The World of Northern Evergreens, Second Edition 9780801463037

Table of contents :
Contents
Preface to the Second Edition
Preface to the First Edition
1. Origin of the Evergreen Forests
2. Identifying the Conifers
3. Reproduction of Conifers
4. The Life and Growth of a Conifer
5. Broadleafs Growing among the Conifers
6. Two Kinds of Trees: Conifers and Broadleafs
7. Life on the Forest Floor
8. Parasites on the Conifers
9. Insects and Conifers
10. Some Mammals and Birds of the Forest
11. Natural and Unnatural Interference
12. The Big Picture
13. Global Warming and the Forests
Index

Citation preview

THE WORLD OF

Northern Evergreens

other books by e. c. pielou After the Ice Age: The Return of Life to Glaciated North America A Naturalist’s Guide to the Arctic Fresh Water The Energy of Nature

THE WORLD OF

Northern Evergreens second edition

E. C. Pielou

Comstock Publishing Associates a division of Cornell University Press ithaca and london

Copyright © 2011 by Cornell University All rights reserved. Except for brief quotations in a review, this book, or parts thereof, must not be reproduced in any form without permission in writing from the publisher. For information, address Cornell University Press, Sage House, 512 East State Street, Ithaca, New York 14850. First published 2011 by Cornell University Press First printing, Cornell Paperbacks, 2011 Printed in the United States of America Library of Congress Cataloging-in-Publication Data Pielou, E. C. The world of northern evergreens / E.C. Pielou. — 2nd ed. p. cm. Includes bibliographical references and index. ISBN 978-0-8014-7740-9 (pbk. : alk. paper) 1. Conifers—North America. 2. Evergreens —North America. 3. Forest ecology—North America. I. Title. QK494.P54 2011 585'.2097—dc22 2011011639 Cornell University Press strives to use environmentally responsible suppliers and materials to the fullest extent possible in the publishing of its books. Such materials include vegetable-based, low-VOC inks and acid-free papers that are recycled, totally chlorine-free, or partly composed of nonwood fibers. For further information, visit our website at www.cornellpress.cornell.edu. Paperback printing

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In Memory of Patrick and Frank

Contents

Preface to the Second Edition ix Preface to the First Edition xi

1 Origin of the Evergreen Forests 1 Conifers and the Ice Age, 1 The Advantages of Being Evergreen, 3 The Advantages of Long-Lived Leaves, 4 Enduring the Cold, 5

2 Identifying the Conifers 7 How Plants (Including Trees) Are Classified, 7 The Ten Genera, 8 The Thirty-Two Species, 15 Conifer Families, 33

3 Reproduction of Conifers 35 Pollen Cones and Pollen, 35 Pollination, 37 A Contrast between Seed Cones and Pollen Cones, 39 Vegetative Reproduction, 40

4 The Life and Growth of a Conifer 43 Wood, 43 Cut Stumps and Whole Trees, 45 Cambium, 50 Leaves, 53 Roots, 55

Outside the

5 Broadleafs Growing among the Conifers 59 Broadleafs in a Harsh Climate, 59 Alders, 65

Poplars, 61

Birches, 64

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6 Two Kinds of Trees: Conifers and Broadleafs 67 Introduction, 67 The Ancestry of “Trees,” 67 The Basic Difference between Conifers and Broadleafs, 68 Gymnosperms Are Woody, 69 The Speed of Living, 70 The Architecture of Trees, 71 Vegetative Reproduction, 72 The Aroma of Conifers, 72

7 Life on the Forest Floor 74 The Soil, 74 Forest Flowers, 75 The Floor of the Boreal Forest, 78 Valuable Dead Wood and Debris, 79 Open Water, 81

8 Parasites on the Conifers 84 The Value of Rot and Decay, 84 Dwarf Mistletoe, 90

Decay Fungi, 84

Rusts, 86

9 Insects and Conifers 93 Insects as Feeders, 93 Beetles, 94 Caterpillars and Pseudocaterpillars, 97 Sawflies, 101 Bugs, 101 Parasitoids, 105 Ants and Others, 106

10 Some Mammals and Birds of the Forest 108 Food and Shelter, 108 Seldom Seen Mammals, 108 Squirrels and Their Relatives, 111 A Rodent and a Lagomorph, 113 Big Herbivores, 115 Carnivores, 117 Big Omnivores, 118 Birds, 119

11 Natural and Unnatural Interference 125 Fire, 125 Forest Succession, 129 Snow and Wind, 130 and Acid Rain, 134 Logging, 135

