (C. brevifolia) has, as the scientific name implies, short needles, up to 1.5 cm long, and the other species have much longer needles. According to Mitchell (1974) the old adage of ascending = Atlantic, level = Lebanon and drooping = deodar works quite well when applied only to the tips of the branches.

CEDRUS ATLANTICA (Endl.) Manetti ex Carr.

Atlantic cedar or Atlas cedar

Origin and introduction The Atlantic cedar is indigenous to Morocco and Algeria in the Atlas and Riff Mountains, between 1000 and 2000 m, where it forms pure stands. It is the only African tree that thrives in Britain and was introduced in 1841. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Climatic and site requirements Forest Research (2018b) stated that the species appears to be hardy at least to −20°C in Britain, but growth and survival are poor in high rainfall areas, so planting should be confined to warmer regions with less than 1500 mm of rain. It does not withstand exposure well but is not sensitive to late spring frosts, and it is capable of withstanding periods of drought. It will tolerate urban pollution better than most conifers. It grows best on soils of poor to medium nutrient status that are dry to fresh in terms of soil moisture. According to Mitchell and Wilkinson (1989) it also grows well on dry calcareous soils. Peats and other wet soils should be avoided. Other silvicultural characteristics The tree occasionally grows to about 30–40 m tall in Britain. Macdonald et al. (1957) believed that it would make a productive species on good sites in the south of England. Growth starts slowly but eventually becomes quite vigorous. It is moderately shade tolerant, but is said to grow faster than the cedar of Lebanon. Like all the true cedars, bare-rooted C. atlantica is difficult to transplant successfully owing to the formation of extensive taprooting. The species has been established in parts of France and Italy as a plantation timber tree since the 1860s (Courbet, 2012). It can either be deployed in pure stands or locally in mixture with other Mediterranean conifers such as pine or fir. The rotation length varies with the site quality and intended timber usage, but would typically be rather longer than that for pine species, at 80–130 years. A target diameter of 45–55 cm is suggested and a target basal area for regular thinning interventions of 28–32 m2 ha−1, reflecting moderate tolerance. A regime of pruning is recommended to produce high-value, finely branched final crop trees. Diseases and pests The Atlantic cedar is susceptible to a similar range of pests and diseases to Lawson’s cypress and Leyland cypress. These include cypress canker, Seiridium cardinale, as well as the cypress aphid, Cinara cupressivora. Peace (1962) recorded that Phomopsis pseudotsugae (now called Phacidiopycnis pseudotsugae) is associated with premature needle fall and sometimes death of whole trees. The root fungi Armellaria mellea and Heterobasidion annosum (fomes) occur occasionally on cedars.

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Flowering, seed production and nursery conditions The tree seldom flowers before it is 50–60 years old. Flowers appear in September or October and the seeds take 2 years to mature. Cones should normally be collected in April before they begin to disintegrate. Seeds should be sown immediately, otherwise viability will be lost quickly, ­especially if they become too dry. Provenance No provenance testing has been carried out in Britain and there are few forest plots on which to base information. The current recommendation from Forest Research (2018b) is that seed should be sourced from the ­native range or from French stands. Using examples drawn from research carried out at INRA (Avignon, France) on Cedrus species, Fady (2003) demonstrated that introduced tree species experience rapid and quite considerable ecological and genetic changes. They seem to evolve quickly into new landraces as a result of selection, genetic drift, population admixture and changes in spatial structure of their mating system. Timber and uses The wood of Atlantic cedar is renowned for its strong and persistent fragrance caused by oleoresins. The wood and cedar oil are known to be a natural repellent to moths and other insects, hence cedar wood is a popular lining for chests and wardrobes in which woollens are stored. The timber remains the primary building construction material in Morocco (Ramsay and Macdonald, 2013). Heartwood is light brown and distinct from the lighter coloured narrow sapwood. Growth rings are very distinct. The wood is straight grained and rather resinous, and the texture is medium to fine. It is easy to work and finishes well. It is durable and suitable for outdoor uses. The density at 15% moisture content is about 560 kg m–3. There is a tendency for the wood to warp when drying. It tends to be soft, brittle and not very strong, lacking shock resistance and toughness. Place of Atlantic cedar in British forestry The Atlantic cedar is a tree that might be suitable for wider cultivation in plantations if climate change proceeds as predicted, particularly on drier,

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CEDRUS ATLANTICA (Endl.) Manetti ex Carr.

(A)

(B)

Fig. 9.  (A) Atlantic cedar, Cedrus atlantica; (B) Cedar of Lebanon, Cedrus libani.

warmer sites in southern and eastern Britain. It is one of very few conifers that will grow well on calcareous soils although finely divided topsoil carbonate may cause problems. One potential use for this species might be in mixed coniferous stands in those areas where Corsican pine is currently the preferred crop, for example in lowland heathland forests. Atlantic cedar is grown successfully in silvicultural mixture with Pinus nigra (both Austrian and Corsican pines) and Spanish fir (Abies pinsapo) in southern France, as at Mt Ventoux. There is a possibility that such mixed stands reduce the impacts of Dothistroma and similar fungal infections on susceptible pines by a ‘host dilution’ effect. This needs to be explored through research for British forestry, as it might allow the current moratorium on Corsican pine establishment to be relaxed where the species was to be used in suitable mixtures (Wilson, 2014). Where an Abies species is being used in mixture in lowland Britain, Greek fir (A. cephalonica) or Turkish fir (A. bornmuelleriana) may be better choices than Spanish fir in terms of growth rate.

CEDRUS LIBANI A. Rich

CEDRUS DEODARA (Roxb. ex G. Don) D. Don

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Indian/Himalayan cedar/deodar

Despite familiarity in British India, this species has seen very little ­attention in British forestry. Specimen trees in arboreta and tree collections typically appear very slow grown. Courbet (2012) reports the species to be of little interest for wood production in France due to its drought sensitivity and vulnerability to late frosts as compared to the Atlantic cedar.

CEDRUS LIBANI A. Rich

Cedar of Lebanon

Although this cedar has been grown as a specimen tree for over three centuries, practically no systematic trials have been carried out into its potential as a forest tree in Britain. Two varieties are recognized by some authorities: cedar of Lebanon and Turkish cedar, although the distinction would probably be considered by Farjon (1998) to be ‘botanical nationalism’.

Origin and introduction The cedar of Lebanon is native to the mountains of Asia Minor, Syria and Lebanon. Farjon (2017) stated that it grows between 1300 and 2100 m elevation. The best place to see the largest numbers of naturally ­occurring trees is the Taurus Mountains in southern Turkey (Drori, 2018). Quite hardy, this light-demanding and calciphile species grows rather slowly. This was the first cedar to be introduced to Britain, in around 1638. According to the Royal Botanic Gardens, Kew, it was partly due to the efforts of the 18th-century landscape gardener, ‘Capability’ Brown, that the species was popularized. He designed more than 170 parks and gardens in England, planting cedars in many of them.

Climatic and site requirements In common with the Atlantic cedar, this species appears to be hardy to at least −20°C in Britain. However, most trees were killed during the exceptionally harsh winter of 1739–1740. Almost all of the huge and ancient-­ looking cedars of Lebanon on lawns of stately homes date from after that year. It has been hardy since then, though young trees are sometimes killed by severe frosts. It is not particular about soil, but according to Brown and Nisbet (1894) it needs a dry, deep, open soil with a permeable subsoil.

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Other silvicultural characteristics The species is a strong light demander and has a wide-branching habit, which some writers say may be detrimental in plantation-grown trees ­unless grown in mixture with other species. However, Brown and Nisbet (1894) stated that ‘if it be at all confined among other trees, it rises with an upright stem like any other coniferous tree’. They also said that it is much too slow growing to be recommended as a timber tree. Pests and diseases This species is largely free of major pathogens, except that it is reported to have some susceptibility to root rot caused by honey fungus, Armillaria spp. It can also suffer from various aphid infestations (Forest Research, 2018b). Others are listed under C. atlantica. Flowering, seed production and nursery conditions Believed to be similar to Atlantic cedar. Provenance There have been very few forest plots and there are no known provenance trials with this species in Britain. The current recommendation from Forest Research (2018b) is, as for Atlantic cedar, that seed should be sourced from the native range. Timber and uses Similar to that of the Atlantic cedar, except that Brown and Nisbet (1894) said that, although the wood is of great durability when grown in its ­native mountains, it is soft and of inferior quality when grown in Britain. However, some older British trees that have occasionally been harvested have attracted high timber prices. This was a major timber production species in the Mediterranean and Near East in antiquity, being used for heavy construction, aspects of ship-building and carpentry (Grove and Rackham, 2003). The timber was reportedly used for the construction of Solomon’s Temple (Evans, 2013) and there are sculptural depictions in Assyria and Egypt suggesting harvest and transport of the timber from Lebanon for high-status building construction. There is evidence of ­organized compartmental management in the Lebanese groves by Roman

CEDRUS LIBANI v. brevifolia Hook f.

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times (Thirgood, 1981; Meiggs, 1982). After the end of the classical period the condition of the groves suffered from over-exploitation and a failure of natural regeneration due to the intensity of browsing by goats under extensive pastoral systems (Grove and Rackham, 2003). Recent years have seen deliberate conservation efforts, despite civil war. Place of cedar of Lebanon in British forestry In common with the Atlantic cedar, this species could increase in importance if climate change proceeds as predicted, particularly on drier, warmer sites in southern and eastern Britain. It might be slightly less a­ ttractive than the Atlantic cedar, which grows faster and possibly with less of a ­tendency to produce large branches.

CEDRUS LIBANI v. brevifolia Hook f.

Cyprus cedar

Courbet (2012) reported the species to be of little interest for wood production in France because of slow rates of growth and vulnerability to late frosts. This would also be likely to hold good in Britain.

CHAMAECYPARIS Spach CHAMAECYPARIS LAWSONIANA (A. Murray bis) Parl.

Lawson cypress

Origin and introduction Chamaecyparis lawsoniana is one of about five species of Chamaecyparis that are found in Eastern Asia and North America. Lawson cypress was discovered and brought to Britain in 1854 by William Murray, the original seed being sent to Messrs Lawson, seedsmen of Edinburgh, after whom the tree was named in Britain. In North America it is known as Port Orford cedar. It is native to the Klamath and Siskiyou Mountains of north-western California and south-western Oregon, mainly at elevations from 1200 to 1800 m. It extends up to 65 km i­nland on seaward-facing slopes but is found mostly within 5–24 km of the coast. Lawson cypress can be confused in the field with western red cedar, but can be distinguished quite easily from it because its leading shoot droops, whereas that of western red cedar is erect.

Climatic requirements In its native range Lawson cypress does best in climates with high precipitation and humidity. It thrives in the western, moister parts of Britain but grows well in drier parts too. The tree is hardy to late spring frosts, e­ xcept when very young, but is not suitable for use on exposed sites and so should be used only in the lowlands. Unlike many conifers, it withstands atmospheric pollution well, which is one reason why it has become such a popular garden tree and hedging species in towns.

Site requirements The tree is not particularly demanding but does best on deep, fertile, welldrained soils. It is unsuitable for use on peat or dry heathery ground and sites prone to waterlogging, and grows slowly on heavy clays. It is one of few conifers that will grow well on calcareous soils.

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Other silvicultural characteristics Lawson cypress will grow to about 25 m tall in Britain. It is very shade tolerant. Natural pruning is extremely slow. It is less susceptible to damage by Heterobasidion annosum than many conifers. In the 1950s it was fashionable to plant Lawson cypress round the outside of lowland woods to use their persistent branches and foliage to minimize penetration of wind and thus to ‘keep them warm’ for game.

Diseases In 2010 a fatal disease of Lawson cypress, Phytophthora lateralis, was introduced into Scotland and later to other parts of the UK. It has decimated the trees in its native home since its introduction in 1952. Neither resistance to the rot nor an effective treatment has yet been found (Zobel, 1990). It kills the roots and can extend up the trunk of affected trees and girdle them, leading to tree death.

Natural regeneration The species produces seed frequently and regenerates freely. It will ­become established particularly under a thinned canopy and where there has been some soil disturbance by mixing the litter layer with the mineral soil.

Flowering, seed production and nursery conditions Lawson cypress is usually a prolific seed producer. It flowers in the spring; seeds ripen in September and October of the same year, and are dispersed up to May. The tree bears seed from an early age, 20–25 years, and the best seed crops are produced at intervals of 2 or 3 years between ages 40 and 60. There are about 463,000 seeds kg–1 (range 176,400−1,322,800), of which 230,000 are normally viable. Cones should be collected as soon as they change from bright green to yellow and the tips of the seed wings are visible and a light brown colour, normally in September. The seed should be sown in nurseries between late February and mid-March: no special treatment is required (Aldhous, 1972). It is also easy to propagate from cuttings.

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CHAMAECYPARIS LAWSONIANA (A. Murray bis) Parl.

Provenance and cultivars In plantations the form of Lawson cypress can vary from trees with excellent, straight, finely branched stems to a multi-stemmed candelabrum-like habit. As no provenance testing has been carried out in Britain, the recommendation is that seed should be obtained from good British stands or from the natural range of the species. Seed collected from hedges or other horticultural sources should be avoided (Forest Research, 2018b). Lawson cypress has a remarkable tendency to vary in cultivation and numerous named cultivars exist: surprisingly, practically all of them have arisen in exotic environments in Europe, rather than its native America. Unfortunately, many plantations in Britain are multi-stemmed, possibly due to the use of unsuitable seed sources. Area and yield There are believed to be fewer than 2200 ha of Lawson cypress woodland in Britain, excluding those in amenity and garden plantings. No British yield tables exist for the species, but those for Thuja plicata are said to provide a good indication of growth potential. Yield classes (YC) range from 12 to 24 m3 ha−1 year−1, with ages of maximum mean annual increment between 50 and 70 years.

Fig. 10.  Lawson cypress, Chamaecyparis lawsoniana.

CHAMAECYPARIS LAWSONIANA (A. Murray bis) Parl.

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Timber and uses The wood, known in the British timber trade as Port Orford cedar, is light yellow to pale brown with no clear distinction between sapwood and heartwood. It has a fine, even texture with a straight grain and a f­ ragrant smell. It is more resistant to decay than many timbers (Ajuong et al., 2014), easy to work to a smooth finish and is stable in service. In its native North America the wood is used for boatbuilding and joinery. The average density at 15% moisture content is about 500 kg m–3. Because of the natural variability of the species, its pollution resistance, the fact that it is so easy to propagate from both seed and cuttings, and its dense impenetrable habit, it is popular as an urban ornamental tree in gardens. It is even more often used for hedging because of its rapid growth. Its foliage is valued by florists. Place of Lawson cypress in British forestry Lawson cypress has always been a minor species. However, the fact that it seeds frequently and regenerates quite freely, withstands pollution well and is easy to propagate both from cuttings and seed in the nursery, means that it is probably grown much more commonly than it should be. This is because on sites where it does well, other species such as western hemlock, western red cedar and grand fir do better. However, its significance is likely to decline with the spread of P. lateralis.

CORYLUS L. CORYLUS AVELLANA L.

Hazel

Origin Hazel is a member of the birch family (Betulaceae). The family has five genera in addition to hazel: the birches, alders, hornbeams, hazel-­hornbeam and hop-hornbeams, numbering a total of 167 species. They are mostly ­natives of the temperate northern hemisphere, with a few species reaching the southern hemisphere in the Andes in South America. Their flowers are typically catkins and often appear before the leaves. The genus Corylus includes about 18 species in the temperate regions of the northern hemisphere. C. avellana is native to Europe and common throughout almost all of the British Isles to elevations of about 600 m; it also occurs in North Africa and western Asia. Hazel was an early colonizer after the last Ice Age (Rackham, 1976). Climatic and site requirements There are no particular climatic limitations in Britain. Hazel tolerates exposure to strong winds well and to urban pollution, but not to salty sea winds. It grows best on damp but not waterlogged soils that are no more than moderately acid and will also thrive on dry, calcareous soils. Soils to avoid are dry, sandy ones and peats. Hazel is frost hardy. Other silvicultural characteristics Hazel is usually seen as a multi-stemmed shrub, 1–6 m tall, but sometimes as a small tree up to 10 m. It is not particularly long lived, perhaps 70–80 years. It is reasonably shade bearing, normally being seen as coppice in the understorey of lowland oak woods, sometimes in ash woods and in hedges. It also forms scrub on exposed limestone. Bluebells are often a good indicator of sites where hazel grows well (Rackham, 2006). Hazel is particularly susceptible to being browsed by deer and cattle, and must be properly protected after coppicing if the new growth is to survive.

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CORYLUS AVELLANA L.

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It used to be a plant of considerable economic importance, which arose partly from its ability to produce numerous straight coppice shoots. It was widely planted or propagated by layering. It was then coppiced, both in pure coppice stands and as a major coppice component in coppice-with-standards systems. As pure-managed coppice about 2000 stools per hectare were planted, and they were cut on a 6–9 year cycle (Hibberd, 1988); when in coppice-with-standards the rotation was 14–16 years. Enrichment of a woodland understorey with hazel can be a useful way of reducing bramble growth in continuous-cover systems. It also provides a useful habitat for birds, including game birds. Flowering, seed production and nursery conditions Hazel flowers in the early spring, before the leaves emerge. The fruits ripen in September and fall from October onwards, retaining their viability for about 6 months. For nursery production they are usually stratified for 3–4 months and sown in early April. However, in Poland, studies by Tylkowski (1999) have shown that the most effective propagation of hazel involves autumn sowing of fresh nuts in the nursery directly after collection. Nuts dried to a moisture content below 12% (on a fresh weight basis) and stored for a few months at −3°C should be soaked in water for 24 h followed by stratification at 3°C for 12 weeks. Nuts that have been stored in sealed containers for up to 6 years, in cool conditions, should be soaked in a gibberellin solution (250–500 mg/litre of Gibrescol, i.e. > 90% of active GA3) before stratification. This treatment significantly stimulates the germination capacity of old seeds. According to Rackham (2006), hazel has fared badly because of grey squirrels; usually all the nuts have disappeared by August while they are still green and immature. It layers easily, which involves leaving a few stems on the stool at the time of coppicing; bending them over to the ground; and, after cutting a slit on the downward-facing side (to promote root growth), pegging the shoots to the ground and covering the pegged area with earth. Roots ­develop from this point and, in time, a new plant. Provenance No detailed work has been carried out, but Worrell (1992) thought that since hazel was an early arrival in Britain after the Ice Age, the survival and growth of continental provenances might therefore be expected to be much poorer than British provenances, particularly when planted in the north and west of Britain.

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CORYLUS AVELLANA L.

Area and yield There were estimated to be some 8600 ha of hazel ‘scrub’ in Britain in 1980 and almost 1500 ha of coppice-with-standards, with hazel as the principal understory species and oak by far the predominant standard tree (Locke, 1987). This represents a considerable reduction since the 1947–49 census when it is believed that there were 12,000 ha of simple coppice and 3500 ha of coppice-with-standards. The yields of hazel coppice were investigated in detail by Jeffers (1956). On a 5-year cutting cycle, with 1700 stems per hectare, yields were about 15 m3 ha–1, and at 10 years about 47 m3 ha–1. For well-stocked hazel coppice the production of usable, trimmed, coppice shoots was about 4.5 m3 ha–1 year–1. Uses Up to the end of the 19th century the many demands on hazel included its uses in the manufacture of wattles for ‘wattle and daub’ plasterwork in timber-framed buildings, sheep hurdles (before wire fencing was common), barrel hoops, garden fencing, thatching spars, fuel for brick kilns and baking ovens, and fascines for laying under roads in boggy areas (Forestry Commission, 1956). It was one of the major coppice species grown in woodlands managed as coppice-with-standards, the c­oppice being cut every 4–8 years. The timber-producing standards usually ­included oak, among other species (Rackham, 1976, 2006). The older wood was sometimes used by joiners and sieve makers, and the charcoal for gunpowder manufacture. The leaves were used for cattle (Schlich, 1891). The nuts, which are produced prolifically from about the age of 10 years, are ­edible. It has been suggested that in Mesolithic and later human settlements in Europe they provided the major source of food later taken by cereals (Roach, 1985). Hazel was still grown widely for nut production in Kent at the beginning of the 1900s. Today about 75% of worldwide nut production comes from the Turkish province of Ordu. On a global scale, according to FAO statistics, Turkey was by far the largest commercial producer of hazel nuts in 2014, followed by Italy. Cultivated varieties of hazel are popular, and many are known and sold as ‘cob nuts’, ‘Kentish cobs’ or ‘filberts’. It is not yet clear whether these are hybrids of C. avellana with C. maxima, or simply cultivars of C. avellana. Details of the morphology of each are shown by Roach (1985). Filberts have long husks, completely covering the fruit. Cob nuts are approximately spherical and the husk is shorter than in filberts. Most of the coppicing that is done today is by naturalists’ trusts and similar organizations. Very limited quantities of produce are sold as thatching spars, and for pea and bean sticks.

CORYLUS AVELLANA L.

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Hazel is commonly used as a mycorrhizal host for black truffles (Tuber melanosporum Vitt.) (Santelices and Palfner, 2010). Hazel is an attractive plant, especially in late winter when displaying the yellow catkins. Because of its tolerance to urban pollution it is useful for planting along road verges. Place of hazel in British forestry Hazel was once a species of considerable economic importance, but today it has little or no value. However, as a native component of traditional woodland systems, it is usually considered a species of great nature conservation significance and is managed for that purpose.

Fig. 11.  Hazel, Corylus avellana.

CRATAEGUS MONOGYNA Jacq.

Hawthorn, Quickthorn

According to the Plant List (2018) there are 424 species of Crataegus. This genus (in the Rosaceae – see p. 67) is native to temperate regions of Europe, Asia and North America. Most species are shrubs, though Mitchell (1974) states that some ‘aspire to tree form’. Hawthorn is native to Britain where it is probably the most common woody plant, occurring naturally along woodland edges (Pollard et  al., 1974). It is also native to most of Europe, north-western Africa and western Asia. It is by far the most common agricultural hedging plant in Britain and occurs commonly in scrub. The white or pale pink flowers each have a single style; they are abundant, and fragrant in May. The dark red fruits (‘haws’) ripen in September and are important for wildlife in  winter, particularly for thrushes and waxwings; these birds eat the haws and disperse the seeds in their droppings. Hawthorn has plentiful, extremely sharp thorns about 12.5 mm long. These, combined with its close branching habit, make it almost completely stock- and humanproof as a hedge. There is a second species of Crataegus native to Britain: C. oxyacanthoides, the Midland thorn, which is mainly confined to heavy soils in mature woodlands in southern England. It frequently hybridizes with C.  monogyna. The two species can quite easily be distinguished by their leaf shapes and by the fact that C. monogyna has a single seed in the fruit, a berry-like ‘pome’, whereas C. oxyacanthoides has two or three seeds. C. monogyna also hybridizes with other species of Crataegus, and hybrids complicate identification. The Enclosure movement between the mid-18th and mid-19th centuries promoted a great deal of hawthorn hedge planting in the lowlands. In upland areas dry stone walling was more commonly used for field boundaries. Climatic and site requirements Hawthorn is abundant nearly everywhere up to 500 m. It is adaptable to all kinds of soils except heathlands and waterlogged soils, but ­appears to thrive best where they are alkaline (Huss et al., 2016). It is very hardy, withstanding atmospheric pollution well, and also wind exposure (though becomes markedly deformed as buds on the windward side are killed). Hawthorn is a very successful colonizer of grassland where it is a major ‘scrub’ species.

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Silviculture Hawthorn occasionally becomes a tree up to 14 m tall, though more often it is a multi-stemmed shrub. C. monogyna is a species of broadleaved woodlands where it typically forms an understorey. It is therefore moderately shade tolerant and also drought resistant. It is also reasonably resistant to browsing, which makes it a good nurse for young oak and some other species, offering protection from browsing by cattle and deer. Huss et al. (2016) described it as slow growing. It tolerates cutting and trimming very well, which partly accounts for its value as a hedging plant, though stems require ‘laying’ every few years, a process described in detail by Pollard et al. (1974). This is because it can re-sprout easily from adventitious buds. Hawthorn is widely planted as a hedge but seldom, if ever, in woodlands. The method and timing of cutting can affect the way the hedge regrows and its thorniness (Bannister and Watt, 1995). Croxton and Sparks (2002), in experimental studies on cutting r­ egimes on haw production, have shown a strong influence of hedgerow management. They found that, in all cases, the yield from uncut hedges exceeded annually cut hedges by a factor of 50.These differences were further ­inflated if yields were considered per unit length of hedgerow. There are estimated to be 72,600 ha of hawthorn in Britain (Table 2, p. 8; Forestry Commission, 2018a). Provenance Trials have demonstrated that local provenance material can be superior to anything available commercially (Jones et al., 2001). Soutar (1991) recommended that seeds or other planting stock should come from the nearest native woodlands. Pests and diseases Like many other plants in the Rosaceae, hawthorn is susceptible to fireblight (Erwinia amylovora), so planting hedges of it round commercial fruit orchards of other susceptible species (e.g. apples and pears, also in the Rosaceae), should be avoided (Berrie and Billing, 1996). Occasionally, in May and June, a rather sinister, ghostly silken webbing covers sections of defoliated hedgerows and, on occasions, individual trees. The culprit is usually a harmless caterpillar of the orchard ermine moth (Yponomeuta padella). The leaves are stripped bare, giving the hedge or tree an eerie appearance. Sometimes webs are so extensive that they can cover nearby objects such as benches and bicycles. The affected hedges are not usually attacked in the following year and there is very little permanent harm.

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Timber and uses The stems are frequently fluted and have spiral grain, but the wood is said to be useful for turnery and as a fuel. The hard, tough timber has been used for tool handles in the past. The timber is dense (ca 520 kg m–3), fine-grained, hard and tough, with a pale pinkish colour. The plants come into leaf early and the prolifically-produced, predominantly white scented flowers make hawthorn hedges a conspicuous and welcome sight in April and May. Its fruits are valued as a source of winter food for game birds, and for many wild birds. In former times they were used for fodder for domestic animals; in periods of severe food shortage they were also used by people, though they are said to be most unappetising! The fresh emerging buds and very young leaves are known as ‘bread and cheese’ and are very palatable to humans. Folklore There is probably no other species about which more myths and folklore exist, extending well beyond Britain. Numerous books and websites deal with this (e.g. Hemery and Simblet, 2014). Hawthorn also has a reputation for medicinal uses; it was especially noted for treating disorders of the heart and circulation system, especially angina (PFAF, 2018).

CRYPTOMERIA D. Don CRYPTOMERIA JAPONICA (L. f.) D. Don

Japanese red cedar, Sugi

The Cupressaceae is the largest conifer family in terms of genera of which there are currently 27 recognized in the Plant List (2018). Prominent trees in the Cupressaceae include western red cedar (Thuja plicata), Lawson ­cypress (Chamaecyparis lawsoniana), Japanese red cedar (Cryptomeria ­japonica) (Savill, 2015b), juniper (Juniperus communis), dawn redwood (Metasequoia glyptostroboides), giant redwood (Sequoiadendron giganteum), coast redwood (Sequoia sempervirens) (Wilson et al., 2016) and swamp cypress (Taxodium distichum). Conifer species of this type occurred commonly throughout the northern hemisphere during the Cretaceous and Tertiary periods, up to the beginning of the Quaternary glaciations 2–3 million years ago (Mai, 1989). The European and western Eurasian representatives became extinct by the process of ‘barrier compression’ against east–west-orientated mountain chains (the Pyrenees, Alps, Carpathians and Caucasus) during the succeeding Pleistocene glacial episodes (West, 1970; Godwin, 1975; Ingrouille, 1995). However, the equivalent species have survived in eastern Asia and North America, where mountain chains run mainly north–south. Japanese red cedar (or Japanese incense cedar, according to Ramsay and Macdonald (2013) or peacock pine (Edlin et al., 1981)) is one of several species being considered as an alternative plantation tree to various more commonly used species if climate change proceeds as predicted, or introduced diseases become more common. Cryptomeria is already being used to some extent by the Forestry Commission in England and Scotland and by Natural Resources Wales, as well as some by private estates. Earlier ­accounts of the species were given by Wilson (2010). Origin and introduction This tree is an endemic giant Japanese conifer related to the two sequoias of North America as well as to T. distichum (Farjon, 2010). According to Farjon (2018), there is one species with two varieties (C. japonica var. japonica (L. f.) D. Don), and the Chinese variety C. japonica var. sinensis Miquel in Siebold & Zuccarini). The Chinese variety is sometimes referred to as C. fortunei. They are distinguished by range and by the morphological differences detailed in the box below. Var. japonica is native only to Japan, where it occurs naturally in pure and mixed stands.

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Key to the varieties (Fu et al., 1999 in Farjon, 2012) 1a. var. japonica. Leaves ± straight at least in proximal 1/2, often recurved apically on leader branchlets, arising at 35–45° to axis on leader branchlets, 45–55° on fertile branchlets, rigid and hard; most pollen cones longer than their subtending leaf; cone scales 20–30, each bearing 2–5 seeds; distal projections of bracts and cone scales 2–3.5 mm. 1b. var. sinensis. Leaves usually strongly incurved throughout, arising at 15–30° to axis on leader branchlets, 30–40° on fertile branchlets, rigid but relatively soft; most pollen cones shorter than their subtending leaf; cone scales ca 20, each bearing 2 seeds; distal projections of bracts and cone scales 1–2 mm.

Japanese red cedar (var. japonica), or sugi as it is known in Japan, is i­ ndigenous only to the central and southern islands of Japan, from Kyushu to northern Honshu, at elevations up to 400 m. It occurs naturally in pure and mixed stands (Vidakovic, 1991). It has also been very widely planted in Japan since the Second World War. The variety has also been introduced for forestry to Taiwan and many provinces of mainland China. Var. sinensis is native to Fujian, Jiangxi, Sichuan, Yunnan and north-western Zhejiang, in China, and has also been introduced for forestry into other provinces of China. In natural conditions it grows in forests on deep, well-drained soils subject to warm, moist conditions at elevations from below 1100 to 2500 m. Typical associated trees in Japan are Fagus crenata, Thujopsis dolabrata, Fagus japonica, Abies firma and Zelkova serrata (Röhrig and Ulrich, 1991). It also grows with a variety of shrubs, including Acuba, Skimmia and Hydrangea, and also tall herbs including Spirea (Elwes and Henry, 1906–1913: p. 3). Wilson (2010) noted that it was a regular inclusion in early 20th century forest gardens in Britain. It has apparently performed conspicuously well at locations across the country, including the Crarae and Kilmun forest gardens in Argyll, where it is among the most impressive species although typically unthinned. Rather larger-scale trial plantings took place from 1930 to 1965 in Argyll, Wales, Cornwall and southern England. Most of the limited experience with the species in Britain has been with the Japanese provenance. The species was originally introduced in 1842 from China and the Japanese variety was introduced in 1861. Troup (1921) noted that Cryptomeria was an important species in British India, especially in plantations around some warmer hill stations such as Darjeeling, Sikkim and Ootakamund in the Nilgiri Hills in the south, at elevations of 1200–1800 m. The timber was used, among other things, for cutting into thin section boards for tea chests (Gamble, 1902).

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Climatic requirements Var. japonica comes from a warm maritime climate and, according to Forest Research (2018b), the best growth in Britain is to be found in areas with more than 1200 mm of annual rainfall but which are sheltered and where the summers are warm. It is described by Ray et al. (2010) as ‘moderately drought tolerant’. There seems, in fact, to be no clear drought limit since it grows reasonably well on the Isle of Wight, and in Alice Holt Forest with 600–800 mm rainfall. The tree does not stand exposure well, especially in spring, and exposed sites should be avoided. The need for warmth means that the best stands are in Wales and south-west England, but it also does well in Wester Ross and Argyll, where it can be a very high-volume producer. Var. sinensis is scarcer than the Japanese variety, but it is thought by some to be better adapted to cool regions.

Site requirements These are not exacting, according to Macdonald et al. (1957). Japanese red cedar grows best on deep, well-drained loams in elevated but reasonably sheltered situations. In windy locations it is prone to foliage scorch and crown fraying, which slow growth. It is not suited to very infertile or dry soils, to peats or to calcareous soils. Heathery sites should be avoided. Other silvicultural characteristics On good sites in Britain the tree will grow to over 30 m tall. In Japan, where it is the national tree, it is renowned for growing to great ages (well over 1000 years) and for the very high-quality timber it produces. In Japan the tree is currently grown on a 40–60-year rotation in plantations, though if large diameter timber is required, it is extended to 80–100 years. Japanese red cedar is sufficiently shade tolerant to be of probable use in continuous cover systems (Hemery and Simblet, 2014). Though there are very few mixed species stands in Britain it may do well with western red cedar, western hemlock and Douglas fir on sites where selective thinning is feasible. As with many such species, pruning is an important component of a management regime to reduce knots, maintain evenly spaced annual rings and to reduce stem taper. In Japan, according to FAO Japan (1997), pruning should start when the diameter at breast height (dbh) is 6–7 cm. By contrast, Macdonald et al. (1957) note that the tree self-prunes well, in a similar way to the larches. Unusually for a conifer, it coppices well and also produces root suckers. In Japan the species is planted at densities that are similar to those used in Britain: 2500–4000 per ha, and on most sites it is weeded once or twice a year for the first 5–6 years after planting. According

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to Elwes and Henry (1906–1913: p. 13) fast-growing plantations are pruned from their 8th to 23rd year, and the earliest thinning begins is at age 12. Natural regeneration Macdonald et al. (1957) mention that sporadic natural regeneration has been recorded, and since then it has become prolific in some Scottish stands. However, Elwes and Henry record that, during the visit of Elwes to Japan, a careful search did not reveal a single self-sown seedling. All young trees originated from suckers or coppice shoots (Elwes and Henry, 1906–1913: p. 13). Diseases and pests Japanese red cedar is susceptible to Phytophthora root disease, including P. cinnamomi. It is also considered to be susceptible to Armillaria root rot (honey fungus). Elsewhere, it has been reported to be affected by juniper blight (Phomopsis juniperovora), which is present in Britain and already widespread on juniper (SilviFuture, 2015). Ohba (1993) stated that ‘strecklings’ (rooted cuttings) are apparently free from needle blight (Cercospora sequoia), which afflicts trees grown from seed. Flowering, seed production and nursery conditions Seeds ripen in September and October during the year of flowering. They should be sown early in spring after stratification (chilling) for 2–3 weeks at 4°C. However, both British-collected and imported seed is reputed to have a very poor germination rate, averaging 12%. There are said to be about 60,000 seeds that will germinate per kilogram. Collections from small British stands may suffer from inbreeding depression due to a narrow genetic base. According to Macdonald et al. (1957) Cryptomeria can be raised in the nursery by standard techniques for cedars, though it is slightly prone to damage by autumn frosts. Japanese red cedar is easy to propagate from unrooted cuttings but is most commonly propagated from strecklings. According to Ohba (1993) there is an average 57% strike rate with strecklings. In south-western Japan more than 200 clonal cultivars have been selected, each suitable for a variety of local conditions. In fact, this species possibly has the longest history of clonal propagation of any tree. There are records of planting rooted cuttings around Kyoto from about 1400 (Ohba, 1993). The attractions of clonal forestry are that both yield and wood quality are predictable. Kimura et al. (2013) believed that vegetative propagation, for example by natural layering, may be an important evolutionary strategy for establishment in

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Fig. 12.  Japanese red cedar, Cryptomeria japonica var. japonica.

harsh environments, such as frequent disturbances by heavy snow. This tends to happen above 1750 m (Taira et al., 1997). Cryptomeria pollen causes allergic conjunctivitis to humans and dogs in both spring and autumn (Fujishima et al., 1995). Provenance Virtually no provenance testing has been carried out in Britain. Forest Research (2018b) recommends that seed should be sourced from the northern part of the natural range within Japan. It is imperative to avoid garden cultivars, which produce natural topiary-like forms. Timber and uses In Japan, sugi and hinoki (Chamaecyparis obtusa) are the most economically important timber species and, of the two, Cryptomeria is the most generally useful and popular tree. It has been planted there from time immemorial. According to FAO Japan (1997) it is the most common plantation conifer, constituting 44% (or 4.54 million ha) of the total area of Japanese man-made forests. In terms of current planting, hinoki is more important. Cryptomeria is described by Edlin et al. (1981) as ‘one of the great timber trees’. The wood is strongly rot resistant, has a natural lustrous gloss and is easily worked. It has long been valued for the beauty of both the tree and the wood, and is widely planted around temples. It was used for making barrels and casks and was said to have revolutionized the brewing and market of saké (rice wine). The wood imparts a peculiarly pleasant aroma to the saké. Outside China and Japan it is very extensively cultivated as an ornamental in warm and cool temperate climates.

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The timber is strong and very durable and for centuries has been used in Japan and China for construction purposes. There is very little experience of processing British homegrown material and attempts are being made to get thinnings from the various older stands tested in the laboratory to assess its timber properties. Most material is probably processed along with that of western red cedar and Lawson cypress without noticing. Peter Quelch has sawn Argyll (Knapdale) material for garden uses and has commented upon its impressive natural durability (Savill, 2015b). It is rather soft and coarse-grained but is easily worked and is used for buildings, bridges, ships, lamp posts, furniture, utensils and paper manufacture (Fu et al., 1999; Farjon, 2012). The density of the wood varies widely from about 300 to 500 kg m−3 at 15% moisture content. The heartwood is a warm reddish-brown colour with some dark streaking, though it can be very variable. In Japan the darkest wood is said to come from the south of the island of Kyushu and is known as Satsuma sugi. The heartwood is typically formed at a cambial age of 20–22 years (Yang et al., 1994), and is fragrant and insect resistant. The sapwood is almost white. There is a suspicion that, in Britain, most has been mistaken by sawmillers for western red cedar with which it is often grown and sold in mixed parcels. Wilson (2010) stated that the timber is lighter in colour but has much the same properties, durability and likely applications. It is described as ‘easy to saw and season, it is favoured for light construction, boxes, veneers and plywood’ (Wilson, 2010). Brown and Nisbet (1894), by contrast, described the wood as soft, white and brittle and unlikely ever to become a timber tree in Britain. It seems possible, however, that they were dealing with juvenile sapwood. A frequent defect found in Japan arises because of the high water content in the tracheids that causes a blackish colour to the heartwood (Kuroda et al., 2009). In Britain, up to the present, the tree has mostly been planted for ­ornament in parks and big gardens. In Japan, where it is a favourite avenue tree, many forms have also been selected for their ornamental values; almost all of them are dwarf varieties. Place of Japanese red cedar in British forestry Sufficient evidence exists that Cryptomeria will perform acceptably well in British forestry, thriving under a wide range of conditions including moderately exposed mountain slopes. It may prove to be a high-volume producer. Streets (1962) has described the history of many introductions to Commonwealth countries. They frequently follow a pattern of initial introductions to gardens and arboreta and then, if successful (perhaps half a rotation later) by small-scale planting on estates and by government ­departments. Cryptomeria has shown itself to be very successful in

Site Haldon, Exeter Alice Holt Forest of Dean Friston Gwydyr, N. Wales Corris, mid Wales Novar, Highlands

Age (years)

Trees ha−1

Top height (m)

Mean dbh (cm)

Basal area (m2 ha−1)

Standing volume (m3 ha−1)

Cumulative volume (m3 ha−1)

General yield class (m3 ha−1 yr−1)

53 65 95 68 73 67 44

398 437 309 556 303 284 648

29.9 24.8 40.5 – 34.6 35.8 24.2

40.9 37.6 64.2 33.8 64.3 64.3 31.1

52.3 48.7 100.1 50.0 98.4 92.3 49.1

499 439 1237 808 1020 990 422

689 875 2028

26 16 22 14 20 26 24

1740 1937 1083

dbh, diameter at breast height; hyphen, not measured; blank cell, not calculated (because, presumably, records of past volume removals were not available).

CRYPTOMERIA JAPONICA (L. f.) D. Don

Table 8.  Data selected from Forest Research sample plots of red cedar in Britain.

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the first stage, the introduction into gardens and arboreta. It now needs to be ­extended into the second stage, involving wider planting. Climate warming should extend the range of sites where it will grow well, such as in western Scotland. It is one of several species that seems worthy of serious trials which should begin with small-scale plantations on the right sites. A few large-scale provenance trials also need to be established, with plots big enough for testing silviculture and wood properties, and should include some Chinese provenances. Wilson (2010) said that it would be necessary to clarify on which site types it might be preferable to western red cedar (Thuja plicata). He felt there were indications that it might do better at higher elevation exposed sites, whereas western red cedar is not usually reliable above 200 m in England and Wales, and lower in Scotland.

CUPROCYPARIS Farjon × CUPROCYPARIS LEYLANDII (A.B. Jackson & Dallimore) Farjon

Leyland cypress

Origin and introduction The tree is a sterile intergeneric hybrid between Cupressus macrocarpa and Xanthocyparis nootkatensis (formerly known as Chamaecyparis nootkatensis). It is usually believed to have originated at Leighton Hall, Welshpool, from cones collected in 1888, though Mitchell (1985) stated it was first found at Rostrevor, in Co. Down, in about 1870. Many clones now exist, including the golden ‘Castlewellan’ form, which also comes from Co. Down. Climatic and site requirements Although very little work has been done on the silviculture of Leyland ­cypress, its requirements seem similar in many respects to those of Lawson cypress. It will thrive on a wide range of soils, including calcareous soils, and does best on moist sites. Leyland cypress is cold hardy throughout Britain and is not frost sensitive. It will tolerate urban pollution and salt spray, but exposed upland sites should be avoided. It is probably best suited to more sheltered sites in western and southern Britain with more than 800 mm of rainfall (Forest Research, 2018b). It is not suited to peats or soils of very poor nutrient status but a­ ppears to grow on alkaline soils. Other silvicultural characteristics Leyland cypress shows very rapid early growth on a wide range of soils of poor to medium fertility. Propagation must be entirely vegetative, and plants are therefore relatively expensive. Leyland cypress is light ­demanding. It has been little used in plantations, and consequently there is little experience with it as a forest tree. Limited experience in Bagley Wood, near Oxford, suggests that after thinning plantations of Leyland cypress they become unusually susceptible to windthrow, for which it ­appears to have a growing reputation. The rapid early growth and moderate tolerance of exposure explains its popularity for hedging, but it is not suited to exposed upland sites, where it is also prone to snow breakage. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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× CUPROCYPARIS LEYLANDII (A.B. Jackson & Dallimore) Farjon

Pests and diseases According to Forest Research (2017), Seiridium canker (Coryneum cardinale) causes scattered twig and branch death on affected trees. Cypress aphid (Cinara cupressivora) also causes similar symptoms of foliage browning, though the causal agent is very different. Leyland cypress is also considered highly susceptible to Armillaria root rot (honey fungus) and Phytophthora root rot (particularly P. cinnamomi). Infection by either pathogen can lead to severe dieback or death. Genetics The natural ranges of the two parent species are separated by about 650 km and so they are very unlikely to hybridize in nature, but they have done so on many occasions when grown together as introductions. Many clones are used in horticulture, but ‘Leighton Green’ has been the most widely used in forestry trials in Britain. Timber The timber is naturally semi-durable, but little of it has ever become available, so it remains relatively unknown. Use in hedging Because of its rapid growth (up to 1 m year−1) and dense foliage, it is used widely as a hedging species. It provides a screen very quickly,

Fig. 13.  Leyland cypress, × Cuprocyparis leylandii.

× CUPROCYPARIS LEYLANDII (A.B. Jackson & Dallimore) Farjon

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but growth is so vigorous that it needs constant attention to keep it within bounds. This characteristic has caused Leyland cypress to be the tree that is responsible for more disputes and litigation between neighbours than any other, because high hedges block light, among other things. It has led to thousands of disputes – more than 17,000 in 2018 according to Drori (2018) – and occasionally to violence and, in the case of Mr Llandis Burdon of Talybont-on-Usk in Powys, to murder. He was shot dead by a neighbour in 2001 after a dispute over the height of a Leyland cypress hedge. The survey published by Kieron et al. (2008) reported that approximately 83% of all coniferous planting in towns was with Leyland cypress and that overall (including broadleaves) 12% was with this species. Place of Leyland cypress in British forestry Leyland cypress has practically no place at all in modern British forestry, but Forest Research (2018b) believes that it could find an expanded role if climate warming progresses as predicted, particularly in western Britain and elsewhere with adequate soil moisture. It will no doubt continue to be the major hedging species.

EUCALYPTUS L’Hérit.

Eucalypts

The genus Eucalyptus is one of well over 100 genera in the myrtle family (Myrtaceae), which dominates the Australian flora. Origin and introduction There are over 700 species of Eucalyptus and almost all occur naturally only in Australia. As exotics they are among the most widely planted, productive and controversial of all species both in the tropics and in the warmer parts of temperate regions. Extensive areas are planted for pulpwood in south-western France, northern Spain and Portugal. Even at these latitudes they occasionally succumb to exceptionally low temperatures and the stems may be killed, though they often regenerate quickly from coppice shoots. In Britain eucalypts have, on the whole, been ­objects of arboricultural rather than silvicultural interest (Macdonald et al., 1957). The first successful introduction, of E. gunnii, was from Tasmania in 1846. Other reasonably successful introductions and possibly useful hybrids have been discussed and listed by Purse and Leslie (2016). Among them are E. glaucescens and E. urnigera. It was appreciated at a very early stage that most of the eucalypts had a limited range in Britain, mostly in coastal regions. Climatic requirements Eucalypts have a reputation for being frost tender and prone to damage or dying in cold winters (partly because they do not form a winter bud) but are capable of growing continuously in warm weather at any time of the year. The risk is particularly high during spells of sub-zero temperatures that follow a mild period; in persistent, very cold, desiccating winds; and also on warm, sunny days when the ground is frozen, ­resulting in high levels of transpiration but poor water uptake and consequent desiccation of shoots (Sheppard and Cannell, 1987). E. gunnii, one of the most frost-hardy species, will survive midwinter temperatures of −10° to −14°C and even short periods of −18°C (Leslie et al., 2012), but they are susceptible to damage from early autumn frosts. Late spring frosts are less important. However, even the most cold-resistant species can be damaged by climatic extremes in British winters, and therefore careful matching of species to site environmental constraints is critical (Leslie et al., 2012). Precipitation requirements for E. gunnii are relatively

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© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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high: at least 800 mm year−1. Leslie et al. (2014) recommended that intensive silviculture be practised after planting to ensure trees are between 1 and 1.5 m tall prior to their first winter, to reduce the extent of damage through frost. Evans (1986) considered that, in general, the use of eucalypts in forestry north of latitude 45° is likely to be very restricted. The south of England is at 50° N, so Britain is probably outside the range where they can safely be grown, but at quite regular intervals occasional enthusiastic Australian visitors manage to persuade research workers to try new species or provenances. The result is that there is a burst of research on eucalypts about every 10–15 years in Britain, which usually ends in failure after the first severe winter, at least in areas that have had cold winters. The species ­believed to be most hardy in the mid-1980s (E. gunnii, E. archeri, E. pauciflora ssp. niphophila and E. pauciflora ssp. debeuzevillei) come from high ­elevations in the mountains of south-eastern Australia and Tasmania (Evans, 1986). The latter two species, while reasonably hardy, grow too slowly to be of interest. In a new rush of activity 20 years later, only E. gunnii from Lake Mackenzie in Tasmania at about 1100 m elevation (41°43′ S or ­approximate equivalent in latitude in the northern hemisphere to Biarritz in south-western France) was considered both sufficiently well-adapted

Fig. 14.  Adult leaves and flowers of cider gum, Eucalyptus gunnii.

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to the extremes of the British climate and to exhibit useful rates of growth (Cope et al., 2008). E. glaucescens is said to be almost as resistant to frost as E. gunnii and has the advantage over the latter of exhibiting excellent stem form. Leslie et al. (2012) said that in the New Forest it showed faster growth than E. gunnii and excellent self-pruning. It is also highly unpalatable to deer. In addition, its rapid growth means that, where survival is patchy, production is still maintained. Site requirements High fertility is not required. Suitable sites are most likely to be in sheltered parts of lowland Britain, particularly south-western England, but even there apparently promising trials have been completely killed in severe winters. Poorly drained sites – including peaty ones – should be avoided, and also those prone to drought. Other silvicultural characteristics Interest in eucalypts arises because of their extraordinarily fast rate of growth if they survive at all. Yields of up to 40 m3 ha−1 year−1 have been recorded in Britain (Purse and Leslie, 2016). Most eucalypts are strongly light-demanding trees and must not be planted in the shade. The majority coppice very well and benefit considerably from thorough weed control when very young. Eucalypts lend themselves particularly well to ‘short-rotation forestry’ regimes which concentrate on the following principles to ensure there is full stocking as soon as possible; this is important given the short rotations of 7–10 years: • Cultivation of the site to improve root growth (stability can be an issue) and also reduce weed competition. • Rigorous weed control through use of herbicides or sometimes mulches. Use of herbicide is facilitated by tree shelters as eucalypts are particularly sensitive to glyphosate. • Rigorous protection (again supporting the use of tree shelters) and timely replacement of dead trees (after the first year, or the original trees will suppress the replacements). Unlike most trees of temperate regions, eucalypts do not have a truly dormant period and so can grow during warm winter weather. Like any fast-growing tree, they use a great deal of soil water during transpiration. They have been planted in some countries to lower water tables

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127

to reduce salination of the surface soil and also, in some tropical countries, as a means of reducing malaria by lowering water levels in swamps where mosquitoes breed. The characteristic of using large amounts of water can also create problems in regions where precipitation is scarce, by lowering water tables and hence reducing the amounts of water available for growing food crops. This reflects poor choice of tree species and land management rather than anything else. Pests and diseases Several species are very palatable to browsing mammals, but probably no more so than most native hardwoods. Provenance Only E. gunnii from Lake Mackenzie in Tasmania (see above) is known to be able to tolerate the climate of the colder areas of Britain (Leslie et al., 2012). There are several subspecies of E. gunnii, some of which are shrubby and multi-stemmed. They offer considerable scope for selection and breeding. E. glaucescens, though appearing promising, is still relatively untested over large areas but has performed well in a number of small trials, some over several decades. There is an enormous interest from pulpwood growers – for example, in Portugal – in breeding highly productive clones of Eucalyptus; in that country, usually E. globulus. Yield Mean annual increments of E. gunnii in Britain of 10–15 m3 ha−1 year−1 on a 12-year rotation have been reported by Evans (1983), and more r­ ecently 18–25 m3 ha−1 year−1 have been claimed for it (e.g. Anon., 2010; Leslie et al., 2012). Growth rates of 40 m3 ha−1 year−1 or more are common in some countries. Purse and Leslie (2016) stated that the biggest determinant of yield (at least in E. nitens) is water availability due to both rainfall and soil water-holding capacity. Yield is little influenced by altitude or average temperature. Timber and uses In Britain recent interest in the use of renewable fuels, including woody biomass grown over short rotations of 7–10 years, has stimulated the

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c­ urrent (2000s) work on eucalypts. They are particularly suited to biomass production because of their relatively high wood density and hence high calorific values. In Britain, eucalypts of 15 years generally have densities in the range of 450–500 kg m–3 at 12% moisture content (Purse and Leslie, 2016). This is intermediate between most conifers, willow and poplar (< 400 kg m–3) and the native hardwoods (> 540 kg m–3). They have suitable chemical characteristics and can easily be harvested all year round using conventional machinery, if a single-stemmed growth form is maintained. E. globulus is widely grown for its short-fibre pulpwood using the Kraft and sulfite chemical processes in Portugal, parts of Spain, France and other warm, temperate countries. It is also valued for fuel and building poles in many places. Few commonly grown species produce acceptable saw timber because their rapid growth causes the build-up of stresses within the stem. These lead to serious distortions after the tree has been felled and sawn. There are therefore poor board recoveries. There are exceptions, however, for example E. delagatensis (Tasmanian oak). The leaves of several eucalypts are sometimes sold for the floristry trade. They also contain an oil that can be extracted by steam distillation. It is used for cleaning, deodorizing and as a decongestant, and it has ­insect repellent properties. Most is produced in China and is dominated by E. globulus. Place of Eucalyptus in British forestry The attraction of eucalypts is that they can grow at rates that are unsurpassed by other species. Rotations of less than 10 years are possible in parts of Britain. That they are not planted on more than a trial basis is because few species or provenances are likely to be truly hardy, except on rare, extremely mild sites. What needs to be determined is whether the return through rapid growth justifies the risk of losing an entire crop. It might be appropriate in some areas of Britain but not in many others. Eucalypts tend to arouse passionate criticism wherever they are planted as exotics because of the amount of water they use and the ­adverse e­ ffects they are said to have on local biodiversity, though this is not ­always true. For the present and the foreseeable future eucalypts are likely to r­ emain species of insignificant importance in British forestry, though ­climate change may eventually alter this.

FAGUS L. FAGUS SYLVATICA L.

Beech

Fagus is a genus in the family Fagaceae which is made up of eight genera and almost 1000 species. Species in two other genera will be familiar to British foresters: Castanea (chestnuts) and Quercus (oaks). The family is widespread in the northern hemisphere and members of it are often ecologically dominant in northern temperate forests. The genus Nothofagus (southern beeches; about 40 species from the southern hemisphere), formerly included in the Fagaceae, is now treated in the separate family Nothofagaceae. Origin There are ten species of Fagus in temperate parts of the northern hemisphere, of which F. sylvatica is native to western Europe. Its range extends from southern Scandinavia to central Spain, Corsica, Sicily and Greece, eastwards to western Russia and Crimea, and westwards to Britain. In the southern part of this range it grows only in mountain forests, at 600–1800 m elevation. In Britain beech is now both widely planted and naturalized at elevations below about 650 m. It is believed to be truly native only to parts of southern England and south-eastern Wales, and more recently Scotland (Sjölund et al., 2017). Elsewhere many county Naturalists’ Trusts regard it as an undesirable ‘alien’ tree. According to Huss et al. (2016), it was the dominant tree in western and central European forests before exploitation by human activity. Climatic requirements Beech does best in moist, rather mild and sunny climates where precipitation is well distributed throughout the year and mists are frequent. Like many shade bearers that lack early vigour to get its leader above the frost level, it suffers badly from both early autumn and late spring frosts and can be impossible to establish on exposed, open ground without nurses. The frost sensitivity is not helped by the fact that it comes into leaf rather early in the spring. It grows well in moderately polluted areas. Beech is very sensitive to wind exposure when young, but once established it tolerates exposure well, though crowns may become deformed. It has been much used for shelterbelts on hill farms, for example on Dartmoor and © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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in Northern Ireland, and is suitable for planting near the sea. Its relative vulnerability to drought means that its suitability to parts of southern and eastern Britain will reduce with climate warming, and it should be limited to soils of good moisture status in these parts of the country. Conversely, the warming climate may see greater productivity on suitable sites in the north. Changing climatic conditions may put beech populations in southern England under increased stress, and while it may not be possible to maintain the current levels of beech on some sites, it is thought that north-western England will remain favourable or even improve. Northeastern Scotland will also become suitable for the species (Ray et al., 2002). Site requirements To grow well beech must have a satisfactory soil moisture regime. It will not tolerate either an excess or deficit of water. In south-eastern England it grows best and is the dominant tree on chalk and soft limestone soils if they are not too dry, but is by no means confined to them. For example, it is frequently dominant on well-drained loams and sands. Beech is not a good competitor for water, and if this is in short supply mortality occurs, especially in regeneration (Rust and Savill, 2000). In a survey of beech and other species in the Chiltern Hills, Clements (2001) found that for growth, the optimum conditions for beech are on neutral soils of high fertility with little free carbonate but ample available water. Within the area where beech will become less climatically suitable with climate warming, it will be least threatened if grown on deep soils on slopes that receive the least amount of sun and heat (i.e. facing north-east). Unsuitable sites are either infertile, dry and sandy, or heavy and waterlogged, on both of which it may suffer badly from drought in dry years such as the summer of 1976. It is rare on ‘neutral’ soils (pH 5.0–6.5) according to Peterken (1981) and does best where the pH is between 6 and 7.5 (Huss et al., 2016). In common with many late-successional species, young beech competes less successfully than many trees with grasses and other ground vegetation when grown in the open. Other silvicultural characteristics Beech seldom exceeds 35 m tall and is not a long-lived tree, 150–200 years being about the maximum. Its outstanding feature is its great tolerance to shade. It is the most tolerant tree of all European broadleaves and has the capacity for dominating stands of trees completely; and, by excluding light, it will exclude even the shrub and herbaceous layers. Brown (1953) has described its performance and silviculture in great detail. It benefits strongly from shelter on exposed sites when young. Stem form is often

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poor, so planting must be dense to give adequate selection. It is valued as an understorey to light-demanding species, for example in shelterwood systems, though on heavy soils hornbeam is better for this purpose. Beech responds well to thinning, even at a relatively advanced age, so that when grown in a shelterwood system the trees left after a seeding cut can put on valuable increment. It coppices weakly but is tolerant to pruning, hence its popularity as a hedging plant. Brown and Nisbet (1894) quoted a widely held continental European belief that beech conserves and protects the productive capacity of the soil better than any other species of tree, and therefore may well be termed ‘the mother of the woods’. In mixed woods a suitable admixture of beech enables all species of forest trees to develop much better than when they are grown in pure woods. This is something that Brown and Nisbet (1894) said was then, as now, unrecognized in Britain. Young beech trees retain their dead leaves over winter, as do trees that are coppiced regularly. This makes it an excellent hedging species. One of the tallest (~30 m) and longest hedges that exists is a beech hedge at Meikleour, near Perth in Scotland. If a thinning or clear felling suddenly exposes previously shaded beech stems ‘sun scorch’ of the bark often occurs. This can result in large patches of bark and cambium dying. In continental Europe, it is ­frequently used as a nurse species in stands of oak, to keep the stems free from epicormic branches. Pests and diseases The most serious threat to beech arises from grey squirrels, which can be very damaging to trees of up to about age 40 years, stripping the bark and cambium from around the bases of stems in particular (Mercer, 1984; Rayden and Savill, 2004). Stressed trees tend to suffer from beech bark disease, especially on wetter clay soils during and after the pole stage. This arises from ­attacks by a scale insect, Cryptococcus fagisuga: later, the wounds caused by the feeding activity of the insect may become infected by the fungus Nectria coccinea (Lonsdale and Wainhouse, 1987), which can kill the bark. Rhizoctonia solani is another fungal soil pathogen that causes damping off of young seedlings of beech (among many other species). There are many fungal diseases that affect old, over-mature trees, including Polyporus and Ganoderma. Natural regeneration In much of continental Europe natural regeneration of beech is common and is managed under a variety of silvicultural systems. It performs well

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in the uniform shelterwood system, but because of its tolerance to shade it can be managed in any of the less regular systems just as well (Huss et al., 2016). According to Emborg (2007), beech has a ‘stop and go’ competitive strategy, in that it can survive periods of severe competition but grows again when conditions improve, slowly reaching a dominant position in the canopy. Beech seed is not dispersed far from the parent trees, so gaps of 20 m or more in diameter are unlikely to be adequately stocked in the middle. The contrast between the apparent ease with which it ­regenerates on the continent and the rarity of natural regeneration in Britain is so remarkable that many authors (e.g. Jones, 1952) have ­ ­ attempted to ­determine the extent to which continental methods might be successful in Britain. Beech is monoecious, with separate male and female flowers on the same tree. Good mast years, from which natural regeneration arises, usually occur at intervals of 5−15 years or more in Britain but much more frequently in many parts of continental Europe, including Normandy. It is generally agreed that a good correlation exists between high levels of seed production and the warmth of the preceding summer, provided that the flowers are not killed by a late spring frost (Matthews, 1963). Most of continental Europe has warm summers, and their rarity in Britain is believed to be the most fundamental obstacle to obtaining regeneration. Jones (1952) considered that so many British beech woods are even-aged because they arise from very occasional exceptionally good seed years. A satisfactory seed fall in the Belgium Ardennes was considered by Weissen (1978) to be 200–600 seeds m−2. Enormous losses then occur from fungal attacks (e.g. by Rhizoctonia solani), from predation by insects (e.g. Tortrix grossana), mice and birds, and from severe winter or late spring frosts (Engler et al., 1978). In a study near Oxford during the good seed year of 1984, Linnard (1987) found a total fall of 1316 cupules m−2 between October and March, indicating a possible fall of 2632 seeds m−2. In fact only 1500 seeds m−2 reached the ground, and of these half were sound: 8% had been damaged by birds, 4% by small mammals and the rest were empty. By the end of March 1985 only 18 sound seeds remained per 1 m−2, of which six had germinated. In much of Germany where natural regeneration is practised, the ­regeneration must be fenced to protect seedlings from roe deer. Apart from damaging the trees, deer browsing encourages herbs and grasses to grow, which make conditions much less suitable for regeneration. The young trees must become established within 15 years, which is the average life of a fence. Though less easily damaged by cold than oak, e­ xposure to temperatures lower than about −6°C can be fatal to beech seed lying on the surface (Jones, 1954). To reduce losses and provide conditions that are suitable for germination and establishment, the ground must be in a ­receptive condition. The surface should be loose so that the seed is easily buried and on germination the radicle can readily penetrate the mineral soil. A hard soil surface, the presence of matted grass or too much litter

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all prevent this. Suitable conditions can be achieved by scarifying the soil, deeper ploughing or preferably both, prior to seedfall (Becker et al., 1978). In Britain cultivation is likely to be the most important means of making the best use of the light seed crops that normally occur. These treatments protect the seed from frost and Rhizoctonia, and the roots from frosts and drought in the spring. They also eliminate much of the competing ground vegetation for periods of between 1 and 3 years, depending on the species of weeds, by which time the young beech should be sufficiently competitive not to require further attention. A very impressive example of successful regeneration following a reasonably good seed year in 1990 is at East Wood, near Stokenchurch in the Chiltern Hills. R. Pakenham (1996) scarified the ground surface with a Danish Ledreborg plough both before and after seed fall, and subsequent establishment was excellent. On acid soils the addition of phosphate may also help seedling establishment. The best regeneration in uniform shelterwood systems in France ­always occurs when a good seed fall immediately follows a seeding felling: if the felling is done after the seedfall, germination may be just as good, but many seedlings are inevitably destroyed in the subsequent work. The use of shelterwood systems may not be so sensible in Britain, where seed years are so uncertain: the use of group selection systems might be better, but they need a great deal of skill. Above all, British foresters who want natural regeneration must be opportunistic and be prepared to do the ­necessary felling when the seed arrives. This creates obvious difficulties in organizing felling and may flood the market with timber, causing prices to fall. Successful regeneration is more likely on some sites than others. Jones (1952) considered the best sites to be where there is a moderately shallow loam (about 50 cm deep) over chalk, with a ground vegetation of low herbs such as wood sorrel, yellow dead-nettle, melick, wood millet and creeping soft grass (Holcus mollis). On the heavier, deeper soils where bramble and coarser herbs are luxuriant, regeneration is more difficult. Clements (2001) found that regeneration of ash is greatest in calcareous soils, whereas beech does better on acid soils. Flowering, seed production and nursery conditions The flowers, which appear in May, are particularly sensitive to damage by late spring frosts; seeds ripen in September and October, and fall up to November, often after a frost. The earliest age at which the tree bears seeds is 50–60 years, but the best seed crops are usually after 80 years of age and at intervals of as long as 5–15 years or more. As stated above, good seed years tend to occur if the July of the previous year was hotter than usual and if flowers were not damaged by frost in the spring (Matthews, 1963). There are about 4600 seeds kg−1 (range 3400–6400), of which 60% normally germinate. Beech seed has to be stored carefully to ensure it does not

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heat or ‘sweat’. It can be sown in the year of collection if the site is not prone to late spring frosts; otherwise it can be stored for short periods by a variety of methods, described by Aldhous and Mason (1994), until sown between early and mid-March of the following spring. Beech seed cannot be kept longer than this. It is one of the most sensitive species in its inability to withstand rough handling prior to planting (Huss et al., 2016). Provenance Little work has been done on beech provenances in Britain, though Evans (1981) stated that Carpathian seed is about average, and the best origins tested come from Soignes forest in Belgium and from The Netherlands. The recommendation from Forest Research (2018b) is that material from good-quality British stands should be preferred for planting, with registered western European seed stands as an alternative. Aldhous and Mason (1994) said that the best sources are from Belgium (Forêt de Soignes); or, if these are not available, any registered seed source

Fig. 15.  Beech, Fagus sylvatica.

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in Belgium, The Netherlands, northern France or north-western Germany. Eastern European provenances should generally be avoided. A strange and counter-intuitive relationship between leaf emergence in spring and elevation was found by Vitasse et al. (2009). Working in southern France, they found that along the same climatic gradient species can exhibit opposing genetic clines: beech populations from high elevations come into leaf earlier than those from low elevations, whereas the opposite trend occurred for ash and oak. Beech is a highly ornamental species and will grow to dimensions that far surpass oak. Several well-known ornamental forms exist, most of which are propagated by cuttings, but copper beeches arise naturally from seed reasonably frequently. Area, yield and rotation Beech occupied about 93,700 ha or 3.5% of the forest area in Britain in the late 1990s (Forestry Commission, 2018a). Mean yield classes (YC) range from 5 to 6 m3 ha−1 year−1 in different parts of the country, and the maximum is about 10 (Nicholls, 1981). Rotations of 70–80 years are normal and should not exceed about 100 years if the aim is to grow white timber. If they are longer, trees suffer from ‘redheart’ and are quite often rotten. Timber and uses The timber of beech is hard, strong, straight grained, resistant to splitting and even-textured. When freshly felled the colour is whitish to pale brown with no clear distinction between sapwood and heartwood. The average density of the diffuse-porous wood at 15% moisture content is about 720 kg m−3. It is easily turned and worked, finishes well and bends exceptionally well, especially if steamed. Visually it is rather a plain, uninteresting wood. These properties make it suitable for the manufacture of utilitarian furniture, especially chairs and upholstered furniture frames, turnery and (formerly) kitchen utensils. It is also suitable for flooring, plywood and constructional work. It is rather short grained and brittle, and not of great value where strength and durability are required. Heat treatment of beech timber improves its stability in use a great deal but also makes the wood more brittle. A frequent defect of beech timber is known as ‘redheart’. It occurs in trees of more than about 100 years old and can considerably reduce the value of the wood. It is probably age-related, and site conditions are also contributory factors. The beech woods of the Chilterns were originally managed for the furniture industry, which was based in High Wycombe and dates

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from about 1870. In the south of England much of the timber that has grown since the spread of grey squirrels in the 1940s is damaged and of little value. In fact, the time of arrival of grey squirrels in any ­locality can be dated quite accurately from the bases of felled beech trees by counting the annual rings back to the first one that shows damage from gnawing. Beech wood is also often used for making an artificial fibre, known as ‘Modal’. It is a semi-synthetic cellulose fibre, similar to rayon, made by spinning reconstituted cellulose. Modal is used alone or with other fibres (often cotton or spandex) in household items such as pyjamas, towels, bathrobes, underwear and bedsheets. Modal is processed under different conditions to produce a fibre that is stronger and more stable when it is wet than standard rayon, yet has a soft feel, similar to cotton. Place of beech in British forestry Beech is the only large and strongly shade-bearing tree native to Britain. As such, it has a potentially important place in silviculture. Unfortunately, the American grey squirrel has become such huge threat to the species that its future as a planted forest tree is in serious doubt. Climate change may reduce the tree’s suitability to many sites in the south of England, but its range is likely to be extended in the north. Though likely to decline in importance, beech will nevertheless remain a significant species.

FRAXINUS L. FRAXINUS EXCELSIOR L.

Ash

Ash is in the Oleaceae family which contains 25 genera including olive, forsythia, jasmine, lilac and privet. Origin There are 69 species and subspecies of Fraxinus distributed around the north-temperate zone. F. excelsior is found throughout Europe up to about 64° N, and in North Africa and western Asia, a similar distribution to Quercus robur. It is native to, and widely distributed over, most of Britain and Ireland, where it occurs at elevations of up to 450 m. Clapham et al. (1985) considered that it may have been introduced into some northern counties. Genetic analyses of ash across Europe have indicated that, following the last Ice Age, the dominant origin of ash that colonized Britain was from the Iberian Peninsula (FRAXIGEN, 2005), in a similar way to other species (e.g. the native oaks and holly). Climatic requirements The whole of lowland Britain falls within its range of temperature and rainfall tolerances, and a combination of exposure and unsuitable soil is probably responsible for setting its altitudinal limit (Wardle, 1961). This is higher than that of oak but lower than beech (Kerr, 1995). The tree does best in milder, moister areas: shelter is especially important when young and, if provided, can result in an increase in growth rate of up to four times (Willoughby et al., 2009). Ash is frost tender but, because it comes into leaf so late, it often escapes damage in the spring. Because of its tolerance to drought and intolerance of spring frosts, it is predicted to fare well under current climate change scenarios, and ­indeed has recently been expanding in range in Europe. Site requirements Really good ash sites are rare and small in extent. It usually does best on calcareous loams (pH 7–8) which are continually moist, deep and well drained. The tree does very well on dry calcareous screes and fertile alluvial © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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soils (Thomas, 2016). It thrives on moist soils where the winter water table is ­between 40 and 100 cm below the surface (Kerr and Cahalan, 2004): ash grows well on slightly damper sites than those where sycamore thrives. It requires sites with a high content of available nitrogen. The species is very site demanding, and if planted on soils that have recently been in arable agriculture it often becomes nitrogen deficient, a problem that needs correcting by an application of fertilizer. Foliar nitrogen levels need to be at least 2.3% of dry weight to maintain growth, and ideally 2.5–3.0% (Bonneau, 1995). Ash will regenerate naturally on a wide range of soils but it seldom does well on many of them. Garfitt (1989) stated that, contrary to popular belief, the ideal site is not a damp valley bottom but a well-drained calcareous soil. This may be a rich deep marl, as in parts of Yorkshire; a shallow soil over hard, fissured limestone, as in the Pennines; or a modest 20 cm of soil over platy limestone brash in the Cotswolds. It is usually absent from sites where the pH of the surface soil is less than 4.2 (Thomas, 2016), and less frequent on other acid soils and in drier regions. The species is not suitable for large-scale planting and is probably only worth persisting with on ideal sites where it will grow to large sizes (Stevenson, 1985). Wood sanicle, wild garlic, dog’s mercury and elder often indicate appropriate conditions. Such sites are typical of woodland types W8 and W9 in the national vegetation classification (Rodwell, 1991). Ash is not suited to heathlands or to the uplands in general, nor to sites subject to prolonged waterlogging or which are compacted. Should it be planted adjoining arable fields it often dies back severely. Ash seedlings may germinate in profusion on heavy wet soils because they only need a small depth of well-drained soil on which to become established. Periodic water shortages during droughts on these sites lead to poor subsequent growth: they should be avoided because ash never does well on them and it usually disappears altogether. Other silvicultural characteristics Ash will grow to 30 m tall or more. It is the tallest tree in the genus Fraxinus (EUFORGEN, 2018b). Though it is a good shade-bearer for its first 7 years or so, it becomes strongly light demanding later, so it tends to be an intermediate successional species, invading gaps in mixed stands rather than forming extensive pure stands. For this reason crowns should always be kept free from competition. It responds to delayed thinning slowly or not at all. All thinnings should therefore be heavy with the aim of keeping crowns entirely free. The crop should be at its final spacing by age 30–35. Kerr (1995) noted that the most frequent management failing with ash is under-thinning. If thinned properly, pruning is relatively important to prevent the development of large branches. Ash competes well with the

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climber Clematis vitalba, where other trees may become smothered, but not satisfactorily with grasses, and so it is not suitable for using on previously unplanted ground unless weed control is thorough. An interesting finding by Kerr (2003) was that ash grows better (at least when young) at closer spacings. This may be a silvicultural characteristic of ash. Analysis of the data showed that there was no intraspecific competition. Ash is rarely successful as a pure plantation species. It is said to be easier to establish than beech and oak, though early side shelter provided by compatible species such as Norway spruce or European larch is essential (Kerr, 1995). Ash is often regarded as a useful component in mixtures. It should be planted in groups rather than intimate mixtures, and good weed control is always essential in the early years. Cherry has a compatible growth rate and is suitable for mixing with it (Pryor, 1985; Stevenson, 1985). Its ability to produce a good root system probably ­explains the relative ease of establishment of ash. Kerr and Cahalan (2004) have shown that ash has the ability to transpire large volumes of water on sites with good water supplies, in common with other fast-growing broadleaved trees such as alder, willow and poplar. It can also tolerate dehydration, but for good growth, sites liable to moisture stress should be avoided. In Denmark scattered ash are grown in beech stands and felled after 70 years, leaving the beech to grow on for another 30−40 years. In Belgium it is often grown with sycamore, cherry, oak, elm, aspen and birch (Thill, 1978). Kent, writing in 1775, said that ash has many enemies: because of the wet, which drips from it, is very noxious to most other plants. And as it shoots it roots horizontally, and pretty near the surface, farmers have a particular dislike to it, because it interrupts the plough.

Similarly, Justice (1765) considered that no other tree would thrive under or near an ash ‘because it exhausts all the nourishment round it’. Many websites state that ash is poisonous to ruminants, but no evidence could be found for this assertion. Forbes and Watson (1992) stated that ‘large masses of indigestible material in the stomach can cause severe illness or death; cases have included ash leaves in cattle’. It is probably the indigestibility of the leaves rather than their being poisonous that causes an occasional problem. Ash is certainly not ­regarded as an undesirable tree on livestock farms; in fact, in Spain at least dried ash leaves were at one time fed to pigs and goats in winter (FRAXIGEN, 2005). Ash is sometimes used in agroforestry systems, being planted at very low densities, with the grass beneath grazed by sheep or cattle. This can sometimes lead to the soil becoming so compacted that the trees grow badly.

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Pests and diseases A potentially devastating fungal disease of ash known as ash dieback, Hymenoscyphus fraxineus (Chalara fraxinea), was recorded in Britain for the first time in 2012, though it probably arrived in the country before 2004–2005 (Wylder et al., 2018). In 2012 it was found to be quite widespread, mostly on young planted trees that had been raised in The Netherlands. The disease has caused the death of over 90% of all ash trees in many European countries (Skovsgaard et al., 2010) and is likely to do the same in Britain. At the time of writing (August 2018) the only possible approach for controlling the disease seemed to be a long-term one, of selecting and breeding from tolerant individuals, which are said to constitute about 1% of the population, having first established that tolerance is likely to be permanent. Work by Sollars et al. (2017) has indicated that genetic markers in British populations of ash indicate possible reduced susceptibility to ash dieback in Britain compared to Denmark. Skovsgaard et al. (2017b) recommended that the general strategy with regard to infection by H. fraxineus should be conservative. Trees that are healthy or slightly damaged should be marked and retained. Where dieback is severe, the suggested approach is to harvest the remaining commercial timber before value depreciation sets in and to regenerate or replant the area with other tree species. In ­forests of high value for habitat conservation, it may be advisable to let natural succession proceed unhindered. Skovsgaard et al. (2017b) concluded that ‘the main advice for the silviculture of F. excelsior in response to dieback is to retain apparently disease tolerant ash trees wherever this is reasonable’. They went on to say: lessons learned so far clearly indicate that ash is least vulnerable within certain limits of site conditions. This emphasizes the overall importance of site-adapted tree species and site-specific silviculture.

The importance of tolerance rather than complete resistance is covered on p. 234. Another threat comes from an exotic wood-boring beetle, the emerald ash borer, Agrilus planipennis. This has caused significant damage to North American ash trees, said to be comparable to Dutch elm disease in the devastation it causes, following its recent introduction into that region. Although there is no evidence that this insect has arrived in Britain, the increase in global movement of wood poses a significant risk of its accidental introduction. Cankers (caused by the fungus Nectria galligena and the bacterium Pseudomonas savastanoi) may damage trees badly if they are grown on ­unsuitable sites. Cankered trees should be removed as soon as possible. Young trees commonly fork, sometimes repeatedly. This may be caused by the ash budmoth (Prays fraxinella), though much more likely causes are late spring frost damage and breakage by birds.

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Ash is almost immune from grey squirrel damage, but young trees are severely browsed by hares, rabbits and deer. Natural regeneration Ash has a remarkable capacity to regenerate naturally both in woodland and non-woodland, as well as on sites where it will not grow well. According to Kerr (1995), natural regeneration is best in woodlands where the canopy is dense and dog’s mercury shaded out. Once established it responds well to canopy opening, if it is not browsed by deer. Ash is relatively unusual among broadleaves in that natural regeneration appears in the second spring after a seed fall (see below). Kerr (1995) pointed out that care is required with removal of a parent crop because, if it is done too early, the seedlings could be smothered by weed growth. Natural regeneration is often so prolific that the species can become invasive. It does well in group selection systems with sycamore and beech, though ash is a strong competitor for water; this, if it is in short supply, can lead to mortality of beech (Rust and Savill, 2000). This is partly because the root biomass of ash is greater than that of beech (and many other species) for equivalent stem diameters. Ash produces a large root biomass soon after germination (Einhorn, 2007), and it is this characteristic that caused Emborg (2007) to describe it as having a ‘rush’ competitive strategy, in that it can establish itself quickly in new gaps and then stay in front of competing species by growing fast. Ash coppices well and sound stools of 1000 years old can occasionally be found. Ash coppices strongly after felling. It is also a prolific seed former, and its seedlings occur wherever the field layer is reduced in density, whether by overhead shade or by dryness, excessive wetness or instability of the soil (Wardle, 1961). Flowering, seed production and nursery conditions Ash is unusual in that an individual tree may be female, male or hermaphrodite (i.e. trioecious). The tree flowers in March and April before the leaves emerge. It is wind pollinated. The winged ‘keys’ mature in August or September and are released from winter to early spring. They are mainly wind dispersed, but can float and be moved considerable distances along waterways (Thomas, 2016). The seed is sometimes described as being ‘doubly dormant’ because, after falling, embryos still require several months to develop; for this they require warm, damp conditions and then need to be chilled before they will germinate. If seeds are collected when green, in July or August, and sown immediately they will germinate the following spring, though rather erratically. They should

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ideally be collected in October and stratified for 16−18 months before being sown in March or April in a neutral soil (Aldhous, 1972). There are about 12,900 seeds kg−1 (range 8600−16,000), of which 70% will germinate and 3000–4000 plants will subsequently be saleable. The earliest age at which the tree bears reasonable quantities of seed is 25–30 years, and really good seed years occur at 3–5-yearly intervals between the ages of 40 and 60. Provenance, improved seed and breeding Recommendations for provenance use by Forest Research (2018b) are that material from good-quality British stands should be preferred, with provenances from northern France as an alternative for use in southern Britain. In 1993 the Future Trees Trust (formerly the British and Irish Hardwoods Improvement Programme) established four ash breeding seedling o ­ rchards (Barnes, 1995) using progeny from 36 selected ‘plus’ trees from around Britain. The aim was to achieve an improvement in ­recoverable volume of between 12% and 15%. It is hoped that these orchards will produce sufficient disease-free individuals for a future breeding programme against H. fraxineus. After three thinnings, in which the less desirable families and individuals were removed, the first improved seed was produced in 2012. Well-managed tree improvement programmes such as this not only help maintain the genetic diversity of native tree populations but also increase the economic viability of woodlands (Boshier, 2007). Some continental authors distinguish between ecotypes of ash that grow on calcareous sites and those that thrive on damp, clayey sites, but such ecotypes have not been identified in Britain. Really superior ash trees with long, clean boles and excellent crowns are, according to Larsen (1946) and Garfitt (1989), invariably male trees, though work in 1991 at Oxford University failed to detect differences in stem straightness between male and female trees. There are some varieties of horticultural interest, such as weeping ash and an entire-leaved form. Area, yield and rotation length Ash is the third most common broadleaved tree in Britain after the oaks and birches. According to the Forestry Commission (2018a), there were over 1,578,000 ha of ash around 2010, representing 5.9% of the total forest area. Mean yield classes (YC) range from 4 to 6 m3 ha−1 year−1 in various parts of the country, with a maximum of about 10. On the best sites ash is more productive than oak but less than beech. Rotations should ideally be about 60 years, which is when sizes of maximum value are achieved: these are shorter than for sycamore.

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Timber The outstanding property of ash is its toughness and elasticity after seasoning, as indicated by its bending ability, flexibility and ability to absorb shock. This, and its ability to withstand pressure, makes it suitable for sports goods and the handles of tools such as hammers as well as for furniture, veneer and flooring. The average density of the ring porous wood at 15% moisture content is about 710 kg m−3. It is easy to work when air dried, has excellent steam-bending qualities and takes finishes well. The timber is not durable outdoors. The wood is ring porous, and its value tends to increase with the rapidity of growth. Timber with 4–16 rings inch−1 is likely to be suitable for most purposes. Faster or slower-grown trees are unacceptable for the more specialized markets, such as sports goods. Ash is said to be twice as valuable as oak, on average, since low-grade oak fetches poor prices. White ash is by far the most valuable, but most ash has some dark colouration in the

Fig. 16.  Ash, Fraxinus excelsior.

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centre, especially when the tree is planted on less suitable sites. According to Evans (1984), this ‘brown’ ash is associated with waterlogged soils and slow growth (possibly due to inadequate thinning). Old, stored ash of coppice origin also suffers frequently, though the physical properties of the wood are unaffected. Evelyn (1678) stated that curiously veined wood could be found. This was prized at one time by skilful cabinet makers. Occasional trees are found with rippled grain, and these can be very valuable. Most occur in open situations rather than in woodland. Ash makes the best firewood of any British tree because its moisture content is low, so it burns well even when green. In common with all deciduous hardwoods, the sapwood of ash is much more at risk from attacks by wood-­boring insects if trees are felled when in leaf. It is, however, easy to treat with preservatives. Place of ash in British forestry With the introduction of H. fraxineus the immediate outlook for ash is bleak. Most existing trees are likely to die over a few years from 2012. They might ultimately be replaced if tolerant individuals are found, but this will take a long time. Meanwhile ash will undoubtedly become a minor constituent of woodlands where it now dominates. Among its attractions are that the tree produces a valuable timber for which there is normally a good and constant demand; rotations are short; the tree is relatively free from attacks by grey squirrels, ­unlike most other broadleaved species; and natural regeneration is easy to achieve. It is a species that might have replaced beech on many southern sites as climate warming proceeds and sites become less suitable for beech. Possible alternatives to ash in a changing climate The incidence of H. fraxineus in ash since 2012 prioritizes alternative hardwood tree species that might be used in its place, particularly for ­production of high-quality hardwood timbers. Ash is currently the third most abundant hardwood in Britain, after oak and birch, extending to some 1,578,000 ha, or 5.9% of national woodland cover. Alongside oak, beech, sweet chestnut and sycamore, it is one of the most important British hardwoods that meets specific requirements for furniture and sports goods. Ash has been favoured in recent woodland creation schemes because of its inherent values for biodiversity and timber, ease of establishment and resistance to grey squirrels. To date, less attention has been devoted to alternative hardwood species than to alternative conifers. However, where ash is grown in ancient semi-natural woodlands, mainly for conservation objectives, the preference

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is to replace it with other site-adapted native species, by infill regeneration or enrichment planting. Hill et al. (2018) have produced a paper on this topic, listing suitable species for the specific woodland types defined by Rodwell (1991). Hill et al. (2018) itemized recommended species that would compensate best for the lost functional traits of ash in each area listed. The most vulnerable ecosystems for ash loss include the Yorkshire Wolds, Cannock Chase, the Fenland, Anglesey, west Ayrshire and south Pembrokeshire. The species included Populus tremula, Alnus glutinosa, Tilia cordata, Acer campestre and Quercus petraea. There are also long-introduced hardwoods, producing valuable timbers, that may be used under suitable circumstances: sycamore (Acer pseudoplatanus), sweet chestnut (Castanea sativa) and common walnut (Juglans regia) (Hemery et al., 2010). However, a range of less familiar hardwoods from temperate deciduous forests in North America, eastern Asia and south-western ­ Eurasia might be considered (Röhrig and Ulrich, 1991) (see Table 9). Some are known to produce valuable hardwood timbers suitable for similar ­applications to those of ash. Many species have only been tested in Britain and Ireland in arboreta and forest gardens, but a few have been extended to small-scale plantations. In this section a selection of these ‘candidate alternatives’ to ash are highlighted, and interested readers are directed to available sources of further information. For species-specific information, Burns and Honkala (1990) have been relied upon and also Forest Research (2018b), Leopold et al. (1998), Macdonald et al. (1957), Streets (1962) and Willoughby et al. (2007b). The maples (Acer spp.), in the family Sapindaceae, offer several potential timber hardwood alternatives to ash. Native field maple ­ (A. campestre L.) (p. 35) and sycamore (A. pseudoplatanus L.) (p. 42) (introduced from continental Europe centuries ago) are most familiar. Norway maple (A. platanoides L.), introduced from continental Europe in 1683, has broad site tolerances and on a modest scale has been used successfully in plantations in southern and eastern England. Read et al. (2009) suggested that other maples of interest are: (1) bigleaf or Oregon maple (A. macrophyllum Pursh), native to the Pacific north-west (Day, 1955; Niemec et al., 1995); and (2) silver maple (A. saccharinum L.), native to eastern North America (Leopold et al., 1998). These two species were introduced to Britain in 1825 and 1735, respectively (Macdonald et al., 1957) and have been tried in a­ rboreta and forest gardens. Maples and planes of interest are medium to large trees, often achieving 20–30 m or more in height, with a spreading crown, smooth or platy bark and broad pinnate/palmate leaves. Native ecosystems are usually temperate mixed hardwood forests with fertile, moist soils. Bigleaf maple is exceptional in arising in a mainly coniferous ecosystem (Turk et al., 2008). Three other possible alternative hardwood timber species are the ­eucalypt species, Nothofagus spp. and alternative oaks (e.g. Quercus pubescens, Q. rubra). These and other possible alternatives to ash are shown in Table 9. Some are dealt with in this book.

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Table 9.  Principal characteristics of possible alternative species to ash evaluated.

English name

Scientific name

Native region

Caucasian wingnut

Pterocarya fraxinifolia

Bigleaf/Oregon maple London plane

Acer platanoides

Hop-hornbeam/ Ostrya ironwood virginiana American Liquidambar sweetgum styraciflua Liriodendron Tulip tree tulipifera 1

Caucasus/Turkey/ Iran Eastern North America Eastern North America Eastern North America

−29 to −35 −17 to −23 −17 to −23 −23 to −29 −23 to −29 −17 to −23 −29 to −35 −23 to −29 −29 to −35

For ash (Fraxinus excelsior) equivalent wood density is 680 kg m–3.

Moist/fresh rich/ very rich Moist/fresh medium/rich Fresh/medium dry mediumcalcareous Moist/fresh medium/rich Moist/fresh medium/ rich Very moist/ moist medium/rich Moist/fresh poor/medium Moist/fresh medium Moist/fresh medium/rich

Shade Height tolerance (m)

Seeds per kg

Wood density (kg m–3)1

High

18–21

11,000

645

Medium

25–30

5,000–8,000

545

30+

450,000

560

Medium

25–30

200–250

800

Medium

25–30

70–250

815

Low

25–30

15,000–20,000

610

High

10–20

60,000–70,000

785

Low

24–26 130,000–200,000

590

Low

45–60

455

Low

10,000–40,000

FRAXINUS EXCELSIOR L.

Shagbark hickory Mockernut hickory

Western/central Europe Acer Pacific northmacrophyllum western America North-eastern Platanus × America × hybrida Mediterranean Eastern North Carya ovata America Carya Eastern North tomentosa America

Norway maple

Minimum temperature (°C) Soil

ILEX L. ILEX AQUIFOLIUM L.

Holly

Ilex is the only living genus in family Aquifoliaceae. There are 400–600 species of Ilex in both tropical and temperate regions. The species include trees, shrubs and climbers, and they are often evergreen. Origin Holly is a widespread, common species found all over Britain except Caithness, Orkney and Shetland, and extends southwards to 34° N in Algeria and Tunisia, northwards to Norway at 64° N and as far east as central China (Peterken and Lloyd, 1967). It usually occurs in the understorey of beech and oak woods, where it may often be the dominant species, especially if there is severe grazing. It is also found in hedges and on rocky hillsides to about 550 m (Clapham et al., 1985). Holly occurs as a solitary tree or in small groups, forming nearly pure stands locally. Suckering from roots or, more rarely, layering from branches often gives rise to clonal clumps. Silviculture It is a large, rather slow-growing evergreen shrub or small tree, seldom taller than 10 m, but rarely to 23 m. Holly has never been managed as a forest tree in Britain. It is a hardy plant, capable of surviving in most conditions, except where it is extremely wet. It is said to be very long lived. Holly is sensitive to prolonged periods of frost, and its natural distribution is largely determined by this. It is found on a wide range of soil types, from acid podzols to chalk and limestone, but not normally on wet soils. Brown and Nisbet (1894) said that it grows on ‘light dry soils, having a large proportion of vegetable matter ’. Marshall (1803) thought that thin-soiled heights seemed to be its natural situation. It is one of the most shade tolerant of all British trees, but to grow to reasonable sizes it requires full light. The tree withstands pollution well. Holly responds to coppicing and to pollarding at 2−3 m by producing vigorous leading shoots, a power that appears not to decline with age (Peterken and Lloyd, 1967). © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Leaves and twigs are palatable to many animals, though the distinctive lower spiny leaves deter them to some extent. Axillary buds of browsed shoots develop to form dense masses of foliage. The height growth of young individuals may be checked indefinitely by heavy browsing, to form low scrub less than 50 cm high. Even small seedlings show remarkable tenacity in the face of repeated nibbling. The lower spiny leaves protect birds from predators as they feed on its bright-red berries. On higher branches (where grazing animals pose less of a threat) the leaves have virtually no spines. Holly is often planted for ornament; numerous cultivars exist with variegated leaves and different coloured berries. Natural regeneration Holly is normally only planted as an ornamental specimen or as a hedge; therefore, practically all holly that exists in woods is of natural origin, seeds having been distributed by birds. According to Peterken (1965), the browsing pressure exerted by large herbivores including deer, ponies and cattle regulates the success of regeneration, at least in the New Forest, though seedlings must have enough light if they are to survive. Flowering, seed production and nursery conditions The tree is normally dioecious (i.e. there are separate male and female trees), and rarely hermaphrodite. The flowers are white and appear in summer. They are insect pollinated. The first conspicuously bright-red or yellow berries are produced at age 20, but maximum production starts after 40 years: good crops then occur every 2–4 years. Shading significantly reduces fruit production. High summer temperatures are probably necessary for the formation of fruit, and if the summer has been cold they do not develop. Good seed years for holly are believed to coincide with good mast years for beech, which Matthews (1963) has shown to be correlated with the July sunshine and air temperature of the previous year, and the absence of severe latespring frost. As tradition indicates, berries are ready for collection by Christmas, when they are widely used for decoration. Seeds never germinate in the first year and need to be stratified in damp sand for 16 months before sowing. There are about 45,000 seeds kg−1, of which about 80% normally germinates. In natural conditions they probably germinate over a number of years.

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Holly is extremely sensitive to transplanting and can easily be killed by this operation unless it is done with great care, preferably near the ­beginning or end of the growing season, in May or September, and ideally in wet weather. Brown and Nisbet (1894) recommended that they should remain in a nursery for 4–5 years.

Pests and diseases A leaf miner of holly (the dipteran Phytomyza ilicis) occurs widely through Britain (Metcalfe et al., 2000). The fly lays its eggs in May or June, at the base of the petiole of a young leaf (on the underside). The larvae initially feed in the midrib, later producing characteristic irregular linear blotches on the upper surface. It is apparently one of the few insects that is able to make use of holly leaves as a food, but has little impact on the growth or vigour of the holly.

Fig. 17.  Holly, Ilex aquifolium.

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Timber and uses The wood of holly is much denser than that of any other native hardwood except box, at about 800 kg m−3. In 1803 Marshall described it as ‘being in good esteem among inlayers and turners; it is the whitest of all woods; its colour approaching towards that of ivory’. The wood is strong, hard and polishes well. It must be well dried and seasoned or else it warps badly. It is highly regarded by cabinet makers. The heartwood of mature trees is used for such things as printing blocks, engravings and turnery. The wood makes a good fuel, burning well even when green. Holly is quite widely used as a hedging plant because it can easily be kept trimmed. There are many horticultural varieties, and those with variegated leaves are particularly popular. Place of holly in British forestry Rather like hazel, holly has little or no economic value today. However, as a native and often ornamental component of broadleaved woodland that regenerates easily from seed, it will continue to be a common species.

JUGLANS L.

Walnuts

Walnuts are in the Juglandaceae, a family of 12 genera and some 89 species that includes pecans and hickories (Carya), which are important nut-producing trees, and wingnuts (Pterocarya). They have large, aromatic, compound leaves. There are 24 species of Juglans worldwide. Only one, J. regia, is indigenous to parts of south-eastern Europe; others are native to eastern Asia and the greatest number is distributed between North America and the Andes. Hybrids of various species of walnuts are increasingly available, e­ specially J. nigra × J. regia crosses from France (Clark and Hemery, 2010). Some of these, such as the hybrid NG23, have excellent vigour and form. However, the wood of hybrid walnuts lacks the dark and often beautiful figure common in its parents, particularly J. regia. They are not considered further here.

JUGLANS NIGRA L.

Black walnut

Origin and introduction Black walnut is native to the eastern USA and south-eastern Canada, and was introduced to Britain in the early 1600s. It is much less common in Britain than common walnut and grows best in south-eastern England, most frequently as a tree in parks and gardens. According to the RBG Kew website (http://www.kew.org/) the very harsh European winter of 1709 damaged common walnut (J. regia) trees to such an extent over continental Europe that in 1720 a ban was imposed on the export of the timber to Britain. This was in the middle of a period when fashionable furniture was made primarily from walnut. An alternative had to be sought urgently and was eventually found in the USA in the shape of black walnut. Climatic requirements A constant hazard in growing walnuts is the risk of damage from late spring frost, though black walnut is said not to be as susceptible as common walnut (Brown and Nisbet, 1894). Its requirement for summer warmth is believed to be high (Macdonald et al., 1957) as it does not grow nearly as well in the north of Britain as in the south. Shelter, even in the English lowlands, is just as important for black walnut as it is for common walnut (see under J. regia, below). The importance of its provision cannot be overstated. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Site requirements Kerr (1993) reported that even within the native range of black walnut ‘probably between one-half and two-thirds of plantations have failed or are unthrifty because of poor siting’, and this, he said, is a recurring theme in descriptions of the silviculture of both walnut species. The lure of producing a high-value timber on relatively short rotations has led foresters to introduce the species throughout the world. To reduce the risk of frost damage, planting should be in a sheltered, mid-slope position with a south or south-west aspect. Chard (1949) noted that ideal sites for walnuts are the lower and outer edges of ‘hanging’ woods, on south-facing slopes above the level at which valley frosts occur. He also observed that box favours similar situations in the few places in Britain where it is indigenous. The soil should be at least moderately fertile, well drained and of ­medium texture, with deep rooting possible; very sandy and very clayey soils are unsuitable. The optimum pH is slightly acid to neutral (pH 6−7). Trees grow well in regions of chalk downland and on limestone hills where there is at least a 60-cm depth of soil over the strongly calcareous horizons or chalk bedrock. These exacting site requirements suggest that it is unlikely to be suitable for large-scale planting on arable land or grassland coming out of agriculture. In its natural habitat black walnut grows on moist, well-drained, deep and rich soils. These sites have many potential plant competitors. The black walnut can grow on them by reducing competition by the production of juglone (see below), in a sort of chemical warfare. Other silvicultural characteristics In Britain black walnut rarely exceeds 30 m tall. Like common walnut, it has a reputation for being difficult to establish. This is because it produces a substantial taproot in its first year from seed. Nursery-grown plants normally have damaged taproots and young trees consequently take a long period to recover after planting in the field. Effective weed control is ­essential for at least the first 3−4 years. Because walnuts do not have a strong central axis and show a sympodial growth habit (i.e. the main stem develops from a series of short ­lateral branches), formative pruning is usually necessary to produce a single stem and a desirable straight bole (see under J. regia, below). However, black walnut is a much straighter and better-formed tree than the common walnut. Trials in the native range of black walnut have investigated interplanting it with European black alder (Alnus glutinosa L.) and autumn olive (Elaeagnus umbellata Thunb.), both of which are nitrogen-fixing ­species. Results in the USA (Campbell and Dawson, 1989) on this s­ilvicultural

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153

r­ egime have been encouraging, as have similar trials with common walnut in Britain (Clark et al., 2008). Juglone In common with many members of the walnut family, black walnut produces an allelopathic chemical known as juglone. It acts to protect the tree from attacks by insects, and it can also inhibit the growth of some plants that grow under its canopy. Black walnut is notable in that it produces far more juglone than any other species. Juglone is a respiration inhibitor. It is present in all parts of the tree but is particularly concentrated in the buds, the outer covering of the nut (the husk) and roots. It is not very soluble in water and thus does not move rapidly in the soil. Toxicity is especially concentrated closest to the tree, under the drip line. This is mainly due to greater root density in this area and the accumulation of decaying leaves and husks. Juglone is so toxic that even minute amounts can sicken, sedate or kill people and animals (Coder, 2011). It exists within the tree as various non-toxic precursors that are found in high concentrations in buds, flowers, fruit and in the phloem. Under oxidative conditions, outside living cells and during and after injury, juglone is formed. In continuing contact with the air or as a result of tissue drying the juglone is decomposed. Some plants are susceptible to juglone poisoning, while others are apparently unaffected by it and grow well around black walnut trees. Sensitive plants include tomatoes and root vegetables. Those that are apparently unaffected include most grasses, oak, red cedar, hawthorn, Robinia, maples and some types of clover. Flowering and seed production The tree flowers in May and June, and the seeds ripen and are ready for collection in October. There are 65–200 seeds kg−1, of which about twothirds will normally germinate. Provenance Recent recommendations available to growers in southern Britain are those made by Kerr (1993) and Evans (1984): northerly seed origins of black walnut from the states of Vermont, Illinois and Ohio are suitable choices for planting in southern Britain. The current (2018b) Forest Research website states: ‘Late flushing varieties identified in French breeding programmes or provenances from the northern part of the natural distribution (New England, southern Ontario) should be preferred’.

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A new series of comprehensive trials was established in 2005 jointly by the Earth Trust and East Malling Research in Oxfordshire and elsewhere, using seedling progeny from seven European countries and 13 states in the USA, but they have not yet yielded any results. Timber and uses The timber of black walnut is similar to that of the common walnut in being highly valued. It is heavy, decorative and strong, and has excellent working qualities. The dark heartwood has historically been used for furniture and in the manufacture of stocks for guns. The density of the wood is about 660 kg m−3. It tends to be darker and more uniform in colour than that of common walnut, and it is the only walnut with traces of purple colouration. Despite producing edible nuts, it is notoriously difficult to extract the edible part from the shell.

Place of black and common walnuts in British forestry The motivation for planting either species of walnut in Britain, if not for edible nuts, is normally to produce a valuable timber on a relatively short rotation. The evidence from trials suggests that they can only be grown successfully on a very limited number of sites in the south of the country, and then only if sufficient shelter is available. Black walnut is particularly limited in this way. Walnuts are likely to become more suited to the British climate if global warming proceeds as predicted. Black walnut often grows more quickly than the common walnut and was described by Mitchell and Wilkinson (1989) as being ‘a far finer tree than the common walnut. It grows a straight long bole’.

JUGLANS REGIA L. Common walnut, Persian or English walnut Some of the information given below on the silviculture of the species has been obtained from an unpublished report by H.M. Steven, dated 1926. It was prepared at the request of the War Office and presumably reflected concern about the shortage of walnut timber for rifle stocks. Origin and introduction The common walnut is native to south-eastern Europe, and westwards to central Asia and China. Its ecology and management in parts of its n ­ atural

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range have been described in detail by Blaser et al. (1998). It is one of the ancient introductions to Britain and is known to have been in cultivation since at least 1562 (Brown and Nisbet, 1894). According to Roach (1985), there is evidence that walnuts were grown in Britain long before that. Today there are probably fewer trees than at any time since the late 16th or 17th century. Walnut is regarded by some European foresters as a ‘forgotten’ species. Campaigns for planting it have been waged in the distant past by John Evelyn and others. Evelyn, in his Sylva (1678, first published in 1664), wrote: In truth, were this timber in greater plenty amongst us, we should have far better utensils of all sorts for our houses, as chairs, stools, bedsteads, tables, wainscot, cabinets, etc., instead of the more vulgar Beech.

In the 1700s walnuts are said to have been relatively common in Surrey, Hampshire and other parts of south-eastern England. Even up to about 1800 planting was pursued with vigour throughout England, but by d ­ egrees both practice and interest has faded. The reason for this is e­ xplained by Marshall (1803), who said that mahogany superseded walnut ‘in the more elegant kinds of furniture; and beech, being raised at less expense … and being worked with less trouble, has been found more eligible for the commoner sorts’. Climatic requirements Mitchell and Jobling (1984) stated that walnut can be found everywhere in Britain except in the Scottish Highlands, but it is most frequent in south Yorkshire, Lincolnshire, Devon, Somerset and Dorset. It is an exacting species, requiring a warm and sheltered site with a long growing season. Though winter cold is not a particular threat, it is liable to suffer badly from late spring and early autumn frosts, and unseasonable frosts in general (Mohni et al., 2009). In Britain, young shoots and flowers are easily damaged by spring frosts of −1°C. There are major differences in flushing and flowering times between provenances, and it has not yet been possible to find one that can entirely escape frequent frost damage on cold sites or in frost hollows. A consequence is that crops of nuts frequently fail in Britain. Early autumn frosts will kill shoots that have not yet lignified. Precipitation is seldom, if ever, limiting in Britain. The importance of shelter, even in the relatively sheltered English lowlands, can scarcely be overstated. Increment is many times faster and tree form much better if shelter exists. Site requirements Walnut is possibly the most site demanding of all potential timber trees in Britain. In 1597 Gerard wrote: ‘The walnut tree groweth in fields neere

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common high waies in a fat and fruitful ground, and in orchards; it prospereth on high fruitful banks; it love not to grow in waterie places’. Low water tables and good drainage over deep (80−100 cm), well-aerated, loamy soils are particularly favourable; a general view is that walnuts do well where beech thrives. The ideal pH range is 6.5–7.5 (Becquey, 1997). Sites to avoid are light, sandy soils and heavy soils in general (Klemp, 1979), and also shallow soils over chalk, peaty soils and damp situations. One of the difficulties of growing walnuts in Britain is the scarcity of available and really suitable sites: most potential ones are used for arable agriculture. Other silvicultural characteristics Walnut will live for 150−200 years and will reach 20–25 m tall. Once past the seedling stage walnuts are intolerant of competition, and mature trees require full light. In forests they should be planted singly or in groups, and the crowns need completely freeing by the age of 30–40 years. Walnut has strong phototropic tendencies so the leading shoots grow towards gaps in the canopy if the trees are shaded. An unusual feature of walnut is that it produces a substantial taproot of up to 50–80 cm in its first year from seed (Boudru, 1989). This means that nursery-grown plants normally have damaged taproots and because of the common practice of undercutting; young trees consequently take a long period to recover after planting in the field. For this reason direct seeding is normally recommended today (see below). The common walnut has a reputation for being difficult to establish and needs ‘arboricultural rather than silvicultural attention’ (Macdonald et al., 1957). Like cherry and the poplars, its growth benefits considerably from careful early weeding. The frost-tender nature of the species indicates the desirability of using nurses when young. Appropriate nurses can result in enormous improvements in both early form and height growth (Clark et al., 2008). However, the strongly light-demanding nature of walnut can make the management of nurses difficult because, unless constant vigilance is exercised, they may suppress the light-demanding walnuts (Mohni et al., 2009). Mixtures with beech and hornbeam (both shade bearers) have been suggested in the past, though recently Clark et al. (2008) have found that Italian alder and the shrubby autumn olive (Elaeagnus umbellata), both nitrogen fixers, are successful in promoting the growth of walnut. The alder has to be removed after about 10 years. To achieve a clear, straight stem of at least 2.5 m in length (but ideally 4–6 m), some formative pruning is normally essential. It needs to be started early and must not be too ‘brutal’ if a loss in tree vigour is to be avoided (Becquey, 1997). Branch collar diameters should not exceed 3 cm on branches to be removed, and pruning should not normally exceed

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(A)

(B)

Fig. 18.  (A) Common walnut, Juglans regia; (B) Black walnut, Juglans nigra.

­ ne-third of the stem length in woodland conditions. Walnuts ‘bleed’ proo fusely after pruning, and it is usually recommended that any necessary pruning is done in February or late summer (July and August), before the leaves fall, to minimize this. Apparently, pruning can cause a change of wood colour in walnut. In woodland conditions common walnut responds very slowly i­ ndeed to delayed thinning. Tree crowns must be kept free at all times to maintain adequate growth rates. Walnut has an extraordinarily high ratio of crown diameter to stem diameter, and hence there should be fewer trees per unit area than with most other species. For example, calculations by Hemery et al. (2005a) indicated that, at 60 cm diameter, there can be 68 walnut trees per hectare, compared with 104 oaks.

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Common walnut produces a protective allelopathic chemical known as juglone (described above under black walnut) but in much smaller quantities than black walnut. The lemon-scented leaves deter insects. When horses were used for transport, walnuts were quite commonly planted for them to rest under, away from the nuisance of flies. Spence and Witt (1930) suggested that this species might profitably be planted bordering arable fields with the prospect of crops of nuts during good years and an annual increment in timber value. Diseases Walnuts are prone to a bacterial blight, Pseudomonas juglandis, when grown together in large numbers. Natural regeneration Walnut has become naturalized in much of southern England but it is never common. Natural regeneration occurs occasionally in hedgerows but only rarely in woods. Seeds are probably spread by squirrels and possibly by jays, which collect and bury the nuts for winter caches. Obtaining regeneration throughout its natural range and in other places where the tree is grown appears to be equally problematic. It is normally planted. Flowering, seed production and nursery conditions Walnut flowers in June, and the nuts ripen and fall in late September or early October. Flowering and nut production normally begin between the ages of 5 and 15 in seedling trees but earlier in grafted varieties developed for nut production. According to Brown and Nisbet (1894), the nuts will generally ripen in parts of Britain lying to the south of the river Forth but only occasionally to the north of it. There are about 65−180 walnuts kg−1, of which 75% will normally germinate if they are stratified immediately after harvesting until the spring and sown when the seed is on the point of germinating. Ideally, bad and small nuts should be rejected. Fertile, near-neutral soil is needed for growing good nursery stock. As mentioned above, walnuts produce large, long, strong taproots and, when young at least, few lateral roots. Fibrous roots are slow to ­develop. This makes them difficult to transplant successfully. Most a­ uthors (e.g. Popov, 1981) state that for these reasons planted seedlings are particularly difficult to establish. Plants from direct sowing of pre-germinated seed are much faster growing and have better root systems. Planting walnut seeds

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directly into tree shelters, where they are wanted as trees and covering them with a little soil, gives good results and can save several years of establishment time as well as the cost of having to buy plants. When nursery stock is used it should be planted out as young as possible. By contrast Aldhous (1972), while recognizing the care that is needed in lifting 2-year-old plants from seedbeds, recommended they should be moved to transplant beds for a further 2 years. If stems exceed about 2 cm in diameter at the base, it is often prudent to ‘stump’ them or cut them off at 2–3 cm above the ground and allow a new shoot to grow. Some authors (e.g. Macdonald et al., 1957) also advocate pruning the taproot. A risk, discovered in Italy, is that roots of micropropagated walnuts will not develop properly, so such trees can lack stability. Provenance Several varieties that produce edible nuts (listed by Roach, 1985) are relatively resistant to late spring frosts and most of these originate from the famous French walnut-growing region of Isère, near Grenoble. Forest Research (2018b) recommend late-flushing varieties identified in French breeding programmes. Provenance/progeny trials run by the Earth Trust (based in Oxfordshire) have confirmed that 5-year-old material from French breeding programmes is very good. Trial sites in Oxfordshire, Gloucestershire and Somerset have indicated that provenances from Tadjikistan, Romania and Slovakia are among the fastest growing and latest flushing (Hemery et al., 2005b). Timber and uses The semi-ring-porous walnut wood was the chief wood of fashionable furniture between about 1670 and 1735, but its use declined rapidly as mahogany, which is less vulnerable to woodworm, became available in quantity. Walnut timber is stable. Though the sapwood is white, soft and comparatively valueless, the heartwood is dark brown and often beautifully figured. It scarcely warps or checks at all, and after proper seasoning shrinks and swells very little. It is easy to work and holds metal parts with little wear or risk of splitting. It is strong without being too heavy (density at 15% moisture content is about 670 kg m−3) and can withstand considerable shock. Its colour is dark, so it does not show the dirt. These attributes make it the most valuable wood for gunstocks, particularly as its uniform and slightly coarse texture makes it easy to hold. The one great defect of the timber is that it is susceptible to attacks by the common furniture beetle (Anobium punctatum), normally known as woodworm. Inadequately seasoned sapwood is particularly vulnerable. The  most

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valuable and attractive timber tends to be slowly grown, and it is comparatively seldom that it can be found in Britain because, on suitable sites, walnut will grow fast. Trees from the mountainous regions of Italy and the area round Grenoble in France have particularly good reputations for producing high-quality timber. In Italy only the heartwood of walnut is considered usable, and it tends to be narrow. Light-coloured heartwood is preferred. Italian research workers believe that soil, growth rate and genetics all affect wood colour. There is considerable current interest in France in planting walnuts for their potentially extremely valuable timber, in the belief that decorative temperate woods are likely to be in much greater demand as ­tropical supplies decline. This echoes what Marshall said in 1803: ‘were the importation of mahogany to be obstructed, the walnut it is probable would become a valuable wood’. The wood is used for making veneers and high-quality furniture. Highly figured veneers are used for cabinet making and decorative panels. Their use was developed and increased considerably after the 1720 ban on the export of walnut to Britain (see above). Walnut was one of the finest woods for making aeroplane propellers in the early days of aviation. The large burrs that are found occasionally on trees are particularly valuable for veneers, producing a fine mottled grain; these are often used on the dashboards of expensive cars. A notable source of these, which have flame figures low in the stem, are the junctions of grafts of common walnut and some Californian species harvested from over-mature (40−60-year-old) trees in Californian nut orchards. Burr wood can be extremely valuable. The ‘curl’ patterns from just below a fork also fetch high prices. The common walnut provides the edible nuts of commerce, 70% of which were grown in China and the USA in 2007. According to Spence and Witt (1930), the great majority of English-grown walnuts suffer from insufficient development of the normal oil content of the nut and excessive moisture in the kernel, probably caused by an insufficiently long and hot summer, particularly in more northern areas. This makes them more suitable for pickling. Pickled walnuts are made from tender, young nuts, collected before they become woody and while they can readily be pierced by a needle without the soft shell that is forming inside the husk being felt – usually towards the end of June or early July. The oil pressed from walnuts sells for high prices in delicatessens. At one time it was used in some parts of France instead of butter and olive oil (Reneaume, 1700–1701). When boiled, the husks of the nuts produce a dark-­ yellow dye, which was once much used for colouring wood, hair and wool. Place of common walnut in British forestry See under black walnut.

LARIX Mill.

Larches

The larches (Larix spp.) are a genus of about 11 species of trees in the family Pinaceae (Farjon, 2017). Most of the well-known conifers of commercial ­importance – including cedars, firs, hemlocks, larches, pines and spruces – are in this family. They all occur within the cooler parts of the northern hemisphere or at high altitudes and, unusually for conifers, larches are all deciduous with strongly dimorphic shoots. The short (spur) shoots are prominent on twigs 2 years old or more, each bearing leaves (needles), and often pollen cones, or seed cones; the lateral long shoots are sometimes produced by current-year growth increments. The leaves are in tufts of 10–60 on short (spur) shoots or borne singly on long 1st-year shoots.

LARIX DECIDUA Mill.

European larch

Origin This tree is native to the Alps and Carpathian regions of south-eastern France, Switzerland, northern Italy, southern Germany, Austria, the Czech Republic, Slovakia, Poland, Ukraine and Romania. It is mostly found at elevations between 1000 and 2200 m (Farjon, 1990). The date of introduction to Britain is not known, but the species was present by 1629. Climatic requirements European larch does best where the atmosphere is reasonably dry and sunny, but high temperatures are not required. It is a species that grows up to the tree line in the Alps and will grow well at high elevations in Britain if the site is not too exposed. The tree often flushes as early as mid-March, particularly in milder regions and at low elevations. It is therefore susceptible to late spring frost damage which, apart from killing foliage, may allow infection by canker. Otherwise, European larch has no particular climatic limitations in Britain. However, it will not thrive in conditions of low rainfall on soils that do not retain water, such as lowland heaths (Macdonald et al., 1957). It is only moderately tolerant to pollution, and not to salty sea winds.

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Site requirements This is an exacting species requiring at least moderate fertility and moist but freely draining soils. Sites to avoid include poor and non-water-retentive sands, peats, heavily leached soils and soils over chalk, and also those carrying dense heather. The tree needs to root deeply, so badly drained sites that are prone to waterlogging must also be avoided. European larch grows best on the middle and higher slopes of hills, provided they are sheltered. Other silvicultural characteristics European larch will grow to over 40 m tall. It is a deciduous, light-­demanding, pioneer species which, according to Anderson (1950), has high water ­requirements after flushing and so needs a continuous supply of soil moisture. It tends to die on drought-prone sites. Thorough weed control in the year or two immediately after planting is important but, after that, European larch is more competitive than many other trees and weeding can be relaxed. This is one reason why it is a popular plantation species on many estates. Because it is so strongly light-demanding, thinning must begin as early as age 12–15 years and must be heavy. Because the trees are very variable in form, selective rather than systematic thinning is essential. Natural pruning takes place rapidly. Its relatively light crown and compatible growth rate makes European larch a useful nurse for oak on sites where it does not grow too fast. Diseases Larch canker, Trichoscyphella wilkommii, is a serious and often complex problem on unsuitable sites (Pawsey and Young, 1969). The species is also relatively susceptible to fomes root rot, Heterobasidion annosum, and to the leaf cast fungus, Meria laricis. It is more resistant than Japanese larch to ramorum disease, Phytophthora ramorum, but is not entirely ­immune to it. Natural regeneration Though seed production is good, natural regeneration rarely persists in great quantities.

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Flowering, seed production and nursery conditions The tree flowers in March and April, and flowers are often damaged by frosts. Seeds ripen and can be collected between September and December, and fall between October and the spring. The earliest age at which the tree bears seed is 20–30 years, and good seed years are usually at 3–5-yearly intervals between the ages of 40 and 60. There are about 158,700 seeds kg−1 (range 92,600−269,000), of which 60,000 are normally viable. For nursery purposes seed is usually sown in mid- to late March. Pre-treatment by stratification for 3 weeks is occasionally necessary if the seed has become dormant (Aldhous, 1972). Provenance Aldhous and Mason (1994) recommended that seed should originate from the Sudeten area of the Czech Republic and Slovakia, collected from between 300 and 800 m in elevation (Lines and Gordon, 1980). If not available, low-elevation stands in Austria and Germany are suitable; alternatively, seed collected from good British stands has proved satisfactory. These sources are much less vulnerable to larch canker than others. Trees grown from seed collected at high elevations in the Alps are susceptible to canker and should be avoided. Unfortunately, many existing British stands have originated from high-alpine seed. Area and yield European larch covered 22,500 ha (2% of the forest area of Britain) in around 2000 and was the eighth most common species (Forestry Commission, 2003). Mean yield classes (YC) in different regions range from 6 to 9 m3 ha−1 year−1 (Nicholls, 1981), with a maximum of 12. The disadvantages of the rather low levels of productivity are to some extent offset by the early age at which profitable production can begin, and by the short rotations. Timber and uses Details are given in the section on Japanese and hybrid larches. The average density of the wood of European larch at 15% moisture content is about 590 kg m−3. Before the leaves fall in the autumn, they turn an attractive bright yellow.

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Place of European larch in British forestry European larch is a relatively minor species in British forestry and this is unlikely to change. Projected climate change will not make conditions any more favourable for its growth.

LARIX KAEMPFERI (Lamb.) Carr. LARIX × MARSCHLINSII Coaz

Japanese larch Hybrid larch

Origin and introduction The natural range of Japanese larch is confined to a small region centred in Nagano Prefecture on the island of Honshu in Japan, between 35° and 37° N, mainly at elevations between 1200 and 2700 m. The species was introduced to Britain in 1861 (Mitchell, 1974). Hybrid larch is a cross between L. decidua and L. kaempferi. It first arose naturally in about 1895 at Dunkeld, Tayside, and was noticed in 1904. Silviculturally it is usually considered similar to Japanese larch, but in terms of growth it is superior to either parent. Climatic requirements In the native range of Japanese larch there is a summer rainfall climate with over 1000 mm a year. In Britain it needs a high atmospheric humidity during the growing season, in contrast to European larch. It does best in hilly districts in the milder, wetter, west coast regions. Areas where the precipitation is substantially below 700 mm should be avoided unless the conditions of soil moisture are particularly favourable. Trees stop growing and young ones may die in periods of severe drought. The range of Japanese and hybrid larches in Britain is not limited by temperature, but these species harden off late in the autumn and are therefore susceptible to early autumn frosts (Macdonald et al., 1957). Both species, but especially hybrid larch, tolerate exposure quite well on good soils, and they withstand salt spray ­remarkably well too. They are also better than European larch at tolerating pollution, though none of the larches is particularly good in this respect. Site requirements In its natural range, Japanese larch is a pioneer species that colonizes recent volcanic ash deposits. Japanese and hybrid larches thrive over a wider

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Fig. 19.  (A) European larch, Larix decidua; (B) Japanese larch, Larix kaempferi.

range of conditions than European larch, but do best on well-drained but moist, moderately fertile soils that are not too heavy. The pH should be in the range of 4.5–5. They are suitable for upland planting if the soil is reasonably well drained. The presence of dense bracken is often used as an indicator of suitable sites. Dry sites in the south and east of England and elsewhere should be avoided, and also badly drained land where the trees may become unstable and, in dry years, suffer from drought. In exposed

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situations, especially where the soil is fertile, the trees tend to lean and twist, to the extent that they are useless for sawn timber production. Other silvicultural characteristics Both species will grow to about 45 m tall, and they are unusual among conifers in being deciduous. In most respects the silviculture of these species is similar to European larch. Their main attractions are their ease of establishment, rapid early growth, amenity value and the possibility of early financial returns. Their fast early growth makes them particularly useful on old hardwood coppice areas and sites where weeds, especially bracken, are a problem. They are valuable for fire belts and sometimes as nurses for more tender species such as beech and oak, but if they are used for this purpose they must be removed early enough otherwise they are likely to suppress the species being nursed. As with European larch, systematic thinning should never be practised with Japanese or hybrid larches because trees are so variable in form. Selective thinning is essential. Natural pruning is good. The trees are strongly light-demanding and consequently need to be thinned heavily: at least one-third of the total stem length should be maintained as live crown. Diseases A fatal and recently introduced disease of Japanese larch is caused by Phytophthora ramorum, which kills Japanese larch trees within 1 year of symptoms first being detected. Until about 2009 the pathogen, known as ramorum disease, had only been recorded on a few other woody species, ­including Rhododendron and bilberry. According to Webber et al. (2010), it was ‘the first widespread and lethal damage caused by P. ramorum to a conifer and the first to a commercial plantation tree in Britain’. Until 2011 the disease was confined mostly to southern Wales and south-western England, where large-scale felling was carried out in an attempt to prevent its spread, but it has since been found elsewhere in England. The future of Japanese larch, as a relatively important forest tree in Britain, is now clearly at serious risk. In fact, these larches are no longer being planted and those that remain are being cleared rapidly. The susceptibility of hybrid larch is still uncertain. Both Japanese and hybrid larches are susceptible to fomes root and butt rot, H. annosum, and also (particularly) to Armillaria spp.; however, both are generally free of the canker that is so damaging to many European larch provenances. If conditions are not favourable these larches do not go into check like spruces but tend to die quite quickly. Larches are, on occasions, damaged by grey squirrels.

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Natural regeneration Though seed production is good, natural regeneration rarely persists in great quantities. Flowering, seed production and nursery conditions Japanese and hybrid larches flower in late March and early April; flowers are often damaged by frost. Seeds ripen and can be collected in September. They are dispersed naturally over the winter. The earliest age at which they bear seed is 15–20 years, and the best seed crops are commonly at 3–5-yearly intervals between the ages of 40 and 60. There are about 253,500 seeds kg−1 (range 125,700−335,100), of which 100,000 are normally viable in Japanese larch, and only half this number in hybrid larch. The treatment of seeds for nursery purposes is the same as for European larch. It should be noted that only seed of first-generation (F1) crosses of ­hybrid larch should be used. Later generations are much less likely to show any hybrid vigour. However, the yield of hybrid seed from open-­ pollinated seed orchards can be very low, and for this reason vegetative propagation from cuttings is normally recommended. Aldhous and Mason (1994) give recommended stem diameters just above the point of root emergence for transplants of different heights. Provenance Most seed of Japanese larch is now collected from selected British stands and this is recommended. If it has to be imported, suitable areas for collection in Japan are given by Aldhous and Mason (1994). Essentially, these are from the central part of its natural range. Hybrid larch seed is usually very difficult to obtain, though work on producing F1 hybrids from ­juvenile stem cuttings has proved successful. Area, yield and rotation length The larches together covered 126,000 ha (4.8% of the forest area of Britain) in about 2000 and together were the fourth most common group of trees in the country (Forestry Commission, 2018a). Larches are not high-volume producers. Mean YCs range from 7 to 11 in different parts of Britain (Nicholls, 1981), with a maximum of little more than about 14 m3 ha−1 year−1. Rotations are short in comparison with many other conifers, at about 45–50 years.

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Timber The timber of European, Japanese and hybrid larches is noted for its hardness, natural durability and strength. It tends to distort on drying and is also resinous. Japanese and hybrid larches are described as being ‘milder’ than European, with a tendency for the early wood to crumble and tear during sawing. Advances in timber engineering methods, such as the ‘gridshell’, now allow small battens of first- and second-grade larch to be finger- and scarf-jointed into structural members for use in largespan roofs, such as the Savill Building at Windsor Park (Wilson, 2010). All three species are used for outdoor work, including transmission poles, the ­exterior of buildings where something more durable than spruce is needed and for boatbuilding. Even early thinnings can be sold profitably for rustic work in gardens. Although relatively durable, preservative treatment can be worthwhile. The average density of the wood of Japanese larch at 15% moisture content ranges between 530 and 590 kg m−3, and of hybrid larch it is about 480 kg m−3. Place of European, Japanese and hybrid larches in British forestry The susceptibility of larches to P. ramorum will almost certainly limit their use for the foreseeable future, if not permanently. Their attractions in British forestry arose from the ease by which they could be established on weedy sites, their rapid early growth and the general usefulness of their timbers, even at small sizes. As the only commonly planted deciduous conifers they also had considerable amenity values, particularly in spring and autumn. The role of Japanese larch could potentially increase in western Britain with climate change, especially if it is planted on mineral soils to diversify spruce forests.

MALUS Mill. MALUS SYLVESTRIS (L.) Mill.

Crab apple

Malus is in the family Rosaceae, which contains almost 5000 species in 91 genera. Many edible fruits are in this family including apples, pears, quinces, apricots, plums, cherries, peaches, raspberries, loquats and strawberries. There are about 40 species of Malus within north-temperate regions. The crab apple is native to, and widely distributed in, Europe and Southwest Asia, including Britain. It probably reached Britain after the last Ice Age from a refugium in the Iberian Peninsula (Kirby and Watkins, 2015). Mitchell (1974) noted that roadside trees may often be seedlings grown from orchard apples, especially if the flowers are very pink. Origin Crab apple trees occur throughout England and Wales but are never common and are rare from central Scotland northwards. They are found particularly in oak woods, along woodland edges, and in hedges and scrub up to elevations of almost 400 m. Climatic requirements The species is not noticeably limited by climate in Britain except by excessive cold and exposure. Site requirements It grows on a wide range of soils, from acid to basic and clays to sands, but is said to do best on the wet edges of forests. It can be found in hedgerows, scrub and copses, along roadsides and also on rough ground. Other silvicultural characteristics It is described by EUFORGEN (2018b) as ‘extremely light-demanding and with weak competitive abilities’. Crab apple grows much more slowly in height than most other trees and reaches rather low final heights of around 10 m (Turok et al., 1998), similar to a few related species, including © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Fig. 20.  Crab apple, Malus sylvestris.

the wild service tree and whitebeam. For these reasons, crab apple cannot be integrated successfully into high forest stands, even in groups. The tree is largely confined to habitats where it can find suitable light conditions for growth, such as the sunny edges of woods and hedgerows. Huss et al. (2016) noted that it is very stress tolerant, also that it will reproduce by suckering (like many other trees in the Rosaceae). In earlier centuries it used to grow successfully in low-density coppice-with-standards systems, where standing volumes of timber were seldom more than 150 m3 ha−1, but these have now become obsolete. In continental Europe the understorey of older pine stands also provides a suitable habitat because, even if the canopy is closed, enough light penetrates it. Young trees can be badly browsed by deer. Threats The main threats are hybridization with cultivated apple varieties (M. domestica) and forest management, particularly as the species tends to grow in forest margins. As the plants mainly occur as isolated individuals, rather than as viable populations, they are ‘susceptible to events’ (IUCN, 2017). Flowering and seed production The tree flowers in May and is insect pollinated. The fruits are normally 2−4 cm in diameter and mature in the autumn. They are almost inedible.

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Apple genetics In the wild the crab apple hybridizes with cultivated apples to the ­extent that Kleinschmit (1998) doubted whether it still exists as a pure wild form. It is now believed to have had little, if any, role in the evolution of the ­domestic apple (Juniper and Mabberley, 2008). The genetic makeup of the apples cultivated today is complex because of the easy hybridization ­between species that has occurred naturally, and over thousands of years of selection and breeding by man (Roach, 1985). However, cultivated ­apples are now believed to be derived primarily from the central Asian species M. pumila (Juniper and Mabberley, 2008). Timber and uses Crab apple trees are seldom straight, so it is difficult to obtain reasonable lengths of timber. Well-grown trees provide wood of excellent quality. It is dense at about 830 kg m–3 (similar to ash, beech and oak) at 15% moisture content and has a fine, uniform texture with no distinct heartwood. Its main virtue is its resistance to splitting, but it tends to distort. According to The Wood Database (2018) the heartwood can vary from a light reddishor greyish-brown to a deeper red/brown. The grain of crab apple sometimes has streaks of darker and lighter bands of colour, similar to olive. Sapwood is a pale cream colour. The wood is not durable unless dried very slowly. When properly seasoned and kept dry it holds its shape well enough to be used for precision work: carving, wood engraving, musical recorders, tool handles and turnery. At one time it was used for making set squares and other drawing instruments. It is believed that the Druids planted apple trees in the vicinity of their sacred groves of oak trees, possibly because they served as hosts for mistletoe, which was of great importance to them. There are a number of ornamental varieties of crab apple, cultivated for their showy flowers and/or fruits, but these are mostly derived from the Japanese M. floribunda. The crab apples themselves are small, sour and virtually inedible, ­except by many wild animals. They are, however, a rich source of pectin which is used as a gelling agent in jams. Place of crab apple in British forestry The crab apple is an attractive small tree that has no economic place in British forestry but has some conservation value.

NOTHOFAGUS Blume NOTHOFAGUS ALPINA (Poepp. & Endl.) Oerst. NOTHOFAGUS OBLIQUA (Mirb.) Blume

Rauli

Roble, Chilean or southern beeches

Nothofagus is a genus in the family Nothofagaceae that is distributed around the southern Pacific rim: in southern South America (Chile, Argentina), New Guinea, Australasia and New Caledonia. It used to be included in the Fagaceae. Origin and introduction According to Mabberley (1990), Nothofagus is a genus of 35 species. Several of them are known in South America as Lophozonia, but the name Nothofagus is retained here. Only two species (N. alpina and N. obliqua) are likely to be of value in Britain. They both occur in Chile and Argentina (Macdonald et al., 1957). Southern beeches are the dominant components of the temperate and subantarctic forests in Chile from 33° to 56° S and on the drier side of the Andes, south of approximately 39° S. Ten species occur in this region, and N. obliqua is one of the most abundant of these. The first detailed summary report on the performance of Nothofagus species as forest trees in Britain was provided by Macdonald et al. (1957). They noted that only Chilean species had proved successful in British forests and that the species of particular interest were rauli and roble (N.  alpina and N. obliqua). According to Elwes and Henry (1906–1913) rauli was first introduced to Britain in 1849 but subsequently died out. It was reintroduced by Elwes in 1902 and, together with roble (which was introduced in 1913), both species attracted considerable attention because of their rapid growth in various arboreta. Further seed collections were made in the 1920s which resulted in plots being established in various parts of Britain. These were supplemented by further seed imports in 1953 which resulted in a series of trial plantations being ­established in different parts of Britain. The good growth of some of these plantations led to the upsurge in interest in Nothofagus species during the 1970s.

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Climatic requirements The most serious limitation to the use of southern beeches in Britain is their lack of cold hardiness. They survive in climatically suitable areas but are easily killed by winter cold. Murray et al. (1986) found that both species develop resistance to cold very slowly in the autumn and are damaged by temperatures of −14°C in midwinter. Most trees are killed by temperatures of −18°C. Their characteristic of coming into leaf early in the spring does not help! Nothofagus trees lose their resistance to damage by frost (i.e. they ‘deharden’) early in the year, in February or March, prior to bud burst in the latter half of April. Such characteristics almost guarantee that southern beech will suffer severely from frost damage at least once during a rotation in most parts of Britain. By contrast, the native beech develops cold resistance rapidly in September, is undamaged by frosts of well below −20°C and does not lose its frost resistance until late April. Frost can cause dieback of shoots and consequent multiple leaders; it can kill the cambium and the resulting damage ranges from irregular swellings to large open fissures, which allow entry to fungal pathogens. These often kill the trees. If individuals could be selected that are 3–6°C more frost hardy than the population mean, they would avoid frost damage in most lowland regions. Observations by Cros and Duval (1990) in France suggested that N. obliqua is more cold resistant than N. alpina, and the same was found by Deans et al. (1992) in southern Scotland. Neither species is tolerant to exposure. Apart from frost damage, both species have done well in areas with rainfall ranging from 700 mm to well over 1000 mm, though N. obliqua is the species found in the drier regions of Chile. They are sensitive to prolonged dry spells, which can cause dieback of the shoots and branches. In Scotland dieback and death, mainly from cold, is common. The species seem most suited to the lowlands and hills up to 300 m in the south and west of Britain, but not to the most severe upland climates, especially sites exposed to cold north and east winds. Site requirements Neither species is too exacting. They grow well on a wide range of soils, from deep sands to heavy clays. Sites to avoid are those with severely ­impeded drainage, shallow soils over chalk, calcareous clays and acid peats. On the more difficult sites N. obliqua may succeed where N. alpina fails, and it tends to produce better results in the south of England (Lines and Potter, 1985). Southern beeches can grow faster than the native beech on all but calcareous soils. Good growth rates are obtained on sites marginal for oak.

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Forest Research (2018b) states that N. alpina is: [an] early flushing species which can be damaged by late spring frosts. For this reason, it is currently only recommended for use close to the western coasts and more generally in the western and milder parts of England and Wales. Best suited to well drained fresh soils of poor to medium nutrient status, it does not tolerate very poor nutrient status soils or compacted/ waterlogged conditions.

Of N. obliqua Forest Research (2018b) states: tolerates warmer and drier sites than rauli (N. alpina) and therefore is better suited to suitable soils in eastern Britain. Best growth is on moderately dry to fresh soils of poor to medium nutrient regime. Not suited to compacted or peaty soils or to those of very poor soil nutrient regime.

Other silvicultural characteristics The species are strongly light demanding. Plants are susceptible to drying out between lifting in the nursery and planting: the use of anti-transpirants on stems is sometimes recommended. Early and thorough weeding is ­essential for good establishment, and growth is best where there is a light overstorey, but this must be removed after 2–3 years. Neither species suffers too badly from grey squirrel damage, even in areas where beech is badly attacked. For nature conservation purposes southern beech is considered better than beech and most conifers (Wigston, 1980). They coppice well if the shoots are in full light. N. obliqua has the better form, being narrower, less heavily branched and nearly always having a single, good straight stem if undamaged by frost. Pests and pathogens Mature stands of both species can suffer sudden dieback from Phytophthora pseudosyringae. Natural regeneration Both Nothofagus species produce reasonably good natural regeneration, and they hybridize with each other. Seed production and nursery conditions Aldhous and Mason (1994) stated that, for nursery production, the seed should be stored dry and prechilled (i.e. imbibed (moist) dormant seed

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exposed to low temperatures of between 2° and 5°C in early March for 3 weeks before sowing). N. obliqua produces about 115,000 seeds kg−1 and N.  alpina 90,000, which will yield between 5000 and 6000 seedlings (Aldhous and Mason, 1994). Germination percentages are usually low, at around 25%. Propagation from cuttings has also been successful. Provenance There is still a serious lack of information about the variability of these species. Many of the earlier imports of seed came from areas with climates that are too unlike the British climate for the trees to succeed, probably from too far north in Chile. The serious stem cankers and dieback that result from cold winters may possibly be overcome if more southerly provenances are used, though the logistical problems of acquiring seed from them would be considerable, because few roads exist in southern Chile. Seed from low elevations, south of 38° S (which is at an equivalent latitude to Lisbon or Athens in the northern hemisphere) was recommended by Tuley (1980). Recommendations for the use of various provenances in Britain are given by Potter (1987). Forest Research (2018b) recommends provenances from the southern part of the natural range or from good-quality British stands. Yield N. obliqua and N. alpina have no equals among broadleaves (except poplars on suitable sites) at their highest levels of volume production, and in some situations they compare favourably with the best conifers. The mean yield class (YC) in Britain is 14 m3 ha−1 year−1 and maximum local YC of 20 occur in places, such as the Forest of Dean. Rotations are ­unlikely to exceed 45 years (Christie et al., 1974). Rates of height growth may be equalled by Japanese larch and among broadleaved species by cherry, sweet chestnut and poplar, but volume production of the two southern beeches is much greater than any of them. Timber The timber of N. alpina of Chilean origin, known as rauli in the British timber trade, is said to be the best of all southern beech species and is comparable to – though much lighter than – beech (about 470–560 kg m−3 at 15% moisture content compared with 720 kg m−3 for beech). It is pinkish in colour with a very fine grain. It is used in making furniture, for barrels, doors, veneers, shingles and floors. One of the main uncertainties with both species in Britain is whether the fast-grown trees will produce timber

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with qualities that have any significant value. Early indications are that this is unlikely. The wood of N. obliqua has a density of about 609 kg m−3 (The Wood Database, 2018). Place of southern beeches in British forestry Neither species has an established place in British forestry, but both have attracted interest because of their potentially high levels of productivity, even on rather poor sites. Though broadleaved, they can grow as fast as many conifers on some sites, but the lack of winter hardiness and doubts about the value of the timber are both major problems. Much more ­research, especially provenance research, and an extended period of trials are needed before they could become widely accepted. Essentially, as Macdonald et al. said in 1957, ‘unless a provenance remedy can be found for their lack of winter hardiness, planting outside the milder localities appears to be hazardous’. They may benefit from climate warming, and may also be suited to a wider range of sites in northern Britain, wherever the site requirements are met (Forest Research, 2018b).

(A)

(B)

Fig. 21.  Southern or Chilean beeches, (A) Nothofagus alpina (rauli); (B) Nothofagus obliqua (roble).

OSTRYA Scop.

Hop-hornbeams

The birch family (Betulaceae), to which Ostrya belongs, has five other genera: the birches (Betula), alders (Alnus), hazels (Corylus), hornbeams (Carpinus) and hazel-hornbeams (Ostryopsis), numbering a total of 167 species. They are mostly natives of the temperate northern hemisphere, with a few species reaching the southern hemisphere in the Andes of South America. Their typical flowers are catkins, which often appear before leaves. Origin and introduction The genus gets its name from the leaves, which closely resemble those of British native hornbeam; and from the bladder-like involucres that enclose the nutlets, which superficially resemble clusters of hops (Humulus lupulus). By contrast with the native hornbeam (Carpinus betulus), which is a member of the hazel family (Corylaceae), the hop-hornbeams are in the Betulaceae. Two hop-hornbeam species are of possible interest in Britain: (1) European hop-hornbeam (O. carpinifolia Scop.), introduced before 1724 (Mitchell, 1974), native to mid-elevation forests in southern Europe, Asia Minor and the Caucasus region; and (2) American eastern hop-hornbeam or ironwood (O. virginiana (Mill.) K. Koch), introduced in 1692, and n ­ ative to south-eastern Canada through the eastern USA into Central America. These are medium-sized trees, reaching 18–24 m tall and often growing in the understorey. The bark is reddish when young and similar to cherry, but becomes grey, loose and flaky as trees age. Another five Ostrya species are found in similar forests in eastern Asia, and two in the south-western USA. There have been introductions to arboreta and forest gardens in Britain and Ireland, but no plantation record. Some trials (e.g. O. japonica at Kilmun) have failed. Climatic and site requirements There is little information available on site requirements of European hop-hornbeam, although its occurrence in mixed montane forests of the Mediterranean region suggests that it might be suitable for the climate of southern England, as with Corsican pine and Atlas cedar, especially given predicted climatic warming. There is a better understanding of site r­ equirements of O. virginiana, based on American experience. A wide range of climates appear acceptable, given that the species grows naturally as far north as Manitoba, Ontario and Nova Scotia. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

177

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OSTRYA Scop.

Preferred soil conditions are moist, with acid to neutral pH, but some use can be made of drier, less fertile and calcareous soils, albeit with slower growth. Edaphic matching in British conditions appears to be with lowland oak or oak–lime–hornbeam woodland (NVC W10) (e.g. on plateau sites with heavy, poorly aerated – London or Wealden – clay soils) rather than calcareous ash woodland (W8), but this remains to be confirmed. Silviculture Ostrya species are late-successional shade-tolerant trees, adapted to the understorey of mixed hardwood forests. They are rather rare in Britain. O. carpinifolia will grow to about 24 m tall and O. virginiana to about half that height. If released by gap formation, O. virginiana can b ­ ecome dominant, displacing commercially valuable medium tolerants. However, it can be useful silviculturally as a managed understorey to prevent epicormics growing on oak. Some silvicultural prescriptions for North American forests seek to reduce or eradicate it. Given the lack of d ­ omestic experience with Ostrya spp., silvicultural prescriptions and yield predictions developed for native hornbeam (C. betulus) are most relevant. Yields are typically low (general yield class, GYC 2–6). At present it is difficult to identify sites where Ostrya might be preferred silviculturally to native hornbeam. Provenance, seed production and nursery practice Ostrya species are monoecious and produce clustered seeds. Vegetative reproduction by stump sprouting can be profuse in nature. In O. virginiana, seed production begins from around 25 years. The seed is doubly dormant and must be stratified for 20–30 days at 20–30°C, followed by 90–150 days at 1–5°C. As the species has been little favoured by American forest managers, there has been limited demand for planting stock. There is little availability from British nurseries at the present time. As there have been no provenance trials of these species in Britain, there is no advice on provenance, but material from the northern part of the range should be hardier. Pests and diseases The only reported problems are stem and butt rots in the Ontario range, caused by common fungi including Stereum, Phellinus and Pholiota.

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Timber properties and utilization As with the native hornbeam, these species produce hard, durable timbers, used since antiquity for tool handles (e.g. plane soles) and similar specialist applications (Korkut and Guller, 2008). This is reflected in the colloquial name ‘ironwood’ given to the American species. The timber varies from white to reddish brown. As the timber is diffuse-porous, the material is considered one of the best substitutes for yew for making archery bows. It is dense and heavy (853 kg m–3 air dried). West and Coronado (2016) regarded the American hop-hornbeam (O. virginiana) as an underutilized ornamental landscape tree. It has several desirable features including exfoliating bark and unique fruit clusters. The recent decline in ash (Fraxinus), as a result of attacks by the dieback fungus, Chalara fraxinea, has increased American hop-hornbeam’s importance as it is a relatively pest-free species.

PICEA A. Dietr.

Spruce

Spruces are in the family Pinaceae, which contains most of the well-known conifers of commerce. Trees in the family often form the dominant component of boreal, coastal and montane forests. There are about 34 species of spruce, which extend between subtropical high altitudes, and temperate and boreal regions of the northern hemisphere. Spruces are most common in the boreal forests, where they are the dominant species across vast tracts of Scandinavia, Russia, Alaska and Canada. There are two European species: Norway spruce (Picea abies) and Serbian spruce (P. omorika). Of these only the former is currently ­important in Britain. The north-western American Sitka spruce (P. sitchensis) occupies the largest area of forest and produces more timber than any other species. Several other spruces have been tried in Britain with varying success. An account of them is given by Savill et al. (2017b).

PICEA ABIES (L.) H. Karst.

Norway spruce

Origin and introduction The natural distribution ranges over much of northern Europe, from the Pyrenees, Alps and Balkans, northwards to south Germany and Scan­ dinavia, and eastwards through the Carpathians and Poland to western Russia, where it merges with the north Asian Siberian spruce, P. obovata. It is a high-mountain species in central Europe and a lowland tree outside permafrost areas in northern Europe. It was probably introduced to Britain before 1500. Climatic requirements Norway spruce is the most climatically tolerant species in the genus. It will grow in both extreme oceanic and central continental conditions. It has no particularly marked climatic limitations in Britain, except those imposed by exposure, which it will not tolerate nearly as well as Sitka spruce. It thrives in regions where rainfall exceeds about 800 mm. It can therefore be used in drier regions than Sitka spruce but not at such high elevations. Though more hardy than Sitka spruce to late spring frosts it can nevertheless suffer from them badly. It will not tolerate salty winds or urban pollution.

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181

Site requirements The tree does best on moist, even moderately waterlogged rushy land of medium to high fertility, including heavy clays and the less acid peats. It will thrive on sites where deep rooting is not possible, but the penalty for this is that it becomes susceptible to windthrow. If the site is too dry it tends to suffer from crown dieback (Day, 1951). Drought-prone sites should be avoided, especially in eastern Britain. On calcareous soils, the species suffers from chlorosis (yellowing of the foliage) caused by iron deficiency. On sites dominated by heather, Norway spruce is unable to compete for nitrogen and consequently grows slowly or scarcely at all.

Other silvicultural characteristics Norway spruce always grows slowly for the first year or two after planting, going into what is often termed ‘post-planting check’. The tree is moderately shade bearing when young and can be used for underplanting, if freed early enough. It recovers well from under-thinning. It is a valuable and commonly used tree in mixtures with hardwoods, especially oak, with which it has a reasonably compatible growth rate, at least in the early years. However, care has to be taken that the spruce does not eventually suppress the broadleaved companion species. Many broadleaves have been lost from this cause. Since the mid-1980s there has been an increasing interest and ­emphasis upon native woodlands and their associated biodiversity (e.g. Kirby, 1988). This has resulted in large sums of public money being spent on converting lowland mixed, or pure coniferous, plantations on ancient woodland sites (known as PAWS) back to something approaching their original broadleaved composition by removing coniferous components. Norway spruce plantations are frequent targets for conversion.

Pests and diseases The green spruce aphid, Elatobium abietinum, causes some damage to Norway spruce but normally it is less severe than to Sitka spruce. Norway spruce also suffers rather commonly from a problem known as top-dying. This can result in decline and ultimately death, especially on the eastern side of Britain. It is susceptible to fomes root and butt rot, Heterobasidion annosum and to various other decay fungi, which can seriously degrade the wood.

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Natural regeneration Viable seed is rarely produced in any quantity in Britain, largely b ­ ecause it does not mature properly. Because of this natural regeneration is unusual.

Flowering, seed production and nursery conditions On the rare occasions when it is successful, the tree flowers in May. Seeds ripen and are usually ready for collection in October, and are dispersed naturally between October and April. The earliest age at which a tree bears seeds is 30–35 years, but the best crops are usually between 50 and 60 years. There are about 195,400 seeds kg−1 (range 103,600–218,300), of which 110,000 are normally viable and will give 80,000 usable 2+0 plants. For nursery purposes seed is usually sown in mid- to late March. It requires no pretreatment if it is sown soon after collection but, if stored, it should be stratified for 3–4 weeks (Aldhous and Mason, 1994).

Provenance In general, southern European provenances, from Poland, Romania, Czechoslovakia and Bulgaria, grow best in Britain. These combine fast growth with late flushing times (Lines, 1987). Many seed-bearing British stands are of French, Alpine and Scandinavian origin. They grow slowly and tend to flush too early and so are sensitive to damage by late spring frosts. Seed should not, therefore, be collected from them.

Area, yield and rotations According to the Forestry Commission (2018a) inventory, Norway spruce had been planted on 60,600 ha, or 2.3% of forest land. This was a reduction from the area recorded in a previous inventory by Locke (1987), probably for the reasons explained above and on p. 10. Mean yield classes (YC) range from 10 to 13 m3 ha−1 year−1 in different regions (Nicholls, 1981), with a maximum of 22. For equivalent YCs rotations of Sitka spruce are around 15 years shorter than those of Norway spruce, and on sites where both will grow Sitka spruce is usually more productive. If a choice between the two species exists it is therefore rare to pick Norway spruce.

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Timber Details are given with the description for Sitka spruce. The average density of the wood of Norway spruce, at 12% moisture content, is about 405 kg m−3, which is somewhat denser than Sitka spruce. The specific gravity is 0.35, and that of Sitka spruce 0.36 (Ramsay and Macdonald, 2013). Christmas trees At one time Norway spruce provided the traditional and most popular British Christmas trees, although its pre-eminent position has been overtaken by various other spruces and silver firs, mainly Abies nordmanniana and A. procera. In 2017 only 15% of all Christmas trees were Norway spruce. The cultivation of Christmas trees has undergone huge changes since about 1990. Once they came mainly from the tops of trees that had been removed in thinnings, but today their production is highly specialized and good quality is all-important. This is ensured by controlling weeds and insects that might cause a loss or discolouration of the needles, and shaping the trees by shearing, often annually from the 4th year. Norway spruce is normally sold as Christmas trees for domestic use at 4–6 years old and the slower-growing firs up to 4 years older. Big trees grown for large buildings or open spaces are obviously older. Pearce (1979) stated that seed sources from south-western Germany are suitable for Christmas trees. No work on this subject appears to have been done since then in Britain, though it has in Denmark by Madsen (1989). Place of Norway spruce in British forestry Norway spruce has a useful place on wet sites, at middle and lower elevations, in regions that have too little rain for Sitka spruce. It is also one of the better conifers for planting in mixture with broadleaved trees ­because it is not too fast growing. It has been widely eradicated from lowland PAWS in recent years, often without replacing the cleared trees with any other species, at least on the publicly owned forest estate (Wilson, 2012). Its use is likely to decline in favour of broadleaved species, but it will nevertheless remain an important species.

PICEA OMORIKA (Pančić) Purk.

Serbian spruce

Characteristics of the species Identification is made easy by its unique habit and its needle characteristics. In cultivation, substantial variation in form of the trees occurs

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from seed. Most trees have a slender stem and short (less than 2 m long) ­ascending or drooping branches forming a narrow, very graceful, spirelike habit. This snow-shedding form is comparable to that of Engelmann spruce (P. engelmannii) in western North America. Serbian spruce is one of the few spruces with flattened needles like those of western hemlock, not the four-sided needles typical of most spruces. The short, 1.5–2.5 cm long needles are lustrous dark green above and the underside has two broad, white stomatal bands. These bands collectively stand out, creating a unique silvery contrast that is very ­effective when the upswept branches move in the wind. Female cones are an elongated egg-shape, 4–7 cm long and pendulous with stiff scales, blue-black when young, cinnamon when mature. Male cones mature in May and ­female cones in September–October. Most remain closed until the following May–June and they may persist on the tree for up to 5 years. Typically, a good coning year is followed by a poor one. Occurrence and introduction The native range of Serbian spruce is in south-eastern Europe. It is ­restricted to western Serbia and eastern Bosnia and Herzegovina in a small area around the River Drina. The species was widespread in Europe millions of years ago, but after the Pleistocene glaciations it survived only in this refugium. It is found chiefly on calcareous soils at 700–1500 m elevation, usually on steep, north-facing slopes. It occurs both in pure stands and in mixture with other species (Farjon, 2017). The total natural extent of Serbian spruce is only about 60 ha, dispersed across 4,076 km². It was originally discovered near the village of Zaovine on Tara Mountain in 1875 and named by the Serbian botanist Josif Pančić in 1877. ‘Omorika’ is its Serbian name. Within this restricted range there are four main localities where it is found (see Savill et al., 2017a). Details of the particular localities where the species can be found in the wild are given in the IUCN Red List (2017). In total there are 25 individual sites found in the four main localities mentioned above. Some of these sites contain as few as 100 trees. Until the middle of the 19th century, the natural range of P. omorika was more continuous and less fragmented than it is today. It was harvested for timber until the early 20th century, but the few remaining small stands are now protected (Jovanović, 1970). Its current distribution is mainly the result of anthropogenic factors such as general forest clearance and selective cutting, pastoralism and wildfires. Fire has perhaps been the biggest threat, and logging has been a subsidiary one. Recent fieldwork indicates that there is a continuing slow decline in the natural extent of occurrence, area of occupancy, quality of habitat and number of mature individuals in some locations. This is primarily due to

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poor natural regeneration and an inability to compete with associated tree species including P. abies and Fagus sylvatica (Farjon, 2017). Serbian spruce was introduced into cultivation in Britain in the late 1880s and specifically to Kew Gardens in 1889. Trees from the original introduction survive at Murthly Castle, Perthshire (Mitchell, 1982). Climate and site Serbian spruce occurs naturally mainly on steep, sometimes precipitous, north- to northwest-facing limestone slopes which overlie igneous ­material. Soils are typically shallow rendzinas and calcareous brown earths. In the wild state, the species is strictly confined to limestone soils and never grows on the slate which is found in parts of the Drina valley. However, when cultivated, it does very well (at least in its early years) on soils that are not calcareous (Elwes and Henry 1906–1913; StirlingMaxwell, 1929). The climate in its natural range is characterized by very high humidity, high precipitation (regularly distributed over the year), deep snow cover which lasts 4–5 months and low winter temperatures. Wardell (1956) concluded that there should be no climatic limitations to its growth in Britain (though it can occasionally be damaged by frost). It is said to be more drought tolerant than Norway spruce, and is in fact unexacting as regards both moisture and nutrients, and in its resistance to exposure and frost. It is reported to be hardy to Zone 5 (cold-hardiness limit between −28.8°C and −23.3°C) (Bannister and Neuner, 2001). Most sources indicate it will tolerate a wide soil pH range, droughtprone soils and polluted urban conditions. Serbian spruce was regarded as being relatively pollution resistant by Kommert (1982) and Dallimore and Jackson (1948). According to Macdonald et al. (1957), growth in Britain differs very little over a wide range of sites, though it has perhaps distinguished itself best on moist moorland soils and on peats where the majority of stands have been established in the past. Macdonald et al. (1957) also concluded very tentatively that it could be planted on calcareous sites in Britain, based on its natural occurrence, but few examples are known. Stirling-Maxwell (1929) described the species as ‘very accommodating as regards soil, thriving better than any other on dry gravel at Kew and seeming equally at home here (at Corrour, Inverness-shire) on good black peat’. On good sites it is unlikely that Serbian spruce will appear impressive in rate of growth when compared with other, more demanding spruces. Although the potential yield is less than that of Norway spruce, P. omorika is characterized by greater drought resistance and freedom from snow and insect damage and is recommended for sites of low to medium quality in the calcareous beech zone in Switzerland (Leibundgut, 1978).

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Habitat and ecology In the native range, associated tree species typically include Abies alba, P. abies, Pinus nigra, Fagus sylvatica, Acer pseudoplatanus, Populus tremula, Sorbus aucuparia, S. aria, Quercus spp. and Ostrya carpinifolia. Sometimes it forms almost pure stands. On rocky outcrops it is co-dominant with Pinus nigra; on steep slopes at high altitudes it grows with Picea abies and Pinus nigra; while on steep slopes at lower elevations it is co-dominant with Fagus sylvatica. Post-fire regeneration is usually very good though limited to steep slopes and cliffs. After visiting sites where the tree grows in the wild, Wardell (1956) concluded that the spruce, which is a relic in the flora of the Balkan Peninsula, succumbs to competition with other species and is poorly adapted to its present environment but holds its own when competition is absent (as on north-facing cliffs or screes) or where it has regenerated itself after fire or exploitation – the sort of areas inhabited by chamois, according to Elwes and Henry (1906–1913). In one site that was disturbed by ordnance during the Bosnian–Serbian conflict in the 1990s, regeneration was prolific (pers. comm., T. Christian, 2017). Silviculture Macdonald et al. (1957) said that Serbian spruce had never been planted extensively in Britain. The earliest larger-scale plantings were at Corrour and Fersit (Inverness-shire) in 1908. It was estimated that, by 1957, less than 200 ha had been planted. More recent data suggest that in 2015 there was around 160 ha of Serbian spruce in public forests in Britain, most dating from the period between 1950 and 1970, but with a small increase in planting in the last few years. There has also been limited adoption on private estates, as at Brahan Estate in Ross-shire, northern Scotland. There is, therefore, little experience of the silvicultural requirements of the species in Britain. The narrow form of the tree suggests that it could be established at greater densities than the conventional 2500 per ha (­potentially around 4000 per ha). Thinning could also be carried out later than normal. Rotation length seems likely to be similar to that for Norway spruce (about 70 years), though as Stirling-Maxwell (1929) stated, ‘the habit is slender, and it makes less wood than the Norway spruce in proportion to height’. Keenleyside (1985) observed that the Corrour trees, then over 70 years old, had very narrow crowns, not more than 2 m in diameter. He said that ‘growth has been very steady but very slow, equivalent to General YC 6’. Serbian spruce grows well in full sun to partial shade on sites protected from winter wind. If grown in too much shade the tree

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becomes thin and leggy and will not thrive. It is considered to be one of the most adaptable spruces and was said by Dallimore and Jackson (1948) to have succeeded better than any other spruce at Kew Gardens ‘in spite of the enervating conditions of a hot dry soil and an impure atmosphere’. It flushes late, thereby escaping injury from late spring frosts. There are some quite impressive plantations in Britain, especially in Galloway, in Wales and around the inner Moray Firth. Dallimore (1945) described experiments at the National Pinetum at Bedgebury and suggested that this species will do well on an acid soil at low elevation. Young trees (35–45 cm high) were planted in 1926. In 1945 they were about 9 m high and 22 cm diameter at breast height (dbh) at 1.5 m above ground, and had a branch spread near the ground of almost 5 m. They had not been injured by winter or spring frosts, nor attacked by ­insect pests. Trees planted in 1931 were 6.0–6.7 m high at age 14, and in 1942 they bore a heavy crop of cones with a fair percentage of fertile seeds. The great value of this species appears to be for planting in places where other spruces are liable to injury by drought or spring frosts. Natural regeneration As already stated, P. omorika is a poor competitor with associated species, and consequently natural regeneration in its native range is unusual ­except on the steepest rocky slopes and on cliffs where broadleaved trees are unable to compete effectively. Regeneration also occurs in the bottoms of wet, shaded ravines (Elwes and Henry, 1906–1913). It is dependent on catastrophic events for good recruitment and healthy subsequent growth to take place. Once established, it often becomes suppressed by Abies alba, F. sylvatica and P. abies (Burschel, 1965). Record and potential in Britain According to unpublished Forest Research records, about 135 experiments and trial plots that included this spruce were established between 1920 and 2017. Of these trials, 27 trials were still active in 2017. Nearly 50% had been planted in the 1950s, with relatively little experimental planting ­between 1970 and 2010. More recently, there has been increasing interest in the potential of the species for diversifying upland conifer forests or as a means of adapting forests to the threats from pests and diseases, so it was included in a new series of operational species trials established in various parts of Britain in 2015. Possibly because of the restricted native range of Serbian spruce, it appears that many of the earlier experimental plantings will have used plants grown from seed collected in British stands of uncertain origin.

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Examination of seed records suggests that relatively little seed was ­imported from the native range before the 1960s; further investigation would be ­necessary to check which, if any, of the surviving experimental plots were of specific native origins and might therefore have value for ex situ genetic conservation. Growth The tree has a moderate height growth rate of up to 30 cm a year and will usually attain an eventual height of 15–18 m and a spread of 6–7 m. In continental Europe, it is regarded as being slightly slower growing than Norway spruce (e.g. Leibundgut, 1978; Altherr, 1981). In Britain, it has achieved YC of between 14 and 16 m3 ha−1 year−1 in the south of England, and 8–12 in the north of England and Scotland (see Tables 10 and 11). Growth at higher elevations has tended to be poor (e.g. 6 m3 ha−1 year−1) at Corrour, Inverness-shire. Basal areas per hectare are often quite high, probably reflecting the narrow crown form of the species, resulting in high stocking densities compared to other spruce species. Pests and diseases Few diseases, except for honey fungus, appear to affect Serbian spruce seriously. Greig (2017), in a letter to the Quarterly Journal of Forestry, stated that it is ‘extremely susceptible to honey fungus (Armillaria spp.)’, quoting sources Greig (1981) and Greig et al. (1991). Gibbs et al. (2002) recorded that, in Thetford forest in the east of England, it was one of the species least affected by fomes (Heterobasidion annosum). Some sources list aphids, mites, scale insects and budworm as potential pests; however, so far there are no reports of these affecting the tree significantly in Britain, though aphids were observed by T. Christian, and were numerous enough to cause damage to the trees (Tom Christian, pers. comm., 2016). This contradicted observations by Klqft et al. (1964), who ­reported that the green spruce aphid (Elatobium abietinum) hardly attacked P. omorika at all. Even the great spruce bark beetle (Dendroctonus micans), which affects P. abies and P. orientalis very severely, has not been recorded as a pest on P. omorika. Genetics and provenance There have been no attempts to compare different provenances of Serbian spruce in Britain, but Ravensbeck and Madsen (1998) described the ­results of a provenance trial in northern Jutland comprising 29 seed lots collected

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Table 10.  Growth of selected sample plots of Serbian and other spruces in different parts of Britain. Cumulative basal area Yield class production (Local yield class) (m2 ha–1) (m3 ha–1 yr–1)

Species

Location

Age

Top height (m)

Serbian spruce

Bedgebury, Kent (plot 1170) Bedgebury, Kent (plot 1175) Thetford, East Anglia Brechfa, Carmarthenshire Forest of Deer, Aberdeenshire Newcastleton, Borders Glen Urquhart, Inverness Bennan, Galloway Bedgebury, Kent

49

22.6

81.6

16

49

23.1

92.1

16

49 45

18.5 22.0

75.7 70.8

14 16 (14)

48

14.3

103.0

8 (10)

57

18.0

87.8

10 (11)

64

18.8

103.9

8 (12)

73 35

27.0 17.2

118.3 67.2

12 (15) 16 (19)

Dawyck, Borders Bedgebury, Kent

53 31

19.7 10.3

93.8 –

10 (12) 12

33

13.5

37.5

10 (9)

39 32

16.7 16.7

48.6 37.2

16 (17) 18 (14)

49

12.6

28.2

4

Oriental spruce Black spruce

Brechfa, Carmarthenshire Red spruce Bedgebury, Kent Englemann Bedgebury, Kent spruce Hondo Bedgebury, Kent spruce Hyphen, no data available.

Table 11.  Comparative performance of five different spruce species after 50–65 years at Kilmun Forest Garden, Argyll.

Species Norway spruce Serbian spruce Oriental spruce Red spruce (Picea rubens) Sakhalin spruce (Picea glehnii)

Age (years) 62 62 62 61 52

Basal area (per cent of Norway Top spruce) height (m) 28.1 29.0 28.1 25.8 14.5

100 112 143 103 80

Yield class (m3 ha–1 yr–1) 18 18 18 16 10

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from plantations within Denmark and one Yugoslavian p ­rovenance. The differences in height were moderate: the ten tallest provenances were p ­ redicted to attain heights 11% taller than a provenance of average growth. Frequency of forking varied considerably but seemed primarily determined by site variation within the trials. Nasri et al. (2008) suggested that ‘current P. omorika populations are shaped by an extreme demographic bottleneck and random genetic drift linked to Quaternary glacial cycles. P. omorika thus belongs to the small group of genetically depauperate tree species’. Outcrossing rates were estimated in a natural wild population and in a cultivated Finnish population of Serbian spruce. The outcrossing rates (which vary from 1 = all outcrossing, to 0 = all selfing) were 0.98 ± 0.03 and 1.02 ± 0.04 in two different years in the cultivated stand, and 0.84 ± 0.05 in the natural stand. The relative self-fertility was also estimated in seven trees in the cultivated population. The results indicated high self-fertility, which agreed with earlier information. Thus, despite its very narrow natural range and morphological uniformity, Serbian spruce is naturally an outcrossing species but is tolerant of high levels of self-crossing when r­equired. Presumably this evolved as an evolutionary adaptation in years when few trees are flowering (Kuittinen and Savolainen, 1992). P. omorika has been planted in Denmark for various purposes (e.g. as a park tree and for Christmas trees) as well as for wood production, ­especially on poor soils and frost-exposed sites. The species is known to ­ hybridize with Sitka spruce and other spruces. In cultivation, it has produced hybrids with the closely related black spruce and also with Sitka spruce. For this reason, seed should not be collected from stands adjacent to mature Sitka spruce in British plantations. P. omorika is widely grown in gardens throughout northern Europe but few of these collections are either comprehensive or well documented. More recently the International Conifer Conservation Programme, based at the Royal Botanic Garden Edinburgh, has made comprehensive introductions of Serbian spruce from throughout its natural range, for its ex situ conservation collections, sampling from the majority of the 25 sites across all four localities. These young trees have since been widely distributed across 61 host locations in Britain and Ireland (pers. comm., T. Christian, 2016). The advice given on the Forest Research (2018b) website is ‘There is little evidence of significant provenance variation; seed from good British stands or from the natural range should be preferred’; however, the Danish work by Ravensbeck and Madsen (1998) suggests that this may need further refinement. There are provenance differences but, as there are no provenance trials in Britain yet, recommendations cannot be given.

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Nursery conditions There is likely to be difficulty in procuring seed from the native range for some years to come and every opportunity should be taken of collecting seed from trees of good form in larger, home-grown stands. Kuhns and Rupp (2000) state that propagation is straightforward as seeds require no pretreatment. However, a slight improvement in germination percentage occurs when seeds are stratified for up to 3 months at 4°C. Meyer (1960), working in Germany, thought that Serbian spruce required 1–2 years longer in the nursery than most other conifers because it is slower growing, but others dispute this finding. Serbian spruce transplants well in spring or autumn from containers or as balled and burlapped (B&B) plants, and establishes quickly under a variety of landscape conditions. There is an untested seed orchard of 1.4 ha near Pitlochry, Perthshire. Uses At present, outside its native range, Serbian spruce is of major ­importance only as an ornamental tree, mainly in northern Europe and North America, for use in large gardens. It is regarded as one of the most attractive spruces because of its elegant form and dark green needles and ability to grow on a wide range of soils (including alkaline, clay, acid and sandy soils), ­although it does best on moist, drained loams. The tree deserves a more prominent place in commercial and residential landscapes. It can be used in groups, as single specimens or even as an evergreen street or avenue tree. It has utility as a natural screen and ­selections with a narrow habit are suitable even for small urban landscapes. Serbian spruce represents a welcome alternative to the all-toocommon Norway spruce. It is also grown to a small extent for timber and paper production, particularly in northern Europe, but its slow growth makes it less important than Sitka spruce or Norway spruce in Britain. Greig (2017) stated, however, that Serbian spruces make excellent Christmas trees owing to their compact form, usually being the first choice if a number of species are available. Chylareck (1966), in a trial of 16 alternative species of Picea in Poland, regarded P. omorika and P. pungens as being best for the country as a whole because they are undemanding as to site, highly ornamental, healthy, ­almost completely resistant to cold, vigorous in growth and producing seeds of high germinative capacity. Its wood is said to be very similar to that of Norway spruce: closegrained, compact, yellowish and easily worked. Density, at about 441 kg m–3

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(Sachsse, 1981) is similar or slightly higher, especially in younger trees (Kommert, 1990). Ramsay and Macdonald (2013) gave several timber properties for small clear samples from five trees. They quoted the specific gravity as 0.36. Norway spruce is given at 0.35 and Sitka spruce as 0.39. Role in British forestry Elwes (in Elwes and Henry, 1906–1913) stated that ‘though the tree is very ­ornamental, I do not expect it can have any value as a forest tree’. This was, of course, before climate change was anticipated. In the light of the evidence above, it seems that Serbian spruce may have some value as an alternative spruce on sites in eastern Britain where Sitka spruce is e­ xpected to suffer from moisture stress. However, other alternatives such as Norway and hybrid spruces (see Savill et al., 2017a) may be more productive candidates for the majority of sites. Serbian spruce has some ­potential for use in spaced-tree silvopastoral systems because of its narrow crown architecture, allowing retention of forage throughout the rotation.

PICEA ORIENTALIS (L.) Peterm. Oriental or Caucasian spruce Distribution and ecology Oriental spruce (P. orientalis) is effectively the eastern equivalent of the European or Norway spruce, found in central Eurasia. The species is considered native to the mountains around the eastern end of the Black Sea, including the central Greater Caucasus and the eastern ends of the Trialeti ridge on the Lesser Caucasus. These areas lie within Abkhazia, Georgia and north-eastern Turkey (Kayacik, 1955; Farjon, 1990). Oriental spruce can also be found in northern Iran, though its numbers there have ­decreased owing to deforestation. The species does not exist in the south of Asia Minor, nor further west than the Melet River, and records from other areas are believed to be erroneous (Kayacik, 1955). Farjon (2017) stated that it occurs in dark coniferous and mixed forests, primarily from 600 to 2100 m asl and reaches its optimum development above 1200 m. Its scarcity below 600 m asl may be more connected with the influence of biotic factors (human interference, competition) than climatic and edaphic factors. The species’ overall distribution, however, is determined mainly by climatic factors, such as restriction due to ­deficient rainfall and low atmospheric humidity. Kayacik (1955) included some data on temperature, rainfall and humidity, and a sketch map. Westwards, Oriental spruce also becomes limited by dense competitive forest dominated by Fagus sylvatica and Abies nordmanniana. It is hardy to

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Zone 5 (cold-hardiness limit between −28.8°C and −23.3°C) (Bannister and Neuner, 2001). Oriental spruce usually grows on brown forest soils but it can also often be found on stony and rocky slopes, and is generally considered ­undemanding of soils. It forms pure stands or is associated in mixed montane forests with A. nordmanniana, Pinus sylvestris and Fagus sylvatica. Oriental spruce-dominated forests may have various types of understorey, of which the Colchic type made up of evergreen shrubs and dwarf trees such as Laurocerasus officinalis (Prunus laurocerasus), Ilex colchica, Buxus sempervirens, Taxus baccata and Rhododendron spp. is particularly notable. Description and growth Oriental spruce is a large, coniferous, evergreen tree growing to 30–45 m tall (exceptionally to 57 m), and with a typical diameter at breast height (dbh) of up to 1.5 m (exceptionally up to 4 m). The shoots are buff-brown and moderately pubescent (hairy). The leaves are needle-like, 6–8 mm long, rhombic in cross section, dark green with inconspicuous stomatal lines. The cones are slender cylindric–conic, 5–9 cm long and 1.5 cm broad, red to purple when young, maturing dark brown 5–7 months after pollination, and have stiff, smoothly rounded scales. The comparative performance of Oriental spruce with some other spruces at Kilmun Forest Garden in Argyll is given in Table 11. Silviculture Much has been published on this species, most in the Turkish language. Urgenc (1960) presented tabular and graphical data on maturity of cones and seeds, after-ripening, seed production and collection, morphology of cones and seeds, conditions for germination and seed quality. All the ­salient points are given in a 6-page English summary. In a discussion on its silviculture, Oksbjerg (1957) described the range and site requirements of Oriental spruce and its growth in stands in Jutland and Germany, concluding that, while its increment pattern in general resembles that of Norway spruce, its height increment is less affected by good and bad seasons, and it is better suited to growth in shade. Selective logging, agricultural land development and insect damage are the major threats to the species, although they are not thought to be causing an overall decline. Oriental spruce occurs in a number of protected areas throughout its range, such as Meryemana Forest (Pontic Mountains, Turkey), Kintrishi, Ritsa and Algeti Protected Areas (Georgia) and Teberda Nature Reserve (Russian Caucasus). Population monitoring, including species-based actions such as selective logging and trade management, are needed.

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Threats As global warming increased, Oriental spruce forests started to experience serious bark beetle problems. More than 200,000 trees died in the native range recently due to bark beetle attack. Tufekcioglu et al. (2008) reviewed existing literature to assess the current status of the spruce stands and concluded that climate change could significantly influence distribution, diversity, structure and stability of Oriental spruce ecosystems. There have been several outbreaks of the great spruce bark beetle, Dendroctonus micans, which have killed millions of trees in the Black Sea region of Turkey since its discovery in 1966. Besides wounded and overthrown trees, apparently healthy trees are also attacked by the beetle, ­usually indicated by resin bleeding. In infested stands, repeatedly ­attacked trees, unattacked trees or trees with aborted attacks have been reported. Alkan-Aknc and Ersen-Bak (2016) investigated the role of some tree vigour parameters in the beetle’s successful establishment on its host. They found that D. micans females became established more often on trees with thicker phloem and on vigorously growing trees. Stressed or destroyed vigorous trees may produce epidemic populations in stands. Trees are sometimes defoliated by Rhizosphaera kalkhoffii (Wilson and Waldie, 1926), in common with most other spruces and many species of pine. The disease is transmitted by splash-borne conidia (Diamandis and Minter, 1980). Record and potential in Britain As noted by Macdonald et al. (1957), though Oriental spruce was introduced to Britain in the 19th century, there have been surprisingly few trials with this species. Forest Research records suggest that there have only been 18 experiments that include it, and most of these have been established in the last few years. Records indicate that from 1920 to 1991 there were no imports of seed from the native range, so all mature stands found in British forests are almost certainly derived from home collections, from small Victorian and Edwardian ornamental plantings of potentially limited genetic diversity. Although Macdonald et al. (1957) suggested that early growth in one trial plantation at Dawyck (200 m asl in the Scottish borders) was comparable with what would be expected from Norway spruce, there have been few other measurements of the long-term potential of this species. Subsequent data from this plot (Table 10) plus other data from a short-lived plot at Bedgebury show that growth rates are quite similar to that of Serbian spruce. A large plot established in 1940 in the Crarae Forest Garden, Argyll, is one of the best performing in that collection. More recent data from comparative plots of a range of species at Kilmun (Table 11) indicate that, at

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YC 18, Oriental spruce had the highest production of a range of alternative spruce species. However, the comparative performance of Sitka spruce on this climatically moist site was equivalent to YC 24. Provenance No provenance testing has been carried out in Britain and there are few forest plots; seed should be sourced from the natural range (Forest Research, 2018b). Seed collections from small British stands should be used with considerable caution because of the risk of an inherently narrow original genetic base. Uses and trade Oriental spruce is an important timber tree in the Caucasus, where it forms extensive pure stands, many of which are managed for production (Farjon, 2017). It has also been introduced as a plantation tree in countries in the eastern Mediterranean. The species is commonly grown as an amenity tree in parks and large gardens in many European countries and in the USA, ­favoured for its attractive foliage and ability to grow on a wide range of soils. A number of cultivars exist in the trade, among which are dwarf forms, forms with yellowish flushing leaves and those with ‘mounding’ habits. Elsewhere it is grown to a small extent for Christmas trees, timber and paper production, though its slower growth compared to Norway spruce reduces its importance outside the native range. Timber Ramsay and Macdonald (2013) gave some limited data on the performance of small clear samples from mature wood of four old trees in Turkey. The specific gravity is quoted as 0.52. This is much higher than Norway spruce at 0.35 but may be an artefact of the sampling method used. The wood is most likely to be comparable to that of Norway spruce, and is used for similar purposes. Among these are construction, flooring, carpentry, furniture manufacture and parts of musical instruments. Place in British forestry Given the likelihood of increased occurrence of drought in parts of eastern Britain, it is possible that this species could be used as an alternative spruce on sites where projections using ecological site classification (ESC)

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suggest that Sitka spruce may become ‘marginal’ or ‘unsuitable’. Limited evidence also suggests that Oriental spruce may also be better suited to such sites than Norway spruce. Its reported ability to tolerate shade suggests that it may also have a role where conversion to continuous cover forestry is being considered. However, the limited experience to date suggests that, for the immediate future, any further planting should be on an operational trial basis, with considerable attention being paid to obtaining material from provenances from the native range of the species. There is a need for further research comparing the yield performance of Oriental and Norway spruces on equivalent site types.

PICEA SITCHENSIS (Bong.) Carr.

Sitka spruce

Origin and introduction The natural range of Sitka spruce is a narrow (ca 80-km wide) but very long strip in the ‘fog belt’ of the Pacific coast of North America, from Alaska southwards to California, spanning over 22° of latitude. It is largely ­restricted to west-facing slopes, seldom above 900 m in elevation, and it extends inland a little up river valleys. Its inland distribution depends upon the penetration of fog banks. The largest Sitka spruces are found in Haida Gwaii (formerly the Queen Charlotte Islands) off British Columbia and in the Olympic peninsula of Washington, where the trees grow 80 m tall. It was introduced to Britain in 1831. Climatic requirements In its native range the climate is maritime, with abundant atmospheric moisture. In Britain Sitka spruce rarely does well where annual rainfall is below 900–1000 mm. It is therefore well suited to higher elevations in the west and north. The much drier east and south should be avoided. One of Sitka spruce’s outstanding attributes is its ability to grow well, without becoming deformed, in regions of high exposure: it is better in this ­respect than any other common conifer. The upper limits of planting are about 600  m on reasonably sheltered sites and 150–300 m lower in exposed western areas. The tree is not tolerant even of mild atmospheric pollution. Lines (1984), for example, showed that in the southern Pennine Hills of England Sitka spruce grows poorly where an experienced silviculturist would have been sure of good growth on apparently similar sites away from industrial pollution. Most provenances of Sitka spruce are susceptible to damage from late spring frosts when young, particularly the southerly ones from Washington and Oregon. Cannell and Smith (1984) have shown that young trees

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(C)

(B)

Fig. 22.  (A) Norway spruce, Picea abies; (B) Sitka spruce, Picea sitchensis; (C) N ­ eedles of both spruces are inserted onto the small woody projections shown in this enlarged drawing of part of the stem.

planted in the British uplands are likely to be subjected to a damaging spring frost once every 3–5 years. In most places they are sufficiently fast growing to get above the frost level quickly enough for this not to be a serious constraint on planting. However, damage in frost ­hollows is likely to be more severe and much more frequent, and Sitka spruce will not survive if planted in them. The species, especially Washington and Oregon provenances, is also susceptible to early autumn frosts, which o ­ ccasionally occur, before they have hardened off sufficiently for the winter (Cannell et al., 1985). Such damage is much less frequent than spring frost damage, occurring on average every 8–11 years. Site requirements Sitka spruce is very accommodating. It grows well on drained peats, is highly productive on gleys and does best on deep, freely drained soils in

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suitably high rainfall areas. The tree is quite site demanding and, unless a planting area is at least moderately fertile, nutrients have to be added (particularly phosphate). It is regarded as a ‘high input’ species in comparison with, for example, lodgepole pine. Dry and shallow soils should be avoided. If these are in the uplands, lodgepole pine or noble fir should be planted rather than Sitka spruce. Like Norway spruce, and several other conifers, the tree will not grow in competition with heather because it is unable to compete with it for nitrogen (Handley, 1963). If heather-­ covered sites are to be planted with Sitka spruce it is necessary either to kill the heather with a herbicide or to provide enough fertilizer nitrogen to enable the spruce to close canopy and suppress the heather. Other silvicultural characteristics Sitka spruce will grow to a maximum height of over 50 m in Britain. It can thrive on wet soils where deep rooting is not possible; though, if these are located in exposed parts of the country, windthrow is a constant and serious risk and a frequent occurrence. Natural pruning is rather slow, which makes young plantations particularly impenetrable and prickly. It is u ­ nfortunate that the most widely planted tree in Britain should also be one of the most unpleasant in this respect. Very deep accumulations of fallen needles are to be seen on most upland acid sites where decay and consequent ­nutrient cycling are poor. This often leads to nutrient deficiencies, but Sitka spruce responds well to fertilizers, even after many years of check. Views differ about the shade tolerance of Sitka spruce. In its native range it is regarded as tolerant (Harris, 1990), but in Britain it is light demanding and cannot, for example, be used for underplanting. Throughout most of its natural range in North America Sitka spruce is associated with western hemlock in fast-growing dense stands. It is one of the few conifers that develop epicormic branches along the stem but they seldom grow to any significant size. Their production is related to light intensity, and roadside trees often develop dense new foliage from base to crown. Thinning can stimulate epicormic branching and could decrease the quality of the wood, though this is not normally a serious problem (Harris, 1990). Pests and diseases The species is susceptible to attack by honey fungus and therefore not usually recommended for planting after a broadleaved crop. It is more susceptible than many conifers to fomes root and butt rot, Heterobasidion annosum, particularly on less acid soils. It is defoliated periodically and quite seriously by the green spruce aphid Elatobium abietinum (Carter,

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1977; Carter and Nichols, 1988). It is much more seriously affected by this aphid than is Norway spruce. Sitka spruce is less palatable to deer than most conifers, but it can nevertheless be severely damaged if the deer have no other choice. Natural regeneration First rotations have been coming to an end increasingly since the 1970s, and it has become apparent that natural regeneration is a reliable means of obtaining a successor crop on many sites on the borders of England and Scotland, and in Wales and elsewhere. It is best in places where crop stability is good and rotations long enough (50 years or more) for the trees to produce large quantities of seed. Re-establishment is often ­almost ­entirely by natural regeneration, and this seems likely to become a common method of replacement in the future, except on sites where new provenances or genetically improved strains of Sitka spruce are intended or a change of species is desired. Regeneration is much more successful in the open or under a light canopy than under a denser canopy. Since the late 1990s there has been increasing emphasis on multipurpose management of forests, and pressure to diversify even-aged monocultures in terms of both species composition and structural diversity. This involves practising what is known in Britain as ‘continuous cover forestry’. Sitka spruce monocultures have been the focus of much of the work into how this might best be achieved (e.g. Malcolm et al., 2001; Mason et al., 2004), though other species have also received attention (e.g. Kerr, 2002; Poore, 2007). The aim would be to achieve most of the conversion by natural regeneration. Research by Mason et al. (2004) concentrated on ­determining the critical level of below-canopy light for survival and growth of young seedlings of a variety of species. Their studies indicated that sufficient light in Sitka spruce plantations is available at basal areas of 25–30 m2 ha−1, which suggests that an irregular shelterwood system with frequent interventions would be a suitable silvicultural system to use. This would provide tree height/gap diameter ratios of greater than two, which is needed by Sitka spruce. Flowering, seed production and nursery conditions The tree flowers in May. Most seeds are mature by September, when they can be collected, and they are dispersed naturally between October and the spring. The earliest age at which the tree bears seed is 30–40 years, but reliable seed crops are usually at 3–5-yearly intervals after the age of about 50 years. There are about 463,000 seeds kg−1 (range 341,700–881,800), of which 320,000 are normally viable and will give about 150,000 usable

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2+0 seedlings (Aldhous and Mason, 1994). Sowing times and seed treatments for nurseries are the same as for Norway spruce. Small, but increasing numbers of genetically selected and improved, micropropagated, clonal Sitka spruce became generally available from the 1990s. The increase in timber volume obtained by planting this material has been estimated by Mochan et al. (2008) to be between 21% and 29% at the end of a rotation, compared with a control of unimproved Sitka spruce of Haida Gwaii origin. Today practically all planted Sitka spruce is of this improved type, even though it is about twice as expensive as plants grown from seed. There is a ‘tested’ seed orchard of 3.4 ha near Elgin, Aberdeenshire. Provenance Genetically improved material is now widely available and is planted on the great majority of sites where the tree is established (see above). In Britain provenances show a cline of decreasing vigour and ­increasing resistance to late spring frost damage from Oregon to Alaskan seed sources, possibly with some ecotypic variation in Haida Gwaii (Lines, 1978c) where vigour and frost resistance are better. The choice of provenance must attempt to compromise on these two characteristics. In general, seed from Haida Gwaii satisfactorily combines hardiness with adequate growth rates and often gives the best growth on all sites. More southerly provenances can be used in milder parts of Britain, and more northerly ones from Alaska may have a place on exposed or elevated sites (Lines, 1987). Area and yield Sitka spruce is planted on almost 664,000 ha (25.1%) of the forest land in Britain, making it by far the most common species in the country. It is well ahead of the next most common group, the pines, at 13.7%. Mean YCs range from 10 to 14 m3 ha−1 year−1 in different regions (Nicholls, 1981), and the normal maximum is about 24. Timber The typically fast-grown wood in Britain is light in weight, non-resinous and rather coarse textured. On drying it is liable to twist. The timber of both Norway and Sitka spruces is preferred for pulping processes. It also provides material for lightweight types of particle board. The potential market for sawn timber is large, but rapid early growth, a tendency to

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spiral grain and, in the case of Sitka spruce, the difficulty of avoiding a rough finish means that only a small proportion is suitable for joinery and other high-class structural work. Grading is, therefore, important. The wood of Sitka spruce is close to the lower limit of strength required for many structural purposes. Any treatment that causes the core of juvenile wood to increase in size is in danger of reducing the strength below an acceptable limit, hence much attention is paid to spacing with this species. There is also scope for selecting and breeding trees with high-density wood (Savill and Sandels, 1983). The average density of the wood at 12% moisture content ranges between 380 and 450 kg m−3 and specific gravity 0.36 (Ramsay and Macdonald, 2013). Suitably graded material is satisfactory for many purposes including trussed rafters, internal framing and partitioning, but it should be protected against rot and insect attack. This is difficult as the heartwood is naturally resistant to pressure treatment with preservatives. The wood of slowly grown Norway spruce is resonant, and because of this it is used for making parts of violins and cellos. Place of Sitka spruce in British forestry Sitka spruce is the most economically important and widely planted tree in Britain. It has been responsible for most of the enormous expansion of the British forest area in the 20th century. It is the best-adapted species for growing in the wet, upland parts of the country, being tolerant of severe levels of exposure, growing well on a wide variety of sites and is at the same time highly productive. Its importance is unlikely to diminish.

PINUS L.

Pines

Pines make up the largest genus in the family Pinaceae, which contains most of the well-known conifers of commerce. Trees in the genus often form the dominant component of boreal, coastal and montane forests. The pines occur on all continents and some islands in the northern hemisphere, mainly in boreal, temperate and mountainous tropical regions. Pines have been introduced into much of the southern hemisphere where they are the basis of major industries. Most species are adapted to fire. In habitats where fires are rare, pines tend to grow on nutrient-poor soils. Most pines are strong light demanders and also very drought tolerant. Certain fire successional species have a ‘grass stage’; that is, the stem of the young seedling elongates little during the first several years (meanwhile developing a large taproot) and bears many long, curved leaves, the plant then resembling a bunchgrass (Kral, 1993). Other species have cones that are long persistent and remain closed, opening only when heated by wildfire (such cones are called serotinous); seeds are released soon after, once the fire is out (Farjon, 2017). About 114 species of pines are found in temperate, subtropical and tropical regions of the world, including 11 in Europe. About 70 species are in the subgenus Pinus (the ‘yellow’ or ‘hard’ pines). These are mostly 2- or 3-needled trees, and about 44 species are in the subgenus Strobus (the ‘white’ or ‘soft’ pines, which have 4 or 5 needles). Two of the latter species, P. peuce and P. strobus, are described below. P. sylvestris, the Scots pine, is the only pine native to Britain and is one of only three native conifers. Other species have been extensively planted, including lodgepole and Corsican pines. In the most recent inventory of woodland (Forestry Commission, 2018a) the total area of Scots, lodgepole and Corsican pines in Britain was estimated at 363,600 ha, or 13.7% of the total forest area, making them the second most common genus of trees in the country after spruces. The importance of pines has diminished considerably since the 1990s owing to the introduction and rapid spread of red band needle blight, caused by the fungus Dothistroma septosporum (Brown and Webber, 2008). Up to this time the disease was a major cause for concern mainly on Monterey pine in the southern hemisphere, but it has spread to other species of pines in both Europe and North America and has seriously reduced growth rates and consequently interest in establishing the species. All species of pine commonly grown in Britain are susceptible to the disease. In the quest to find species that might be useful in Britain when the effects of climate change become more apparent than they are in the early part of the 21st century, a number of pines have been suggested, several of which are dealt with in this section. One that has not is P. pinaster (­maritime 202

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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pine), which is unlikely to be accepted in Britain unless improved varieties of the species can be found, with greater straightness and more resistance to pests and diseases (Savill, 2015a). Because of its susceptibility to red band needle blight and potential attack from the pine wood nematode (Bursaphelenchus xylophilus), its sensitivity to frost damage and the scarcity of suitable sites (coastal, with acidic sandy soils), it is unlikely ever to become a major species in British forestry partly because other pines (e.g. Corsican and Scots) are better and faster growing. As Brown and Nisbet said in 1894: The maritime pine possesses little sylvicultural interest for Britain except as a means of forming woods for shelter along coast districts, so as to enable other species of crops to thrive better under their lee.

The situation has not really changed in the ensuing 120 years (Savill, 2015a). The oldest living tree in the world, P. longaeva (the Great Basin bristlecone pine), was believed to be 4844 years old in 2012. Many of the most productive and widely planted timber trees are pines. They also produce pulp and resinous products including rosin and turpentine, and some species have edible seeds (Mabberley, 2008).

PINUS CONTORTA Douglas ex Loudon

Lodgepole pine

Origin and introduction The range of lodgepole pine extends from south-eastern Alaska and interior Yukon in the north, to Baja California (Mexico) in the south and extends eastwards as far as the Black Hills of South Dakota (i.e. 33° of latitude and 33° of longitude). The species is most abundant in the northern Rocky Mountains and Pacific coastal region. It is found from sea level to about 3600 m and covers a huge variety of climatic and soil conditions, which has led to some subdivision of the species. Lodgepole pine was first successfully introduced to Britain in 1853. Climatic requirements It grows well under diverse but generally upland conditions. Appropriate provenances are resistant to winter cold, late spring frosts, salt-laden winds and air pollution, and will withstand great, even extreme, exposure in Britain. Lodgepole pine often used to be planted in the uplands as a last resort, where no other tree would thrive because of severe climatic and site conditions.

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Site requirements Some provenances will grow fast on the poorest soils such as deep acid peats, hard boulder tills and upland heaths. Performance can be outstanding compared with other species on such sites, provided there is adequate phosphorus in the soil (which tends to be in short supply). Lodgepole pine is much more tolerant of competition from heather than Sitka spruce; if this is likely to be a problem, lodgepole pine is often a natural choice, at least for a first rotation. On fertile lowland sites it becomes a coarse and unattractive tree and several other species are much more productive and much better formed.

Other silvicultural characteristics Lodgepole pine used to be commonly planted at high elevations on the poorest western and northern soils, mainly peats, because it is relatively undemanding and will grow well with only low inputs of fertilizers. Var. latifolia will grow up to over 29 m tall according to Johnson (2003). It has the reputation for drying out peat in some areas (Pyatt and Craven, 1979). It was also widely planted as a nurse, to provide shelter and to suppress weeds, usually in mixture with Sitka spruce. However, from the late 1960s this practice was largely abandoned as establishment techniques for spruce improved. Difficulties were also quite frequent, in that the pine grew so much faster than the spruce that the latter was eventually suppressed and killed. Some of these mixtures have led to the discovery of a previously unknown nursing effect of the pine on Sitka spruce on some peaty sites deficient in available nitrogen. This was first described by O’Carroll (1978) and has been much more widely observed since, especially with Alaskan origins of lodgepole pine (Lines, 1987). Spruces in such mixtures often grow at rates that are many YCs higher than if they are grown pure. The mechanisms involved are not yet understood, though they clearly involve improving the nitrogen nutrition of the spruces and may be connected with mycorrhizae. Lodgepole pine is susceptible to snowbreak and windthrow on wet soils, and then (unlike spruces) the timber decays within about 1 year, so it must be salvaged quickly. This can be difficult if windthrow is extensive. The most vigorous provenances are coarse and suffer from basal sweep. These problems lead some foresters to prefer to leave areas unplanted if the alternative was pure lodgepole pine (Davies, 1980). Transplants of inland provenances are particularly susceptible to being killed if they are temporarily waterlogged (Aldhous and Mason, 1994).

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Pests and diseases In spite of its virtues, lodgepole pine also has many serious problems, some caused by stress brought on by the extreme environments in which it tends to be grown. It is vulnerable to damage by many organisms: the pine beauty moth, Panolis flammea (Stoakley, 1979); pine sawfly, Neodiprion sertifer; pine looper moth, Bupalus piniarius (Bevan and Brown, 1978); pine shoot beetle, Tomicus piniperda (Bevan, 1962); deer; red band needle blight, D. septosporum; and, on infected sites, by Heterobasidion annosum. In its native home it is the main host of the mountain pine beetle (Dendroctonus ponderosae) which causes huge amounts of damage during summer. The beetles spread a blue-stain fungus which spreads though the wood of much of the tree. Natural regeneration Natural regeneration can be prolific, especially on burnt sites, and will mean that mixtures with other species are likely to develop in places. Flowering, seed production and nursery conditions Pinus contorta ssp. contorta (the ‘south coastal’ provenance) flowers in May and June, and seeds ripen 18 months later, between December and March. The cones are variably persistent and variably serotinous (Farjon, 2017). Seed is dispersed in the spring. The earliest age at which the tree bears cones is 10–20 years, but the best seed crops are usually at intervals of 2 or 3 years after the age of 30–40. Seed is usually collected in October and November. There are about 298,000 seeds kg−1 (range 245,000−364,000), of which 270,000 are normally viable, giving 135,000 2+0 transplants. If the seed has been stratified or pre-chilled it can be sown between late February and midMarch; if it has not, it should be sown in late March (Aldhous, 1972). Provenance Three main interfertile races are usually recognized as subspecies of lodgepole pine. They differ markedly in ecology and morphology, and can lead to enormous differences in growth rates and yields of timber. Subspecies contorta grows along the Pacific coast of north-western America from British Columbia to California in bogs, on sand dunes and on the margins of pools and lakes. It is short, shrubby and of poor form but is the fastest-growing subspecies in Britain. Before about 1980 it was the most widely planted provenance in Ireland, when there was a widely held

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­ elief that the quantity of wood grown was all that was important, not b the quality. Subspecies latifolia is found in the intermountain systems from central Yukon to eastern Oregon and southern Colorado, and subspecies murrayana grows mainly in the Cascade and Sierra Nevada mountains of Oregon and California (Critchfield, 1957). The latter two subspecies are well-formed, slender, tall, straight trees, but grow much more slowly in Britain than ssp. contorta. A constant dilemma has been to decide how far to compromise on form while retaining vigour. In Britain, Skeena River and other interior provenances are recommended, except for use in nursing mixtures, when slower-growing Alaskan origins are better (Forest Research, 2018b). The north coastal provenances possibly have an advantage in that they are less susceptible to damage by the pine beauty moth (Leather and Barbour, 1987). The correct choice of provenance is complex, but a detailed guide is provided by Lines (1985a). In the future interprovenance hybrids may offer possibilities for combining vigour and good form. The current advice from Forest Research (2018b) is that south coastal origins should not be used, while Alaskan provenances are best suited for use in nursing mixtures. If pure stands are being planted, then Skeena or other interior provenances should be used; expert advice will be helpful. Area and yield For many years in the latter half of the 20th century lodgepole pine came second to Sitka spruce in the extent of new planting, which was a reflection of the poor quality of the land that was then being acquired for afforestation. By about 2000 the total area was estimated at 134,000 ha or 6% of the forest land in Britain. In 2018 it was 99,800 ha or 3.8% of all forest in Britain. The extent of planting today is negligible. Mean yield classes (YC) range from 6 to 10 m3 ha−1 year−1, with a maximum of about 14 for coastal Oregon and Washington provenances, though YCs as high as 18 are commonly found in Ireland. Timber The timber of lodgepole pine is similar to that of Scots pine, except that it contains a greater proportion of heartwood and it is more resistant to penetration by preservatives. It has a straight grain, low distortion on seasoning, high stability and a comparatively uniform texture. A smooth finish can be obtained, even on wide-ringed timber, and it should make a suitable joinery wood. Its value may, of course, be reduced considerably by the poor form of many trees of ssp. contorta, and by knots, though a very clever bit of marketing in the 1960s suddenly made ‘knotty pine’ a

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fashionable wood for kitchens and some types of furniture. The average density of the wood at 15% moisture content is about 470 kg m−3. Place of lodgepole pine in British forestry Lodgepole pine will grow on the poorest and most exposed sites in Britain where few other trees will survive. Considerable areas of it were established in the second half of the 20th century. However, this is more of historical than current interest because such extreme sites are seldom planted today and several have been cleared of trees and left to revert to blanket bog. As explained in more detail in the section on Scots pine, the importance of pines in general has diminished greatly since the 1990s owing to the introduction and rapid spread of red band needle blight, caused by the fungus D. septosporum. While this disease seldom kills the trees, it can reduce the rate of growth significantly.

PINUS MURICATA D. Do

Bishop pine, Obispo pine

Origin and introduction Bishop pine is found in more than 15 scattered colonies in the ‘fog belt’ along the coast of California, Baja California in Mexico and on the nearby Cedros and other islands from 0 m to 300 m elevation. It was introduced to Britain in 1846. It is one of the closed cone (serotinous) pines, and cones are retained on the trees. There are two needles per fascicle. Climatic requirements These are uncertain, but the tree is close to or slightly beyond the limit at which it can safely be grown in Britain. Its requirements appear similar to those of Monterey pine, though bishop pine is slightly less demanding. It is largely confined to low elevations in south-western England but may be reasonably safe south of a line from the Dee to the Wash, according to Everard and Fourt (1974). Quite large areas have been satisfactorily established by the Forestry Commission at Wareham Forest in Dorset. Site requirements These are not clearly known. Bishop pine is believed to be more tolerant of poor soils than Monterey pine, but calcareous soils should be avoided.

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Other silvicultural characteristics and yield The tree is often multi-stemmed and coarse. It grows up to 26 m tall. It needs singling and pruning early, often by 3 or 4 years, and later needs high pruning if a reasonably straight single stem is to be produced. It often suffers badly from the pine budmoth Rhyacionia buoliana, and also from red band needle blight, caused by the fungus D. septosporum. The northern ‘blue’ variety of the species has shown promise as a potential timber tree in Britain, with growth rates of up to 2 m year−1 when young, even on poor sandy soils (Farjon, 2017). YC of 20 m3 ha−1 year−1 can readily be achieved. In a review by Lally and Thompson (2000) on the performance of the species in Ireland, they state that a provenance from Stewarts Point (Sonoma coastal) performed significantly better than all other material, although stem form was inferior to other provenances. According to Fraser et al. (2016) P. muricata is one of the most susceptible pines to Dothistroma needle blight.

Provenance The species is quite variable over its natural range: there is a southern ‘green’ form and a northern ‘blue’ or glaucous form. The latter is better shaped and appears to be more frost hardy (Tuley, 1979). Other varieties are sometimes recognized.

Place of bishop pine in British forestry The main source of interest in this otherwise unattractive species is its potential for fast growth. It is unlikely ever to become an important tree in Britain.

PINUS NIGRA J.F. Arnold

Black pine

Origin and introduction P. nigra has a discontinuous distribution in central Europe and the northern Mediterranean region from southern Spain to Turkey and the Crimea (35°–49° N and 3° W–34° E). It is predominantly a mountain tree but also occurs at sea level along the shores of the Adriatic. According to Farjon (2017), this species has received an excessive number of described names, many of them invalid under the International Code of Botanical

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Nomenclature (ICBN).1 Farjon (2017) recognized only two subspecies, each with several varieties: • ssp. salzmannii (Corsican pine) • ssp. nigra (Austrian pine) Only ssp. salzmannii is grown as a timber tree in Britain. It originates from Corsica, southern Italy and Sicily, and was introduced in 1759; ssp. nigra, from Austria, central Italy and the Balkans, will also grow in Britain.

PINUS NIGRA J.F. Arnold ssp. NIGRA 

Austrian pine

Austrian pine is much more at home on exposed chalk and limestone than is Corsican pine. It is one of few conifers with this attribute, and it grows well even where the soil is dry and shallow. It can be useful as a nurse for beech on such sites. It will not thrive on wet, heavy soils, but tolerates sea winds and industrial pollution well. It has a coarse and poor form, and largely because of this it is virtually useless as a timber tree. Little information has been published about provenances suitable for use in Britain, but most seed is currently being acquired from Slovenia. There is a tree in Co. Wicklow that is 43 m tall (Johnson, 2003). Place of Austrian pine in British forestry Austrian pine is only worth planting as a short-term nurse where limestone, salt-laden winds or pollution rule out other conifers, and then only as a shelterbelt or for amenity.

PINUS NIGRA J.F. Arnold ssp. SALZMANNII (Dunal) Franco

Corsican pine

Climatic requirements Until the advent of red band needle blight, Corsican pine grew well in the south and midlands of England, the coastal fringe of southern   Farjon (2017) provided a link to the ICBN Code he referred to (http://ibot.sav.sk/ icbn/main.htm). This is the ‘Vienna Code 2006’, which was superseded by the ‘Melbourne Code’, adopted in 2011 (when the ICBN was renamed the International Code of Nomenclature for algae, fungi, and plants) and published in 2012, and again in 2018 by the ‘Shenzhen Code’ (https://www.iapt-taxon.org/nomen/main.php).

1

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(B)

(A)

(C)

(D)

Fig. 23.  (A) Austrian pine, Pinus nigra ssp. nigra; (B) Bishop pine, Pinus muricata; (C) Lodgepole pine, Pinus contorta; (D) Corsican pine, Pinus nigra ssp. salzmannii.

Wales and along the eastern coast of Scotland. These are all districts of low summer rainfall and high temperatures and duration of sunshine. In  wetter, cloudier districts of the north and west it is usually fatally attacked by a dieback fungus, Gremmenielta abietina (Read, 1967). For this

PINUS NIGRA J.F. Arnold ssp. SALZMANNII (Dunal) Franco

(A)

(B)

(C)

(D)

Fig. 24.  (A) Scots pine, Pinus sylvestris; (B) Macedonian pine, Pinus peuce; (C) Weymouth pine, Pinus strobus; (D) Monterey pine, Pinus radiata.

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reason, uplands are normally avoided, but if planted it should be confined to warmer south and west-facing slopes, brashed early and thinned heavily in an attempt to prevent infection by improving air circulation. As a guide, Lines (1987) suggested elevations of no more than 300 m in south-western England as being safe. Wetter upland sites, over 150 m in northern England and lower further north, carry some risk of dieback. In milder parts of the lowlands it grows well on sites that also suit Scots pine. It does best in areas of low rainfall and tolerates smoke better than most conifers. It will also grow better than Scots pine on coastal sand dunes subjected to salty winds. Corsican pine can be difficult to establish in places where frequent and severe late spring frosts occur. The risks of damage can be minimized by planting on ploughed ground or otherwise maintaining bare soil round the plants, as a means of preventing temperatures from falling too low. Site requirements Corsican pine will grow on most soils except moist or compacted ones. It grows well on a wide range of mineral soils but with least risk on sands and heavy clays in the midlands, south and east of England. Though more successful than Scots pine on chalky soils, it does not thrive on them. It tolerates exposure and pollution and will grow well on sand dunes provided it is not directly exposed to salt spray. Other silvicultural characteristics Corsican pine will grow to larger sizes than the native Scots pine: over 40 m tall. Conventional bare-rooted plants are notoriously difficult to establish. They are sensitive to bad handling and especially to the roots drying out. High and even complete losses with them were common before the 1980s, and this encouraged the use of container-grown (Japanese paper pot) plants. To ensure reasonable survival with undercut transplants or plants in pots, planting should be carried out as late as possible in the spring. Very little growth occurs 1–2 years after planting, so careful weed control is necessary to prevent smothering by herbaceous vegetation. Like most pines, the tree is strongly light demanding. Though the form is generally good, branching can be heavy and the species does not self-prune well. Timber quality can therefore be considerably improved by pruning, though this is seldom done. Damage from grass fires is rare in the pole stage or later owing to the thick bark.

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Pests and diseases One important attribute is that Corsican pine is seldom attacked by rabbits and hares, unlike Scots pine which was often planted in the same areas. In common with most pines, the species is more susceptible to attack by fomes heart and butt rot, Heterobasidion annosum, than many conifers. At Thetford Forest stumps of clear-felled crops used to be removed from the ground in the worst-infected areas in an attempt to reduce the levels of infection to the succeeding crop. Corsican pine is rarely attacked by the pine shoot moth, Rhyacionia buoliana, or the fungal resin-top disease, Peridermium pini, both of which can cause severe damage to Scots pine in East Anglia. It is sometimes susceptible to attack by the aphid Schizolachnus pineti and the pine looper moth Bupalis piniarius. Red band needle blight, D. septosporum, causes major losses in increment. Corsican pine is very susceptible to the disease, to the extent that all planting of it was suspended on Forestry Commission land in 2008 (Brown and Webber, 2008). The disease and mortality are particularly severe in dense, unthinned, pole-stage stands where humidity levels are high. Heavy thinning and pruning reduce the extent of infection, because they improve air circulation and reduce the humidity, which promotes infection. Natural regeneration Natural regeneration is comparatively rare because the species tends not to produce much seed until it is over 60 years old, which is beyond the normal rotation age in Britain. Flowering, seed production and nursery conditions The tree flowers in late May and early June; seeds ripen between December and March 18 months later, and are dispersed in the spring. They are normally collected in January. The earliest age at which the tree bears reasonable crops of seed is 20–30 years, but the best seed crops are usually at 3–5-yearly intervals between the ages of 60 and 90 years, which tends to be after the normal commercial rotation age (of 45–60 years). There are about 57,000 seeds kg−1 (range 31,000−86,000), of which 55,000 are normally viable. Most seed is produced in the south-east and east of England. If required for the nursery it should be sown between late March and early April. No pre-treatment is required (Aldhous, 1972).

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Provenance Seed from Corsica grows best in Britain (Lines, 1987), and trees from this source have excellent form. Many British plantations of Corsican origin exist from which seed could be collected, particularly in East Anglia (Kennedy, 1974). Forest Research (2018b) currently recommends these sources. Area and yield In the late 1990s stands of Corsican pine occupied an area of 45,350 ha or 2% of forest land (Forestry Commission, 2018b) and almost exactly the same in 2018 (Forestry Commission, 2018b). Mean YCs range from 9 to 13 m3 ha−1 year−1 (Nicholls, 1981), with a maximum of about 20. The potential mean should be 18 or more on a wide range of sites (Fourt et al., 1971). Rotations of maximum mean annual increment are 45–60 years. In suitable areas, such as the Brecklands of East Anglia, Corsican pine is always much more productive than Scots pine, which it was replacing until the introduction of red band needle blight. It is now no longer planted. Timber and uses Corsican pine makes heartwood much more slowly than Scots pine, but the timber is otherwise similar and can be used for the same purposes. The wood is stable, with intermediate strength: stronger than spruces but weaker than larches. It is suitable for use in building, roofing, flooring and interior framing, provided the correct grade is selected. Large knot whorls are potential zones of weakness. The timber is not durable but can easily be treated with preservatives. Treated timber is used for making sleepers, poles and fences, and these should last for 50 years or more. Freshly felled logs are very susceptible to attack by blue-stain fungi and should therefore be extracted, converted and dried with a minimum of delay, though the stain does not affect the strength properties of the wood. The average density of the wood at 15% moisture content is about 510 kg m−3. Place of Corsican pine in British forestry Corsican pine was, until the advent of red band needle blight, regarded as the most profitable conifer for planting in much of lowland Britain, particularly on light soils in the east of the country. It was predicted to do well under forecast climate change scenarios. The disease has caused interest in the species to diminish greatly.

PINUS PEUCE Griseb.

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Macedonian pine

Origin and introduction The species is a five-needled pine that is closely allied to the Himalayan P. wallichiana. It is native to the Balkan peninsula – Montenegro, Macedonia, western Bulgaria, Albania and northern Greece – where it occupies a total area of no more than 30,000 ha, ranging from 1100 to 2300 m in elevation. It was introduced to Britain in 1864.

Climatic and site requirements Very little experience exists with the tree in Britain, but what there is (­together with a full account of the tree’s performance in its natural habitat) has been given by Lines (1985a). It appears likely to grow well on a wide range of soils, including peats, in such inhospitable places as the central Highlands of Scotland. It withstands exposure and atmospheric pollution well. Farjon (2017) stated that in its natural range it is usually found on north-facing slopes on siliceous soils and rarely on carbonate soils. Other silvicultural characteristics Macedonian pine is potentially a big tree, growing up to 40 m tall. Difficulties with it arise in the nursery and establishment phases, which is probably why it has received so little attention up to now. Early growth is slow. Until the fifth or sixth year the trees go through a ‘grass stage’, having a dense, bushy form before making strong vertical growth. The bark remains thin for up to 30 years and so is potentially susceptible to being stripped by deer, and the young trees could suffer in fires. The American P. palustris (longleaf pine) also goes through a ‘grass stage’, which is believed to be under strong genetic control. Although the length of time individual seedlings remain in this stage is influenced by the environment, it can last for as long as 25 years and often for 15 years. Reasonable stands will reach breast height in about 8 years. Generally seedlings of P. palustris remain in the grass stage until they reach 2.5 cm at the root collar, and they invariably begin height growth upon reaching that size. The control of competition is a major factor in stimulating fast diameter growth: if it is good, height growth can begin at the end of the second year (Walker and Wiant, 1973). It is possible that a similar mechanism may operate in P. peuce.

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Diseases and pests The five-needled Macedonian pine is notable for its resistance to diseases that affect so many other pines. Unlike Weymouth pine, the Macedonian pine is resistant to attacks from the blister rust, Cronartium ribicola, and to several other pests and diseases of pines. These include red band needle blight, D. septosporum, which is affecting all other pines grown in Britain, and the pine wood nematode, Bursaphelenchus xylophilus, another potential threat to many pines. In fact, it seems likely to be a remarkably hardy and healthy tree in Britain. Seed production, nursery conditions and provenance Seeds are fully ripe in September but are often completely stripped in August, at least in small plots, by squirrels. Lines (1985b) stated that one of the major disadvantages of Macedonian pine is the tendency to poor or delayed germination, which is partly due to incomplete embryo development. Usually some seeds germinate in the first spring but many do not germinate until the second. Limited provenance testing suggests little variation in performance so suitable seed sources are either from the native range or from good British stands (Forest Research 2018b). Yield There is increasing evidence that the species could be a high-volume producer in Britain compared with other pines. A feature is that, in comparison with other species, basal area growth can be up to 50% greater for a given height. This makes it of interest where the risk of windthrow is high. Some further details are given by Savill and Mason (2015). Timber and uses Preliminary studies in Britain indicate that an important attribute of the wood is its stability compared with other common coniferous timbers, though its strength is poor. Its density at around 12% moisture content is about 350 kg m−3, which is considerably lower than Scots pine, at about 510 kg m−3. It is considered to be a valuable ornamental tree and is much planted in Scandinavian parks and gardens.

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Place of Macedonian pine in British forestry Forestry in the uplands of Britain is often regarded as being too ­d ependent upon very few species. There has been concern for some time to find possible alternatives, particularly to Sitka spruce. Macedonian pine is a possible choice especially because of its resistance to red band needle blight and high-volume production at relatively low heights. Though its potential has been recognized for some time, it never gets beyond this stage. Climate change could provide the impetus for more work.

PINUS RADIATA D. Don

Monterey pine, Radiata pine

Origin and introduction Monterey pine is native to three small areas in the ‘fog belt’ on the coast of California, round Monterey and Cambria, between 35° and 37° N, and also to three small islands off the coast: Santa Cruz, Santa Rosa and Guadeloupe. It has no economic importance in its native range, but as an exotic it is one of the most widely planted of all trees and is the basis of huge timber industries in New Zealand, Australia, Chile, south-western Europe and South Africa. It was introduced to Britain in 1833.

Climatic requirements Monterey pine is probably beyond the limit of where it can safely be grown, even in the mildest parts of southern England. Its requirements are not well understood, but it may be hardier to low winter temperatures than is generally believed: tolerance to frosts of −6°C in summer and −14°C in winter have been found in New Zealand (Menzies and Chavasse, 1982). It probably needs a long, warm growing season. Green (1957) considered that it required an accumulated temperature above about 1400 day°C (i.e. the sum of the mean monthly temperatures above 6°C × days in the month). Much of southern England has these temperatures, but r­ecent experience suggests that even higher levels may be needed, possibly over 1700 day°C, such as are found in parts of Dorset, Somerset, Devon, Hampshire and Cornwall, at low elevations. Rainfall is not limiting. In unsuitable climates trees of any age may suddenly go yellow and needle retention is reduced; they may die quite abruptly.

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Site requirements Monterey pine has grown well on deep, dry and infertile sandy soils in the south of England, and grows on loams and clay loams. Wet and shallow calcareous soils over chalk should be avoided. Other silvicultural characteristics, timber and yield Trees up to about 35 m tall occur in Britain. Growth is rapid where the tree survives; it is the fastest-growing pine, and this is the cause of the interest in the species. YCs of 18–22 m3 ha−1 year−1 are common in south-western England, with maximum mean annual increment occurring before age 30. The average density of the wood at 15% moisture content is about 480 kg m−3. In common with bishop pine, the cones of Monterey pine are serotinous, remaining closed and on the tree until opened by the heat of a forest fire; the abundant seeds are then discharged to regenerate the burnt forest. The tree is coarsely branched, so pruning is necessary. Diseases Monterey pine is attacked very severely by red band needle blight, D. septosporum. Up to the 1990s the disease was a major cause for concern, mainly in the southern hemisphere. Pitch pine canker, Fusarium circinatum, is also a serious potential disease that has recently been introduced into western Europe but is not yet present in Britain. It is also attacked by the pine shoot moth, Rhyacionia buoliana. Provenance Forest Research (2018b) states that very limited provenance testing has been carried out in Britain; seed could be sourced from cold-hardy strains identified in breeding programmes in other countries (New Zealand) or possibly from British stands of good form. Place of Monterey pine in British forestry This species is currently of negligible importance in Britain, but interest in it has arisen in the past because of its rapid rate of growth on the right site in the few places where it survives. These are mostly coastal regions in the south and south-west. The species should benefit from climate warming in Britain if the serious disease problems can ever be overcome and a wider range of sites in southern and lowland Britain should become more suitable.

PINUS SYLVESTRIS L.

PINUS STROBUS L. 

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Weymouth pine (UK), Eastern white pine (USA)

The species is native to and widespread in north-eastern North America and is found occasionally in Mexico and Guatemala. It grows at low elevations in the northern part of its range and in the mountains in the south, in regions of cool, humid climates. It was introduced to Britain in about 1705 and named after George Weymouth, who first discovered the tree growing in Maine. The name was subsequently appropriated by the unrelated 1st Viscount Weymouth who, at the time, was First Lord of Trade and Foreign Plantations. He called it the Lord Weymouth Pine and planted it extensively on his family’s estate at Longleat in Wiltshire. Its site requirements are said to be quite exacting (Elwes and Henry, 1906–1913): good loamy hardwood sites are needed, or deep sandy or sandy loam soils. The timber tends to be less dense than that of many pines, at about 420 kg m−3. The timber was once used extensively by the Royal Navy for masts, from trees felled in its native range. Weymouth pine will grow to quite large sizes in Britain. Johnson (2003) recorded two trees of over 40 m tall (one in Gloucestershire, the other in Lancashire).

Diseases Weymouth pine, like Macedonian pine, is a five-needled pine. It is a species that appeared to have potential in Britain, because of the impressive growth of occasional isolated trees, but it is only rarely planted because fatal infections of the blister rust, Cronartium ribicola, soon attack even small stands. The disease is highly virulent and trees are susceptible from the seedling stage onwards. In its natural range blister rust causes high losses. If planted at all it should only be on a small scale and, even then, preferably in mixtures. The disease is common in five-needled pines and was first noticed in 1892. The main alternate host of the rust in Britain is the blackcurrant, Ribes nigrum, and the risk of infection has precluded the use of the pine.

PINUS SYLVESTRIS L.

Scots pine

Origin Scots pine is one of three native British conifers and the only one of any commercial significance. It is the world’s most widespread conifer after Juniperus communis (which is also a native species), extending from southern Spain to Norway, across the whole of northern Europe and

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Siberia, almost to the Pacific Ocean. Farjon (1998) stated that over 140 subspecies, varieties and forms have been described in an ‘orgy of botanical nationalism’, but they are barely distinguishable from each other. Within Britain the Caledonian pine, P. sylvestris var. scotica, is regarded by some as the name of the native tree of the Highlands of Scotland, though it is not currently accepted by taxonomists at the Royal Botanic Gardens, Kew, and Missouri Botanical Garden (Plant List, 2018). The Caledonian pine is found between latitudes 57°57′ N and 56°22′ N and longitudes of 5°03′ W and 5°38′ W (Carlisle and Brown, 1968). According to the Forestry Commission (undated), native Caledonian pinewoods are found at 84 sites in the north and west of Scotland and cover around 180 km2.

Climatic requirements Scots pine is an adaptable species but does best in the drier eastern districts. It does not grow well where exposure is excessive, though Caledonian pine occurs at 625 m in the Cairngorm Mountains. It is not normally regarded as a tree for high elevations unless there is adequate topographic shelter. It is frost hardy, drought tolerant and windfirm but will not withstand atmospheric pollution or salty winds from the sea.

Site requirements Scots pine grows best on light, non-calcareous, heathland soils, especially sands, gravels and other well-drained sites, at lower elevations. On more fertile sites it will grow vigorously, but stem form is often poor. It has a relatively short life on calcareous soils where it is sometimes used as a nurse. Deep wet peats on exposed, elevated sites should be avoided. Other silvicultural characteristics Scots pine will grow up to a maximum of about 35 m tall. The oldest tree recorded in Britain was 395 years (Carlisle and Brown, 1968). Lines (1987) described Scots pine as a typical light-demanding, pioneer species. It is frost hardy and grows rapidly when young. It will regenerate in open stands, but if the regeneration is to succeed it must be released from competition within 10 years otherwise it will not respond. Scots pine is useful as a nurse for broadleaved trees, especially for beech on frost-prone calcareous soils, because it does not grow too fast. The tree does not selfprune well and to obtain clear timber without loose, dead knots, pruning is necessary. Scots pine is tolerant of low levels of available nutrients. Its

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mycorrhizae allow it to compete successfully for nitrogen on heathlands dominated by heather, Calluna vulgaris, where spruces would fail. The cellulose of pine needle litter, like that of spruce, is relatively resistant to decomposition by fungi (Carlisle and Brown, 1968) and can accumulate over time on the forest floor. The water draining from such woodland is very acidic (pH 3.7) compared with that of larch litter (pH 4.2−5.3) and birch litter (pH 4.7−4.8). This process can contribute to the acidification of streams and rivers to the detriment of fish spawning (Nisbet, 2001). Acidification led the Forestry Commission to introduce Forests and Water Guidelines, originally in 1988 (Anon., 2011). Diseases It suffers from resin-top disease, Peridermium pini (Pawsey, 1964), particularly in north-eastern Scotland, and from red band needle blight, D. septosporum. Natural regeneration Where Scots pine is truly at home, such as in parts of the New Forest, natural regeneration can be both prolific and invasive if it is not browsed too much by deer and sheep. Flowering, seed production and nursery conditions The tree flowers in May and June; seeds ripen in September and October of the following year, and are dispersed between December and March. The best seed crops are usually produced at 3–5-year intervals after the age of 60. Seed is usually ready for collection in January. There are about 165,000 seeds kg−1 (range 74,000−245,000), of which 140,000 are normally viable. The treatment of seed in nurseries is the same as for Corsican pine. Provenance For the native pinewoods of northern Scotland, the appropriate local seed origin should be used. Elsewhere, seed from British seed orchards is available and should be preferred. Area and yield In about 2018 there were an estimated 218,200 ha (or 8.2% of the total British forest area) covered with Scots pine, making it the second most

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common species after Sitka spruce. However, this figure gives a false impression of current levels of planting, which are almost negligible. Its susceptibility to red band needle blight, rather slow growth and long rotations compare badly with more exacting species. It was, for example, being rapidly replaced by Corsican pine in eastern England. Mean YCs range from 8 to 11 m3 ha−1 year−1 in different parts of Britain (Nicholls, 1981), with a maximum of 14; rotations vary between 50 and 60 years. Timber The wood of Scots pine, known in the timber trade as deal or redwood, has been the standard utility timber of northern Europe for generations. Its average density at 15% moisture content is about 510 kg m−3, though it varies with provenance. It combines adequate strength with light weight, and is easy to nail and work. Clear, narrow-ringed wood can be of excellent quality for joinery and most is suitable for building: wood of this quality is most commonly imported from Scandinavian countries. It is easily treated with preservatives and is useful where a high standard of treatment is required, as in railway sleepers and fencing. An excellent veneer can be made from knot-free material. Place of Scots pine in British forestry Scots pine was a traditional species for planting in Britain because of its ability to survive and grow under difficult conditions and the fact that it is a native tree. However, the introduction and rapid spread of red band needle blight caused by the fungus D. septosporum in the 1990s has reduced interest in establishing the species. Only the native Scots, or Caledonian, pine has seen a resurgence in planting that coincides with the general interest in establishing local races of native species.

PLATANUS L.

Plane

The planes (Platanus spp.) are in a genus of about 6 or 7 species in the family Platanaceae which is confined to the northern hemisphere (Mabberley, 2008). The main current interest in planes as forest trees is that they offer some timber-producing hardwood alternatives to ash, which is threatened by the dieback disease caused by Hymenoscyphus fraxineus. The London plane (Platanus × hybrida, also known as P. × acerifolia and P. × hispanica) is a fertile hybrid between western plane (P. occidentalis), native to eastern North America, and oriental plane (P. orientalis), native to south-eastern Europe/Balkans (Feng et al., 2005; Grueva and Zhelev, 2010). It is believed to have arisen in the Oxford Botanic Garden around 1670 (Macdonald et al., 1957) and has been widely used, mainly as an urban street tree. More than half of London’s trees are planes (Drori, 2018). Silviculture There is little experience of plantation silviculture with London plane in Britain. Most trees have been established along urban streets and in parks, and for landscaping, where 30 m or more height is frequently achieved. Their habit of shedding bark allows them to cast off particulate pollutants, while their huge, stiff leaves make them excellent shade trees in urban settings. P. × hybrida is light demanding and must be well thinned in plantations to permit adequate crown development. It is frost-hardy but sensitive to exposure by cold, drying winds. Based on experience with the parent species P. occidentalis (Burns and Honkala, 1990), plantations in North America are fast growing, yielding 7–15 m3 ha−1 year−1 over conventional rotations, and potentially much higher in shorter-rotation coppice woodlots. EUFORGEN (2018b) stated that P. orientalis grows in a wide variety of soils and is tolerant to high levels of air pollution, compacted soils, root disturbances and strong winds. Very wet, and alkaline soils should be avoided, as should heavy clays. Brown and Nisbet (1894) said that it should not be planted ‘where the lime tree does not prosper’, as it is somewhat more sensitive to late spring frosts. Though they are able tolerate a degree of drought (Anon., 2008), they do best in moist soils and cannot grow in the shade. They are characteristically found along stream sides in areas with Mediterranean climates. The preferred natural habitat is in stream or river valleys and wetlands in warmer temperate regions, but they can tolerate dry conditions after establishment. All are large trees, and will generally reach 20–50 m high. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Urban plane trees are frequently pollarded, and as a result they ­acquire very distinctive shapes, with club-like ends to the branches. They are often sought after, especially in continental Europe. The American plane (P. occidentalis), known as sycamore, has been grown as a biomass crop in the USA (Ranney et al., 1980). Nursery production and provenance The Platanus species are monoecious and produce seed balls (2 cm in diameter) containing a mass of light seeds (~450,000/kg). The dispersing seed balls accumulate urban pollutants, which can cause allergenic effects, as can the hairs shed from the leaves in spring. Allergies are frequently and incorrectly ascribed to pollen (Mabberley, 2008). Seed production begins before the age of 10 and is optimum from 50 to 200 years for P. occidentalis (Burns and Honkala, 1990). Natural regeneration has occasionally been observed. Some plants are produced by nurseries for the horticultural trade in Britain. There have been no provenance trials for planes in Britain and no guidance is available on provenance selection. Pests and diseases Traditionally, the main disease of the planes was anthracnose (Apoignomonia veneta) (a fungal disease of leaves) which affects London plane less severely than other species. Anthracnose mainly causes the death and premature fall of leaves, and shoot dieback. American plane (P. occidentalis) is said to be much more susceptible to the disease than P. orientalis. London planes differ in susceptibility according to the clone that is used. Most trees recover eventually. More recently, canker stain (Ceratocystis fimbriata f. platani) and Massaria disease (Splanchnomena platani) have emerged as serious threats to planes growing in continental Europe and the latter is now known to occur in Britain. Timber Platanus timber is one of several species known as lacewood in the timber trade. When quarter sawn it has a distinctive and highly decorative ­appearance of dark reddish-brown flecks against a lighter background. The timber is relatively hard and strong, but brittle and not particularly durable. It is not suitable for physically demanding applications such as hardwood flooring or sports goods, for which ash is currently favoured. Hardwood pulp and woodfuel are potential markets for smaller-­dimensioned and inferior-grade material. It is occasionally used for furniture making.

PLATANUS L.

225

Place of plane in British forestry According to Forest Research (2018b), the species is likely to benefit from climate warming and it may prove a useful forest tree on warmer sites in eastern lowland Britain, but it is likely to remain, perhaps, the most ­important tree for planting in towns and cities for amenity and shade.

POPULUS L.

Poplar, Cottonwood

The Salicaceae, the family to which the poplars belong, comprises some 54 genera and 1269 species according to the Plant List (2018). Populus is a genus of about 35 species of trees native to most of the northern hemisphere. The majority are dioecious (i.e. with separate male and female trees). According to Jobling (1990), the genus can be divided into six sections (i.e. taxonomic ranks below genus, but above species), the geographical ranges of which are set out in Table 12. This table does not give a complete list of species, and some of those included (shown in brackets) are not discussed further in the text. The section Leucoides i­ncludes a few species of poplar with very large decorative foliage, several of which (­including P. lasiocarpa) can be grown in Britain, but none a­ ppears to be of any importance for timber production. The sections Turanga and Abasco are confined to central Asia, the southern Mediterranean region, North Africa and Mexico, and are therefore of no interest to British forestry. This means that, for practical purposes, we are concerned only with the poplars of the remaining three sections (Populus, Tacamahaca and Aigeiros) and of the hybrids between them. The divisions between the sections have become blurred by the production of hybrids. Many poplar species interbreed freely, and the presence of numerous natural hybrids presents the taxonomist with difficulties in defining the limits of species. This situation is complicated by the occurrence of leaves with different shapes within species and evidence that some ‘species’ are themselves intersectional hybrids. The identification of most cultivated poplars is, therefore, extremely difficult and frequently impossible from morphological characteristics alone. Introduced poplars (including hybrid clones) have been widely used in Canada, in much of continental Europe and in Britain for the production of lightweight timbers for match-making and box manufacture (Morley and Balatinecz, 1993). In broad terms, one can distinguish three periods of interest in poplars in British forestry over the last century. The first predated the Second World War when various poplar species and varieties were tested as part of a concerted effort to find species that could be suitable in Britain (MacDonald et al., 1957). These poplar trials were often unsuccessful because of disease or poor adaptation to British sites and climate. The second occurred in the 1950s and 1960s when there was substantial private investment in poplar plantations in southern England using selected clones to grow veneer logs for splint production for matches. This specialist market disappeared in 1978 when the sole British match manufacturer decided to rely only upon imported logs (Tabbush and Beaton, 1998). The third period began in the late 1980s, coinciding with attempts to reduce overproduction of arable and other crops under 226

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

Table 12.  Geographical distribution of poplar sections (species in parentheses are not discussed further in the text). Adapted from Peace, 1952. Section

TACAMAHACA Balsam poplars AIGEIROS Black poplars LEUCOIDES Bigleaf poplars TURANGA Subtropical poplars

North America

–

Eastern Asia

Mexico

– –

alba × canescens tremula

(tremuloides) deltoides

(balsamifera ssp. balsamifera)

(trichocarpa) (× candicans) (angustifolia)

nigra and its varieties

(deltoides and vars.) (fremontii, etc.)

–

(heterophylla) –

– alba (tomentosa) tremula (tremula var. sieboldii) (adenopoda)

– –

(laurifolia) (yunnanensis) (suaveolens) (simonii, etc.) –

POPULUS L.

ABASCO Mexican poplars POPULUS White poplars Grey poplars Aspens

Europe and North Africa to western Asia

– –

lasiocarpa (wilsonii, etc.) –

–

(euphratica)

Empty cells: while there are species in the section, they are not of any relevance to British forestry. Hyphen, absence of species in the section.

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the EU Common Agricultural Policy and to provide farmers with an alternative source of income from fast-growing trees. In lowland Britain, there was interest in using newer hybrid poplar clones to provide this alternative income (Tabbush and Beaton, 1998). Unfortunately, these plantings were also seriously affected by disease, resulting again in a loss of interest in poplar growing. There was also a period in the UK, in the 1950s and 1960s when poplars were strongly advocated as ‘biological pumps’ for drying out wet, waterlogged land. Needless to say it was a failure because poplars will not thrive on waterlogged land. Poplars are a characteristic feature of many lowland landscapes in East Anglia and the Midlands and have been used to mark field boundaries and watercourses, forming shelterbelts or windbreaks. A recent National Forest Inventory (NFI) report (Anon., 2014) suggested that there are 13,635 ha of poplar woodland in Britain, of which over 80% is located in England, mostly in the southern counties. In recent years, there has also been increasing interest in the potential role of aspen and aspen hybrids in Scottish forests, either as part of a ­diversification strategy for conifer forests or to establish broadleaved crops on marginal agricultural land (e.g. Worrell, 1995a,b; Harrison, 2009).

POPULUS NIGRA L.

Black poplar

Black poplar is Britain’s rarest large native tree. It is potentially long-lived and tall, up to 35 m, and has survived either as single trees or small groups along river and stream sides and on floodplains. This type of land has mostly been drained and cleared for agriculture over the centuries (Peterken and Hughes, 1995). It has only survived at all up to the present, mostly in east and central England, because it was considered an attractive and useful timber tree. It was propagated from cuttings taken almost exclusively from male trees because females were considered a nuisance due to the plumed seeds that have large quantities of fluff attached (Tabbush, 1996; Winfield et al. 1998). This practice has led to a narrowing of the genetic base because cuttings were probably taken from relatively few specimens. The British black poplar belongs to the subspecies betulifolia. It is on the verge of extinction in large parts of western Europe (Cagelli and Lefèvre, 1995). The maximum number of trees in Britain and Ireland is ­believed to be about 7000 (Cottrell, 2004), of which as few as 600 may be female. It hybridizes with other species of poplars. Molecular analyses have demonstrated that the British population has low diversity, compared to that in other European countries, and that there is a great deal of clonal duplication. Modern molecular methods provide the opportunity for conservation efforts to be concentrated at the level of the clone rather than of the individual tree. Work by a number of conservation organizations is attempting to ensure that the species is saved in Britain.

POPULUS TREMULA L.

229

Silviculture is similar to that for the hybrid poplars. Historically this species was widely used as beams in the construction of farm buildings. Timber The timber of black poplar is often described incorrectly. Unlike most other poplars, it has a very distinct heart/sapwood interface with white/ creamy sapwood, and the heartwood is a light olive-green colour. It is regarded as a decorative timber and was once widely used in cottage joinery. It takes paint very well.

POPULUS TREMULA L.

Aspen

Origin According to Worrell (1995a), aspen is probably the world’s most widely distributed tree. Its latitudinal range extends from as far north as trees will grow in Norway (71° N) and southwards to Greece and Italy at 39° N, and its longitudinal range goes to eastern Russia and Japan. Aspen is native throughout Britain, and northern and central Europe, as a widespread but infrequent species. It is most common in the north and west in many different types of woodland, and is particularly common in the Scottish Highlands and Islands, where it occurs on a diverse range of sites, from sea cliffs to near the tree line. It is widespread but uncommon, occurring in two distinct habitat niches: • In the English lowlands, it occurs as an occasional component of mixed broadleaved woodland on better sites, often growing to considerable dimensions. • In the Scottish uplands, it is found as individuals and multi-clonal stands as a component of Scots pine, birch and oak woodlands. It can grow on some rather poor sites, including peats and skeletal rocky outcrops, and can persist over long periods of time by suckering. Flowering is thought to be occasional. Despite its widespread occurrence in Scotland, large stands of aspen are rare and the total size of the resource is thought to be around 500 ha (MacKenzie, 2010).

Climatic requirements The enormously wide natural range of aspen demonstrates that it is tolerant of a huge variety of climatic conditions. In Britain it will grow in

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exposed situations and is cold hardy and frost resistant. It will grow at higher elevations than any other poplar. Site requirements It thrives on a wide range of sites, from slightly dry to wet soils of poor to rich nutrient status (Forest Research, 2018b). It will tolerate wet conditions and droughts but is most common on sites where other poplars also thrive: river banks and in gulleys beside streams. Aspen will grow well on acid sandy soils and neutral to alkaline clays, but according to Worrell (1995b), it grows fast and to its biggest dimensions on freely drained mineral soils, particularly on flushed fine sand/clay; its possible use as a component of forest plantations should be ­restricted mainly to such sites. Peats should be avoided. Other silvicultural characteristics Aspen is a relatively poorly formed tree in Britain. It is short lived (50–100 years) and will grow to 20 m tall. Though regarded as a light demander, it is unlike many other poplars in that it is sufficiently shade tolerant to be able to form a stable part of a woodland canopy with a wide variety of other species, either as single trees or in small clonal groups derived from suckers, or on the fringes of conifer forests. In this respect, it does particularly well in mixture with species that cast only a relatively light shade, including Scots pine and birch. In woodlands dominated by dense-canopied shade-bearers it requires gaps created by disturbance of some kind to maintain itself. Aspen self-prunes well, even in relatively open stands. Aspen will not coppice after about 5 years of age, but it produces suckers freely and prolifically, sometimes up to 40 m from the parent tree, and forms quite extensive clonal colonies in a rather similar way to cherry. Clonal colonies can be eliminated by girdling the parent tree 4–5 years ­before felling it. This leads to suckers from these trees dying out, partly due to the lack of light. Contradictory statements can be found in the literature about its palatability to deer. In southern England it appears to be unattractive and consequently often escapes browsing damage completely, while Worrell (1995b) stated that in Scotland its apparent palatability to grazing animals has led to the widely held view that aspen is scarcer today than in the original forest cover. This difference possibly arises because aspen is more palatable than the main alternatives in much of Scotland (often spruce, pine or heather) but less so than alternative broadleaved species in many English woodlands.

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231

Aspen is said to be host to a large number of invertebrate species (possibly around 190) – including a unique and endangered population of saproxylic insects that depend on decaying wood – and also to a number of hole-nesting birds. Diseases In common with other poplars, aspen can be attacked by leaf rusts, Melampsora spp., which can result in serious defoliation, while older and larger trees can be killed by the stem rot Phellinus tremulae. Bacterial canker is also more damaging to aspen than to many other poplars. However, ­information about the disease susceptibility of aspen is relatively scanty. Natural regeneration Natural regeneration from seed is rare and has never been recorded in Britain. As mentioned above, regeneration is mostly achieved by suckering. Flowering, seed production and nursery conditions The production of fertile seed is rare in Britain though its frequency is ­unknown. In Scotland seed matures from the middle of May to the beginning of June and is wind dispersed. The viability of freshly collected seed is reported as high, but at room temperature it declines to zero within about 2 months. It should, therefore, be sown immediately after collection. On suitable sites germination occurs in less than 24 h. Aspen can be grown and will regenerate from seed but is normally propagated from root cuttings, root suckers or leafy softwood cuttings from the current year’s shoot (Worrell, 1995b). Cuttings from dormant woody stems, which are used for other poplar species, are not successful with aspen. Provenance A large amount of genetic variation has been reported in continental populations of aspen, but no work of this kind has been carried out in Britain. Limited trials of a small number of Scottish clones have shown good survival and reasonable growth from native material, but there are selected Scandinavian clones that can give faster growth rates (Forest Research, 2018b). In contrast, Worrell (1992) speculated that since aspen was an early arrival in Britain after the Ice Age, the survival and growth of continental

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provenances might be expected to be much poorer than of British provenances, particularly when planted in the north and west of Britain. Growth and yield In comparison with other poplars, the average growth rate of aspen is slow, and it does not attain large dimensions. This seems to be a particular characteristic of British aspens where, in comparison with those from continental Europe, trees are rather poor. On the continent aspen is a significant timber-producing species (Worrell, 1995b). Timber and uses The timber of aspen is similar to that of other commonly grown poplars, though possibly of finer texture than faster-grown kinds. At about 430 kg m−3 it is not dense, being similar to willow. It is used for the same purposes as other poplars. Its timber is currently acceptable in Britain as chipwood and could complement the production of poplar timber in the lowlands. Historically aspen has had two specialized uses: making arrow shafts and clogs. Aspen is an excellent amenity species, with bronze spring foliage and a strikingly yellow autumn colour. These features, and the rustling action and sound of the leaves in a breeze, make it appealing. Place of aspen in British forestry Aspen is regarded by Forest Research (2018b) as a species that is unlikely to be affected by climate warming and which could find an expanded role as a broadleaved component of conifer plantation forests in upland Britain. It also has a potential place in native woodland establishment and restoration, where suitable planting stock is available. It has been neglected until recently in much of Britain. Timber production from aspen is likely to remain very limited.

HYBRID POPLARS  

(POPULUS (Aiton) Smith; P. × CANADENSIS Moench; P. × EURAMERICANA Guinier, etc.)

The origin of some poplar hybrids goes back many decades or even centuries, for example cv. ‘serotina’ was originally identified in France in the early 18th century while cv. ‘robusta’ was first propagated near Metz in 1895.

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233

Poplars are the fastest-growing broadleaved trees native to the northern hemisphere. Selected clones said to be capable of producing up to 26 m3 ha−1 year−1 are propagated vegetatively from cuttings or sets. Those in current commercial use are crosses between known individuals of the European black poplar, P. nigra, and the three North American species: balsam poplar, P. balsamifera (from eastern North America), black cottonwood, P. trichocarpa (from western North America) and eastern cottonwood, P. deltoides, and also of hybrids of P. trichocarpa and P. deltoides (known as Populus × generosa). These clones have generally been produced in breeding programmes at research centres in Belgium, France, The Netherlands and Italy, and then introduced to Britain after testing in their countries of origin. The adaptation of such material to the British climate has often been problematic, as shown by the history of 11 new Belgian clones introduced to Britain in 1985 (Jobling, 1990; Potter et al., 1990; Tabbush and Beaton, 1998). These clones were the result of nearly 20 years of disease screening, testing and selection for resistance to the fungi Melampsora, Marssonina and Discosporium. Screening for disease resistance was done before selection for morphological and production characteristics. The new clones differed from those that had previously been planted in Britain in that they were either P. trichocarpa × P. deltoides (= P. × generosa) or P. deltoides × P. nigra crosses, with parents originating from similar latitudes to those of Britain. They seemed promising in initial trials and were quite widely planted. However, new pathotypes of damaging fungi, particularly Melampsora rust, appeared in 1994 and 1997 (Tubby, 2005), and by the summer of 2005 several of the Belgian clones had been killed in various parts of the country. As a result, there was a severe loss of confidence in the planting of poplars as timber trees in Britain. However, 18 hybrid poplar clones are still registered as approved material for use in British forestry (Forestry Commission, 2018; see also Table 13). Diseases and pests of poplars Poplar species and hybrids can vary considerably in their resistance to cankers and leaf rusts, which can cause both premature defoliation and death of susceptible trees. Poplars suffer from a number of potentially fatal diseases, the most serious of which is probably Melampsora rust: a leaf disease characterized by the presence of bright orange pustules on the underside of leaves. Melampsora spp. affect trees in different ways. Leaves on infected trees shrivel and fall prematurely. The rust can also reduce frost tolerance. A combination of these effects can lead to seriously reduced increment, shoot dieback and often tree death, depending on the timing and severity of the infection. Infection early in the growing season has a more adverse effect than infection in August or September. Sometimes infection occurs late enough for it to have little measurable effect upon growth. Melampsora

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HYBRID POPLARS

Table 13.  List of ‘tested’ hybrid poplar clones registered as approved material for forestry purposes (Forestry Commission, 2018b). Clone name

Parentage

Balsam spire (aka ‘TT32’) Beaupre Columbia River Fritzi Pauley Gaver Gelrica Ghoy Gibecq Hazendans Heidimij Hoogvorst I-78 Primo Raspalje Robusta Scott Pauley Serotina Trichobel

Populus tacamahaca × P. trichocarpa P. trichocarpa × P. deltoides P. trichocarpa P. trichocarpa P. deltoides × P. nigra P. deltoides × P. nigra P. deltoides × P. nigra P. deltoides × P. nigra P. trichocarpa × P. deltoides P. deltoides × P. nigra P. trichocarpa × P. deltoides P. deltoides × P. nigra P. deltoides × P. nigra P. trichocarpa × P. deltoides P. deltoides × P. nigra P. trichocarpa P. deltoides × P. nigra P. trichocarpa

apparently thrives in areas where night time temperatures are moderate, days are warm (15°–26°C) and dew periods are long. Infection by bacterial canker Xanthomonas populi via natural openings, such as leaf or bud scale scars, can lead to the death of branches and to the development of perennial cankers on the trunk. Dothiciza is another serious canker, caused by the fungus Discosporium populeum. The poplar leaf spot caused by Marssonina brunnea can cause problems for some clones. Work on both poplar and willow clones being grown either for timber or biomass production has shown that establishing plantations with mixtures of clones results in significantly more dry-matter production than single-clone plots. This is ascribed, at least in part, to reduced attacks by foliar rusts. Disease onset is delayed, build-up is reduced and disease levels at the end of the season are significantly lower than in monoclonal stands (Dawson and McCracken, 1995; McCracken and Dawson, 1996; Tubby, 2005). According to Tubby (2005), mixtures are likely to be most ­effective if the component varieties are disease tolerant rather than disease resistant (‘tolerance’ here implies that although the pathogen can infect the host plant only superficial damage is caused). Disease tolerance is generally controlled by many genes and is more stable than outright disease resistance, which tends to be controlled by a single gene. A consequence of single-gene resistance is that small changes in the pathogen can overcome this defence, with serious consequences. For this reason,

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235

recent French ­advice recommends that, when planting poplar for timber production, the area covered by a single clone should not exceed 3 ha (Paillassa, 2018). Most poplars are susceptible to bark stripping by grey squirrels. Defoliation can be caused by sawflies and leaf beetles, and serious damage to the timber may result from the larvae of an agromyzid fly, which bores long tunnels in the stem cambium. An aphid, Pemphigus bursarius, ­migrates in summer from black poplars (especially Lombardy poplars) to the roots of lettuces, sowthistles and related species; a second species, Pemphigus phenax, migrates to carrots (Blackman, 1974). Another aphid, Dysaphis apiifolia, similarly attacks celery. Quite serious economic damage can be caused if poplars are planted for shelter in regions where such crops are grown. Climate and site requirements of cultivated poplars Hybrid poplars are near their climatic limits as forest trees for economic timber production in Britain. On suitable sites, they grow best in parts of southern Europe, such as the Po valley in Italy or western Turkey, where summers are hot and dry. In Britain, they should be confined to sheltered sites in lowland regions. About half of all British hybrid poplar grown for timber production is carried out in south-eastern England and East Anglia (Anon., 2014). As Peace (1952) wrote: ‘Most poplars are e­ xtremely ­responsive to climate, particularly to the summer temperatures. Within reasonable limits, provided soil moisture is available, the hotter the summer the faster will poplars grow’. Most poplars are hardy to winter cold throughout Britain but can be damaged by late spring frosts. They are reasonably tolerant of exposure, which explains their popularity for planting as shelterbelts on fruit farms. They are favoured for this purpose because they do not transmit any diseases to the fruit trees, though they do to some field crops (see above). Clones of Populus trichocarpa are better suited to the cooler north and west, while black poplar hybrids are favoured by the warmer conditions in the south of England. Although some poplars have the capability to use relatively infertile sites, including better quality peats, cultivated varieties are very exacting in their site requirements. There are few suitable sites available for production as most are now used for highly profitable agriculture. They need fertile, base-rich, loamy soils, or rich alluvial or fen soils, which are well drained but also aerated and moist, even in conditions of summer drought, usually with a water table within 1–1.5 m of the surface. Banks of streams and the alluvial soils of river valleys are suitable. Most poplars will grow reasonably well on a much wider range of soils but, in Britain, not fast enough for profitable commercial production. Sites to avoid are

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HYBRID POPLARS

(A)

(B)

(C)

Fig. 25.  (A) Grey poplar, Populus × canescens; (B) Black poplar, Populus nigra; (C) Aspen, Populus tremula.

those with acid soils, shallow soils and soils where there is stagnant or slowly draining water. In nature, poplars grow in single-species stands or in mixtures with conifers (e.g. spruce and pine) or shade-intolerant/ medium-tolerant hardwoods (e.g. ash, birch, rowan, oaks). On better sites, poplar may form a pioneer stand before succession to mixed hardwood forest. Many poplar species reproduce readily by coppice regrowth or root suckers, allowing recovery from herbivory or short-rotation biomass harvesting (Stiell and Berry, 1986; Weih, 2004).

HYBRID POPLARS

237

Silviculture of cultivated poplars Hybrid poplars are strongly light demanding and must be established at wide spacings if being grown for timber. They were traditionally planted in pure stands at final crop spacings at about 5–7.5 m but Jobling (1990) recommended a wider spacing of 8 × 8 m or 155 stems ha−1. This produces trees of 45 cm diameter at breast height (dbh), with no thinning, on rotations as short as 22 years (Beaton, 1987). At such wide spacings regular pruning up to 5–6 m height is necessary if the wood is not to become ­excessively knotty. Jobling (1990) considered that most poplars intended for sawlog production should be pit-planted into carefully prepared ground and felt that failures in plantations, where not due to an inappropriate site choice, were likely to be caused by poor site preparation abetted by inadequate weed control. Poplars are most intolerant of competition from weeds and grow extremely slowly when young unless almost complete weed control is practised. These wide spacings make it possible to combine growing poplars with the cultivation of annual crops, in an agroforestry regime (Beaton, 1987). Alternatively, poplars can be interplanted with a range of broadleaved species to create a two-storied mixed forest, as practised in woodland creation schemes in parts of The Netherlands, where the poplars are used to provide quick woodland cover and an early financial return. Seed production and nursery practice of cultivated poplars Hybrid poplars are normally propagated from cuttings in open nursery beds. For the majority of clones these are taken from ‘ripened’ hardwood cuttings (i.e. from dormant woody stems). Details are described by Jobling (1990). Propagation from seed is rare. Yield, timber and uses of poplars In early woodland inventories, yield classes (YC) of up to 14 m3 ha−1 year−1 were normal with older poplar clones, but the new ones were said to be capable of 18–22 m3 ha−1 year−1 compared with up to 40 m3 ha−1 year−1 in parts of southern Europe such as the Po valley and western Turkey. Rotations of between 30 and 40 years were anticipated. Christie (1994) published yield tables that went up to 26 m3 ha−1 year−1 on a maximum mean annual increment (MMAI) rotation of 35 years with trees reaching 45 cm dbh in 16 years. Perhaps because of the unusual silviculture used with hybrid poplars (e.g. planting at wide spacing and pruning) few sample plots have been ­established. Five established between 1927 and 1936 used old clones such

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as ‘serotina’ and the local YCs that were achieved ranged from 8–12 (Forest Research, unpublished data). A series of plots was established at two sites (Kent and South Wales) in the late 1950s using ‘balsam spire’ and a P. trichocarpa clone. These examined the effects of different spacings and thinning ­regimes upon volume production; the local YCs recorded were higher, varying from 10–19 in south Wales and 15–23 in Kent (Forest Research, unpublished data). More details of these trials can be found in Jobling (1990, Table 7.1). The quality of timber of different poplar clones can vary considerably. However, the wood of most poplars is the best of all temperate timbers for peeling into thick veneers. It is soft, light coloured, of low but variable density (about 300–550 kg m−3 at 12% moisture content) and fine textured, but quality varies according to species, region and conditions of growth. One of the main assets of poplar wood is that it can be peeled with no prior treatment, owing to the high moisture content of recently felled wood. The wood is valued for some special purposes because it is tough for its weight and has the unusual attributes of bruising rather than splintering when subjected to abrasion (which makes it valuable for pallets), and of smouldering rather than igniting when violent friction or heat is applied, hence its use for brake blocks and flooring round fireplaces. The wood is usually white and free of taints and smells, and is therefore valuable for use in contact with food (e.g. vegetable crates and high-quality packaging for cheese). The timber is not durable and the heartwood is resistant to penetration by wood preservatives (The Wood Database, 2018). It is, however, a good sawtimber when dry, being stable and taking a good finish. It can be used for joinery and even for making some musical instruments. Large areas of poplar were established in the 1950s and 1960s for the match and horticultural veneer markets, but these declined because ­imported matches were cheaper than those made of British-grown poplar, and crates made from veneer have mostly given way to plastic containers for fruit and vegetables. There are other uses for veneers, however, such as separators for loads of bricks when they are being transported. There has been considerable recent interest in growing poplars as short-rotation biomass crops for the production of energy, particularly in countries that do not have large reserves of fossil fuels (Hummel et al., 1988), and as a species in ‘agroforestry’ systems (Beaton, 1987). Poplars are also used for providing shelter round orchards and for screening and ornament. Because poplar roots are often close to the ground surface they can be troublesome near buildings, blocking drains and damaging paving. It is usually recommended that trees are not planted within 40 m of such structures.

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HYBRID ASPEN POPULUS × WETTSTEINII Hämet-Ahti = (POPULUS TREMULA L. × P. TREMULOIDES Michx.) Populus × wettsteinii is a hybrid between aspen and the closely related quaking or trembling aspen (P. tremuloides) of North America. According to Tullus et al. (2007) these crosses were first made in Germany in the 1920s and the first plantations were established in Sweden and Finland in the 1950s when the aim was to grow timber for match production. The production of hybrids was supported through a breeding programme, but this activity ceased in the 1960s following the decline in match production in Sweden (Rytter and Stener, 2005). During the same period, limited trials of hybrid aspen were carried out in Britain (MacDonald et al., 1957) but, despite promising growth rates, the plants proved extremely vulnerable to bacterial canker. In addition the propagation techniques used at that time made it difficult to produce the hybrid through cuttings, so that the test ­material was raised from seed, with seedlings being imported from Denmark. From the 1980s onwards, interest in hybrid aspen in Scandinavia ­revived, partly as an alternative crop for surplus agricultural land (Rytter and Stenner, 2005), but also as a fast-growing broadleaf for the production of pulpwood or for biomass (Hytonen et al., 2018). Almost all breeding work with this hybrid has been done in Scandinavian countries and the Baltic States (Tullus et al., 2012). Advances in propagation technology have made it possible to reproduce selected clones cost-effectively through the use of root cuttings or by micropropagation. Following the upsurge in interest in the planting and management of native aspen in Scotland (e.g. MacKenzie, 2010), some limited trials of hybrid aspen have been carried out using either Swedish or Latvian clones.

Climate and soil requirements of hybrid aspen The site preferences of hybrid aspen are similar to those of its parent species (Tullus et al., 2012), so that it can thrive on a wide range of sites, from slightly dry to moist soils of poor to rich nutrient status (Forest Research, 2018b). It will tolerate wet conditions and droughts but is most common on sites where other poplars also thrive: river banks and in gulleys beside streams. Aspen will grow well on acid sandy soils and neutral to alkaline clays, but according Worrell (1995b), it grows fastest and to its biggest dimensions on freely drained mineral soils, particularly on flushed, fine sand/clay. Its possible use as a component of forest plantations should be restricted mainly to such sites. Peats should be avoided. Hybrid aspen will tolerate lower pH than most other poplars (specifically P. trichocarpa) (Bohlenius et al., 2016).

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Silviculture Tullus et al. (2012) stated that planting in Scandinavia and the Baltic States is carried out at a density of 1100–1600 plants per ha, usually on cultivated ground. The hybrid is straighter stemmed and self-prunes better than ­either parent species. It was envisaged that the species would be grown on short rotations of 20–30 years to produce pulpwood. After clear felling a new generation would arise from root suckers which would be managed through thinning regimes to produce a mixture of small-dimensioned ­material for biomass and larger material for pulp (Tullus et al., 2012). In other European countries (e.g. Germany), hybrid aspen is planted at higher densities (4000 stems per ha) to produce biomass for energy on very short rotations (5–10 years) (Leisebach et al., 1999).

Pests and diseases P. × wettsteinii is reported by Tullus et al. (2012) to be more resistant to damage and diseases than either parent. Although hybrid aspen has been grown for some years in Britain (mainly in Scotland), the trials have been limited in scale and in most cases also fenced, therefore yielding relatively little information about browsing (at least by larger mammals). Some authors, however, note that hybrid aspen is browsed by voles (Liesebach et al., 1999) and roe deer (Tullus et al., 2012). Both parent species are well known to be susceptible to browsing by many ungulate species, and it is therefore likely that the hybrid will also prove to be very palatable. In North America, P. tremuloides is widely reported to be heavily browsed by the black-tailed deer (Odocoileus hemionus) and elk (Cervus elaphus), to the extent that there are concerns that the species may be eliminated in some areas.

Yield and timber properties Tullus et al. (2012) reviewed a number of studies from northern Europe where the stem wood production of hybrid aspen was compared with the parent species, and reported that the yields were about twice those ­obtained with the parent species. They anticipated that the mean annual increment in southern Scandinavia could be in excess of 20 m3 ha−1 year−1 on a 20–30-year rotation. The impact of tree breeding was shown using data from Sweden where hybrid families created in the 1940s and 1950s had produced about 16 m3 ha−1 year−1; whereas it was anticipated that the most recent clonal selections could produce about 25 m3 ha−1 year−1 (Tullus et al., 2012; their Figure 3).

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Results from the limited recent trials of hybrid aspen in Scotland also show impressive early growth. Thus, in the trial at Kilmichael in Argyll where Harrison (2009) reported height growth of seven Swedish clones to be about three times that of native Scottish aspen, this difference has been maintained until 21 years (see Table 14). Survival was more variable with three Swedish hybrid aspen clones, having achieved less than 10%, but two other clones had 100% survival. A set of selected Latvian hybrid aspen clones was included in a series of experiments evaluating the potential of a range for use in short-­ rotation forestry in Scotland. These experiments were sited either on former agricultural ground or restocking sites from Aberdeenshire in eastern Scotland to Mull in western Scotland. After 6 years the hybrid aspen material was both the second tallest treatment across all sites, and had a high survival (see Table 15), with the performance being better than that of more typical Scottish forestry species such as silver birch and Sitka spruce. The uniformly short fibres of hybrid aspen, with narrow diameter and thin cell walls, are ideal for producing high-quality paper. It is also used for producing energy-wood. The species has proved to be one of the fastest-growing hardwoods in northern Europe, suitable for the production of pulp- and energy-wood using the principles of short-rotation forestry (Tullus et al., 2012).

Table 14.  Height and diameter growth of Swedish hybrid aspen clones after 21 years compared with native aspen and clones of hybrid poplar in Kilmichael, Argyll. Treatment

Height (m)

Dbh (cm)

6.6 17.3 16.9

8.9 31.0 23.6

Aspen (mean of 39 Scottish clones) Hybrid aspen (mean of 7 Swedish clones) Hybrid poplar (mean of 3 clones) dbh, diameter at breast height.

Table 15.  Height and survival of Latvian hybrid aspen clones after 6 years compared with red alder, silver birch, common alder and Sitka spruce. Results are pooled across five experimental sites in Scotland. Adapted from Parratt (2018, Figure 8). Treatment Hybrid aspen ex Latvia Red alder Silver birch Common alder Sitka spruce

Height (m)

Survival (%)

3.8 4.2 2.1 2.7 2.1

92 63 96 85 87

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Place of hybrid poplars in British forestry The species discussed here are currently unlikely to be deployed in place of the native poplars, aspen (P. tremula) and black poplar (P. nigra), ­either within existing native woodlands or when creating new native woodlands for objectives of habitat restoration and biodiversity conservation. Caution should be exercised when establishing non-native poplars close to natural populations of the native species, to minimize the risk of cross-pollination/invasiveness. The history of the use of hybrid poplars and aspen within British forestry appears to follow a repeated ‘boom and bust’ cycle with waves of enthusiasm for the fast growth and good form of these hybrids being dampened by the subsequent effects of disease eliminating material that is not well adapted to the vagaries of the British climate. However, the predicted ­advent of warming temperatures as a result of climate change could mean that some of the limiting factors that have previously constrained their performance may be less critical. There appears to be a case for a ­modest expansion of the use of these hybrids, particularly in the a­ fforestation of agricultural ground. Their major application is likely to be in the initial formation of new plantations for timber and woodfuel production, carbon sequestration, soil conservation and flood regulation. Past ­ experience ­indicates that it is essential that any deployment of h ­ ybrid clones is undertaken using a mix of clonal material, to minimize the p ­ otential ­effects of disease. These species can either be deployed to form pure broadleaved stands for management under short-rotation forestry systems, to act as nurses to coniferous crops (e.g. Sitka spruce) or to provide an upper canopy where broadleaves are being established on agricultural ground. The former ­approach can be applied under ­either u ­ pland or lowland conditions to produce biomass for energy purposes. The most relevant species for such situations are likely to be the hybrid poplars in lowland Britain or hybrid aspen on sheltered ground in Scotland. The ‘nursing mixtures’ approach with conifers is more likely to be applicable to upland areas, to enhance the biodiversity value of productive spruce plantations or to ­improve their long-term soil sustainability on sites where rotational nutrient depletion is a factor. Here, hybrid aspen might be an appropriate choice.

PRUNUS L. The family Rosaceae, to which Prunus belongs, is large and consists of trees, shrubs and herbs. Among the trees are most of the temperate regions’ fruit trees, including plums, cherries, peaches, apricots and almonds, as well as apples, pears and quinces. There are approximately 100–160 genera. The genus Prunus contains over 200 species, many of which are found in the northern hemisphere. It includes all the fruit trees mentioned above. Two species are native to Britain, and the wild cherry or gean is by far the more important. Its scientific name, P. avium, can be translated from the Latin as ‘bird cherry’. The other species, P. padus L., is rather confusingly named ‘bird cherry’ in English. Bird cherry is usually a small, suckering tree, which grows to a maximum of 15 m tall, most commonly in Scotland and northern England. It is rare in southern England (Leather, 1996). It occurs on acid soils, especially in valleys and wet fen woodland. In drier areas it grows where the groundwater is calcareous. The tree is hardy but not tolerant of exposure to strong winds. It is not considered further here.

PRUNUS AVIUM (L.) L.

Cherry, or gean

Origin Cherry is native to Europe, North Africa and western Asia. It occurs throughout Britain but its regional distribution is patchy, and it is less common in Scotland than elsewhere. It is typically found in small groups, often on the margins of woods. This may be because of the more favourable growing conditions found on banks along the boundaries of woods, or possibly because it was planted there. Climatic requirements Cherry is essentially a lowland species, seldom being found above 300 m in elevation. It tolerates exposure badly, becoming deformed. It is very winter hardy, but flowers can be damaged by late spring frosts. Site requirements Cherry grows best in deep, light, silty, fertile soils with a good water supply, but will grow reasonably well in many soil types and in pHs ­ranging from © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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5.5 to 8.5. This has made it a common species for planting in farm woodlands. It does well on the deep soils developed in the thicker layers of drift over chalk and limestone, and on deep-flushed soils on lower valley slopes. Parts of the Sussex and Wiltshire downs, the Chilterns and the Cotswolds are particularly suitable. Soils to avoid are: shallow ones where the weathering bedrock is within 40 cm of the surface; sands; exposed sites; and sites prone to waterlogging. Sites for establishing cherry orchards for fruit production are on light, well-drained soils such as fertile, clayey, alluvial soils (brickearths) and light-to-medium loams where there is good under-drainage. Fruiting trees do not thrive on cold, heavy clays or badly drained soils of any kind. Other silvicultural characteristics Cherry is a medium-sized tree that very seldom grows taller than about 25 m. It is rather short-lived and liable to windthrow and heart rot after 60 years. It is easy to establish and is productive for a broadleaved species. Rotations are short, and the tree has attractive flowers and autumn colour, and a much sought-after timber. It is strongly light demanding, except when very young, and a pioneer species (EUFORGEN, 2018b). It has good apical dominance and relatively weak phototropic tendencies. This usually results in it developing and retaining a single straight leading shoot. Because most trees are well formed, stocking levels can be lower than with other more variable broadleaves. They are often planted at spacings of about 3 × 3 m. Partly because of this, branches, which are produced in whorls, tend to grow large. They are also retained for many years after their deaths and will form dead knots in the wood unless timely pruning up to a height of at least 5 m is carried out. Some people ­believe that pruning can cause a change in colour in cherry wood, though this is possibly ascribable to infection by silver leaf disease (see Diseases and pests section, below). Pruning should always be done in summer so that the wounds have time to recover, otherwise the tree may become ­infected with silver leaf disease. Young plants are sensitive to competition, and full weed control can more than double early height growth. Cherry grows faster than almost any other species in tree shelters. The tree coppices ­rather poorly, but prolific vegetative regeneration from root suckers ­occurs, so that clumps of cherry are often clonal (see Natural regeneration section, below). Cherry has a useful place as an early-maturing component in mixtures with other broadleaves. Height growth is well matched to that of the larches for the first 45 or 50 years (i.e. all of a larch rotation). It may suppress oak when planted in mixtures, but mixtures with ash are also appropriate, the cherry being removed 10–15 years before the ash or possibly at the same time.

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There is often a race against time to achieve sawlog sizes before rot sets in. Thinnings must be heavy and regular to develop unimpeded crowns and suitable diameters early, since recovery from suppression is poor in stands over 40 years old. It is often regarded as an ideal species for ­woodland edges, where crowding is not a serious problem and the benefits of its attractive appearance are most obvious. A major virtue of cherry is that it is not often damaged by grey squirrels, possibly because the bark can only be peeled horizontally round the stem and squirrels are more familiar with bark that can be peeled from top to bottom. Deer are serious pests and can be very damaging. The fruits are eaten by many small mammals and by birds, which is good for wildlife conservation but a nuisance to those who want to collect seed. Other aspects of the silviculture of cherry have been described in ­detail by Pryor (1985, 1988). Diseases and pests Cherry leaf spot disease, caused by the ascomycete fungus Blumeriella jaapii, is regarded as the worst problem of wild cherry in some European countries, and breeding to develop resistance to it is common. It causes premature defoliation, reduced vigour (especially diameter growth) and reduced winter hardiness, which can result in death. Cherry sometimes suffers from a bacterial canker, Pseudomonas syringae pv. morsprunorum, which characteristically causes exudations of gum from the bark. It is much more common in orchards of fruiting cherries and plums than in wild woodland trees, because the latter have a much wider genetic base. According to Roach (1985), English cherry orchards suffer from it more than those in the drier areas of France and Italy. It is said to be particularly associated with sites where soils are deep and well drained and, possibly, where soil potassium levels are low (MAFF, 1980). To minimize the risks of bacterial canker in any planting scheme, Kerr and Rose (2004) recommended that the best approach is to replicate the natural pattern of the species in British woodlands, where it is usually found singly or in small groups in mixed broadleaved woodland containing a maximum of 20% cherry. Silver leaf disease, Chondrostereum purpureum, is a fungal disease that infects wild cherry (among a number of other species) through wounds, including pruning wounds. It can kill trees. Since the fungus produces most of its infectious spores in autumn and winter, pruning should always be done in summer so the wound has time to recover. Cherry is also attacked by a number of virus diseases. The cherry blackfly, Myzus cerasi, forms colonies in spring on new growth: it causes leaves to curl, shoots to stunt and may result in dieback.

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Natural regeneration Cherry produces suckers in great numbers after felling, so small, dense, clonal clumps can often be found. Regeneration from seed disseminated by birds also occurs as rare scattered individuals, and presumably also among the suckers that regrow, but it is not possible to distinguish ­between a sucker and a seedling without uprooting the trees. Those that survive beyond the juvenile stage are mostly on the edges of woods and in big gaps. Flowering, seed production and nursery conditions The tree flowers from early April to mid-May; fruits ripen in June and July, and are commonly collected for seed in September. Cherry normally flowers at less than 10 years old and produces good seed crops every 1–3 years, especially after the age of 30. In natural conditions they are disseminated by birds and small mammals, and seed orchards need to be protected from birds by netting. There are about 5000 seeds kg−1 (range 3200–6600), of which 80% normally germinates. Aldhous (1972) and Aldhous and Mason (1994) stated that cherry seed should be kept cool

Fig. 26.  Cherry, Prunus avium.

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and in airtight containers between the time of extraction from the fruit and sowing or stratification. It can be sown immediately after collection, or stratified for 4 months and sown in March or early April. Genetically improved wild cherries were becoming available for the first time in 2012. Their use is expected to reduce rotations from the current 50–65 years by as much as 10 years. The improvement was undertaken by conventional breeding after selecting parent trees for form, vigour and disease resistance. Micropropagation is subsequently used. Provenance According to Forest Research (2018b), there is limited knowledge of provenance variation, and it recommends that seed from seed orchards or good British stands should be used. Material from eastern and southern Europe is not adapted to British conditions and should be avoided. In continental Europe different races and even subspecies have been recognized, but no work of this kind has been done in Britain. Cherries cultivated for their fruits are derived from two species: the sweet ones are Prunus avium, and the acid Morello varieties are P. cerasus. Duke cherries are considered to have arisen from crosses between the two (Roach, 1985). For many years up to 2012, wild cherry planted in Britain was of continental origin, unknown quality and uncertain adaptability. Seed was collected from stands bred to produce heavy crops of large fruits and to have a wide, open and strong branching habit for ease of picking. These are often called ‘jam factory’ cherries by foresters. By contrast, trees selected and bred for timber production have light branching and vigorous apical growth. Kerr and Rose (2004) described a well-intentioned attempt by East Malling Research (EMR) in Kent to breed and commercialize well-formed, canker-free stock in the 1990s. They tested five clones of cherry that were subsequently sold as ‘Wildstar ’ cherries. Four years after planting only one of the clones turned out to be canker-free and to have good form. The other four showed no superiority, either in form or resistance to the canker Pseudomonas syringae pv. morsprunorum, compared with unimproved stock. The relative failure of this enterprise was ascribed to an inadequate period of testing. A number of clones have been developed and are available in France but they have not been tested in Britain. Yield and rotations Average yield classes (YCs) of 6–10 m3 ha−1 year−1 are found on most sites, which is high for a broadleaved species. Rotations, at about 60 years, are

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only 5–10 years longer than for most conifers and much shorter than for most broadleaved trees. Timber Cherry is sought after for furniture in particular and for veneer, turnery, cabinet making and decorative joinery. Demand for wild cherry timber is high and greatly exceeds supply. Top-quality wood is hard to find and expensive. Brown and Nisbet (1894) stated that ‘it is comparatively seldom that a tree of any really useful size is brought into the market’. Unfortunately, the same is true today. The best cherry has a uniform, honey-coloured wood and a silky grain. Butts with a variable colour and ‘green’ rings are worth only a fraction of the value of the best timber. The average density of the wood at 15% moisture content is about 630 kg m−3. It is not durable outdoors. The timber is described by Desch and Dinwoodie (1981) as being falsely ring-porous because of the aggregation of vessels at the beginning of each annual ring. It is tough (but not as tough as ash) when seasoned, and moderately durable, but liable to attack by furniture beetles. It usually works, polishes and glues reasonably well, but unless it is dried carefully and slowly it is inclined to shrink on drying and is rather prone to bowing. Place of cherry in British forestry Interest in cherry arises from the high value of its timber, its fast early growth and its attractive flowers and autumn colour. It will remain an important but nevertheless minor component of broadleaved woodlands.

PSEUDOTSUGA Carr. PSEUDOTSUGA MENZIESII (Mirb.) Franco

Douglas fir

Origin and introduction The genus Pseudotsuga is in the family Pinaceae, which contains most of the well-known conifers of commerce: the cedars, firs, hemlocks, larches, pines and spruces. Trees in the family often form the dominant component of boreal, coastal and montane forests. There are six species of Pseudotsuga: four in eastern Asia and two in western North America (Farjon, 2017). Douglas fir was first described by Archibald Menzies (1754–1842) a Scottish surgeon and plant collector, who discovered it on Vancouver Island during the 1790 voyage by Captain Vancouver to the north-eastern Pacific Ocean. It has a wide natural distribution and is a species of great commercial importance in its native North America. It occurs along the Pacific coast from northern British Columbia to northern California, the Rocky Mountains and Mexico, and was introduced to Britain in 1827. There are two intergrading subspecies of Douglas fir: ssp. menziesii (coast Douglas fir) and ssp. glauca (Rocky Mountain Douglas fir), sympatric in southern British Columbia and north-eastern Washington (Lipscomb, 1993). Climatic requirements The species is moderately accommodating but tends to grow best in the wetter western parts of the country, and satisfactorily though more slowly in the lower rainfall areas of south-eastern and eastern England, provided there is sufficient soil moisture. It can be damaged by late spring frosts in low-lying places, though less severely than the spruces (Macdonald et al., 1957). Douglas fir will not tolerate serious exposure to wind, becoming badly deformed; it tends to be more prone to windthrow on unsuitable sites than many conifers. The tree requires reasonably sheltered conditions for healthy growth. Douglas fir is one of the least tolerant of all species to atmospheric pollution. Site requirements Douglas fir is usually regarded as a species best suited to the middle and lower slopes of valleys of at least moderate fertility. It needs a deep, welldrained soil in order to develop a good root system. The soil can be clayey, if © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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on a slope, and the tree will grow well on sandy soils. Adequate moisture and soil aeration are essential. Douglas fir is therefore unsuited to heavy, waterlogged soils where rooting is restricted. If planted on such sites it b ­ ecomes unstable; it is one of the least tolerant of all species grown in Britain to the anaerobic conditions resulting from flooding. It is also unsuited to calcareous soils and, like Sitka spruce, will not grow in competition with heather because it cannot compete with it for nitrogen and so becomes deficient. On sites where it will thrive, especially in the west, Douglas fir is one of the most desirable species as it is both productive and has a valuable timber. In its native range, both subspecies (menziesii and glauca) tend to occur at progressively higher elevations from north to south. The principal limiting factors are temperature in the north of the range and moisture in the south. Consequently, Douglas fir is found mainly on southerly slopes in the northern part of its range, and on northerly exposures in the southern part (Farjon, 2017). Other silvicultural characteristics Most of the current tallest trees in the different countries of Britain and Ireland are Douglas firs of 60 m or more. Only Wales is an exception, with a grand fir being the tallest tree. Mabberley (2008) mentioned a specimen of 133 m that was felled in British Columbia in 1895, and a tree of 138 m has been claimed. The theoretical maximum height a tree can grow to is said to be about 140 m. The tree is fast growing, and its stems have a tendency to sweep when young. Grass control is essential for successful establishment of young plants, but the species is more competitive with weeds than many other trees and eventually casts a heavy shade. Its litter decomposes readily on most sites. Douglas fir is difficult to grow in mixtures with other species, ­especially broadleaves, because it tends to suppress them. However, it is sufficiently shade bearing to be useful for planting beneath well-thinned canopies, especially for enriching scrub and in neglected woodland, but it is not so shade bearing that it will form a lower storey to another species, unless it has a very light canopy. As with larch, systematic thinning should never be practised with Douglas fir because trees are so variable in form. For this reason, selective thinning is essential. Unless stems are pruned the timber will be knotty because branches tend to be persistent. Pests and diseases It is less susceptible to attack by Heterobasidion annosum than many conifers. The woolly aphid, Adelges cooleyi, which feeds in spring and early summer,

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causes yellowing and deformity of the needles and can severely check the growth of some provenances when the trees are young. However, in general, Douglas fir remains healthy and well adapted to many parts of Britain. Natural regeneration Natural regeneration occurs only occasionally on a scale sufficient for it to be a reliable means of crop replacement, in stark contrast to Sitka spruce in many areas. This is partly because trees are usually felled before seed production becomes sufficient. Flowering, seed production and nursery conditions The tree flowers in April; seeds ripen and are ready for collection in September, and are dispersed naturally from then until late March. Cones should be collected when they are a light golden brown or yellow colour. The earliest age at which the tree bears seed is 30–35 years, but the best seed crops occur at intervals of 4–6 years, usually from the age of 50 or 60. This may be beyond the length of normal commercial rotations. There are approximately 70,000 germinable seeds kg−1, which will give 38,000 usable 2+0 plants (Aldhous and Mason, 1994). Seed collected in Britain is often infested with a seed wasp, Megastigmus spermatrophus (Forestry Commission, 1961), which can seriously reduce the quantity of sound seed (Kennedy, 1974). For nursery purposes seed is usually stratified or pre-chilled for 3 weeks or more and sown in late March. Provenance For the western parts of Britain the best origins come from low elevations with high rainfall in the state of Washington, from the western foothills of the Cascade Mountains, westwards along the 46th parallel to the low coastal range and north as far as Forks in Clallam County (Lines, 1987). Material from the southern Washington Cascades can be used on suitable soils in drier zones of eastern Britain (Forest Research, 2018b). Area and yield According to the Forestry Commission’s (2018a) inventory, Douglas fir had been planted on 45,600 ha, or 1.7% of forest land, in around 2009. Mean yield classes (YC) range from 10 to 15 m3 ha−1 year−1 in different regions of Britain (Nicholls, 1981), and the maximum is 24. Rotations of

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Fig. 27.  Douglas fir, Pseudotsuga menziesii.

maximum mean annual increment are similar to those of Sitka spruce, ranging from 50 to 65 years. Timber Knot-free Douglas fir is one of the world’s finest coniferous timbers, valued for veneering, interior and exterior joinery, and decoration. The timber is light reddish-brown in colour and dries with little degrade. It is fairly stable and presents no special difficulties in sawing and working, though it has a tendency to split when nailed. It is only moderately durable out of doors, and the wood is much less permeable to preservatives than Scots pine and tends to ‘bleed’ in service. The average density at 15% moisture content is about 530 kg m−3. Douglas fir timber is often sold as ‘Oregon pine’. It attracts higher prices than most other conifers. Markets for small sizes are similar to those for spruce and pine, and for large timber (60–90 cm diameter at breast height, dbh) there is a distinct market for knot-free structural beams for major building projects (Wilson, 2010). Little clear joinery wood is produced in Britain, but what is available is suitable for carpentry and structural work or for use in the round as transmission poles and fence posts. Place of Douglas fir in British forestry Demand is likely to ensure that it retains its place as an important but nevertheless relatively minor species in British forestry. The main limitation to its use is the availability of suitable sites for growing it.

PTEROCARYA Nutt. ex Moq. PTEROCARYA FRAXINIFOLIA (Poir.) Spach Caucasian wingnut Caucasian wingnut (Pterocarya fraxinifolia) is one of six species of Pterocarya within the wider walnut family (Juglandaceae – see p. 151). Its native range is within montane forests of the Caucasus region, Turkey and northern Iran, from 400 to 1200 m asl (Avsar and Ok, 2004). The other wingnut species are found in China and Japan. Caucasian wingnut is a medium-sized tree, reaching 25–30 m tall. It has a short, thick bole and rounded, spreading crown (to 20 m wide). However, this describes open-grown specimens, and stem form in forest stands may be superior. The bark is grey, smooth when young, becoming furrowed with greater age. The leaves are simple pinnate, as in walnut, and can superficially resemble those of ash, although they reach 60 cm in length. The species was introduced to Britain after 1800 and is found in tree collections and arboreta. It often occurs as multi-stemmed clusters or ‘stools’. Some trial plots were established in Britain during the 1980s. The species was considered in a paper on alternatives to ash by Wilson et al. (2017). Climatic and site requirements Caucasian wingnut favours temperate, moist, forest conditions and can be damaged by frosts exceeding −30°C but appears to be able to tolerate −20°C. It is believed to be relatively tolerant of exposure and drought once established. It does best in moist, fertile alluvial or riparian soils (Ebrahimi et al., 2005; Maharramova, 2015), rarely being found more than 200 m from open water, and forms a component of mixed stands containing alder, oak, elm, maple and beech. It can tolerate a range of non-ideal soils, including compacted and acidic types. It is most likely to be ecologically compatible with site conditions in lowland valley ash woodlands and plantations (NVC W8 – see Rodwell, 1991) and lowland oak–ash woodlands (NVC W10). Its suitability for upland ashwood sites (NVC W9) is uncertain, but it might tolerate more sheltered examples. Provenance, seed production and nursery practice Caucasian wingnut is a wind-pollinated monoecious species producing ‘turbinate’ nuts that are dispersed 20–30 m from the parent (on a ‘helicopter rotor’ principle) and can be carried further by flowing water. This © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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differs from the related hickories and walnuts, which require vertebrate seed vectors. Wingnut ‘drupes’ usually have two wings and, when m ­ ature, are 3.5–4.5 cm wide and 1.0–1.5 cm long. Seed production begins from 18–25 years in maiden trees but can be earlier in specimens reproduced vegetatively from root or stump sprouts, the preferred mechanism. Seed is partially dormant, requiring cold stratification for 5–7 weeks (Cicek and Tilki, 2008). Suckers and cuttings can be rooted. There is limited information on nursery practice or provenance variation and no guidance on provenance selection for British or Irish conditions. Silviculture Caucasian wingnut is a light-demanding species capable of rapid early growth on suitable sites (Ayan and Syvacyodlu, 2006; Sohrabi et al., 2008). It can live for up to 250 years. In the native range it is reported to achieve 9–12 m in height after 10 years and 30 m after 30 years when established from seed. Stem diameter can reach 80–100 cm in 60–80 years, which might be a typical rotation for productive silviculture, comparable to that for ash. A trial plot at the John F. Kennedy Arboretum in Co. Waterford, Ireland, was 8 m tall after 21 years’ growth (hence GYC 2–4). Vegetative sprouts can reach 4 m tall after 6 years’ growth. The species tends to weak ­apical dominance, poor stem straightness and crown asymmetry. Caucasian wingnut produces suckers profusely and, as a consequence, is potentially invasive under suitable conditions, which may constrain its British ­deployment. It grows either as a many-stemmed tree or with a ‘cup’ of heavy limbs arising from a short, lumpy bole. It branches low down. Trees in unmown areas are well known to be hidden in huge thickets of suckers. Pests and diseases The species is believed to be relatively resistant to pests and diseases in Britain, including honey fungus (Armillaria); however, the basis of evidence to date is very limited. Timber properties and utilization Wingnut produces a soft, reddish-white timber, less dense and strong than that of walnut or hickory (density is 610 kg m−3) (Dogu, 2007). The timber is durable when dry, but rapidly decays if exposed to moisture. This makes it comparable with alder and poplar. Wingnut timber might substitute for certain applications of ash: light construction (non-exposed uses), furniture making, decorative carpentry, marquetry and veneers. It can also be

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used to produce plywood. Authors from Turkey and Iran have considered timber properties in detail (e.g. Kantay et al., 1999; Gungor et al., 2006; Dogu, 2007). Place of Pterocarya in British forestry At present, in Britain, P. fraxinifolia is found mostly in botanic gardens or as street or ornamental trees. It makes an excellent large shade tree for parks, schools, golf courses and other spaces where it is said by Johnson and More (2004) to be ‘almost vandal-proof’.

QUERCUS L. Quercus is a genus in the family Fagaceae, which is made up of eight genera and almost 1000 species. Species in the family are described by Mabberley (1990) as ‘strongly tanniferous’. Trees in two other Fagaceae genera will be familiar to British foresters: Castanea and Fagus. The family is widespread in the northern hemisphere and members of it are often ecologically dominant in northern temperate forests. The genus Quercus contains about 600 species (Mabberley, 1990), which are confined to the northern hemisphere: 27 are European species. Most of the species require too much warmth to do well in Britain in the current and predicted climates. There are several sections in the genus, i.e. a taxonomic rank below genus, but above species. They are: • Quercus – White oaks, including the native British oaks, Q. petraea and Q. robur, and the American Q. alba. • Mesobalanus – Downy oak, Q. pubescens, Hungarian oak, Q. frainetto and Algerian oak, Q. canariensis. • Cerris – Turkey oak, Q. cerris. • Protobalanus – American evergreen live oaks e.g. Q. virginiana. • Lobatae – American red oaks, including Q. rubra. Most oaks occur in more southerly latitudes than Britain, in warm temperate and even tropical montane climates. By far the greatest concentration is in Mexico, which has about 125 endemic species and a further 75 that extend either north into the USA or south towards Colombia. Only four oaks have any potential in Britain, of which two are native: Q. petraea, sessile oak, and Q. robur, pedunculate oak.

QUERCUS CERRIS L.

Turkey oak

Origin Turkey oak is indigenous to southern Europe and south-west Asia. Its time of introduction to Britain is uncertain, but it was in cultivation by 1735. The tree is superficially similar to the two native species. It grows to considerable sizes and, unlike native oaks, usually has excellent form. There is apparently a widely held belief that Turkey oak will hybridize with pedunculate oak, but unpublished molecular work by Kevin McGinn at Reading University in 2009 concluded that this is unlikely to have a

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s­ cientific foundation. He believed that the great variation in leaf shape of Q. cerris creates much confusion. Climate and site requirements Turkey oak will tolerate strong winds, but it is essentially a lowland tree. It grows well in a wide range of soils provided they are moist, though not in dry sandy ones or organic soils. It is unusual in that it will thrive in calcareous soils, even where they are shallow. It can grow under a light woodland canopy or in the open. Other silvicultural characteristics Turkey oak is a fast-growing and attractive tree that will reach about 40 m tall. A really good-looking, well-formed oak should always be suspected of being a Turkey oak. It grows faster than both sessile and pedunculate oak, and coppices well. Natural regeneration Turkey oak seeds regularly, and regeneration is common. Macdonald et al. (1957) believed that, if the tree were more widespread it would speedily naturalize itself, as sycamore has done. Pests and diseases Turkey oak is the alternate host of the knopper gall wasp, Andricus quercuscalicis, which became common in the 1980s and caused concern about seed production and consequent regeneration of the pedunculate oak. The galls form on the acorns of pedunculate oak. They are typically convoluted and make the acorns infertile. The wasp has a complex life cycle involving the development of an asexual generation on pedunculate oaks, alternating with a sexual generation, which develops in tiny galls on the male flowers of Turkey oak (Speight and Wainhouse, 1989). Knopper galls only occur on pedunculate oaks if Turkey oaks grow close by. Timber and uses Jones (1959) stated that, from the very first, references to Turkey oak have almost universally condemned it as a timber tree, so it has been

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planted mainly for ornament. The timber is hard and brittle, and has an even wider band of sapwood than the two native oaks. It seasons poorly, shrinks considerably on drying and has a limited natural durability. The quarter-sawn wood shows the silver grain figure characteristic of the two native species. It is suitable only for rough work such as shuttering. Its average density varies between 780 and 870 kg m−3 (BRE, 1972), more than 100 kg m−3 higher than that of the two native oaks, but similar to beech. This makes it an ­attractive fuelwood, since there is an almost perfect correlation between the amount of heat produced and wood density. It was widely planted as a fuelwood in some parts of Europe in the 1800s (e.g. Majer, 1984). Place of Turkey oak in British forestry The main virtues of Turkey oak are its resistance to exposure and adaptability to calcareous soils. It can be of value as a windbreak, especially in the south of England. Unfortunately, its timber is much inferior to that of most hardwoods, and this limits its potential uses considerably. Also, as an alternate host of the knopper gall wasp, which can destroy acorns of pedunculate oak (see above), it is probably better removed from British woodlands than encouraged in them.

QUERCUS ILEX L.

Holm oak

Origin and introduction The evergreen holm oak was introduced to Britain in about 1500 from its native lowland Mediterranean region. It has never been used as a forest tree, but is very useful for shelter, especially in coastal districts (e.g. Holkam on the Norfolk coast and round Osborne House – Queen Victoria’s home on the Isle of Wight), as it withstands sea winds very well (Macdonald et al., 1957). Mitchell (1974) noted that it is common in parks and gardens everywhere. It is a slow-growing, smallish tree, rarely more than 20 m in height, but will grow to a diameter that exceeds 1 m. The ­occasional very cold winter will cause leaves to die, but trees usually ­recover the following spring. The acorns provide an important source of food for pigs from which Spanish ham, jamón ibérico, is prepared in its native region. The timber is very dense at 800–960 kg m−3 and consequently makes good firewood. Place of holm oak in British forestry The use of holm oak is unlikely to extend beyond its current position, in parks and for shelter near the sea.

QUERCUS PETRAEA (Mattuschka) Liebl.  QUERCUS ROBUR L.

Sessile oak Pedunculate oak

These two species have been described by Muir et al. (2000) as ‘a classic example of a taxonomic group that has significantly challenged existing species concepts’. Doubts have been expressed by taxonomists for many years as to whether they are in fact separate species. However, Muir et al. have demonstrated, by using microsatellites in molecular analyses, that they are definitely separate taxonomic units. Intermediates between the two species are quite common, and these are hybrids. These two oaks represent the most iconic of British (or perhaps English) trees. They are among the tallest, most massive and longest-lived trees, though the records for all these characteristics go to other species. Some have strong historical associations, and oak is even used as the logo of many organizations, including the National Trust. Because the similarities are greater than the differences, the two species are dealt with together below, and Table 16 shows the main differences. Detailed accounts of the two British oaks have been written by Jones (1959), Morris and Perring (1974) and Newbold and Goldsmith (1981), from which much of the following information originated. Origin The oaks are native to Europe, including Britain and Ireland, and to western Asia. Within Britain pedunculate oak is dominant mostly in the lowlands of the south, east and central parts of England, while sessile oak is characteristic in the north and west of England, Wales, Scotland and Ireland. The ranges of the two species frequently overlap, and planting over many centuries has obscured the differences in the natural ranges. Sessile oak has a more limited range than pedunculate, in regions where the climatic requirement resemble those of beech (Huss et al., 2016). Climatic requirements It is widely believed that sessile oak is more tolerant of environmental stresses than pedunculate oak, at both ends of the spectrum. Sessile oak is said to be less frost tender than pedunculate oak, though both species suffer badly from late spring frosts, when temperatures of −3°C will kill new foliage. They usually avoid such frosts by being among the last trees to come into leaf, normally in mid- to late © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Table 16.  Main differences between sessile and pedunculate oaks. Character

Sessile oak

Pedunculate oak

Site/soil preferences

Acid; freely draining

Frost susceptibility Drought resistance

Slightly less frost tender Relatively resistant

Shade tolerance Early growth Form of tree

More shade tolerant Relatively slow Straighter, taller, longer clear bole; persistent main stem Less liable to produce them due to earlier flushing One week earlier than pedunculate oak Less likely due to earlier flushing Possibly less susceptible Relatively small ca 300 kg−1 High lactones, low phenolics: good for maturing whisky

Wetter, heavier and more alkaline More frost tender Not resistant. Tendency to die back Less shade tolerant Relatively fast Low branching, short bole, main stem not persistent More likely due to later flushing One week later than sessile oak More likely due to later flushing Possibly more susceptible Relatively large ca 273 kg−1 Low lactones, high phenolics: less good for maturing whisky

Epicormic shoots Leaf emergence Defoliation by caterpillars Acute oak decline Acorns Oak lactones and phenolics in wood

April. Sessile oak grows best on south-facing slopes where summer warmth can stimulate fast growth. Oak woodland is rare above 300 m altitude in Britain (Jones, 1959). Low wind-trimmed scrub, usually of sessile oak, is often found in exposed situations in the western hills. There are exceptions: Wistman’s Wood, at 365–420 m on Dartmoor, is composed of pedunculate oak scrub. Site requirements Pedunculate oak is dominant and grows best mostly in the English lowlands on heavy, moister, richer and especially basic soils; while sessile oak, which is more drought resistant, is characteristic of deep, freely draining, drier, siliceous soils in the north and west. The ranges of the two overlap on the damper acid sands and planting has obscured the differences in the natural ranges. Both species tolerate an extremely wide range of soils and often grow together. They are much less site demanding than many other broadleaved trees. Pedunculate oak tolerates some waterlogging. Its strong, deep-­rooting system allows the species to evade droughts on many soils and makes it windfirm. Shallow, badly drained soils and compacted soils should

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be avoided. Though it will grow well and root deeply on heavy soils, pedunculate oak benefits from drainage on certain sites (Evans and Fourt, 1981). Sessile oak is intolerant of flooding. Throughout the lowlands of central, southern and eastern England it is generally restricted to base-­ deficient sands and gravels where it will do well, but sites of at least moderate fertility are required for acceptable rates of growth. Soils that should be avoided are drought-prone and sandy soils, which give rise to a high proportion of trees with shaken timber (see p. 265), and those with erratically variable water tables, in addition to soils that are shallow, badly drained and compacted. Within their ranges in Britain the oaks are usually the dominant native trees except that, within the lower part of its altitudinal distribution, beech is liable to be a serious competitor with sessile oak for dominance. Beech is much more responsive to higher levels of available nutrients in the soil than oak; therefore, on soils of relatively low nutrient status, oak tends to have the advantage. Other silvicultural characteristics Among the most impressive oaks in Europe are the sessile oaks in the French forests of Reno-Valdieu and Bellême in Haute-Normandie. Trees of about 240 years old range in total height from 38 to 45 m, with 20–28 m to the first branch: they each contain 10–20 m3 of timber. In Britain oaks greater than about 27 m tall are not common, and trees taller than 33 m rare. The two species do not differ in respect of the height that they can attain in Britain. Contrary to popular opinion, the British oaks are not ‘climax’ species and have none of the characteristics of such species. For example, neither as seedlings nor as adult trees are they really shade bearing. The widespread apparent ‘climax’ role of the oaks is due to: • their ability to outlive most competitors, once established; • their ability to grow on a wide range of soils; and • good seed dispersal, which enables them to spread. Oaks have a considerable pioneer capacity: seed is dispersed widely by birds and mammals, and their large reserves make seedlings competitive with most grasses. They can root quickly in the autumn, withstand summer droughts and can persist in grass until the roots have developed enough to allow rapid shoot growth. Unlike most later successional trees, the oaks are in no way dependent on shade from other trees. In natural conditions they regenerate by species alternation: for example, birch may replace fallen oak trees in northern oak woods, which are in turn replaced

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by other oaks, but because the oaks are so long-lived the forest remains predominantly of oak. Birch is regarded as an excellent nurse for oak. In Britain sessile oak is a successional tree in the lower part of its altitudinal range, where the very shade-tolerant beech becomes a competitor for dominance on the more fertile sites. Beech is a stable climax species, being capable of regenerating and self-replacing under the shade of other trees. Many of the behavioural traits of both pedunculate oak and sessile oak are characteristic of trees in warmer climates. They come into leaf too late to make full use of the growing season and even then are sometimes damaged by frosts. The large fruits are readily killed by desiccation as well as by frosts, and radicles often germinate in the autumn. Seedlings have the ability to shoot and die back repeatedly when growing in adverse conditions, while the taproot thickens and accumulates reserves until it is capable of producing a long, strong shoot in a single season. This is also a feature commonly found in warm climates, as is the ability to produce epicormic shoots, which is an adaptation to fire. Coppicing ability is good, especially in the south, and has probably been selected for by man to some extent through many generations of this treatment in Britain. The grasses Deschampsia flexuosa and Holcus mollis are both said to ­inhibit the development of seedlings, and controlling them with herbicides is necessary. Young oak trees frequently need some formative pruning if they are to grow straight and unforked. Really badly formed trees can be stumped (i.e. cut off just above ground level) and will usually regrow quickly as straight stems. Natural pruning is not very good, so for the production of high-quality timber high pruning is usually necessary. Both species are relatively intolerant to shade but, according to Jones (1959), sessile oak is considerably more shade bearing than pedunculate oak. Many of the differences between the two species, such as the faster early growth of pedunculate oak and the greater suitability of sessile oak for growing in high forest, are in keeping with this difference. Pedunculate oak has been grown far more commonly than sessile in plantations, possibly because it seeds more frequently and the acorns ­retain their viability for slightly longer when in store. Sessile oak is frequently reported to have a better form than pedunculate oak, tending to be straighter and taller, with a greater length of clear bole. Its main stem tends to persist through the crown of the tree; the branches are relatively straight and horizontal; the branching regular with a gradual decrease in size of successive orders of branches and narrow ­angles between branches; and the foliage is uniformly distributed, forming a dense crown. By contrast, pedunculate oak has a tendency for the main trunk to disappear in the crown; the branches are irregularly divided; the angles between branches wide; and the decrease in size from main branches to twigs is abrupt. The foliage and small twigs tend to be in clusters, resulting in an open crown.

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In France, in recent years, there has been much emphasis on reducing the very long and expensive rotations on which oak trees have traditionally been grown. The outcome of 30 years’ work was published by Jean Lemaire, in 2010 as Le chêne autrement. The book was translated into English under the title Oak: fine timber in 100 years. Detailed prescriptions are provided throughout from site selection to silviculture (Lemaire, 2010). Many of the old oak trees in Britain were grown as standards over coppice, and in these conditions the trees are likely to be significantly shorter than those grown throughout their lives in high forest (Savill and Spilsbury, 1991). As Marshall (1803) observed, presumably about widely spaced trees: ‘Oaks which endure for ages, have generally short stems; throwing out, at six, eight, ten, or twelve feet high, large horizontal arms; thickly set with crooked branches’. Good comparisons of standards and high forest trees growing in reasonably close proximity can be seen in parts of Germany, such as Iphofen forest in Bavaria, where the differences in height at similar ages are striking. Constant problems in producing high-quality oak timber are caused by the growth of epicormic shoots and the economic losses associated with shakes in the timber, both of which are believed to be highly heritable (Kanowski et al., 1991). They are discussed below. Epicormic shoots Epicormic buds that are dormant beneath the bark develop into whiskery shoots when the space and light available to a tree is suddenly increased by thinning and also in response to restriction of the crown or defoliation of the tree by caterpillars. Epicormic shoots produce small knots in the wood, which can decrease its value, except if they originate from the centre of the stem so that it yields what is termed ‘pippy’ oak, which is decorative and hence valuable. Pedunculate oak is said to be more prone to producing epicormic shoots than sessile because it is defoliated more readily (see under Natural regeneration, below). Suppressed trees die from the top downwards, and as they do so new epicormics form at successively lower levels. Generally, the better developed the crown, the smaller is the tendency to form epicormic shoots; growers of high-quality oak should always aim to maintain deep-crowned trees. Jones (1959) recorded that when the ratio of diameter of crown to diameter of bole is increasing there is little or no tendency to form epicormic shoots, and the tendency varies with the rate of increase or decrease of the ratio. Wignall et al. (1985) found that thinning oaks in summer resulted in less epicormic production than spring thinning. The conventional and widely accepted way to minimize epicormic shoot production is to thin lightly and often, ensuring that the crowns have room to grow. In continental Europe a lower storey of shade

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(A)

(B)

(C)

(D)

(E)

Fig. 28.  (A) Red oak, Quercus rubra; (B) Sessile oak, Quercus petraea; (C) Turkey oak, Quercus cerris; (D) Pedunculate oak, Quercus robur; (E) auricles at base of pedunculate oak leaf.

bearers, usually beech, hornbeam or lime, is often grown with oak to shade the stems and hence suppress the growth of epicormic shoots. Morisset et al. (2012) showed that that oak silviculture to minimize epicormics can be optimized by the early selection of crop trees with few epicormics and/or eventually a heavy first thinning that identifies and ­removes individuals prone to the development of multiple epicormics.

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Shake Oak and sweet chestnut are the two species grown in Britain that suffer seriously from ‘shakes’ or internal splits in the timber. The splits do not extend to the bark and so are invisible in standing trees. They occur most commonly on drought-prone sites (Henman, 1984). Water stress is thought to trigger shakes in trees that have a predisposition to the problem. Savill (1986) found that earlywood vessels with greater diameters than average provide the main predisposition. Vessel size has subsequently been found to be a highly heritable characteristic of oak trees (Kanowski et al., 1991), so that theoretically it will be possible to select (or to breed) trees with small vessels. In fact, this is currently being done by the Future Trees Trust (see section on Provenance and breeding, below). Fortunately, it has been found that, in any population, the trees coming into leaf latest in any year are also those with largest earlywood vessels, and hence are the most predisposed to shake (Lechowicz, 1984; Savill and Mather, 1990). This simple way of recognizing shake-prone trees makes it possible to remove them during early thinning operations, leaving shake-free individuals to grow to the end of the rotation. If trees are marked for removal during the 2–3 week period of flushing in any year, most of those with a predisposition to shake can be recognized and taken out over three or four thinning cycles. The reason for the connection between flushing time and the possession of larger-than-average-sized vessels is obscure, but it is possible that the same auxin (indole-3-acetic acid) is responsible for both. Pests and diseases A few serious new pests and diseases of oaks have appeared in recent years. Many have been introduced by the ornamental plant trade. The risks associated with the international transfer of material have been known for a long time (e.g. Gibson and Jones, 1977), and they are heightened by the increasing speed and frequency of travel. Some already existing pests and diseases, and also some new ones, are detailed below. Grey squirrels Oaks of between about 20 and 40 years old are a favourite of grey squirrels, which strip the bark from the upper stem and branches. Unless control can be carried out effectively it is probably not worth attempting to grow oak for timber. Oak processionary moth (Thaumetopoea processionea) A native of central and southern Europe, this was first recorded in The Netherlands in 1991 and was introduced from there into Britain in 2006. Caterpillars attack both species of native oak. Their hairs carry a toxin that

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can be blown in the wind and cause serious irritation to the skin, eyes and bronchial tubes of humans and animals. Populations of the caterpillars vary widely from year to year and peak occasionally. They can defoliate trees completely, although the trees normally recover the following year. Control measures are extremely difficult to apply effectively and are consequently expensive. They were essentially abandoned in Britain in 2011 and a policy of ‘containment’ was introduced instead. This was changed in 2017–18 when the Forestry Commission (2017) adopted a strategy of designating outbreaks as being in ‘core zones’ or ‘control zones’. In the latter, the aim was ‘to slow the rate of spread and reduce the impact of the processionary caterpillars’, and in the former, the Forestry Commission supported ‘landowners with advice and communications coordination’, but little or nothing else. The spring climate in Britain is unpredictable and is believed by some not to be suited to the long-term survival of the processionary moth. If this proves to be the case then fears about its spread can be allayed, but if it does gain a good hold it may be a considerable problem. In its native southern Europe it is controlled by natural enemies, according to Mabbett (2012). Knopper gall wasp This is described in the section on Q. cerris. It causes some damage to acorn crops where both Turkey oak and pedunculate oak grow in the same ­locality. It does not affect sessile oak. Powdery mildew and other fungi Powdery mildew, caused by the fungus Erysiphe alphitoides, was introduced to Britain in 1908 and is a common foliar pathogen of oak throughout Europe. It attacks young leaves and soft shoots, particularly of young trees, covering them with a felty, white mycelium, and eventually causes them to shrivel and blacken. Mild, overcast conditions promote development of the disease, which usually appears in summer when warmer conditions prevail. It is seldom fatal and trees usually recover completely as they grow. A large number of other fungal diseases have been listed for oak by Peace (1962) and by more recent authors. In normal circumstances few are particularly threatening. Acute oak decline This worrying and relatively recent problem tends to affect pedunculate oak more than sessile oak. It appears to pose a major threat to trees that are more than 50 years old, across central, eastern and south-eastern England and parts of Wales, as well as neighbouring parts of the Continent. The condition can kill a tree in as few as 4 or 5 years. The syndrome, described by Denman and Webber (2009), is not yet completely understood but is thought to be caused by an interaction between an

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(as yet) unknown bacterium and the native jewel beetle, Agrilus biguttatus. The beetles lay eggs on the bark of sickly trees and the resulting larvae then tunnel through the bark to feed on the phloem and cambium, and this can girdle infested trees. Symptoms include a dark, tarry, fluid bleeding from splits in the bark on stems, and as affected trees ­approach death there is a notable thinning of the canopy and dieback of the branches. The timber is rendered useless after attacks by the larvae of the beetle, which does not attack healthy trees. Sometimes trees appear to recover though the timber is then blemished. Gaertig et al. (2002), working in Germany, believed that a lack of ­adequate soil aeration could be a primary cause of oak decline, because fine-root formation is reduced in the absence of aeration and this reduces the stress tolerance of the trees. It may also be caused by a shortage of soil moisture. There was a serious drought in southern England in 1921 and a period of oak decline developed from 1923 onwards (Day, 1927). On the Continent there was a series of severe droughts from about 1910, and from then up to 1930 oak decline caused consternation. There have been several very dry years from 2009, particularly in south-eastern England, and these could be a contributory or even the main cause of the recent outbreak. It seems to occur most seriously in the driest parts of the country and mostly on pedunculate oak: the species that requires moister soil conditions. It could be a consequence of climate change. Natural regeneration Natural regeneration of both native oaks is usually accomplished in Britain with difficulty, if at all. Some writers (e.g. Rackham, 1980) believed that this is a feature of the last 150 years and that it has been caused by the abandonment of the coppice system of management and increases in the numbers of small mammals, pheasants, squirrels, pigeons and deer, which feed on acorns and browse on seedlings. In addition, the introduction of the oak powdery mildew (see above) in 1908 may be responsible for killing shaded seedlings (Newbold and Goldsmith, 1981). Others, perhaps the majority, believe that oak regenerates in the same way today as it did in former times. It has never been easy. Experience in continental Europe and in the New Forest has been that fencing to protect regeneration from deer browsing is essential. Without this form of protection ­regeneration will not survive. There are generally intervals of at least 2 and sometimes up to 10 years or more between heavy acorn crops. In the west germination occurs freely almost anywhere but is best under litter, which has a very suitable microclimate and also offers some protection from predation and frost damage. Temperatures of −6°C for a few hours will kill most acorns (Jones, 1959). In the drier parts of south-eastern England acorns on the surface fail to

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germinate because they become desiccated (Watt, 1919). The dormancy of acorns is never deep, and in suitable conditions in autumn some radicles germinate immediately upon falling, but the epicotyls (i.e. the shoots) ­always remain dormant until they have been chilled over a winter. Dry ­autumns can be damaging in that they cause the death of roots that emerge from acorns on the surface. If summer droughts occur the acorns that germinate late usually die quickly. Burying acorns by scarifying the soil often results in earlier germination and much more prolific regeneration, in just the same way as has been described for beech, and was once widely practised for regenerating the two oak species. It can be an advantage to create conditions that encourage autumn germination of the radicle so that the seedlings can begin growth early in the spring. High winter water tables result in high mortality of acorns and seedlings, especially of sessile oak. This is an important reason for retaining a light canopy during the early establishment phase on heavy soils, because water tables usually rise on clear-felled sites. Maximum height growth of first-year seedlings occurs in about 30% of full light (equivalent to open-canopied woodland or small clearings), but at least 50% is needed in later years (Jarvis, 1964). Young oaks are well adapted to survive some browsing by wood mice, Apodemus sylvaticus, and bank voles, Clethrionomys glareolus, but they do not survive complete defoliation over successive years. Several insects that feed on oak leaves have co-evolved with oaks and time their emergence in spring to the brief period when young oak leaves are most palatable. Defoliation of seedlings by the winter moth Operophtera brumuta, the green oak tortrix, Tortrix viridana (and other lepidopteran caterpillars which come down from the crowns of mature oaks before pupation) is one of the main reasons why oaks will not regenerate well under parent trees. Established trees are able to withstand repeated attempts at attack by producing chemical defences (tannins) in their leaves and because they produce several flushes of new leaves each year. For maximum growth and reproduction most caterpillars must complete feeding before any ­appreciable amounts of condensed tannin is laid down in the leaves. For this reason pedunculate oak, which comes into leaf about 1 week later than sessile oak in the New Forest, is attacked more easily by defoliating caterpillars. Tannin contents vary widely from tree to tree. Successful natural regeneration occurs when the canopy is opened rapidly once seedlings are established on the ground because they mostly avoid attacks by defoliators: hence, the use of the uniform shelterwood system is usually considered essential. It is practised impressively in parts of Normandy. Attempts to manage oak by selection or group systems, ­except by using very large groups, are doomed to failure. The concept of an intimately mixed, all-age climax oak woodland is impossible, contrary to the beliefs of many officials in County Naturalists’ Trusts who frequently

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attempt, and fail, to create such woodland. One of the major problems with oak regeneration in Britain is that people expect it to happen at no cost and with little effort. This is virtually impossible. Flowering, seed production and nursery conditions Pedunculate oak flowers early in May and sessile oak in late May. Acorns ripen between September and November, when they are ready for collection. The earliest age at which woodland-grown trees bear seed is 40–50 years, but the largest amounts are usually after the age of 80 years. Reasonable quantities are produced on average every 3–5 years and good ones at about 8-yearly intervals, or longer. Sessile oak is not such a prolific seed producer as pedunculate oak, possibly because it tends to grow mostly at higher, colder elevations and on rather infertile soils. A consequence of this has been that pedunculate oak has probably been planted on many sites where sessile oak would do better. Also, Jones (1959) stated that acorns used to be transported long distances (e.g. from Sussex to Cumberland) and often did not survive the journeys. The greater ease of storing and transporting pedunculate oak acorns, their larger size and perhaps their more frequent and copious production, all helped to favour pedunculate oak at the expense of sessile oak. With sessile oak there are commonly 300 acorns kg−1, 85% of which will normally germinate, while pedunculate oak produces larger acorns (250 acorns kg−1), of which 80% normally germinates (Aldhous and Mason, 1994). For nursery purposes the acorns should be stored in a cool, well-ventilated place from January onwards. If signs of shrivelling are ­observed, they should be sprinkled periodically with water until sown in late March. They can be sown in the autumn they were produced, in well-drained soils, but only if bird predation is unlikely to be severe: protection can be provided by covering the seedbeds with 7–10 cm of extra soil, which is removed in March (Aldhous, 1972). In some years it can be difficult or impossible to obtain seedlings from nurseries. The irregularity of seeding and the virtual impossibility of storing acorns in a viable condition for more than a few months are serious problems to those concerned with planting. Attempts at preserving acorns in a viable condition for long periods on a large scale have always come to nothing. Among those that have been reported the earliest was probably by Ellis (1768) who found that cleaned and dried acorns enclosed in bees’ wax remained viable for at least 1 year: the success of which, if properly followed, may in a few years put us in possession of the most rare and valuable seeds in a vegetating state from the more remote parts of the world, which in time may answer the great end of the improvement and advancement of our trade with our American colonies.

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This is a reference to the long duration of sea voyages in the time of sailing ships, when seeds lost their viability in transit. Because of the short period of viability and irregular production, a great deal of the oak seed used in Britain has been imported from continental Europe. Another attempt at long-term storage was made in 2017 (McCartan et al., 2016) by Forest Research. Their work to date indicated that it is possible to store acorns for 8 months and maintain at least 50% viability. Conservation The native oaks have 423 species of insects and mites associated with them, far more than any other British plant except the willows, which have 450 (Morris, 1974; Kennedy and Southwood, 1984). This accounts for the considerable value placed on oaks by everyone interested in nature conservation. Some of the invertebrates, such as the caterpillars of the Tortrix viridana moth and the winter moth, Operophtera brumata, have slightly ­adverse effects on the growth of oaks in occasional years by causing ­severe, if not complete, defoliation of trees soon after the first leaves flush in spring. Most invertebrates cause virtually no measurable damage to healthy trees. The caterpillars are immensely important sources of food for fledgling blue and great tits in spring and can be crucial to their survival (Savill et al., 2010). Provenance and breeding There has been comparatively little work on this subject in Britain. However, the most recent recommendation comes from Hubert (2005) who reported on two series of provenance trials established in 1990 and 1992. They contain continental and British provenances of both native oak species. In terms of height and survival, plants from British ‘selected’ (see pp. 76–77) stands have consistently shown good height growth and survival on most sites. Near-continental sources are average in performance, and Danish and eastern European provenances are poor. British nonselected seed has a variable performance. Scottish provenances grow slowly when moved south but are recommended on more-testing Scottish sites where frost damage is a risk. Much work on the adaptability of the oaks to climate change has been carried out by Kremer in France. Some of his main findings are summarized on pp. 4–5. The Future Trees Trust established a series of eight Breeding Seedling Orchards (Barnes, 1995) across Britain and Ireland in 2001 from the progeny of the best-formed, shake-free, sessile and pedunculate oaks that could be found in Britain, Ireland, parts of north-western France and The Netherlands. It will probably be at least 2030 before any useful

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quantities of improved seed come from these orchards. Even when in full production the quantities of seed available will be far too small to fulfil anything but a small fraction of demand. A means of cheap and easy multiplication, such as micropropagation from acorns, is needed urgently if improved oaks are to be made widely available. Unfortunately, this has not yet proved possible on a commercial scale. Area and yield Sessile and pedunculate oaks together occupy 219,100 ha or 8.3% of forest land in Britain, making them the third most common group after Sitka spruce and the birches (Forestry Commission, 2018a). Mean yield classes (YC) are low, averaging only 3–5 m3 ha−1 year−1 (Nicholls, 1981), with a maximum of 8. Rotations for sawn wood or veneer are long, commonly 120 years or more (but see Lemaire, 2010). The best veneer trees in the Spessart region of Germany are grown on 400-year rotations. For economic reasons oak is often grown with a productive conifer, such as Norway spruce or European larch, which will provide some financial ­return to the owner after 50 or 60 years, leaving a pure crop of oak for the remainder of the rotation. Timber and uses There is no reliable way to distinguish between the strongly ring-porous timbers of the two native oaks. It is durable, hard, strong, heavy and ­attractive, though rather difficult to work. Good oak timber can fetch extraordinarily high prices and is used for furniture, panelling, high-class joinery, veneers and barrel making. In earlier centuries it was the main shipbuilding timber and it also provided the beams for many of the great buildings in Britain, including York Minster and Windsor Castle, as well as framing for agricultural and domestic buildings. Its decorative appearance, particularly when quarter sawn (which shows the silvery medullary rays to best effect), made it the dominant timber used for high-quality furniture from 1600 to about 1680, when it began to give way to walnut. Its a­ ttributes of natural durability of the heartwood and great strength make it valuable for outdoor work such as fencing, gates and mining timber in the lower grades. In common with sweet chestnut, tannins in the wood (which confer the durability to oak) have a slightly corrosive ­effect on metals, and the wood becomes stained black when in contact with iron. For this reason, for indoor work, oak is always secured with wooden pegs, brass screws or other non-ferrous metals. The average density of the wood at 15% moisture content is about 720 kg m−3 (Farmer, 1972). Oak timber is characterized by having a large number of sapwood

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rings, usually ­between 20 and 30 (Savill et al., 1993). Sapwood is far less durable than heartwood, and in this respect oaks compare unfavourably with sweet chestnut, which has a somewhat similar timber but only 2–3 sapwood rings. The valuable veneer oak grown in the Spessart region of Germany sells for very high prices. Generally light-coloured oak is rare and, because of this, it fetches the highest prices. Narrow annual rings also confer greater value on logs. Only the best oak trees pay for themselves, and these can be very profitable indeed, but oak timber can be remarkably difficult to sell profitably if it has any of the defects to which it is remarkably prone. Knowles (1821) described three of them as ‘foxy’, ‘doaty’ and ‘quaggy’ oak: its tendency to be reddish in colour (foxy), stained yellow with blackish spots (doaty) or shaken (quaggy). Timber merchants believe that oaks with ‘stringy’ or ‘stripy’ bark are much easier to saw than those with ‘chequered’ bark. Oak also produces large, heavy branches and epicormic shoots. These features all present problems in succeeding in the modern quest for bulk, standardized materials. Even its natural durability is less of an asset today, when most timbers can be treated with preservatives quite easily. It has been known since at least the time of Theophrastus (371–286 BC) that oaks, like most broadleaved trees, should not be felled when leaves are on the trees because the wood is much more easily attacked by fungi and insects (Theophrastus, c. 200 BC/1916). The reason for this is that, during the growing season, the sapwood contains sugars and other carbohydrates which make it susceptible to attacks. These carbohydrates are translocated mainly to the roots during the dormant season. An immensely important historical product of oak, especially between 1780 and the late 1800s, was the bark, from which tannin for curing leather was extracted. Its use declined rapidly towards the end of the 1800s when imports of oak bark and bark of other species from the Continent, India (particularly Acacia catechu and Uncaria gambir), Brazil (Libidibia coriaria) and elsewhere replaced British oak (Brown and Nisbet, 1894; Tyler, 2008). Synthetic tanning agents, particularly chromium sulfate, were also introduced. Really good-quality oaks provide the preferred European timber for making barrels in which wine and whisky are matured. Work by Mosedale and Savill (1996) on variations in levels of tannins and oak lactones in the wood of oak found that sessile oak is characterized by lower levels of total phenolics (tannins) but, by a factor of 10, greater concentrations of oak lactones than pedunculate oak. These differences indicate that differences in flavour of whisky or wine that is matured in oak barrels are likely, sessile oak imparting the preferable taste. These results support the findings of earlier studies.

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Place of sessile and pedunculate oaks in British forestry Sessile and pedunculate oaks are the most characteristic and common native broadleaved trees. In spite of the many problems associated with growing valuable trees, they will undoubtedly continue to play a major part in British forestry. Because of climate change, much more attention than in the past needs to be devoted to matching the species of oak to the site where it is to be grown, to minimize the problems associated with acute oak decline.

QUERCUS RUBRA L.

Red oak

Origin Red oak has a wide distribution in the eastern USA and the extreme south-east of Canada. It is the most northerly of the eastern American oaks and grows from valley bottoms up to the lower- and mid-slopes of hills and mountains. It is a major species in the hardwood forests of eastern North America and was introduced to Britain before 1724 (Mitchell, 1974). Red oak is also an important exotic species in some European countries, especially The Netherlands, Belgium and northern France. It is the third most common species in Hungary, after hybrid poplars and Robinia pseudoacacia. An excellent book on the species and its silviculture has been produced in France by Timbal et al. (1994). Silviculture The species is hardy in Britain but, like the two native oaks, is occasionally damaged by late spring frosts. It will grow up to 35 m tall. It is far more tolerant of heavy clays and of base-deficient acid soils than the indigenous oaks and will make useful trees on these soils where no indigenous broadleaved species of equal value could be grown. It grows rapidly even on poor acid soils, though not on peats, and does best on acid sandy loams. It will not grow well on calcareous sites. The French believe it has a place on dry, acid sites for which there is no productive broadleaved species at present. It could, for example, be planted in some of the drier parts of lowland Britain on sites too poor for sweet chestnut and where atmospheric pollution is too high for conifers but where reasonably rapid growth is required. In common with pedunculate oak and sessile oak, the timber of this species has a reputation for being shaken. Assuming the causes of shake are the same (see p. 265), the use of red oak on well-drained and drought-prone sites is likely to produce timber that will commonly be shaken. On such sites red oak should probably be regarded as an amenity species and certainly not as one for producing saw timber unless shake-­ resistant varieties can be produced. Red oak is decidedly more shade bearing than Q. petraea and will form a good understorey in pine stands. It coppices well and is known for its crimson autumn colour for which it is, perhaps, most often planted. Unlike the native oaks, epicormic branches are not a serious problem, but it has a tendency to fork rather badly and so must be singled and pruned early. Frequent heavy thinnings are also considered necessary in France.

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Macdonald et al. (1957) stated that it is not a long-lived tree and will not grow to the dimensions of the native species. Natural regeneration Seed is produced best in the south of England, but even there it is not plentiful. Macdonald et al. (1957) believed that the failure to produce viable seed in adequate quantities is probably the reason why it has remained a minor species in Britain. Flowering and seed production The tree flowers in May; seeds, which take 2 years to ripen, are ready for collection in September and October. The earliest age at which the tree bears seed is 30–40 years. The greatest quantities are produced at intervals of 2–4 years from ages 50–80. There are about 280 seeds kg−1 (range 165–564), of which 80% will normally germinate. Provenance According to Forest Research (2018b) there are very few provenance trials in Britain and few forest plots; seed should be sourced either from southern Canada or from good stands in north-western Europe. In France, where it is an important tree, there is a large programme of selection and breeding. Pests and diseases Forest Research (2018b) states that records suggest red oak is susceptible to Phytophthora root rot. Armillaria root rot (honey fungus) may also kill it, especially if it is weakened by drought or other causes. Area and yield There are said to be about 700 ha of Q. rubra in Britain, though it is widely planted in many parts of western continental Europe. Growth rates are usually much faster than those found in the native oaks. In The Netherlands they range from yield classes (YC) of 3–9 m3 ha−1 year−1 (Bastide and Faber, 1972), up to 9.4 m3 ha−1 year−1 in Belgium

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(Laurent et al., 1988) and over 10 m3 ha−1 year−1 in Hungary (Rédei et al., 2002/2004). In Britain similar levels have also been found. Timber The heartwood is light to medium brown, commonly with a reddish cast. Quarter-sawn sections display a prominent ray fleck pattern. The grain is straight, with a coarse, uneven texture. The vessels are so large and open that it is possible to blow into one end of the wood, and air will come out the other end, provided that the grain runs straight enough (The Wood Database, 2018). Tyloses are absent. In Britain the timber is traditionally thought of as being inferior to that of the native oaks for furniture and decorative work, because its colour and texture is less attractive. It tends to be avoided by much of the British timber trade. It is nevertheless in demand in France and the USA for furniture making and is also used for flooring and interior joinery. Untreated timber is unsuitable for exterior work because of its lack of natural durability, though it can easily be impregnated with preservatives. It is at least as strong as the native oaks (FPRL, 1964). The average density of the wood at 15% moisture content is about 790 kg m−3. The Wood Database (2018) states: Arguably the most popular hardwood in the United States, red oak is a ubiquitous sight in many homes. Even many vinyl/imitation wood surfaces are printed to look like red oak. Hard, strong, and moderately priced, red oak presents an exceptional value to woodworkers—which explains why it is so widely used in cabinet and furniture making.

Place of red oak in British forestry Red oak is a species that is still of minor importance in Britain, even though it grows well and is quite widely planted by Britain’s continental neighbours. It is most commonly used for its spectacular autumn colour but may have a place on dry lowland sites, especially if climate change proceeds as predicted. Regrettably, on such sites, it is likely to suffer from shake, though a breeding programme could probably reduce this risk.

ROBINIA L. ROBINIA PSEUDOACACIA L.

Robinia, Black locust, False acacia

Robinia is in the Fabaceae family, which includes peas and beans. It is the third-largest family of flowering plants, after the Orchidaceae and Asteraceae, having about 19,000 species in 750 genera and constituting about 7% of all flowering plants. The family includes many important food and crop plants, including soybean, chickpea, alfalfa, groundnut and carob. Some species are ­invasive weedy pests in some parts of the world, including gorse, broom, lupin and robinia. The distribution of the Fabaceae is worldwide, except in Antarctica and the high Arctic. The trees are mainly found in tropical regions, and the herbaceous plants and shrubs are predominantly outside the tropics. Nodules form on the roots of many Fabaceae. These contain Rhizobium bacteria that enable the hosts to fix atmospheric nitrogen, making them ­essentially independent of soil nitrogen for growth. Trees in only one other genus growing in Britain, Alnus, fix nitrogen. Origin and introduction There are at least four North American species of robinia. R. pseudoacacia is native to the eastern and mid-western USA between latitudes 35° and 43°N, where there are two separate populations, one each side of the Mississippi valley. It was introduced to Europe by a French botanist, J. Robin, and is first mentioned in Britain in the 1640s. William Cobbett (1825) was a huge advocate of the species, describing it as ‘the tree of trees’, mainly on account of its very durable wood. However, his enthusiasm often outran his discretion (Macdonald et al., 1957). The tree did not perform nearly as well in Britain as he predicted. In natural conditions robinia is a pioneer on disturbed soils or burned sites, this being facilitated by its nitrogen-fixing abilities. In Europe, the tree is found at altitudes from sea level up to 1600 m; however, in many European countries, robinia is considered invasive (EUFORGEN, 2018b). Climatic requirements Robinia is noteworthy because of its ability to tolerate severe frosts: it is among the few nitrogen-fixing trees adapted to frost-prone areas. As a © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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forest tree it is really only suited to the milder and more sheltered parts of England, as it needs summer warmth to grow well. Site requirements It will grow on a wide range of soils with pH varying from 4.6 to 8.2 but does best on well-drained calcareous loams. Robinia grows reasonably well on dry and infertile sandy soils. Its main asset in Britain is as a nurse on such sites, especially those poor in nitrogen, such as strip-mined areas and sandy heathlands. It will tolerate drought and air pollution (Hanover, 1990) but not waterlogging or soil compaction (EUFORGEN, 2018b). It is essentially a tree for use in the lowlands. Other silvicultural characteristics In Britain robinia is a medium-sized tree, up to about 25 m tall. It has a reputation for being a low-volume producer with bad form, and so is unlikely ever to be grown as a timber-producing species in its own right. Like all nitrogen-fixing plants, the tree is a strong light demander. It is seldom planted in Britain today because of the nuisance caused by its prolific suckers and the unpleasantness of its vicious thorns (Macdonald et al., 1957). Root suckers grow particularly well from exposed roots following soil disturbance, so the species has a value in the control of erosion on slopes and in gullies. Its broad site tolerance and silvicultural properties have resulted in wide acceptance in parts of continental Europe. In Hungary it is now a species of major economic importance (Rédei, 2002), as well as in Romania and France, where it is valued both for timber and nectar production by beekeepers and also sometimes as a forage species (Keresztesi, 1978), though the young leaves in particular are said to be poisonous to humans and livestock (Voss, 2018). Robinia is now rivalling poplar as the second most widely planted broadleaved species in the world, after the eucalypts. Diseases In the USA young trees, particularly in the diameter at breast height (dbh) range of 2–12 cm, are badly attacked by the borer Megacyllene robiniae, especially on nutrient-poor sites, to the extent that in 1913 planting of the species was virtually given up. Thirty years later it returned to use because its tolerance to acid soils and its nitrogen-fixing properties made it ideal for planting on strip-mined sites, particularly in the Appalachians.

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Natural regeneration In its native habitat robinia is an important colonizing species in areas of disturbed forest. It is not long-lived but regenerates readily from seed, coppice and root suckers. The latter are particularly prolific after the parent tree is felled. Seed production and nursery conditions There are about 53,000 seeds kg−1, of which 70% will normally germinate. For nursery purposes seed pods have to be picked from the trees. In  common with those of many legumes, the seeds of robinia have impenetrable coats, which can delay germination. A satisfactory method of pre-treatment is to put the dry seed into a container with five times its volume of freshly boiled water and leave it to cool. Aldhous (1972) stated that this is more reliable than the alternative of soaking in cold water for 1 week. It should be sown at the end of March or early April. Provenance and breeding No provenance research has been carried out in Britain, but the species has been the subject of intensive breeding programmes in some countries, especially in eastern Europe, including Hungary. According to Hanover (1990), natural variation in numerous traits has often been observed and many cultivars have been described (including a popular garden variety, R. pseudoacacia ‘Frisia’). There is a high degree of polymorphism in the species, most of which resides within seed sources with low geographic variation. Clonal selection, early pruning and close spacing have been effective in producing straight-stemmed black locust in plantations, ­especially in eastern Europe. In Hungary, for example, a large array of tall clones is in commercial use (Keresztesi, 1983), based on seeds from trees originating from Long Island in New York State. There is clearly scope for testing some of these origins in Britain. Robinia is the source of the inaccurately named but much sought-after ‘acacia’ honey. In Hungary a breeding programme to extend the flowering period has resulted in the selection and recognition of several clones that show promise both for forestry and apiculture (abundant flowering, high nectar content and long flowering period). Growth and yield Growth, especially when young, is extraordinarily fast in some parts of the world. Trees can reach 3 m tall in one growing season and average 0.5–1.5 m

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height and 0.2–2 cm diameter growth per year. On fertile sites yields of more than 14 m3 ha−1 year−1 (9.5 dry t ha−1 year−1) have been r­ eported over a 40-year rotation with only moderate management. On poor sites, such as strip mines in the USA, oven-dry biomass yields range from 3.1 to 3.7 t ha−1 year−1 (Hanover and Mebrahtu, 1991). Timber The timber is naturally extremely tough (similar to ash), very durable and, according to Cobbett (1825), it is ‘absolutely indestructible by the powers of earth, air, and water’. This natural durability was a huge asset in the days before pressure impregnation of timber with preservatives. When ships were made of wood, they frequently rotted within 4 or 5 years of being completed, when made of many other timbers. In the 19th century it was used for shipbuilding in the USA. Robinia has an ­unusually narrow (ca 0.5 cm) sapwood. It is dense (slightly heavier than oak, at about 740 kg m−3 at 15% moisture content) and hard, which makes

Fig. 29.  False acacia, Robinia pseudoacacia.

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it suitable for outdoor work. The wood is usually straight grained and fairly coarse textured. It has a strong tendency to warp. The colour is naturally yellowish green. It has excellent properties for steam bending, and high-pressure steaming can change the colour to golden yellow, yellowish brown, light brown or dark brown. The larger logs can be used for veneers, and it is also used for structural wood and furniture. Place of robinia in British forestry Robinia could be a useful nurse or pioneer species on ‘stressed’ sites such as reclaimed areas and road cuttings where only subsoil exists on the surface, though several species of alder would normally serve this purpose just as well. Because of its thorns, it is a difficult tree to work with and is likely to remain one of minor importance.

SALIX L.

Willows

Species The Salicaceae is a large family, comprising some 54 genera and 1269 species according to the Plant List (2018). Salix is a genus within the family and contains about 35 species of trees, native to most of the northern hemisphere. Some 450 species of willow occur worldwide, most of them in the north-temperate and Arctic regions. It is generally agreed that 18 species and 27 interspecific hybrids are native to Britain and Ireland. Stace (2010) stated that 68 combinations of hybridization are known in the British Isles, of which 20 are hybrids between 3 species. This tendency to hybridize, ­especially among sallows (S. caprea), makes members of the genus very difficult to identify. Many willows are shrubs. The tree willows native to Britain are S. pentandra, S. caprea, S. fragilis and S. alba. Their main characteristics are shown in Table 17. None of them is of any importance as a productive forest tree, but numerous species and varieties of willows are commonly planted for ornament and a few for more specific purposes, such as for baskets and cricket bats. One of the most popular is the weeping willow, Salix × sepulcralis. S. alba ssp. caerulea is the cricket bat willow (see p. 285). Most willows are dioecious (i.e. male and female flowers are on separate trees). They are distinguished from poplars by their sympodial growth (i.e. apical meristems are terminated and growth is continued by one or more lateral meristems, which repeat the process). Poplars are ­almost always monopodial, with terminal buds that continue growing. In Salix, buds are protected by a single bud-scale, whereas in poplar there are several scales. So, while there is seldom any difficulty about distinguishing between poplars and willows, the situation with species is very different, especially with Salix. Salix is less amenable to subdivision than poplar, but most authorities (e.g. Meikle, 1984; Stace, 2010) accept that there are three subgenera: • Subgenus Salix (true willows), including S. alba, S. fragilis and S. pentandra. • Subgenus Vetrix (sallows and osiers), including S. caprea, S. viminalis and S. purpurea. These are mostly shrubs, but some can become small trees. • Subgenus Chamaetia (dwarf arctic or alpine willows). These subgenera are neither homogenous nor well defined.

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Table 17.  Characteristics of the native willow species in Britain (adapted from Michell, 1974 and Huss et al., 2016). Features shown in italics are particularly useful for identifying the species. Species

Tree size and form

Branches

Leaves

Flowers

Tree, 10–25 m with branches quite steeply ascending at 30–50°, forming a narrow, silvery-grey crown

Site requirements and distribution

2–5

Along lowland riversides and valleys. Less common than S. fragilis

3–4

Damp woodlands, scrub and hedges. Ideally where water table is high. A fast-growing pioneer

SALIX L.

Catkins on leafy 5–10 cm long and Tall, rather stalks. Males often at least 6 shapeless, yellow, females times as long ascending slender, green but as broad, finely main branches. soon fluffy white serrated and Shoots slender, with seed. Both covered with white light grey to 4–6 cm. (hence name) pink (see Fig. 30) silky hairs on both surfaces; petiole ca 5–8 mm Grey at first with long Male flowers Bushy, open Goat willow, Shrub, rarely expand to show hairs; soon smooth, crown with Pussy willow a small tree. silver hairs for shiny, deep low, ascending Salix caprea L. Thick, short, weeks before the red-brown. branches knobbly shoots dense stamens Variable in size and upright, emerge. In full and toothing. shrubby flower during Petiole dark red, growth March very hairy, 1 cm long White willow Salix alba L.

Flowering month

Continued 283

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Table 17.  Continued. Species

Tree size and form Tree, 10–25 m; branches spreading widely at 60–90°, forming a broad crown

Bay willow Salix pentandra L.

Tree to 10 m. Broad, low, domed crown, dense and dark in leaf

Leaves

Broadly conic crown with long, slender, upswept branches and widely spaced, rather pendulous leaves

Male catkins yellow, 4–5 6–15 cm and often 3–5 cm. Female about 7 times as green, 10 cm long as broad, usually asymmetric, twisted at apex, coarsely serrated, underside hairless (except when very young), paler green than top surface; petiole 1–2 cm with 2 small glands at top 5–7 Male catkins 10 × 4.5 cm, deep (the latest cylindrical, glossy green above, of all 2–6 cm on leafy, glaucous beneath. willows) hairy stalks Similar to, but smaller than those of S. fragilis

Site requirements and distribution Abundant beside lowland rivers in southern England, often pollarded to prevent stems splitting

Native in N. Wales and north of Midlands. Uncommon by rivers

SALIX L.

Crack willow Salix fragilis L.

Flowers

Flowering month

Branches

SALIX L.

285

Site requirements Willows are often the first woody plants to become established in wet habitats, and they can be valuable for stabilizing the banks of waterways. They tolerate the low soil oxygen levels around roots in wet places by producing numerous long adventitious roots, to increase the surface area for absorption. Like poplars, the roots can be troublesome near buildings, blocking drains and damaging paving. It is usually recommended that they are not planted within 40 m of such structures. Most tree willows require deep, rich soils for reasonable growth, but they are not long lived. They will grow on a wide range of soil types with a pH optimum of 6.5 and a range of 5.5 to 7.0. Water consumption of short-rotation coppice willow is high, like that of all fast-­growing crops, and can reach 4.8 mm m−2 day−1 in midsummer (Bassam, 1998). An annual rainfall of 600–1000 mm is considered ideal for willow growth (DEFRA, 2002) unless groundwater is available, as along riversides. Silviculture Most willows are quite short lived, at 50–80 years. During the establishment phase willows are more intolerant than many species to competition from weeds. Most of the pollarded willows that line the banks of English lowland rivers are S. alba. Female trees of S. alba ssp. caerulea have, since about 1820, provided the wood for making cricket bats (Warren-Wren, 1965, 1972). About 2000 ha of cricket bat willow are now grown in Britain on 12–18 year rotations, mostly in Essex (Evans, 1984). In common with poplars, they do best at wide spacings on stream and river banks where there is moving (aerated) water. Willows also have potential, though little-used, values in silviculture in being planted as ‘sacrificial’ trees in places where deer browsing and rubbing is a problem. Deer prefer willows to most other trees, possibly because of the salicin content of the bark; this, like aspirin, can relieve pain and irritation. They may benefit, especially when the felt on antlers is being shed. Planting them for this purpose can relieve the pressure on more commercially valuable species. Willows are hosts to more invertebrates than any other ­native trees: 450 species of insects and mites are a­ ssociated with them. They are just ahead of oaks, which have 423 (Kennedy and Southwood, 1984). Species of willow, especially S. fragilis and S. alba, are the woody winter hosts of a damaging aphid of carrots, Cavariella aegopodii (Jones and Jones, 1974).

286

SALIX L.

Diseases A potential threat to growing satisfactory trees for cricket bats is ‘watermark’ disease, caused by the bacterium Erwinia salicis. Infected trees wilt and die back, and the wood becomes brittle and unsuitable for the manufacture of bats (Preece, 1977; Patrick, 1991). It is controlled by propagating from disease-free parent trees and by the prompt destruction of infected trees. E. salicis is a problem that is not confined to cricket bat willow: it ­attacks other species of willow too. There is an ever-present risk that, as with poplars, foliar rust diseases caused by Melampsora spp. will kill, or at least severely reduce, the productivity of willow clones. This means that no single clone should be used ­extensively or for long periods. For example, in the 1980s S. aquatica gigantea was being very widely promoted in most countries where it would grow as a biomass crop (Pohjonen, 1987). It eventually became very susceptible to Melampsora and was abandoned as a species to be dealt with seriously. It is becoming increasingly obvious that no single clone can be used ­either extensively, or for long periods, because of risks of diseases. Most clones used for biomass production eventually become susceptible to a­ ttacks by rusts. A means of at least partially overcoming Melampsora attacks is to

Fig. 30.  White willow, Salix alba.

SALIX L.

287

establish plots that contain mixtures of clones. This can result in significantly greater dry matter than in monoclonal plots. Increases of 20% or more have been quoted for trials in Northern Ireland by Dawson and McCracken (1995) and McCracken and Dawson (1996). The increase was ascribed, at least in part, to reduced attacks by the rusts and delayed onset of attack, meaning that the build-up of the disease is reduced, and disease levels at the end of a season are significantly lower than in monoclonal stands. These benefits result from the influence of lower densities of susceptible plants, the barrier effect of disease-resistant plants and induced resistance owing to non-­ virulent pathogen biotypes. Unfortunately, recommendations for using clonal mixtures are not included in current planting guides for growers, such as DEFRA’s (2002) publication ‘Growing short rotation coppice: best practice guidelines for applicants to Defra’s Energy Crops Scheme’. Growth and yield One of the main differences between traditional coppice and modern, high-yielding, short-rotation coppice concerns the level of input or intensification. The latter requires regular fertilizer inputs (especially ­ ­nitrogen from the second year after establishment), herbicide and pesticide treatments to maintain the high yields. Intensive short-rotation coppice is a high-cost/high-return system, compared to the low-cost/low-return one in most traditional existing coppice. The dry matter content of winter-­harvested material is generally in the range 40–55% at harvest, but end-users generally need 70% dry matter, so it must be dried before use. Maximum commercial yields of short-rotation coppice in Britain average more than 10 oven-dry t ha−1 year−1, depending on planting density and genotype (Wilkinson et al., 2007). Experimental yields have reached more than 18 oven-dry t ha−1 year−1 using new genotypes, some of which are now commercially available (DEFRA, 2002). Propagation Almost all willows will root easily from cuttings, and this is the main method of propagating them. S. caprea is an exception, being less easy to propagate from cuttings than other species. The seeds of willows are recalcitrant, or highly perishable (Gosling, 2007). Timber, uses and area The wood of large trees is white, straight grained and light in weight at about 450 kg m−3 at 15% moisture content, though cricket bat willow is lighter at 340–420 kg m−3. It is not durable out of doors.

288

SALIX L.

The wood is used for purposes where lightweight, easy working and resistance to damage by impact are required: hence its value for cricket bats and at one time for the wooden parts of artificial limbs. Before the days of plastics and other similar materials willows were enormously important in the economy of the country, for the production of basketwork for numerous purposes: hurdles, screens and containers of all kinds. Their very pliant stems account for this use. The leaves were once used for animal fodder. Short-rotation coppice Willows make good bioenergy crops because their rapid early growth ­results in high yields in a few years. The ease of vegetative propagation and ability to re-sprout for many coppice cycles adds to their attraction for this purpose. First-year height growth after coppicing can reach up to 4 m. Members of the subgenus Vetrix (shrub willow) are used primarily for bioenergy production in short-rotation coppice systems, particularly clones of S. viminalis and S. dasyclados. Willows grown for short-rotation coppice are planted densely and harvested on a 2–5-year cycle after coppicing the growth from the first (establishment) year. Each stool produces up to 20 shoots and normally remains productive for at least 30 years (or around 10 coppice cycles). The recommended planting design for short-rotation coppice willow is to plant in double rows, with spacings of 0.75 m and 1.5 m between double rows. Within each row the plants should be spaced 0.59 m apart (Bullard et al., 2002; Tubby and Armstrong, 2002), giving a density of about 15,000 ha−1. Willows are intolerant to weed competition during the establishment phase. Broad-spectrum contact herbicides are usually recommended to remove perennial weeds prior to cultivation, and residual soil-acting herbicides can be applied after planting (Tubby and Armstrong, 2002). Following cutting at the end of the establishment year, the vigorous, dense re-growth of up to 4 m per year is sufficient to suppress subsequent weed growth. The total area of short-rotation coppice in about 2006, planted under all grant schemes, was said to be about 6000 ha, the great majority of which was willow. Place of willows in British forestry Willows have practically no place today in productive forestry in Britain, except as short-rotation biomass crops where they are used on a significant scale, and to contribute to biodiversity, mainly in upland forests.

SEQUOIA Endl. SEQUOIA SEMPERVIRENS (D. Don) Endl.

Coast redwood

Sequoia sempervirens is in the Cupressaceae family (see p. 113). Coast redwood, the world’s tallest tree, is a large, long-lived evergreen conifer, capable of exceeding 2000 years in age and 110 m in height (Fig. 31). Young trees generally have conical crowns, becoming columnar and more open with age. Bark in young trees is orange-brown and stringy, becoming darker, drier and more fissured with age. The mature foliage is deep green, hard and slightly prickly, with both scale and normal leaves, the latter pectinate (i.e. with parallel ridges), oblong, pointed and 10–15 mm in length. White stomata occur on the upper side of leaves, and along pale blue-white or green bands beneath. Terminal buds are slender, pointed and incurving with pinkish-green loose scales. Origin and introduction The species is confined to a 725-km strip along the coast: the ‘fog belt’, of central and northern California, with a minor northward incursion into southern Oregon, and it is rarely found more than 75 km inland. Coast redwood typically grows from sea level up to 750 m asl, only locally ­extending up to 900 m. It can form pure stands naturally, elsewhere growing in natural mixtures with Douglas fir, Sitka spruce, grand fir, western hemlock or Lawson cypress. Some 80,000 ha of protected old-growth coast redwood forest still survive, although there are a further 260,000 ha of commercially managed (largely second rotation) redwood-­ dominated forest. A further 300,000 ha of forest land falls within the natural occurrence range of this species. It extends with some breaks from 42° N in the extreme south-west of Oregon to south of Monterey in California at almost 36° N. Its occurrence is confined to areas within about 60 km of the sea, normally at elevations below 300 m. The species was introduced to Britain in 1843 (Mitchell, 1974). Climate and site requirements Its natural range is characterized by mild temperatures, quite high rainfall (620−3050 mm) (Morgan, 2009) and intense summer fogs, so that evapotranspiration is low. Within this range it grows mostly on soils derived from sandstones and best where these are overlaid by alluvial deposits, where it forms pure stands or occurs mixed with Douglas © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

289

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fir, Lawson cypress and other conifers. In Britain, coast redwood generally grows well on fertile soils in the moister, south-western regions (Macdonald et al., 1957), though impressive specimens can be found in most lowland parts of the country provided there is an adequate supply of soil moisture. It will tolerate neither waterlogged sites nor droughtprone ones. If soils are suitable the tree grows best in well-watered valley bottoms and on the lower slopes of valleys in south-western England and south Wales where precipitation exceeds 1250 mm. Sites to avoid are ­exposed areas, very acid soils, heavy gleys, peats and sites exposed to salty winds from the sea. It cannot tolerate serious atmospheric pollution. Rather like the southern beeches and the walnuts, a limitation to the use of coast redwood in Britain is its lack of cold hardiness. It is easily damaged by frosts and consequently can be difficult to establish in the open. The species must be approaching its climatic limit in some areas: for example, it seldom grows well in Scotland and is rare in most neighbouring parts of continental Europe. It is a species that could possibly be grown more widely in Britain with climate warming, not least because it produces a high-quality timber. Other silvicultural characteristics The tallest tree in the world is a coast redwood (at 115.6 m in 2011) in California’s Redwood National Park. Trees with heights exceeding 60 m are abundant in Britain and heights of 90 m are frequent. Early height growth is rapid, with 45 m achievable by 50 years and 67 m by 100 years. Bark in young trees is orange-brown and stringy, becoming darker, drier and more fissured with age. Coast redwood is reasonably shade bearing, in that it will not die even if quite heavily shaded, but neither will it grow. Like most trees, it needs almost full light to grow fast but is best established in forest conditions. It is considered to be a shade-tolerant, late-successional species. Young trees are sensitive to weed competition, so careful weeding is necessary until they begin to grow rapidly. The trees do not self-prune well, hence dead knots can be a problem in timber if pruning, preferably high pruning, is not carried out. A rare feature among conifers is the ability to produce coppice shoots, and redwoods do it with great vigour. On suitable sites, such as at Longleat in Wiltshire, stands can be renewed satisfactorily by this means. This coppicing ability and the thick, soft, fibrous red bark are adaptations to protect the tree from fires, which are common in the southern and eastern parts of its natural range. The bark can be punched without causing injury to one’s hand. The tree is also said to produce root suckers (Mabberley, 1990). Coast redwoods can grow almost as tall (120 m) as the theoretical maximum height a column of water can withstand without breaking.

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291

Growth and yield The main interest in this species is its potential for rapid growth and high-volume production, but this only happens on really good sites. According to Edlin (1966), it grows much faster than Sitka spruce on the best sites. One stand at Leighton in Powys had a standing volume of 2152 m3 ha−1 at age 94 and was said by Macdonald et al. (1957) to have the highest volume standing on the ground of any plantation in Britain. At 149 years of age, the stand had a cumulative volume of 3312 m3 ha−1, which far exceeds any other species shown in the Forest Management Tables (Hamilton and Christie, 1971). Table 18 gives more details and other examples. Natural regeneration Natural regeneration occurs quite freely in Britain, and the tree produces copious advance seedling regeneration under a forest canopy, but seedlings are easily injured by frosts. Unusually for a conifer, it is capable of some degree of vegetative reproduction, derived from the sprouting of old cut stumps or layering of fallen logs of more mature specimens. These means of vegetative ­reproduction can form a very significant component of overall natural ­regeneration. Natural regeneration of coast redwood seedlings can also be secured from around 50 years of age, but this may well include a significant component of root-crown and stump sprouts. Flowering, seed production and nursery conditions In their natural range redwoods start to bear seeds at age 5–15 years (Burns and Honkala, 1990), though little of it is viable at this early age. Flowering in Britain takes place in February, but the male flowers are often killed by frost. One reason why redwood is seldom planted in Britain is that it is difficult to raise plants in any quantities from seed, ­because often well under 10% is viable. This may be because frost and wet weather conditions, which are common during the flowering period, prevent pollination. Continuous rain during flowering washes pollen from the male strobili and little is therefore available to reach the receptive ­female strobili. Dry periods during flowering permit better pollen dispersal and improve seed viability. Morgan (2009) stated that seed collected from the tops of trees (which is very expensive) has more than 90% viability, presumably because flowers in the treetops have a better chance of being pollinated. Plants can be raised from cuttings taken from young trees, without much difficulty, and also by micropropagation.

292

Site Brechfa, Wales Forest of Dean Leighton Park (1) Leighton Park (2)

Age (years)

Trees ha−1

Top height (m)

Mean dbh (cm)

Basal area (m2 ha−1)

Standing volume (m3 ha−1)

Cumulative volume (m3 ha−1)

General yield class

 53  56  77 149

215 266 162 196

30.1 37.7 41.2 37.3

 66.1  55.1  88.2 128.2

 73.8  63.4  99.0 253.1

 865  865 1489 3312

1785 1757 2554

30 28 26 24

dbh, diameter at breast height. The empty cell indicates that this plot was never thinned, so the standing volume was the same as the cumulative volume.

SEQUOIA SEMPERVIRENS (D. Don) Endl.

Table 18.  Selected data for coast redwood from Forest Research sample plots in Britain.

SEQUOIA SEMPERVIRENS (D. Don) Endl.

293

Provenance According to Forest Research (2018b), only very limited provenance testing has been carried out in Britain, but more northerly provenances are likely to be more cold hardy. However, the original geographic origin of older planted specimens in Britain is frequently unknown or cannot be reliably established from surviving records. Timber The heartwood turns red soon after felling (hence the name of the tree), with the red colour ranging from light pink to maroon. The greatest virtue of the wood is its dimensional stability in service. It is light, with an average density of about 420 kg m−3 at 15% moisture content, and is non-resinous. The timber has a reputation for being strong, and the heartwood, especially darker-coloured heartwood (but not the sapwood), is usually naturally durable, because of its very high tannin content. Its main drawbacks in use are said to be its softness (Gale, 1962), the tendency for unpruned trees to contain many dead knots that fall out after sawing and for the timber to splinter unless really sharp tools are used. It also dents easily. The tree often produces burrs, some of which can be valuable. Apparently ‘curl’ is the most common figure. Morgan (2009) stated that the central parts of logs grown in Britain tend to produce the soundest heartwood without dead knots, but would also include more of the less-desirable j­uvenile core. In its native range the trees have supported a large timber industry. Coast redwood is used in America in house building and, because of its durability, for such things as shingles, cooling towers, silos, greenhouses and farm buildings. The fact that there is no resin in the timber, unlike that of many other conifers, makes it less of a fire risk in buildings. The best timber is used for panelling, cabinet making, decks and hot tubs. The timber is virtually unknown to the conservative British timber trade and consequently is not sought after. Place of coast redwood in British forestry In 1966 Edlin said that most British foresters exclaim that the coast redwood is ‘a tree of great promise’ but that is as far as it ever goes. The situation is unchanged more than half a century later. It is still a minor species: one that is planted more for ornament than for production. Its main potential values in forestry are its great productivity, and the dimensional stability and durability of its timber. Many misconceptions about the species exist in Britain, partly because it is often confused with Wellingtonia or giant sequoia, Sequoiadendron giganteum, a tree with much less sought-after timber; and partly because coast redwood is known as the ‘giant sequoia’ in the USA.

SEQUOIADENDRON Buchholz SEQUOIADENDRON GIGANTEUM (Lindl.) Buchholz Wellingtonia, Giant sequoia, Giant redwood Sequoiadendron giganteum is in the Cupressaceae family (see p. 113). Origin and introduction The name by which the tree is known in Britain is in honour of the first Duke of Wellington (1769–1852), Commander-in-Chief of the British Army and twice Tory Prime Minister. The species was introduced to Britain by William Lobb (for the Veitch Nurseries in Exeter) in 1853, a year after the Duke of Wellington’s death, having been discovered in 1850. It occurs naturally in a number of scattered locations along a 420-km strip that is no more than about 24 km wide, extending along the western slope of the Sierra Nevada in central California. It is found in middle-­ elevation mixed coniferous forests. Climate and site There is very little experience of growing this tree in Britain except as specimens in parks and gardens. Macdonald et al. (1957) stated that on limited evidence it is less badly affected by exposure and frost than the coast redwood and that it will grow in most parts of Britain. It is slightly more tolerant to atmospheric pollution than the coast redwood. It will grow on most soils in Britain except wet ones. Wet acid peats, in particular, should be avoided. It grows best in deep, well-drained sandy loams, in places where underground water is available to maintain the tree during dry periods. Adequate soil moisture is critically important for successful establishment, survival and growth. In its native range fully grown trees are, therefore, often concentrated near streams. Except for the presence of moisture, soil apparently plays only a minor role in influencing the distribution of the species, judging by the considerable variability in parent material in natural stands, and the fact that Wellingtonia grows vigorously when planted in diverse soils around the world. In its native home Wellingtonia grows in what is described as a ‘humid climate, characterized by dry summers’ (Weatherspoon, 1990) and where most precipitation falls as snow in winter. Low temperatures limit its upper elevational range, as do severe winters in regions where

294

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

SEQUOIADENDRON GIGANTEUM (Lindl.) Buchholz

(A)

295

(B)

Fig. 31.  (A) Coast redwood, Sequoia sempervirens; (B) Wellingtonia, Sequoiadendron giganteum.

the species has been introduced. Its distribution at lower elevations is limited mainly by deficient soil moisture during the growing season. Other silvicultural characteristics The Wellingtonia has never gone beyond the trial-plot stage in Britain, but it is quite widely planted in parks and gardens. Losses can be high after planting. The tree is a strong light demander throughout life, being less tolerant of shade than the coast redwood. Height growth of seedlings is slow during the first few years if there is competition for light and moisture, but seedlings in the open perform well and can outgrow any associated tree species provided weed control is thorough. Height growth averaging 80 cm a year for the first 10 years is common. Wellingtonia normally regenerates by seed. Trees up to about 20 years old may produce coppice shoots after an ­injury, and trees of all ages may shoot from the stem when old branches are broken, but unlike Sequoia sempervirens mature trees do not coppice from cut stumps or grow from root suckers. Wellingtonia bark is fibrous, furrowed, non-resinous and may be 90-cm thick at the bases of the trunks of big trees. It provides significant protection to the cambium from fires. Lower branches are killed fairly easily by shading, but the trees tend to retain most of their dead branches,

296

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so if timber production is an objective, pruning is necessary to avoid dead knots being included in the wood. Even very large trees have a reputation for being extremely wind firm. Accounts of the species and its ecology have been written by Hartesveldt et al. (1975), Harvey et al. (1980) and Burns and Honkala (1990). It is described as a fire subclimax tree that is relatively resistant to fires, which set successions back to an earlier stage. In its native habitat it grows with Abies concolor var. lowiana, A. magnifica, Pinus ponderosa and other species. Diseases Heterobasidion annosum and Armillaria mellea can be serious root pathogens and sometimes kill trees. Natural regeneration There is no experience of this in Britain. Flowering, seed production and nursery conditions The tree flowers in March and April in Britain. Cones bearing fertile seeds have been observed on trees as young as 10 years of age, but the large cone crops appear much later. They mature in 18–20 months. The cones are serotinous and typically remain on the tree, green and closed and ­retaining viable seeds, for up to 20 years. They also continue to photosynthesize and their peduncles grow, producing annual rings that can be used to determine cone age. Estimates of percentage germination of seeds removed from green cones range from about 20% to 40%. In natural conditions some seeds are shed when the cone scales shrink during hot weather in late summer, but most seeds are liberated only when the cone dries from the heat of a fire or when damaged by insects. Hot air produced by a fire dries cones, resulting in the release of enormous quantities of seed over small areas. The fire creates a seedbed that is favourable for seed germination and seedling survival. Seed dormancy in a soil seed bank has not been observed in Wellingtonia (Burns and Honkala, 1990). The most important requirement for germination is an adequate supply of moisture and protection of the seed from desiccation. Cuttings from juvenile material root quickly and mostly without difficulty. As with most species, only limited success has been achieved from rooting cuttings of older trees aged 30 years or more.

SEQUOIADENDRON GIGANTEUM (Lindl.) Buchholz

297

Provenance Provenance tests in Germany have shown differences in cold hardiness and early growth among populations (Weatherspoon, 1990). No trials have been carried out in Britain. Yield The species is famous for the huge dimensions and great age to which it grows: the biggest tree, located in Sequoia National Park, California, is known as the ‘General Sherman’ tree and is estimated to be 2500 years old. Its height to the top of the trunk is 83.8 m, and its volume is estimated at 1487 m3 (Burns and Honkala, 1990). It is often claimed as the tree with the greatest volume in the world. This volume is slightly greater than the cumulative production of a hectare of yield class (YC) 20 Corsican pine at age 80. The Wellingtonia has been planted widely and often successfully in many parts of the world, according to Weatherspoon (1990). As in California, plantations on suitable sites outperform most other species. An 80-year-old Wellingtonia plantation in Belgium, for example, grew at an average annual rate of 36–49 m3 ha−1. Many foresters see considerable potential for Wellingtonia as a major timber-producing species of the world. Its performance in three parts of Britain, in plots managed by Forest Research, is shown in Table 19. Timber and uses The wood is one of the lightest coniferous timbers, at about 340 kg m−3 at 15% moisture content, but is extremely durable. It is dismissed by Patterson (1988) as being ‘not useful commercially’. This is possibly a view typical of the British timber trade. In the USA the wood from old trees has the reputation of being ­fibrous, brittle and having a low tensile strength and so unsuitable for construction; but immature trees are less brittle. Recent tests on young plantation-grown trees have shown the timber to have similar properties to those of coast redwood, including an absence of resin. This is resulting in some interest in cultivating Wellingtonia as a high-yielding timber tree, both in California and also in parts of Western Europe, where it may grow better than coast redwoods. Outside its natural range, both in the USA and in many other countries, Wellingtonia is highly regarded as an ornamental and shows promise as a major timber-producing species.

298

SEQUOIADENDRON GIGANTEUM (Lindl.) Buchholz

Place of Wellingtonia in British forestry Wellingtonia is known more for the great sizes and ages to which it will grow than for its use as a timber tree. It is unlikely to be planted for more than ornamental purposes in Britain, though it could be a major timber-­ producing species.

Table 19.  Data selected from Forest Research sample plots of giant redwood in Britain.

Site

Top Mean Basal Standing Cumulative General area Age Trees height dbh volume volume yield (years) ha−1 (m) (cm) (m2 ha−1) (m3 ha−1) (m3 ha−1) class

Brechfa,  40 Carmarthenshire Bedgebury,  38 Kent Leighton Park, 120 Powys

701 21.4 37.7

78.4

563

 868

18

541 22.0 39.6

66.5

594

1062

18

 54 40.7

65.2

622

dbh, diameter at breast height.

124

20

SORBUS L. Until relatively recent years, three native species in the traditional genus Sorbus were recognized in Britain: S. aria (whitebeam), S. aucuparia (rowan) and S. torminalis (wild service tree). In 1991 Robertson et al. argued that the genus Sorbus should be divided into five genera: Sorbus, Cormus, Chamaemespilus, Torminaria and Aria. Sorbus itself now contains only species with pinnate leaves (see International Dendrology Society, 2018). As a result, two of the three British species have been moved to other genera: S. aria has become Aria nivea, and S. torminalis is now Torminaria torminalis. They are dealt with under those genera in this book. There are also several native but very rare British and Irish species in addition to these three. They occur only in restricted localities in south-western England, Wales, Arran and Ireland. Many are difficult to separate from each other. They are believed to be the result of hybridization events between the three common species that have led to the formation of new species (Robertson et al., 2004; Chester et al., 2007).

SORBUS AUCUPARIA L.

Rowan, Mountain ash

Distribution Rowan is native to British woods, scrub and mountain areas, and will thrive at elevations up to almost 1000 m on sites where few other broadleaved trees will grow at all. Rowan can easily be distinguished from ash by its alternate leaves and the serrated margins of the opposite leaflets. Climate and site The species is present in most of Europe, from Iceland and north (not arctic) Russia to the mountains of central Spain and Portugal, Corsica, Italy, Macedonia and the Caucasus (Raspé et al., 2000). It is found throughout Britain but most commonly on lighter soils in the north and east. It is rare in much of lowland England (Clapham et al., 1985) and usually absent on clays and soft limestones. Rowan is one of the least site-demanding trees. It is cold hardy, frost tolerant and can withstand severe exposure. It grows best on soils of poor-to-medium nutrient status and of slightly dry to moist moisture status. Wet soils should be avoided, but it is found on more infertile soils and at higher elevations (to over 900 m) than whitebeam. © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Silviculture Rowan is a small tree that seldom grows more than 15 m tall and with a lifespan maximum of 150 years (Raspé et al., 2000). In common with whitebeam, it is a colonizing, light-demanding species that can be found in the establishment phase of almost all kinds of woodlands in Britain and in open areas. It is a reasonably strong light demander and coppices well. It is palatable to browsing animals such as deer, and this causes multi-stemmed trees to be common. An aphid, Rhopalosiphum insertum, ­migrates from rowans in summer to the roots of oats, other grasses and reeds, and can cause some economic damage. Natural regeneration The seeds of rowan are disseminated by birds, and natural regeneration (though never prolific) is reasonably common. The trees also produce suckers from the roots, like several other trees in the Rosaceae. Flowering, seed production, nursery conditions and genetics Rowan is a self-incompatible species, which makes it an obligate out-crosser. The tree first flowers in May and June at about age 10 years, and is insect pollinated. It produces plentiful seed from age 15, either every year or every other year. The fruits ripen between July and August, and seed treatment for propagation in the nursery is the same as for whitebeam. There are about 5000 seeds kg−1 of fruit. In natural conditions, dispersal is by birds. Bacles et al. (2004) carried out a fascinating piece of work involving the genetic variability of rowan in relation to habitat fragmentation in the severely deforested landscape of Moffat Dale in the southern uplands of Scotland. They showed that, contrary to popular belief, high levels of genetic diversity have been maintained in this landscape. Values are only marginally lower than those found in non-fragmented continental European populations. It is believed that seed dispersal of rowan between populations by birds is very effective and that the interpopulation dispersal of seeds by birds is actually enhanced by habitat fragmentation. This is because birds feeding in small remnants are likely to move regularly ­between these isolated populations in the landscape, as seed supplies at any one site quickly become exhausted. Provenance There is no knowledge of provenance variation in rowan, so seed should be sourced from British stands of good form wherever possible. This view

SORBUS AUCUPARIA L.

301

is supported by Worrell (1992) who speculated that, since rowan was an early arrival in Britain after the Ice Age, the survival and growth of continental provenances might therefore be expected to be much poorer than British provenances, particularly when planted in the north and west of Britain. Timber and uses The pale yellowish-brown sapwood and deeper-brown heartwood is strong, hard and tough but not durable out of doors. The wood takes an excellent finish, and because of its hardness, it dulls tools quickly. The density of the wood is about 670 kg m−3. The tree does not grow big enough to provide large-diameter logs but is sometimes used in turnery, furniture, craftwork and – in earlier centuries – for tool handles and ­engraving. It is similar to apple wood. Occasional larger-diameter stems can be valuable. The unripe berries of Sorbus contain sorbic acid. Synthetic forms of it are widely used in the food industry as preservatives, protecting food from mould and other infections (Drori, 2018).

Fig. 32.  Rowan, Sorbus aucuparia.

302

SORBUS DOMESTICA L.

Place of rowan in British forestry Today rowan is best known for its ornamental value both in wild, upland parts of Britain, and in the form of numerous named cultivars in towns and gardens, where it has the additional virtue that it does not grow too big. It produces numerous, attractive, small white flowers in May and handsome scarlet fruits in the autumn.

SORBUS DOMESTICA L.

True Service tree

Origin and introduction S. domestica is native to western, central and southern Europe, North Africa (Atlas Mountains) and South-west Asia (east to the Caucasus) (EUFORGEN, 2018b). It appears to be very similar to rowan but is, in fact, very rare in Britain and everywhere else that it occurs, being found mainly in a few parks and large gardens (Mitchell, 1974). It was thought not to be native to Britain until Hampton and Kay (1995) found two long-established populations on a sea cliff site in south Glamorgan. The tree’s exact natural range is unclear as it has been cultivated and distributed by human beings since Roman times. Climate and site The service tree grows best in fresh and rich soils. Because it is a weak competitor with other species, it is usually found on warm, poor and sometimes extremely dry sites. The tree is winter hardy but does best in mild climates: in wine-producing areas, according to Huss et al. (2016). In central Europe, the tree occurs on south-facing slopes below 650 m, whereas in the Mediterranean it is found at higher elevations. Silviculture S. domestica will grow up to 15–20 m tall, rarely to 30 m. It frequently adopts a shrubby habit. The tree is light demanding and thus a weak competitor in mixed stands (like the wild service tree, Torminaria torminalis). The true service tree is light demanding and drought tolerant. The species has a reputation for being difficult to establish and young trees are prone to stem cankers that may potentially kill them. Weed control is necessary at regular intervals until grass and other weeds no longer represent a problem for the true service trees or until canopy closure.

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Service trees are liable to be severely browsed by deer if unprotected. They produce suckers quite freely, like the wild service tree. Timber and uses The tree’s dense and tough wood is suitable for special products such as mechanical parts, measuring sticks, inlays and instruments; however, wood production is very small due to its low abundance. It consistently fetches high prices. As with other Sorbus species, the fruits of the service tree are an important source of food for various birds and animals. The fruits are also good for making jams, juices and alcoholic drinks, and are used in traditional medicine. The tree is valuable as an ornamental: the spectacular autumn foliage contributes substantially to the attractiveness of the forest and is sufficient reason for planting some true service trees (Skovsgaard, 2017a).

TAXUS L. TAXUS BACCATA L.

Yew

Yews (genus Taxus) are in the family Taxaceae, which contains six genera and 31 species according to the Plant List (2018). There are nine species of Taxus, all of which are very closely related: so closely that they are sometimes regarded as varieties of a single species. Interest in T. baccata and other species of Taxus has increased enormously since about 1990 at the conclusion of a series of successful clinical trials for treating various cancers with the alkaloid commonly known as taxol. The chemical is found in all parts of the tree except the aril. Taxol was considered to be one of the most promising anti-­ cancer drugs to be discovered in 20 years. This led to a great deal of destruction of wild yew trees. Initially T. brevifolia (the Pacific yew) in the western USA and Canada was targeted, and soon afterwards, T. baccata. Tissue culture is now an alternative means of taxol production, but awareness of the vulnerability of the species has been raised, and major conservation efforts are now in progress. Yew is regarded as an endangered species in much of continental Europe because intensive land use, ­including forest management, has caused a decrease in populations of the species. Diameters well in excess of 3 m have been recorded on some truly spectacular trees. This makes them favourite subjects for talented photographers such as Thomas Pakenham (1996). The tree has a reputation for living to a greater age than any other species that will grow in Britain. Individuals well in excess of 1000 years old are known. Much of what is written below comes from an excellent review of the species by Thomas and Polwart (2003). Origin There are nine closely related species of yew in the north-temperate ­region. T. baccata is native to Europe, including the British Isles, the Atlas Mountains and from Asia Minor to Iran. In the British Isles, it grows from sea level to 425 m in England and Wales, but is not considered indigenous to Scotland. Within Britain it is native to the chalk downs in the south of England, limestone in the north and oakwoods on other soils. Individual yew trees are widespread, occurring naturally in woods and on cliffs. It has been extensively planted as an ornamental and in churchyards for many centuries but is never very common. 304

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Climatic requirements Yew is a species of the lowlands and middle hills. It grows best in the high humidity of mild oceanic climates. In England it particularly thrives in areas with relatively mild winters, abundant rainfall and where mist frequency is greater than average, as in the coastal regions of Hampshire and Sussex and lowland parts of the Lake District. Conversely, severe winter cold and drying winds in the spring appear to restrict yew growth on ­exposed sites. Yew is intolerant of severe and prolonged frost. Sensitivity to frost in early spring limits its northerly oceanic distribution in Europe. It is tolerant of drought. Yew will withstand smoke and pollution, and has a high resistance to damage by sulfur dioxide, so it will grow in towns, unlike most other evergreen conifers. It is, however, sensitive to air-borne salt spray, which kills the foliage.

Site requirements Yew will grow on most soils so long as they are not waterlogged. The species sometimes forms pure woods in sheltered places on chalk in the south-east of England and on limestone in the north-west. The yews in Kingley Vale in West Sussex are described by Natural England as constituting one of the finest yew forests in Western Europe. It includes a grove of ancient trees that are among the oldest living things in Britain. Stands of yew are typically associated with limestone slopes carrying shallow dry rendzinas (Rodwell, 1991). It does better in drier, more ­exposed conditions than are suitable for beech. Yew grows equally well on thin, warm chalk soils, limestone pavement and well-drained calcareous fen peat. Along valley bottoms yew woodland may extend on to somewhat deeper and moister soils. Yew is generally absent from wet soils, such as wet acidic peat and wet clay.

Other silvicultural characteristics Yews are not tall trees and will occasionally grow to a maximum of about 25 m, but in terms of diameter growth and longevity they have few equals. In a suitably oceanic climate, yew is gregarious and often forms pure, dense, even-aged stands. These are of ‘striking floristic poverty and uniformity’ especially on warm, sunny, south-facing slopes over shallow limestone soils (Rodwell, 1991). In more continental conditions it increasingly becomes an isolated understorey tree. It grows slowly, which is why

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Fig. 33.  Yew, Taxus baccata.

it is not planted commercially; and, unusually among British trees, it is very shade tolerant. Yew typically has a short stem that is deeply fluted. Owing to its thin bark, it can be badly damaged or killed by fires. Old trees are able to reproduce vegetatively by branches touching the ground (due to their weight) and producing adventitious roots. These can survive independently after the death of the main trunk. Yews will also coppice and produce plentiful epicormic shoots from stems and branches. Diseases Yew is unusually susceptible to fungal infections if roots are damaged and can be killed by Phytophthora spp. Natural regeneration Yew seed is disseminated by birds, which eat the red arils surrounding the seeds. Regeneration appears quite frequently as occasional understory seedlings, though it is never common. In mixed yew-rich, beech–ash woods with an understorey of yew, rapid growth can be promoted by felling the overstorey (Forestry

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Commission, 1994). The yew will then be able to outcompete hardwood regeneration and dominate. However, once yew woodland is cut, it is more likely to regenerate to ash or birch in the first instance, with yew as a scattered understorey; thus management should be restricted only to ­occasional harvesting of yews. It is difficult to establish successfully under a beech canopy. Flowering, seed production, nursery conditions and genetics Yew can be propagated artificially from seed and cuttings, by grafting, layering and micropropagation. It is dioecious (i.e. there are separate male and female trees) and reaches maturity relatively late (ca 70 years of age). It flowers in the spring, producing minute male catkins. The fruit is a hard seed, partly embedded in a pulpy, conspicuous and bright red, berry-like aril. Seed should be collected from the female trees in November, the arils removed by crushing and the seed stratified for 16 months before sowing in March (Aldhous, 1972). There are about 4400 seeds kg−1, which can be stored for 5–6 years by drying at room temperature to about 10% moisture content and then storing at 1–2°C. Working on Scandinavian populations of yew, Myking et al. (2009) demonstrated distinct differentiation, partly caused by inbreeding and lower variation than central European populations. These characteristics are typical of marginal populations of a tree species and possibly apply to British populations too. It appears important not only to ensure regeneration for maintaining the population sizes but also for promoting natural selection and adaptation to future climates, and to minimize genetic drift and associated differentiation in the long term. Provenance No provenance research has been carried out on yews in Britain. Many cultivars and varieties are recognized, of which perhaps the best known is the fastigiate Irish yew, T. baccata ‘hibernica’. It was originally discovered in 1780, as a female tree, at Florence Court in Co. Fermanagh, and has been widely propagated as an ornamental from cuttings since then. In 1927 male fastigiate yews were discovered in Sussex. Yew as a poisonous tree Yew has been directly implicated in the deaths from heart failure of ­humans and other mammals. Cattle, and especially horses, can suffer quick and fatal effects from eating yew twigs, bark or foliage.

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All parts of the yew tree are potentially poisonous except the arils. This is caused by the presence of large quantities of toxic alkaloids, primarily taxine, which are found in the seeds, bark and foliage. However, despite popular opinion and the views of early writers, yew is not invariably toxic to mammals. In fact, the tree is susceptible to browsing and bark stripping by rabbits, hares, deer and domestic animals including sheep, goats and sometimes cattle. Grey squirrels will browse on bark and twigs. Yew has also been used as an animal fodder in continental Europe and elsewhere. It seems that animals can develop at least a partial immunity to yew if regularly fed small quantities. Small quantities and habitual access seem to be the key, because there are many records of cattle browsing freely and with no ill effects in pastures with yews, if plenty of other types of fodder are also available. Animals let into fields with yew in winter have been recorded as dying, presumably because yew was the only green food readily available. Access to cut branches has led to many domestic animal deaths. Dried or wilted foliage is reputed to be even more poisonous than fresh foliage but, according to Cooper and Johnson (1984), they are ‘as toxic as the fresh plant’. Growth and yield The growth of yew is slow compared to most other trees, even under ­optimum conditions and after eliminating potential competitors. Even the oldest individuals do not attain great heights. Part of the reason for the slow growth is the high energy investment in defence, which increases the resistance of the wood to fungal and insect attacks. Timber and uses Yew produces a strong, decay-resistant, non-resinous wood. It is among the densest of all coniferous timbers, at least 670 kg m−3 and up to 800 kg m−3. This is high for a gymnosperm, being similar to beech and oak. The wood is one of the most durable of temperate timbers. It is hard, heavy, flexible and impermeable. It is famed as having been the main timber used for making longbows: its suitability for this purpose arises from the prominent spiral thickening of the tracheids, which give the bows their essential elasticity (Sporne, 1974). Apart from its historical value, yew is used for many purposes, from knife handles to expensive and decorative furniture, and veneers for panelling. The freshly cut heartwood is bright tan to red-brown or purple, becoming a warm brown or golden brown upon exposure and with age. Patches of dark purple, mauve and brown streaks, together with tiny knots

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and clusters of in-grown bark, combine to give the wood an attractive appearance. Large-dimensioned material is not readily available because most trees are small to medium-sized and are of poor form. Supplies of veneer-quality wood are also limited and are consequently expensive when available. Yew burrs are rare but, when obtainable, they are suitable for conversion into high-class veneers. The wood has a reputation for being extremely stable when dry. The best timber is said to come from trees that have grown on lime-rich soils. Yew would not be nearly as common as it is, were it not for its tolerance of repeated pruning, as demonstrated by its widespread use in topiary and hedging. Unlike other coniferous hedging plants used in Britain, its ability to produce epicormic shoots after a severe cutting means that bare stems soon become green again. Place of yew in British forestry Despite the many uses of the wood, including for fine furniture, it is unlikely that any yew woodlands have been deliberately planted for timber production in the past or that they will be in the future. About half the yew woodlands in Britain have been designated Sites of Special Scientific Interest.

THUJA L. THUJA PLICATA Donn ex D. Don

Western red cedar

Thuja plicata is in the family Cupressaceae (see p. 118). Origin and introduction There are five species of Thuja distributed in Eastern Asia and North America. Western red cedar is native to western North America, from Alaska (at 56°30′ N) to California (40°10′ N) and from the Pacific coast eastwards to western Montana and Idaho (Minore, 1990). It extends mostly from sea level to 1800 m, though above 1500 m it is a low shrub. This species occurs in both maritime areas of the Coastal, Cascade and Olympic Mountains and further inland in the central Rocky Mountains. Coastal populations of western red cedar occur from sea level up to 1200 m asl, whereas interior populations typically occur from 320 m to 2300 m asl, a wide elevational range. Western red cedar is a major constituent in Douglas fir forests on the banks of watercourses and in marshy valley bottoms. It was introduced to Britain in 1853. About 80% of the standing volume is considered to be in Canada (British Columbia), with the remaining 20% within the USA. Western red cedar has been used as a plantation species in several parts of the temperate world, including Europe. Climatic requirements Generally, the species is most productive in regions with warm, moist, temperate climates, avoiding extremes of wind exposure, summer drought or winter cold. Forest Research (2018b) advises that the species is most suitable for intermediate climates with rainfall in excess of 800 mm, which would encompass most of mainland Britain away from very dry lowland areas in the east. In its natural range western red cedar is restricted to areas with abundant rainfall or snow, high humidity and cool summers. It can withstand low winter temperatures and is moderately resistant to late spring frosts. Western red cedar does not tolerate exposure well but is nevertheless reasonably windfirm on dry sites, but not on wet ones. It can withstand mild maritime exposure but not urban pollution. In Britain it does best on relatively sheltered sites, such as the lower slopes of valleys, in regions with at least 800 mm rainfall (especially at low 310

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elevations in the west) but it grows acceptably well where precipitation is lower. It should not be planted at elevations much greater than 200 m, and lower in Scotland and the north of England. According to the distribution map produced by the Botanical Society of the British Isles (BSBI, 2012), western red cedar is much more common in southern England and Wales than elsewhere in Britain. The use of western red cedar on sites with a detailed aspect method of scoring (DAMS) score in excess of 15 – which is very exposed – is not advised in Britain. Within the natural range, a wide spectrum of climates is encountered, ranging from a minimum of 700 mm annual rainfall in the interior to an extreme of 6600 mm in some hyper-oceanic areas. Minimum winter temperatures are much lower than for the redwoods, reaching −30°C in coastal areas and −47°C in the interior. Although relatively cold tolerant, the species is susceptible to damage, especially from late spring frosts. Site requirements Western red cedar occurs naturally on a very wide range of soils, including some gleys and peats. Optimum soils are brown earths of ‘medium’ to ‘very rich’ ecological site classification (ESC) soil nutrient regime (SNR) and ‘fresh’ to ‘moist’ ESC soil moisture regime (SMR) (Pyatt et al., 2001). However, Thuja will tolerate a wider range of non-optimum soil conditions than many species including gleys, calcareous soils and (potentially) some better shallow peats. Very infertile and very dry soils remain inherently unsuitable for growth of this species. Optimum soils are deep, moist but freely draining brown earths of poor to moderate fertility. There is, therefore, a marked overlap with the favourable ‘lower slope’ site conditions normally regarded as being most suitable for productive growth of Douglas fir and grand fir in Britain. Western red cedar is at its most competitive on the fertile, heavier-­ textured, moist lowland soils in southern England, especially on calcareous clays, even periodically waterlogged ones. It requires soils that are not too acid. It will succeed on both wet and rather dry shallow soils and is one of the best conifers for calcareous downlands, ideally under a light canopy (Forestry Commission, 2018b). Deep, loose and infertile sandy soils should be avoided, as should drought-prone sites. Other silvicultural characteristics Western red cedar is potentially a large conifer, typically growing to a maximum of about 40 m tall in Britain. In its native habitat it has a ­reputation for being very long lived, often achieving 800–1000 years in nature. Top heights in the range 50–60 m are frequent, and diameters can reach 5–6 m

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in older specimens. Although there are small areas of pure western red cedar forests in the natural range, the species typically grows in mixtures with a range of conifers including western hemlock, Douglas fir, Sitka spruce, Pacific silver fir and grand fir. Bigleaf maple and black cottonwood are hardwood associates. It is a markedly shade-tolerant species, which will grow faster than other shade bearers on heavy lowland soils, although early growth is sometimes slow. It is a particularly valuable species to grow in mixtures, either with other conifers or with broadleaved trees, as the narrow crown does not interfere with neighbours. It will keep pace with several species such as the oaks, beech, ash and cherry, without outgrowing or suppressing them, and will live with faster-growing trees. Western red cedar is useful for enriching and underplanting, and is also valuable in pure stands. A frequent defect of the tree is that it often has a fluted, buttressed and rather swollen stem base. This is usually associated with unpruned trees. The best western red cedar comes from pure stands. In mixtures, less wood is recoverable and it is of lower value, often with a pronounced taper and a lot of butt swell. In Britain and Ireland western red cedar should be seen as a shade-­ tolerant, late-successional species, best suited to established forest conditions. Optimum conditions for establishment will be as enrichment interplanting or underplanting within mature conifer stands of open structure (e.g. of pine, larch, spruce or Douglas fir). Establishment in small felling coupes on more sheltered sites, with adjoining retained stands, should also be possible. While single-species stands may be created under such conditions, preference should be given to establishment of silvicultural mixtures including these species – either nursing mixtures with oak, beech and sycamore, or rotational mixtures with Douglas fir, western hemlock and Abies firs. Western red cedar is regarded as a particularly compatible nurse for high-quality oak stands. It is unsuitable for establishment on exposed upland or clearfell sites without side shelter and, preferably, top shelter. It may be possible to create suitable conditions for establishment by using ‘nurse stands’ comprising faster-growing broadleaves (e.g. birch, aspen). In some pure stands, red cedar has previously been established at 1.5–1.7 m spacing (3500–4000 stems ha−1), whereas current (2018) advice and grant requirements are typically for productive conifers to be established at 2.0 m spacing (2500 stems ha−1) (Wilson et al. (2016). It is highly palatable to deer and fencing will usually be necessary to protect it. Good weeding is also considered beneficial, at least to avoid over-topping. Pests and diseases Often the bottom 2 m of a stem may contain rot. Affected stems are frequently swollen up to the point at which the rot ends. Small dead knots

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commonly rot; to avoid this, early pruning is advisable. Since the foliage is in demand by florists, pruning can often be done profitably at an early age. Western red cedar is vulnerable to browsing by deer and is occasionally attacked by grey squirrels. The major pest and disease of western red cedar is attacks by the fungal pathogen Didymascella thujina which causes Keithia leaf blight under nursery conditions and can restrict planting stock availability. Growing trees are susceptible to honey fungus (Armillaria) decay and fomes (Heterobasidion annosum) root and butt rot, especially on sites of higher soil base status, with a record of previous hardwood or conifer crops. The cypress aphid (Cinara cupressivora) can cause foliage browning, confusable with scorch. Recently, a specimen of western red cedar in central Scotland was found to be suffering from infection by Phytophthora ­lateralis, more commonly seen in Lawson cypress. Natural regeneration Natural regeneration is prolific in some areas from trees of about 50 years old, but there is very little experience of managing it in Britain. As a shade-bearing species it could prove valuable in continuous-cover silvicultural systems. Flowering, seed production and nursery conditions The tree flowers from late March to early April and seeds ripen in September. The earliest age at which the tree bears seeds is 20–25 years, but the best seed crops are produced between the ages of 40 and 60. Western red cedar is normally a prolific seed bearer every 2 or 3 years. Seeds are very small and light but, surprisingly, do not travel as far as those of hemlock, spruce or Douglas fir. There are approximately 913,000 seeds kg−1 (range 447,000−1,307,000), of which about half are normally viable. Cones should be collected as soon as they change from bright green to yellow, and the tips of the seed wings are visible and a light brown colour. This is normally in September. Seed should be sown between late February and mid-March and needs no special treatment (Aldhous, 1972). Seedlings were difficult to raise at one time because of attacks by the fungus Didymascella thujina (Keithia blight) in the nursery (Forestry Commission, 1967), but this disease can now be treated with cycloheximide fungicides. Dry, refrigerated storage for at least 7 years is possible without loss of viability. Germination is typically good without the need for stratification. In the nursery, spring sowing is recommended, with partial seedbed shading. Seed may need to be pelleted for compatibility with automated sowing equipment designed for larger-seeded species. Containerized stock can be produced in less than 1 year, but 2-year bare-rooted stock is

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Fig. 34.  Western red cedar, Thuja plicata.

preferred in the coastal range, partly on economic grounds. From British nursery experience, the species is more sensitive than spruce and pine and may require a longer period of development before it is ready to plant out, generating higher plant costs. There are many named varieties of western red cedar, selected for their ornamental values. It is possible to produce large volumes of red cedar clones from stem cuttings, which may root untreated. Provenance Recommended origins for Britain are from the northern slopes of the Olympic Mountains in Washington, and some Vancouver Island sources show promise (Lines, 1987). With some seed sources the stems tend to fork and the lower boles are fluted, though excellent form can also be found. The only comprehensive trial of western red cedar was a series of six experiments with 13 seed sources that was established in the early 1960s. The seed sources tested ranged from northern British Columbia to Oregon and results at 15–25 years provided the basis for Forest Research’s current (2018b) recommendation for the use of western Washington or Vancouver Island provenances of this species (see Table 20).

Table 20.  Selected data from Forest Research sample plots with western red cedar.

Haldon, Exeter* Alice Holt, Surrey Forest of Dean Friston Gwydyr, N. Wales Corris, mid-Wales Novar, Highlands

Age (years)

Trees ha–1

Top height (m)

Mean dbh (cm)

Basal area (m2 ha–1)

Standing volume (m3 ha–1)

Cumulative volume (m3 ha–1)

General Yield class

53 65 95 68 73 67 44

398 437 309 556 303 284 648

29.9 24.8 40.5 – 34.6 35.8 24.2

40.9 37.6 64.2 33.8 64.3 64.3 31.1

52.3 48.7 100.1 50.0 98.4 92.3 49.1

499 439 1273

689 875 2028 808 1740 1937 1083

26 16 22 14 20 26 24

1020 990 422

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Site

*The red cedar plot at Haldon was established by underplanting under European larch. dbh, diameter at breast height; hyphen, not measured.

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Yield Yield classes range from 12 to 24 m3 ha−1 year−1, with ages of maximum mean annual increment between 50 and 70 years. There have been relatively few sample plots established with this species, mostly in the southern half of England and Wales. Selected data from some of the plots (Table 20) indicate the acceptable level of productivity that can be obtained from western red cedar over a wide spread of climatic zones, ranging from south-eastern England to north of the Moray Firth in Scotland. Operational experience also suggests that yields from this species on suitable sites generally ranges between general yield class (GYC) 12–26 m3 ha−1 year−1 or more, but typically GYC 18–22 m3 ha−1 year−1. Yield models for western red cedar are presented in FC Booklet 48 (Edwards and Christie, 1981) and red cedar was also considered in a review of a range of alternative conifers by Aldhous and Low (1974), where it was found to be capable of outgrowing Sitka spruce on more sheltered sites in southern Britain. Thinning practice in British stands of western red cedar has typically been deficient due to the difficulty in selling smaller-­dimensioned material and a perception of a slow response by retained trees. The traditional aim was to produce a dense stand of uniform straight material with minimum marketable diameter at breast height (dbh) of 30 cm as quickly as possible. With improved outlets for the smaller-diameter material and decorative foliage, consideration should be given to earlier thinning to develop a more varied, irregular structure. Many stands containing this species will now be managed under continuous-cover systems. Buttressing and ‘bell bottom’ are typical stem defects in western red cedar, especially once diameter exceeds 60 cm, but processors can still make effective use of such material by ‘sawing around’ the defects to recover boards. Timber and uses In its native western North America, the timber is sometimes known as ‘arborvitae’ and is particularly noted for its natural durability, due to the presence of tropolones in the heartwood (DeBell et al., 1999). In Britain the relatively fast growth and short rotations make it less durable, because there is often a much greater proportion of sapwood. The average density at 15% moisture content is about 390 kg m−3. The market for sawlogs may be high but, because of the reddish-brown colour of the wood and its chemical constituents, small-dimensioned western red cedar is not ­acceptable for pulp and most other wood products, including chipwood. Young stems make ideal fencing rails that can be used without preservative treatment. There is a limited but lucrative market for poles (rugby posts, flag poles, ladder poles, etc.), if they are grown slowly enough. It is

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also used for garden furniture and for making garden sheds and beehives, and it can easily be sold for fencing and allied uses. The wood has a distinctive smell, reminiscent of wooden pencils. Within its native range the traditional shingles for roofing houses are made from western red cedar because of the ease with which the wood can be split. It is also used in Britain for this purpose, and for wall cladding in houses and bungalows. A difficulty with red cedar logs is that the bark is very stringy, which makes it difficult to remove because it tends to block peeling ­machinery. The timber is moderately naturally durable, strongly reddish in colour, of rather low density and mechanically relatively weak. Ramsay and Macdonald (2013) cited specific gravities of 0.30–0.35, and Patterson (1988) and Savill (2013) reported it as having a density in the range 390–420 kg m−3 at 15% moisture content. For western red cedar there is some evidence that natural durability is higher in old-growth material from the native range than in much younger plantation-grown material from Britain. There is often strong visual differentiation between heartwood and sapwood, which has an outer layer of paler – and almost certainly less durable – sapwood. Some processors will discard this material, favouring heartwood on visual and durability criteria. There is currently a serious lack of information about the wood properties of this when grown under British conditions. Research at Edinburgh Napier University is assessing the mechanical properties and density of western red cedar. The study is using material from three pure even-aged stands, located at southern (Forest of Dean, England) central (Clocaenog Forest, Wales) and northern (Loch Ard Forest, Scotland) sites (see Wilson et al., 2016). Place of western red cedar in British forestry Western red cedar is currently a minor species in British forestry (Forestry Commission, 2018b) but, with projected climate change, it may find a role in diversifying relatively sheltered upland conifer forests. Among its ­attractions are the natural durability of its wood and its shade-bearing character, which gives it a potential place in continuous-cover systems. Western red cedar has been deployed fairly widely in British and Irish forestry since its introduction in the 1850s (Bothwell, 1988). There are no published figures for the overall stocked area, but this might amount to some 1000–2000 ha overall. It is most commonly found within private estates and Forestry Commission woodlands in south-western England, Wales, the Marches and, to a much lesser extent, in Argyll. There is a notable stand at Avondale in Co. Wicklow, Ireland (Carey, 2004). There are more occasional deployments in eastern Britain, as at Darnaway (Moray and Nairn), Kyloe (Northumberland) and Weasenham (Norfolk). The majority of these stands date from before the 1960s, with a reduced level of deployment since, until a recent noticeable revival since 2010 (e.g. to

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r­ eplace larch in Wales). There are some very fine pure, pre-Second World War stands, for example at Gwydyr Forest in Snowdonia. A considerable proportion of the stocking is in mixed stands, most often with Douglas fir, western hemlock and oak (with which red cedar was quite often used as a nurse). A number of private forestry estates have, over time, developed a significant record of growing, processing and marketing western red cedar, particularly in south-western England and the Welsh Marches.

TILIA L.

Lime or Linden, Basswood

Lime has recently been moved from the Tiliaceae to the Malvaceae family. The Malvaceae contains some 243 genera and at least 4225 species of herbs, shrubs and trees. Representatives occur in all except the coldest parts of the world but are most numerous in the tropics. A number of species are economically important, including cotton (various Gossypium species), cacao (Theobroma cacao), linden (Tilia species), durian (Durio species), Hibiscus and okra (Abelmoschus esculentus). There are 23 species of Tilia in north-temperate regions, including five in Europe. The two native British species, Tilia cordata and T. platyphyllos, are dealt with below. A third species, the common lime, T. × europaea L., is a fertile hybrid between the two and is the most common and widely planted lime in parks, towns and gardens. The two native limes hybridize where their distributions overlap, as in the Derbyshire Dales and the Wye Valley. Until Pigott’s (1988, 1991) reviews of the ecology and silviculture of the species, remarkably little had been written about the limes. Much of the information below is from these two sources, which should be consulted for more detail.

TILIA CORDATA Miller

Small-leaved lime

Origin and distribution Small-leaved lime is native to England (except Northumberland and Cornwall), as far north as the Lake District, to Wales, but not to Scotland or Ireland. It extends into continental Europe from northern Spain to Norway and eastwards to beyond the Urals. It is a relatively rare tree but nevertheless much the commoner of the two indigenous limes and was the last major tree to reach Britain after the Ice Age. Its distribution is scattered and uneven, but it is found predominantly in long-established woodlands, ancient hedgerows, on cliffs, among rocks, and in wooded ravines and river gorges in England and Wales. It is not native to Scotland or Ireland. According to Pigott (1991), it may also be regarded as a collective species, which extends from Western Europe to Eastern Asia and includes at least seven species or subspecies. The current distribution of small-leaved lime in Britain is thought to be limited by cool summers. According to Pigott (1991), it was rarely planted in parks and towns until recently, and even more rarely naturalized; but since 1960 the species has acquired new popularity and has been planted in many places, © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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though it is still regarded as one of the more overlooked native trees in the UK (Bugg and Ashton, 2012). In prehistoric times small-leaved lime was probably much more common than it is today: it first appeared in eastern England in 5600 BC and dominated the forests of lowland England by 4500 BC before ­declining from about 3000 BC (Bugg and Ashton, 2012). Its decline was due to its ­selective removal by man and a general cooling of the climate by about 2°C, which prevented seed development in the cooler parts of the country. If predictions about climate change prove correct, a warming climate should therefore benefit the species and allow it to grow satisfactorily on lowland sites in southern and eastern Scotland. Climatic requirements Small-leaved lime is tolerant of very low temperatures in winter. There are no reports of damage in Britain even during the coldest winters. It ­escapes frost damage during the growing season by coming into leaf late and by the early onset of leaf-fall and dormancy. The leaves are not fully ­expanded until mid-May, about 10–14 days later than large-leaved lime. Except for ash, this is later than any other common native tree, i­ncluding large-leaved lime and T. × vulgaris. In common with the large-leaved lime, it is normally restricted to valley bottoms because of its intolerance of a dry atmosphere in summer. Most sites where old coppices of small-leaved lime occur on stagnogleys (soils that are waterlogged in winter and hard and dry in summer) are in the drier parts of England, where the average annual rainfall is less than 700 mm. Small-leaved lime is reasonably resistant to urban pollution from SO2, NO2 and ozone, but probably not as resistant as T. × vulgaris, which is by far the most commonly planted lime in large industrial cities. Smallleaved lime does not tolerate exposure to strong winds. Site requirements Small-leaved lime is normally restricted to valley bottoms. Its distribution in England and Wales is patchy. It will grow on many types of soil, including podzols, brown earths and calcareous soils, and does best on neutral or slightly alkaline soils (Radoglou et al., 2009). It will also grow on soils with a wide range of textures: from those with a high proportion of clay and silt to those containing mainly sand. In southern England it thrives on stagnogleys developed on compacted, fine-textured materials, which are frequently non-calcareous (stagnogleic brown soils) but may overlie limestone or other calcareous subsoils. On these sites it can

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c­ ompete successfully with oak. In woodlands that provide a variety of soil types, the tendency for distinct patches of small-leaved lime to be correlated with fine-textured soils may be striking. Only in the west and north, where the rainfall exceeds about 850 mm, is small-leaved lime found on limestone as well as on acid soils. Small-leaved lime grows best on fresh to moist soils with a medium to rich soil nutrient regime (SNR). Sites to avoid are nutrient-poor soils, waterlogged soils, peaty soils and dry soils where the tree grows e­ xtremely slowly. Woods on most of the sites where lime grows have been converted to agricultural uses since the Neolithic age. Other silvicultural characteristics The small-leaved lime is a medium-sized tree that will grow to 20–30 m tall. It is one of few shade-tolerant native trees, although estimates of its degree of tolerance vary. Some class it as being similar to beech, while others regard it as only moderately tolerant and ranking below beech, hornbeam, wych elm and large-leaved lime. Small-leaved lime possesses all the attributes of a dominant tree of the upper canopy of woodland. Seedlings and saplings are relatively tolerant to shade; the species has a ‘waiting and persisting strategy’, according to Radoglou et al. (2009). Mature trees are as tall as, or taller than, most competitors; they possess a monolayer canopy, cast a deep shade and are long lived. Small-leaved lime is excluded by beech wherever beech can grow strongly. It is used in parts of Europe for shading the stems of oak to prevent epicormic shoot growth. Small-leaved lime is a deep-rooting tree and, on deep soils, it is scarcely affected by periods of drought if (as is often the case) it can gain access to soil moisture at depth. Although it has a long history of being used in forestry, this has been almost exclusively as a coppice species. Coppice shoots may eventually become separate trees with independent root systems. Unusually among trees, stools retain their vitality for many centuries. According to Pigott (1991), they appear to be almost indestructible. Even conservative estimates indicate ages greater than 1000 years for large groups. New stools can easily be produced by layering. Shoots are also produced when old stems collapse naturally. Large groups of genetically uniform stems are a feature of the tree at its northern limit in Britain. Soils under lime tend to be much richer in nitrogen and phosphorus, and have higher populations of earthworms, than under other broadleaved species. This soil-improving property may partly be explained by its deep-rooting ability, which enables the tree to gain access to nutrients lower in the soil profile than many other species can reach, in a similar way

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to that described for birch (p. 70). In addition, the lime aphid, Eucallipterus tiliae, may cause an enrichment of nitrogen (Mabberley, 1990) arising from the large amounts of honeydew they produce. They are said to deposit up to 1 kg of sugars m−2 each year. These possibly stimulate the growth of nitrogen-fixing bacteria around the trees and thus enhance nitrogen availability at the cost of some carbohydrate taken from the phloem by the aphids. Lime has often been planted in mixture with other species in continental Europe for its soil-improving litter, which decays rapidly. Pests and diseases The limes are remarkably free of diseases to which other trees are prone. They are among few broadleaved species that are not damaged by grey squirrels stripping the bark, and they are notable for being resistant to honey fungus (Armillaria spp.). However, seedlings and saplings are browsed in preference to many other species by deer, sheep, cattle, voles and other animals. This species is, however, less likely to become infested with lime aphid, Eucallipterus tiliae, than large-leaved lime or T. × vulgaris. Natural regeneration Natural regeneration is not common except in the south of England ­because viable seed is seldom produced. Seedlings rarely persist, however, because they are so palatable to browsing animals such as bank voles. Climate change might result in regeneration becoming much more common. Flowering, seed production and nursery conditions The limes are among few British trees that are insect rather than wind pollinated and they, with sweet chestnuts, are among the last trees to flower, usually in June or July. The flowers are sweet scented, but not as strongly as those of large-leaved lime or T. × vulgaris, and are pale whitish-green in colour. Contrary to popular belief, small-leaved lime produces viable seed in most years in southern England. In the north seed is usually sterile ­because the climate is seldom warm enough for the pollen tube to grow, and consequently fertilization of the embryo does not occur. The first flowers normally open in mid-July, and flowering is completed in about 10 days (this is 10–15 days after flowering of large-leaved lime and T. × vulgaris).

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The trees first produce seed between 20 and 30 years of age, and large amounts of seed particularly follow summers in which July and August are sunny and warm. There are about 29,000 seeds kg−1, of which 50–60% germinates if collected in central and southern England or 0–15% in northern England. A high proportion of seed collected no later than September and placed immediately in moist soil in the open will germinate in the following April. If air-dried to a moisture content of about 9–10% and then sown and left in the open, only 10–15% of the seeds germinate in the following April and most of the remainder 1 year later, 18 months from harvest. Provenance No provenance trials have been undertaken in Britain, so plants should either be of local origin or sourced from stands of good form in Britain or in Western Europe. Yield Pigott (1991), on the basis of an analysis of growth in a Forestry Commission sample plot in Hampshire, stated that during the first 50 years growth is comparable to stands of Class 1 in the former Soviet Union, or more than 5 m3 ha−1 year−1. It is believed that on the best sites limes will grow at something like 8 m3 ha−1 year−1. Timber and uses The wood of lime is soft, light (about 560 kg m−3 at 15% moisture content), white when freshly cut and has a uniform, fine texture. Its main uses are based on its softness, stability and ability to resist splitting. The wood is perfect for making musical instruments as it does not warp; it is used for piano and organ keys, and at one time it was in considerable demand for the sounding boards of pianos. It is easy to machine and is used widely in turnery and carving, and in particular gesso and gilt work. It is not durable out of doors but can easily be treated with preservatives to make it so. From prehistoric times to the 18th century the fibrous inner bark of lime was used for rope making. T. × vulgaris and large-leaved lime have been widely planted in parks and in towns as street trees, but small-leaved lime much more rarely ­because it has too wide a crown to be favoured as an urban street tree.

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(A)

(B)

Fig. 35.  (A) Large-leaved lime, Tilia platyphyllos; (B) Small-leaved lime, Tilia ­cordata.

Place of small- and large-leaved lime in British forestry From about 1970 planting small-leaved lime has often been recommended as an aid to encouraging biodiversity in woodlands, and many small patches have been established. Neither of the native limes has been treated as a commercial crop in Britain since the 19th century, and they have rarely been planted for commercial reasons since at least the 17th century. As species that are likely to benefit from climate change, the position of limes as timber trees might be re-evaluated.

TILIA PLATYPHYLLOS Scop.

Large-leaved lime

Origin Though the status of large-leaved lime in Britain has been in doubt for a long time, Pigott (1988) stated that on the basis of pollen analysis it is definitely a native tree. In continental Europe it is a more southerly species than small-leaved lime, extending from a single location in southern Sweden to central Spain, northern Greece and southern Italy, and eastwards to Poland and the western Ukraine. In Britain it is also found in more southerly regions than small-leaved lime.

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Climatic requirements Like small-leaved lime, it is a rare tree and restricted to valley bottoms because of its intolerance of a dry atmosphere in summer. It comes into leaf in April, normally 10–14 days earlier than small-leaved lime. Damage caused by late spring frosts is rare on large-leaved lime. Site requirements Both native species of lime are essentially trees of the lowlands and lower ranges of hills. Large-leaved lime occurs predominantly on rendzinas ­derived from limestone, or basic igneous rocks, being found mostly with beech, ash, sycamore and yew. Other silvicultural characteristics Large-leaved lime grows to 20–30 m tall. In most respects the tree is similar to small-leaved lime. Differences include the fact that large-leaved lime does not produce many shoots from the base of the trunk, a feature of small-leaved lime. It grows well in Britain and is said by Huxley et al. (1992) to be the only species of Tilia that reliably produces viable seed in areas with cool summers. Pests and diseases In common with other lime species, large-leaved lime is remarkably free of most of the diseases to which other trees are prone and is not damaged by grey squirrels. However, it is frequently infested with lime aphids, Eucallipterus tiliae, as is T. × vulgaris. The honeydew they produce can be a nuisance on cars and on pavements in urban areas. Natural regeneration See under small-leaved lime. Flowering, seed production and nursery conditions Contrary to popular belief, both species, but especially large-leaved lime, produce viable seed frequently in southern Britain. There are normally

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about 7500 kg−1, of which 70% will germinate. The two native limes tend to hybridize where their distributions overlap, as in the Derbyshire Dales and the Wye Valley. The tree normally flowers in early July, and the flowers last for about 10 days. Similar to T. × vulgaris, but in contrast to small-leaved lime, they are quite strongly and pleasantly scented. Seeds ripen in October. Timber and uses Limes are attractive trees and favourites for planting along roads in towns, presumably because of their sweet-smelling flowers and the fact that they can be pollarded and heavily pruned without coming to much harm. Large-leaved lime and T. × vulgaris are particularly suitable as street trees because of their narrow crowns. The timber is similar to that of small-leaved lime: light (544 kg m−3 seasoned), soft, with uniform texture and diffuse-porous. Place of large-leaved lime in British forestry See under small-leaved lime.

TORMINARIA Opiz TORMINARIA TORMINALIS Dippel

Wild service tree

The scientific name of this species was, until recently, Sorbus torminalis (see p. 299). It is in the Rosaceae (see p. 67). Origin The wild service tree has received a lot of publicity in recent years. This arises largely because of its potentially very valuable wood in continental European markets, especially in France and Germany (Huss et al., 2016). Growing this relatively rare tree presents new and interesting challenges to foresters and many have taken up the challenge. A great deal of literature on the species has been produced by Skovsgaard and others, in Sweden and Denmark.The wild service tree is found in managed forests over much of northern Europe, the Mediterranean region and North Africa. It is native to England and Wales, from Cumbria and Lincoln southwards. Almost everywhere, it is rare, patchy and local in distribution. It is not native to Scotland or Ireland. The presence of the species is usually regarded as an indicator of ancient woodland or an ancient hedge, because regeneration from seed is unusual. Recent changes in silvicultural practices in Central Europe have created forests with closed canopies, and tree species more suited to open and sunny forests have declined in area and abundance. This has led to increased isolation of populations of many rare insect-pollinated, fleshy-fruited species with a naturally scattered distribution, like the wild service tree (Angelone et al., 2007). Climate and site Precipitation in Britain should not limit the distribution of the wild service tree but low mean annual temperatures might. The species is said to require mean annual temperatures of between 10°C and 17°C, but in lowland England they are not always within this range and they are ­invariably below it in Scotland and Wales. The wild service tree shows a marked preference for soils derived from clays or harder limestones (Roper, 1993). It usually grows on clays similar to those favoured by wild cherry. In Lower Saxony the species is well adapted to dry, calcareous, but nutrient-rich sites as a timber tree, and it is particularly common near coasts (Meyer, 1980). It will tolerate © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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short-duration droughts but apparently not late spring frosts. Both dry sandy soils and wet peaty ones should be avoided. Other silvicultural characteristics The wild service tree is often rather inconspicuous, seldom taller than 20–25 m at age 100, with stems 50–70 cm diameter, shorter than companion species. It will, exceptionally, reach 30 m in height and 1 m diameter at 200 years old. It typically occurs as a scattered and uncommon tree in woodlands and is believed to be an indicator of ancient woodland (Peterken, 1974; Huss et al., 2016). It never forms pure stands, and populations of more than 100 trees are unusual (Nicolescu et al., 2009). Where it is truly at home, as in parts of France, it is a colonizing species, but it is also reasonably shade tolerant. In fact, there is disagreement about the extent of its shade tolerance: it is described by Demesure-Musch and Oddopu-Muratorio (2016) as a light-demanding species, often outcompeted by other hardwoods, in particular beech. Others, such as Pyttel et al. (2013) say that it is ‘highly shade-tolerant’ and can persist beneath the canopy of oaks. However, often when overtopped by other trees, wild service tree rapidly weakens and wastes away, but even a small opening in the canopy is enough to allow their development. It is also described as a ‘nomad’, post-pioneer species often found growing as a minor component of oak and beech woodland. Where it is truly at home, it can easily colonize forest clearings and low-density forest stands, thanks to efficient seed dispersers. In Britain it is light demanding and a weak competitor with other species. In Italy the tree is regarded as being characterized by slow growth and adapted to low-fertility soils (Piagnani et al., 2006). A feature of the tree is that it usually occurs at low densities throughout its range (0.1–30 individuals ha–1). According to Huss et al. (2016) the species can be valuable when mixed with sessile oak. The wild service tree does not self-prune well so needs artificial pruning if clear timber is to be produced, and it has a tendency to fork ­rather often (Nicolescu et al., 2009), requiring formative pruning. Schmeling (1981) commented that, as a timber producer, it is not really suited to high-forest conditions because its slow height growth and light-demanding nature not only prevent it keeping up with faster-­ growing species, but also cause it to become suppressed and eventually to die. Hence, to survive in dense woodlands, competition must be reduced by consistent thinning to permit adequate crown development. It  probably does best as a standard in systems that are coppiced every 25–40 years, or in the open. The decline in coppicing over the last 150 years has probably contributed to the current comparative rarity of the species.

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Possibly because of its rarity and the value of its timber, a large amount of research has been conducted throughout Europe since about 2000 on the genetics and reproductive biology of the species. Apparently, the fruits are sought out by many birds and a few mammals. The flowers attract insects (including some uncommon beetles) to their nectar and pollen in spring. The leaves are eaten by caterpillars of the moths Bucculatrix bechsteinella and Phyllonorycter mespilella (neither of the species appear to have English names). Hemery et al. (2010), in a review of the effects of climate change on broadleaved trees, stated that the range of the wild service tree may shift north to north-eastwards with predicted climate change, but that the process is likely to be slow due to the present scattered distribution, poor sexual reproduction and poor seed dispersal, as well as to a low competitive ability in comparison with other species. They believed that if natural dispersal is not assisted by humans, a reduction in range is more likely. Natural regeneration Successful regeneration of the wild service tree from seed is rare in Britain because newly germinated seedlings need full overhead light, which usually is not available. The wild service tree coppices well and older individuals also produce masses of shade-tolerant root suckers for a distance of 3–8 m around the stem, especially after the parent tree dies or has been felled. These grow fast for a few years, but unless they are then freed they become suppressed and may die. As a result of the suckering small clonal patches of the tree occur quite commonly (Hoebee et al., 2006), in a similar way to wild cherries. Flowering, seed production and nursery conditions The wild service tree, like lime (Tilia cordata), is a species that is on the edge of its natural range even in the south of England. The summers are seldom long enough or hot enough for seeds to mature. This is possibly the main reason why the species has been so little used. Though it fruits every 1–2 years, germination percentages are low. A seed wasp, Torymus druparum, can be damaging to any potentially viable seed (Winter, 1978). The tree flowers in May and June, and the seeds mature in September and October. Methods of micropropagation have been developed for it. Root suckers, if collected, are slow to grow to usable-sized plants in the nursery because they tend to have such inadequate root systems when small.

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Fig. 36.  Wild service tree, Torminaria torminalis.

Genetics Possibly because of its rarity, and the value of its timber, a large amount of research has been conducted throughout Europe since about 2000 on the genetics and reproductive biology of the species. A study by Rasmussen and Kollmann (2004) in Denmark showed that self-pollination is not efficient and that inbreeding depression occurs: the wild service tree is a predominantly outcrossing species. They concluded that successful reproduction of the wild service tree is reduced at its northern limit of distribution. The low gene flow between populations might eventually lead to extinction. Rasmussen and Kollmann (2006) also found, in a genetically analysed subsample of regeneration, that 98% of offspring were of clonal origin, from suckers, or ‘root sprouts’. They suggested that the 2% of suckers that differ may be a result of somatic mutations in that they differed in only one locus, or that sprouting from a more distant parent tree could have caused the variation. The estimate of clonal diversity (0.17) was very low compared with the centre of the species’ range in Switzerland where it was 0.42 and Poland where 0.435 has been reported. There is evidence that populations with 11 adult trees may be genetically identical, and this may

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seriously limit the species’ ability to adapt to changing environmental conditions. Rasmussen and Kollmann (2006) concluded that T. torminalis is able to persist at the present northern limit of distribution mostly ­because of clonal growth, but they expect that low genetic variability and poor sexual reproduction make the existing populations vulnerable to ­environmental change and habitat destruction. Belletti et al. (2008), in Italy, found that most (~62%) of genetic variation is found within populations, but a significant proportion (~24%) occurs between regions and 14% between populations. They concluded that local populations are often threatened by deforestation, habitat fragmentation and plantation establishment with reproductive material of ­unknown origin. In the short term, it seems important to establish at least one in situ reserve area in each of the regions in which the studied area has been divided, especially for populations with the highest levels of genetic variability. The preservation of genetic diversity is essential, both to promote adaptability of the populations to changing environmental conditions and to preserve a large gene pool for future genetic improvement. The finding that a high level of genetic variation exists between populations suggests that to preserve genetic diversity more reservoirs should be established, to protect as many stands as possible. Wild service tree is a diploid species (2n = 34). It can hybridize with at least two other species: whitebeam (Aria nivea) and rowan (Sorbus aucuparia). Hybridization with whitebeam commonly occurs, especially where the natural ranges of these species overlap. Most of the hybrids are triploid (3n = 51) and a few are tetraploid (4n = 78). Hybrids reproduce mainly by apomixis (Demesure-Musch and Oddopu-Muratorio, 2016). Hybridization occurs predominantly as wild service tree (father) to whitebeam (mother). It is rarely followed by cytoplasmic introgression. As a consequence, interspecific gene flow should not significantly affect the ­diversity dynamics within the wild service tree. The rate of self-fertilization is estimated at less than 1% in open-pollinated progenies and is very variable across mother trees. This very low rate of self-fertilization supports the hypothesis of a partial self-incompatibility system as in S. aucuparia. Patterns of pollen exchange in wild service trees show two main trends: preferential mating between neighbouring trees due to local pollen dispersal, combined with long-distance dispersal (up to 2.5-km dispersal events have been documented). This results in a low pollination effective size: on average, only six effective pollen donors contribute to the pollen cloud of a given mother tree. But, at the same time, a proportion of the pollen donors are far away from the mother tree (Demesure-Musch and Oddopu-Muratorio, 2016). Most seeds are dispersed in the close vicinity of the mother tree; 174 m on average between an established seedling and its mother tree. But at least 17% of the seedlings collected in the middle of a 470-ha forest stand originated from outside that stand.

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Demesure-Musch and Oddopu-Muratorio (2016) reported the distribution of wild service trees in aggregates of 150–300 m in radius, corresponding to individuals more genetically related than expected by chance. These aggregates are likely to correspond to successful colonization of favourable sites by sibling trees. In contrast, genetic surveys at regional and larger scales revealed rather high levels of within-population diversity, for both nuclear and cytoplasmic markers. The level of differentiation between populations observed with cytoplasmic markers is surprisingly low compared with other European scattered broadleaves. For wild service trees, neither pollen- nor seed-mediated gene flow effect was predominant (genes are propagated on average at equal distance by seed and pollen). The wild service tree occurs as a scattered species and its genetic ­diversity can be considered at risk in certain conditions. It is sensitive to competition, so there may be a risk in dense, regular high forests which contain a high proportion of long-lived species. The high level of interspecific competition could impede the regeneration process and lead to local extinction. Thus, the species may be in great danger if no new sites are available for colonization and establishment. As for all forest trees, habitat fragmentation could induce a strong decrease of genetic diversity through reduction of population size and interruption of gene flow. Habitat fragmentation could result either from forest destruction or by management not directed to favour wild service trees. This species is particularly threatened by habitat fragmentation owing to its low population density. The demand for wood could induce introduction in forests of allochthonous (i.e. moved from its original site of formation) material, or seed could be collected from a limited number of unknown seed trees. Therefore, wild service tree conservation should not be practised at the local level (several hectares), but rather at the landscape level or even on a regional scale, though it is not possible to indicate a critical population size below which the populations would be threatened. For long-term sustainability of the genetic resources of the species, competition from neighbouring trees must be controlled and wild service trees need to be released at each thinning intervention. The forester must be aware that seed trees from the neighbouring areas will also contribute to regeneration. Most importantly, regeneration of wild service trees must be established before that of ‘social’ broadleaves. In this way, the young wild service tree seedlings are given a competitive advantage against oaks or beeches. It is also important to ensure a regular distribution of wild service trees in small clusters or as single individuals. The local disappearance of a few trees is not harmful to the population because seed flows allow colonization over long distances. But the forester must ensure the presence of favourable sites for new establishment.

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Pests and diseases The wild service tree typically occurs as a scattered and uncommon tree in mixed broadleaved woodlands. It never forms pure stands, and populations of more than 100 trees are unusual. Trees that grow in such forests are adapted to closed forest conditions and will regenerate in closed forests or in small gaps. Seedlings must survive the much more competitive conditions beneath canopies of more mature trees. Such species are generally unsuited to plantation conditions and seldom, if ever, make good plantation trees. In Britain, wild cherry and ash are species that are rarely found in natural ‘monocultures’ and succumb to several diseases if they are planted too extensively. There are many other examples of (mostly tropical) species that occur naturally at low frequencies making poor p ­ lantation trees, including all the true mahoganies, which succumb to attacks by the shoot borer Hypsipyla (Savill et al., 1997). Before wild service trees are ­deployed on a wide scale, they should be tested in trial plantations over, preferably, more than one rotation. The family to which Torminaria torminalis belongs (Rosaceae) has trees which are notoriously susceptible to diseases and pests. Apple and pear orchards, for example, have to be sprayed several times each year to maintain them in a healthy condition. Spraying with fungicides or insecticides can hardly ever be justified on forest stands. It is known that wild service trees can be affected by fireblight (Erwinia amylovora) and silverleaf (Chondrostereum purpureum) which can be progressive and sometimes fatal (Forest Research, 2018b). The species can be badly damaged by deer (especially roe deer) and rabbits. Timber and uses The sapwood and heartwood of the wild service tree are the same colour. The wood of young trees is yellowish-white to light reddish. Old trees have a darker wood, reddish-yellow to red-brown in colour, and fetch much lower prices than younger, light-coloured wood. The wood of the wild service tree is fine-grained, very dense and has good bending properties. It was used in the past to make screws for winepresses, billiard cues, musical instruments and turnery. Today, its main use is for decorative veneers. It is one of the most valuable hardwoods in Europe. In the 1990s, it fetched the highest timber prices per cubic metre in continental Europe. The wood is dense (about 750 kg m−3), strong and elastic. It is hard and tough and is difficult to split. It has a tendency to shrink and to crack and warp when drying, but once it is dry it is stable. It has a low natural durability when exposed to the elements.

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The wood is similar in most respects to that of rowan according to Elwes and Henry (1906–1913) and is used for the same purposes. It is also very like pear wood and is sometimes, rather enigmatically, marketed as ‘Swiss pear’. It is valued today for furniture; for making veneers (for which light-coloured wood is preferred); for musical instruments, especially flutes, pianos and harpsichords; and, formerly, for measuring and drawing equipment. A minimum diameter of 25 cm is required for most purposes. The wood is relatively easy to work but, because of its high density, sharp tools must be used. It has a decorative grain and colour. Because of this, work is being undertaken in France (Lanier et al., 1990) to determine the relationships between wood quality and site, and the silvicultural methods required to encourage its growth. Cobbett (1829) described the fruit as ‘a thing between a sloe and a haw. It is totally unfit to be eaten’. Others disagree, recommending bletting, which involves storing the fruit in a cool, dry place until it is almost rotten. It is then said to taste like a medlar or a ‘luscious tropical fruit’ (Mabberley, 2008; PFAF, 2012). The leaves and bark were reputed at one time to have medicinal properties; these are described by Evelyn (1678). The fruits are used to produce liqueurs, especially in Germany and Austria, and they were also traditionally used as a herbal remedy for colic. Place of the wild service tree in British forestry Interest in the wild service tree arises from its rarity, the challenge it presents in being grown to large sizes and the potential value of its timber. It is a small, rather badly formed tree which does not self-prune well, has poor sexual reproduction and poor seed dispersal. It also has a low competitive ability in comparison with other species that are likely to grow with it, and is said to be unsuited to high forest conditions. It is likely to suffer from many diseases and pests to which the Rosaceae are prone if planted too extensively. Its yellow autumn colour is spectacular. However, it will never be anything but a minor species and then only in southern Britain.

TSUGA Carrière TSUGA HETEROPHYLLA (Raf.) Sarg.

Western hemlock

Origin and introduction Western hemlock is in the family Pinaceae (see p. 202). There are nine species of hemlock, which occur from North America, through eastern Asia to Vietnam. Western hemlock grows naturally from the Alaskan coast to central California and from sea level to 2250 m. It is particularly abundant along the coasts of Washington and Oregon, and is the dominant species in British Columbia and Alaska along the Coast Mountains, on the coastal islands and the northern Rocky Mountains, especially on well-drained soils in areas with mild, humid climates. It was introduced to Britain in 1851. Climatic requirements Western hemlock thrives in a mild, damp climate where frequent fog and precipitation occur during the growing season. There are no marked regional differences in performance within Britain, though it does best in the humid west where precipitation exceeds 1000 mm. It can also be productive in quite low-rainfall areas but is less suited to the drier east and south-east than Douglas fir. It is mildly susceptible to damage by late spring frosts but recovers well. In exposed areas it is defoliated by strong winds and is more likely to suffer from dieback of the leading shoot on exposed, frost- or drought-prone sites than any other common conifer, especially when young. Its ability to grow replacement shoots can result in multi-stemmed trees. Though generally windfirm, hemlock is not a deeply rooting tree. Its stability can probably be ascribed to the low drag coefficients in winds, caused by the flexible branches, which stream out in the wind (Raymer, 1962; Walshe and Fraser, 1963). Site requirements Hemlock grows best on deep, moist, well-aerated soils such as acid brown earths on the lower slopes of valleys in upland forests, but it grows acceptably well on both wet and dry sites and the better peats. It will not grow on calcareous soils and requires a soil pH in the range of about 4.0–6.3. Soil texture is relatively unimportant, but height growth becomes slower with © Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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increasing soil clay content, probably due to reduced soil aeration or the inability of roots to penetrate compacted soils. According to Packee (1990), western hemlock is poorly suited to sites where the water table is less than 15 cm below the soil surface. Although it will survive on dry soils it grows poorly and tops may die back in years of drought. It thrives particularly well on moderately fertile sites high in available nitrogen, at low elevations, but will grow in wetter regions on less fertile soils. It will also grow on windthrown trees and old stumps. Other silvicultural characteristics In Britain hemlock will grow to 30 m tall and rarely to 40 m. In its native habitat it is a climax species and, like most such species, it is strongly shade tolerant. It is more shade bearing than any other species commonly grown in Britain and is easier to establish under shade than in the open. Within its native range only Pacific yew and Pacific silver fir are considered to have equal or greater tolerance to shade than western hemlock (Packee, 1990). Malcolm et al. (2001) believed that unless greater use is made of shade-tolerant species like hemlock, the application of silvicultural systems such as single-tree selection will not be possible in Britain. It is difficult to establish pure on bare ground and does better with a nurse. Like the spruces, it does not compete well with heather.

Fig. 37.  Western hemlock, Tsuga heterophylla.

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According to Aldhous and Low (1974), on financial grounds, hemlock is often second choice to Sitka spruce and grand fir, though it could have a place on soils too infertile for grand fir and too dry for Sitka spruce. In eastern Scotland and north-eastern England, it is a good alternative to Scots and lodgepole pines if shelter is adequate. The risks of damage by drought and frost mean that it should not be used for afforestation of open ground, but it can succeed under light shade. The roots, especially the fine surface roots, are easily damaged by harvesting equipment; and, because of its thin bark and shallow surface roots, western hemlock is susceptible to damage even from light ground fires. It is also badly affected by SO2 in polluted environments. Diseases Hemlock is more susceptible than many species to being attacked by fomes root and butt rot, Heterobasidion annosum, and infected sites should be avoided. However, honey fungus, Armillaria spp., seldom kills trees. Treating stumps and wounds with chemicals can reduce the rate of infection. Natural regeneration Natural regeneration is often good on a wide range of sites, even in dense shade, to the extent that regeneration of other species is unsuccessful. It responds well to release after a long period of suppression. Advance regeneration that is 50–60 years old commonly develops into a vigorous, physiologically young growth stand after complete removal of the overstorey. Because western hemlock can thrive and regenerate on a diversity of seedbeds (including the surface of fallen logs), natural regeneration can be obtained through a variety of silvicultural systems, ranging from single-­tree selection to clear felling. Careful harvesting of mature stands can easily result in adequately stocked young stands if enough regeneration is present. Flowering, seed production and nursery conditions The tree flowers in April; seeds ripen and can be collected in September, and are dispersed naturally from October to May. The earliest age at which the tree bears seeds is 20–30 years, but the best seed crops are usually at 3-yearly intervals between 40 and 60 years. There are approximately 573,000 seeds kg−1 (range 417,000−1,120,000), of which about half

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are normally viable. Cones should be collected as soon as they change from bright green to yellow, and the tips of the seed wings are visible and a light-brown colour. In nurseries seed should normally be sown between late February and mid-March. It needs no special treatment before sowing, according to Aldhous (1972). Western hemlock seeds remain viable only into the first growing season after seedfall.

Provenance Vancouver Island sources are recommended by Lines (1987) as good general-purpose origins. Where the risk of frost damage is small more southerly ones can be used. The stems of trees originating from parts of coastal British Columbia and south-eastern Alaska are often badly fluted and may be genetically predisposed to the condition (Julin et al., 1993). Fluting reduces the value of the timber considerably. Where such trees are found in Britain collection of seed from them should be avoided.

Growth and yield On the best sites western hemlock grows rapidly. Yield classes range from 12 to 24 m3 ha−1 year−1. Packee (1990) stated that in its natural range western hemlock forests are among the most productive in the world and capable of carrying very high volumes of standing timber.

Timber and uses In North America hemlock is marketed together with Abies procera and A. amabilis as ‘Hem-Fir’. The timber is usually described as being intermediate between Douglas fir and spruce. It is non-resinous, and most people say the colour is white (though Wilson (2010) described it as ‘strongly’ coloured). The grain is typically fine and straight. It has good working qualities. Hemlock has a high moisture content when freshly cut and must be seasoned slowly to avoid distortion, but it is stable when dry. At 15% moisture content its density is about 500 kg m−3. In common with spruces, it is difficult to get timber preservatives to penetrate the wood. Western hemlock is used for constructional work, in situations where its relatively low strength and decay-prone timber are acceptable, and for interior woodwork and other purposes where a fairly high grade of softwood is needed. It is also a major source of pulp in North America and is described as having good-to-excellent pulping

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characteristics. It is a source for groundwood, thermomechanical, kraft and sulfite pulps. Place of western hemlock in British forestry Western hemlock is an important commercial species along the Pacific coast of North America but only a minor species in Britain. Its capacity for natural regeneration means that it is likely to persist in many ­upland forests, and as a strong shade bearer it might find a useful place in ­continuous-cover systems.

ULMUS L. The Ulmaceae is generally considered to include about 9 genera and 45 species. Apart from the elms, the family also includes the Eurasian genus Zelkova which is occasionally found in arboreta and large gardens. Taxonomy and origin According to the International Dendrology Society (2018), Ulmus comprises 20–45 species, distributed across the temperate northern hemisphere and into subtropical Asia and Central America. Six species are believed to be European, and the International Dendrology Society states that there are anywhere between one and six species and additional ­hybrids among the British species, depending on taxonomic viewpoint. A current view is that there are a limited number of species (U. glabra, U. minor, U. procera) with one frequent hybrid (U. × hollandica) and several to many distinct clones of uncertain origin which are probably best treated as cultivars. Apart from the distinct wych elm, U. glabra, their classification is incompletely worked out and difficult for the person dealing with the trees in the field. Most are probably U. procera, of which many strains have been planted over the centuries in different parts of the country. A few are sometimes recognized as separate species or subspecies. Because of the ease by which U. procera can be propagated from suckers it is certain that within many localities clonal material has been used quite extensively.

ULMUS GLABRA Hudson

Wych elm

Distribution The wych elm has a widespread distribution in Northern Europe and is the only elm native to Ireland. It has a wide natural distribution, from Ireland eastwards to the Urals, and from Norway in the north to Greece. Unlike English elm, it is found in mixed woods throughout Britain and Ireland, but most commonly in the north and west. Silviculture and ecology Wych elm is normally rather a small tree, seldom more than 20 m tall. It is most commonly found as a natural rather than planted component of 340

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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mixed broadleaved woods, and in hedgerows and by streams, particularly in shady, damp valleys on fertile soils (Huss et al., 2016). It is more of a woodland tree than the English elm. It does best on sites where the relative humidity is high and the soil is fertile, moist, not too acid and well drained. A high level of fertility is not so important. Dry sites should be avoided. It is moderately shade bearing and casts a dense shade. Like English elm, it is good at tolerating both exposure from sea winds and atmospheric pollution in towns. Wych elm produces suckers, but much less prolifically than English elms, so regeneration is normally from seed. It coppices well and tends to be a multi-stemmed tree.

Dutch elm disease Wych elm is susceptible to Dutch elm disease but appears to be slightly more resistant to it than the English elm. This may partly be because of its relative rarity and the fact that it grows in woods rather than on agricultural land, so it often escapes the notice of the beetles that carry the disease. Trees can still be found quite easily. Work by Peterken and Mountford (1998) has demonstrated that the population of wych elm in Lady Park Wood, Forest of Dean, had changed between 1972 (when Dutch elm disease struck the area) and 1993. Most of the canopy and subcanopy stems were killed, but a few, slow-growing, subcanopy individuals survived unscathed. Subsequent seedling regeneration and growth of suckers from rootstocks of infected trees was substantial and vigorous. By 1993 the number of wych elms had increased by about 40%, although Dutch elm disease continued to afflict vigorous, exposed individuals. Peterken and Mountford (1998) stated that the elm population ­appears to be differentiating into: (1) a large, high-turnover subpopulation of fast-growing but repeatedly diseased maiden individuals and sprouts, which appear mainly in regeneration gaps; and (2) a small, low-­turnover subpopulation of slow-growing individuals rooted in suboptimal dry, ­secluded sites. The same result was observed by Hahn and Emborg (2007) at Suserup forest in Denmark.

Flowering and seed production The flowers appear before the leaves in early spring. The winged fruits mature in late spring and the best germination occurs if they are sown ­immediately, though normally well under 50% of the seed germinates.

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(A)

(B)

Fig. 38.  (A) English elm, Ulmus procera; (B) Wych elm, Ulmus glabra.

Timber and uses The wood of wych elm is very similar to that of English elm (see p. 344). The average density at 15% moisture content is about 690 kg m−3, heavier than English elm. Brown and Nisbet (1894) believed that the tree forms a beautiful ­object in the landscape and ‘cannot be surpassed in the general picturesque ­effect and grace of its outline’. Place of wych elm in British forestry In common with English elm, wych elm has no current place in commercial British forestry. It is likely, however, to remain a valued but minor component of broadleaved woodland.

ULMUS PROCERA Salisb.

English elm

Origin and distribution Once believed to be native, the English elm is now thought to have originated from Italy. Gil et al. (2004) carried out a survey of genetic diversity of elms and showed that English elms are genetically the same and are ­derived from clones of a single tree, the Atinian elm in central Italy. Material cut from it was once widely used for training vines. Clones were originally taken from Italy to Spain by the Romans, and from Spain to Britain. U. procera is not a tree of dense forest, but rather one of open landscapes and hedgerows.

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Silviculture Where it exists, English elm will grow to about 35 m tall, but within most of its range it is now found only as suckers of 4–5 m tall. The English elm was rarely found in woodland but was widely planted when hawthorn hedgerows were being established during the enclosure movements from 1550 to 1850. English elms are particularly well adapted to growing in hedgerows, being able to establish vigorously from suckers, which spring up in profusion from roots of felled trees. English elms are strong light demanders, and they are among the best trees at tolerating atmospheric pollution, so were consequently popular urban street trees. They also tolerate salty winds from the sea. They grow best on fertile, generally heavy, lowland soils. They flower in early spring, before the leaves emerge. The fruits are usually sterile. Galls on elm leaves are caused by several species of aphids, some of which migrate in summer to the roots of economic plants and cause damage: Eriosoma lanuginosa to the roots of pear trees, E. ulmi to currants and gooseberries and Tetraneura ulmi to grasses and cereals (Blackman, 1974). Evans (1984) estimated the yield class (YC) at between 4 and 6 m3 ha–1 year–1.

Dutch elm disease Dutch elm disease is named after the country in which it was first identified in 1921. It did not originate in The Netherlands and neither is it specific to the cultivar known in Britain as Dutch elm (U. × hollandica – a hybrid between wych elm, U. glabra, and field elm, U. minor). A great deal of information is available about the disease on the Forest Research website1 but, in summary, the current virulent strain that has ­resulted in so much mortality of elms is caused by an ascomycete fungus, Ophiostoma novo-ulmi, and is spread from tree to tree mainly by the large elm bark beetle, Scolytus. It has been estimated that 25 million trees were killed in Britain alone in the 1970s (Gil et al., 2004). Trees usually die within a few months of becoming infected. Unfortunately, interest in elms today is largely academic because, since the advent of the virulent strain of Dutch elm disease, it has been impossible to plant the species with any prospect of it surviving. Young suckers remain common in hedgerows, but once they reach diameters of about 10 cm and heights of 5 m the bark becomes thick enough for beetles to breed under, and they are soon attacked and die. The disease has caused 1

  https://www.forestry.gov.uk/fr/treespecies, accessed 23 January 2018.

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a profound change to the character of the English midlands in particular, where elms were once a notable feature of the agricultural landscape. Work on the selection and breeding of elms resistant to Dutch elm disease has been carried out since the 1930s, especially in The Netherlands (Burdekin and Rushforth, 1996). This approach is showing some promise and a few allegedly resistant clones have been produced, but much still needs to be done before they can be planted with any confidence. In Britain there are some Dutch elm disease ‘control’ areas, including Brighton. The town is largely protected from the disease by the English Channel to the south and the South Downs to the north. These two barriers prevent the large elm bark beetle getting to the area. The local authority is also both conscientious and vigilant in removing infected sections of trees immediately if they show symptoms of the disease. Some 15,000 mature trees lining streets and in parks still survive in Brighton. The largest numbers in Europe are in Amsterdam and The Hague. Timber and uses The ring-porous wood of elm has an interlocking grain, and because of this it is notable for resisting strains that cause other timbers to split. Its value arises mostly from this attribute for making seats of chairs, hubs of wooden wheels, jetties, piers and lock gates. It was also a traditional timber for making coffins. It is resistant to decay when permanently wet, and this property led, in mediaeval times, to lengths being hollowed out to make water pipes (Edlin, 1965). It was also used for timber framing but is less durable than oak for this purpose. Today the main use of the dwindling stocks of elm timber is for making furniture. The average density of the wood at 15% moisture content is about 560 kg m−3. It has a tendency to shrink and swell and is difficult to work. Elm also has the reputation for being shaken quite commonly from ground level up to the branches. Elm leaves were, and in some countries still are, used as fodder for many domestic animals and were lopped or ‘shredded’ for this purpose. Place of English elm in British forestry Because of Dutch elm disease English elms have no current place in British forestry.

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Wilson, S.McG. (2010) Minor conifers in Britain: potential for silviculture and timber utilization. Quarterly Journal of Forestry 104, 29–42. Wilson, S.McG. (2011) Using alternative conifer species for productive forestry in Scotland. Forestry Commission Scotland, Edinburgh. Wilson, S.McG. (2012) Retaining timber potential after PAWS restoration. Quarterly Journal of Forestry 106, 105–118. Wilson, S.McG. (2014) Living with climate change: Mediterranean trees and agroforestry in Britain. Quarterly Journal of Forestry 108(2), 90–101. Wilson, S.McG., Mason, B., Savill, P. and Jinks, R. (2017) Noble hardwood alternatives to ash. Quarterly Journal of Forestry, 111(3), 66–182. Winfield, M.O., Arnold, G.M., Cooper, F., Ray, M. le, White, J., Karp, A. and Edwards, K.J. (1998) A study of genetic diversity in Populus nigra subsp. betulifolia in the Upper Severn area of the UK using AFLP markers. Molecular Ecology 7, 3–10. Winter, T.G. (1978) A seed wasp affecting the wild service tree (Sorbus torminalis). Arboriculture Research Note 3. Department of the Environment, London, pp. 2. Wood, R.F. and Nimmo, M. (1962) Chalk downland afforestation. Forestry Commission Bulletin No. 34. HMSO, London, pp. 75. Worrell, R. (1992) A comparison between European continental and British provenances of some British native trees: growth, survival and stem form. Forestry 65, 253–280. Worrell, R. (1995a) European aspen (Populus tremula L.): a review with particular reference to Scotland. I. Distribution, ecology and genetic variation. Forestry 68, 93–105. Worrell, R. (1995b) European aspen (Populus tremula L.): a review with particular reference to Scotland II. Values, silviculture and utilization. Forestry 68, 231–243. Wylder, B., Biddle, M., King, K., Baden, R. and Webber, J. (2018) Evidence from mortality dating of Fraxinus excelsior indicates ash dieback (Hymenoscyphus fraxineus) was active in England in 2004–2005. Available at: https://doi.org/10.1093/­ forestry/cpx059, accessed 12 July 2018. Yang, K.C., Chen, Y.S., Chiu, C. and Hazenberg, G. (1994) Formation and vertical distribution of sapwood and heartwood in Cryptomeria japonica D. Don. Trees 9, 35–40. Young, C.W.T. (1978) Sooty bark disease of sycamore. Arboricultural Leaflet No. 3. Forestry Commission Research Station, Wrecclesham, UK, pp. 8. Zlatanov, T. (2006) Successional trends in pedunculate oak (Quercus robur L.) dominated forests along the upper reaches of Tundzha river – Southern Bulgaria. Centralblatt fur das gesamte Forstwesen 123(4), 185–197. Zobel, D.B. (1990) Chamaecyparis lawsoniana (A. Murr.) Pad. Port-Orford-Cedar. In: Burns, R.M. and Honkala, B.H. (technical co-ordinators) Silvics of North America: 1. Conifers. US Department of Agriculture, Forest Service, Washington DC, pp. 88–93.

FIELD KEY FOR IDENTIFICATION OF COMMON FOREST AND WOODLAND TREES This key has been devised for use only in British forests and woodlands – not in parks and gardens. If used outside woodlands it could prove very misleading and other publications should be consulted instead, such as A. Mitchell’s (1974) Trees of Great Britain and Northern Europe (Collins). This key should cover at least 95% of all trees likely to be encountered, native and exotic, that are capable of growing to heights of 20 m or more. A few generally much smaller but common broadleaved species have been included too, namely holly, rowan, hazel and field maple. The choice of the 28 broadleaved species and 14 conifers included in the key (about half of the total number covered in this book) has inevitably been arbitrary. To use the key, first decide whether the tree is broadleaved or coniferous. Broadleaved species typically have: • Leaves that are usually flat and thin; some are hard, dark and evergreen, but not needle-like nor linear. • A deciduous habit – the leaves fall in autumn (except holly). • Poor apical dominance: the stem divides into many branches long before the top of the tree (poplars and cherry are often exceptions). • The trees produce seeds from flowers or catkins (though female catkins of alder are hard and cone-like). Conifers typically have: • Leaves (needles) that are many times longer than broad, or sometimes scale-like leaves. • An evergreen habit (larches are exceptions). • Good apical dominance – the trunk is continuous from the bottom to the top of the tree. • Seed produced in cones (yew is an exception). When you are sure which type of tree you are looking at, use the appropriate key, with a leaf or branchlet in your hand, to determine which of two or more descriptions fits best. Each description has a number or name to the right of it. If a number, find this in the left-hand column, and continue in this way till you find the name of the tree. Characteristics shown in italics are particularly useful for identifying the species.

© Peter Savill 2019. The Silviculture of Trees Used in British Forestry, 3rd Edn (P. Savill)

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Key for Broadleaved Species

KEY FOR BROADLEAVED SPECIES 1

(a) (b) (c) (d)

2

(a)

(b)

3

(a) (b) (c)

4

(a) (b)

5

(a)

(b) (c) 6

(a) (b)

7

(a)

(b)

Leaves compound, consisting of several distinct leaflets growing from the leaf stalk. Leaves with lobes or deep indentations around the margins. Leaves simple, without lobes or deep indentations. Evergreen with glossy dark-green, hard leaves; 6–8 cm long, in lower parts of crown, with 6–8 spreading, sharp, yellowish spines.

2

Leaves and leaflets opposite. Leaflets usually more than 6 cm long. Bark light coloured, smooth at first, becoming evenly and shallowly fissured in older trees. Buds black and velvety. Seed winged. Leaves alternate, leaflets opposite, regularly toothed, not more than 6 cm long. Flowers white; fruits red. Small tree to 15 m.

Ash (Fraxinus excelsior)

Leaves longer than broad and lobed, usually arising in clusters at the ends of twigs. Bark evenly fissured. Crown large. Bears acorns. Leaf about as broad as long, with 5 (occasionally 3) lobes pointing outwards. Leaves opposite. Seeds winged and in pairs. Leaves similar to (b) but alternate, often with fewer than 5 lobes.

4 – Oaks

Leaves wedge-shaped at base; petiole long, 1–2.5 cm. Acorns in clusters of 2–6, on very short stalks of 5–10 mm. Leaves with auricles at base; petiole short, 3–7 mm. Acorns in clusters of 2–3, on long stalk of 4–8 cm. (Intermediate forms between these two sometimes occur, indicating hybrids.)

Sessile oak (Quercus petraea)

Leaf stalks often red, to 15 cm; leaf up to 18–26 cm, with 3 main lobes and 2 minor basal lobes; very coarsely and unevenly toothed; leathery and dark. A large tree to >30 m. Leaf stalk to 15 cm; leaf 12–15 cm, similar to (a) except leaf is thin-textured, bright, shiny green, with finely pointed lobes and teeth. Leaf stalk to 5 cm; leaf to 8–12 cm; teeth rounded. Usually a small tree, rarely to 20 m.

Sycamore (Acer pseudoplatanus)

Leaves with 3–5 pairs of lobes, rather like a maple, but alternate, 8–10 cm. Buds globular. Leaves of sucker shoots and ends of long shoots deeply palmately 5-lobed; those of short shoots almost circular, broadest below the middle. All leaves persistently hairy white beneath. Petioles flattened.

Wild service tree (Torminaria torminalis) White poplar (Populus alba)

Leaf tips broadly rounded or slightly indented, 7–10 cm; margin slightly wavy, irregularly and very shallowly toothed, dark green, with 7 pairs of veins. Buds on 3-mm stalks. Female catkins hard and cone-like. Generally found near streams or in damp places. Leaf tips pointed.

Alder (Alnus glutinosa)

3 7 Holly (Ilex aquifolium)

Rowan (Sorbus aucuparia)

5 – Maples 6

Pedunculate oak (Quercus robur)

Norway maple (Acer platanoides) Field maple (Acer campestre)

8

Key for Broadleaved Species

377

Holly (upper crown)

Holly (lower crown)

Ash

Rowan

Sessile oak

Pedunculate oak

Sycamore

Norway maple

Wild service tree

White poplar

White poplar (under surface)

Field maple

Alder

378

Key for Broadleaved Species

8

(a) (b)

Leaves very hairy and roughened above. Leaves smooth or at least not roughened above.

9 10

9

(a)

Bark smooth, silvery for many years, becoming darker and ridged with age. Leaves quite large shouldered with abrupt long points, 10–18 cm × 6–9 cm, very unequal at base, ca 17 pairs of veins and very harshly hairy above. Leaf stalks short (