Air Pollution

12 The Big Picture 138 Introduction, 138 Forest Regions, 138 Grow Where? 141

What Controls Which Species

13 Global Warming and the Forests 143 Introduction, 143 The Physics of Climate Change, 143 How Will Climate Change Affect the Forests? 145 Fire and the Forests, 147 Insects, Lightning, Wind, and Snow (Again), 148 The Value (If Any) of Predictions, 149

Index

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Preface to the Second Edition

The world has changed since 1988 (the year when the first edition of this book appeared). At last it is dawning on governments that forests are more than just a source of timber. They provide, as well, indispensable “ecological services.” Were they to disappear, climate change would speed up because the world would lose its greatest carbon sink. Ways to estimate the monetary worth of ecological services have recently been devised. So far, they have been carried out in detail in only a few places in the world. For example,* a closed-canopy forest in Kenya (East Africa) was found to supply $320 million in services, every year, from 1600 square miles (about 4100 km2). As the true worth of forests comes to be appreciated, naturalists’ knowledge is regarded with more respect than it was in the days when their activities were looked on as no more than an enjoyable hobby. Their expertise has become useful and widely appreciated. The purpose of this new edition is to introduce new material on the evergreens in northern North America and to bring the earlier book up-to-date. Some particulars: I have described the contrast between conifers and broadleafs (formerly, and less precisely, known as “hardwoods”) in much more detail. The enormous gap between these two kinds of plants is * Jen Fela, “Reforestation Key to Economic Growth in Kenya,” in Frontiers in Ecology and the Environment, vol. 8, no. 2, 2010, p. 63.

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obscured by labeling them all simply as “trees.” It conceals their great dissimilarity. They have been evolving divergently from each other for more than a hundred million years (see chapter 6). New chapters are devoted to the forest floor (chapter 7) and to the geographical extents of different forest ecosystems (chapter 12). The effects of logging are discussed in chapter 11, and how global warming is affecting the forests and vice versa, in chapter 13. Some paragraphs have been added on animals whose connections with their special habitats are unusually close, for example, caribou, some grizzly bears, and beavers. And much else besides. No branch of science, and that includes natural history, ever remains static. E. C. Pielou Comox, British Columbia

Preface to the First Edition

For many people, certainly for the majority of North Americans with homes in the northern half of the continent, coniferous trees constitute a large fraction of all the “living material” they will see in a lifetime. Naturalists, hikers, backpackers, canoeists, cross-country skiers—in fact all those whose work or recreation takes them outdoors in northern North America—are accustomed to seeing coniferous trees by the tens of millions, whether they consciously notice them or not. Outdoor people have a wide spectrum of interests. Specialists tend to specialize in “interesting” items: a birder is more likely to concentrate on owls than on starlings, and the average plant-hunter finds orchids more fascinating than crabgrass. Because of this preoccupation with the hardto-find, the beautiful, and the unusual, most of the commonest objects in nature are apt to be ignored. They are simply there, part of the background. But to assume that because a thing is common it is therefore uninteresting is a mistake. For most outdoor people the fact that they will encounter rank upon rank of coniferous trees in excursion after excursion in the future neither pleases nor displeases them. They don’t even think about it. If there are innumerable coniferous trees in your future, why not take advantage of the fact, look at the trees more closely, and learn something about them? Knowledge cannot fail to bring interest and appreciation. Learning to identify the different species of coniferous trees is only a beginning. Once you know the trees, many things can be observed if you xi

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know what to look for. There is, however, a world of difference between seeing and interpreting. The ability to interpret is the hallmark of the true naturalist, and developing that ability is one of the pleasures of being a naturalist. The well-informed naturalist understands and enjoys a thousand things that the uninformed one doesn’t even notice; and the more people who understand and enjoy the woods, the more there will be to protect them. E. C. Pielou Denman Island, British Columbia

THE WORLD OF

Northern Evergreens

Chapter 1

Origin of the Evergreen Forests

Conifers and the Ice Age Of all the people who enter a northern forest, only a handful ever ask themselves these two questions: Where has the forest come from? And why are the great majority of its trees conifers rather than deciduous, broadleafed trees —“broadleafs” for short. The answers are by no means obvious. The questions have engaged the interests of ecologists and motivated years of research. What has been investigated and measured is not, for the most part, observable on a hike in the woods, but the hike would certainly be more interesting for somebody knowing about the questions, and the answers that have been discovered so far. Consider the first question, where have the trees come from? The only certain answer is that the trees in the area we are concerned with, the area that was ice-covered at the end of the last ice age, must have descended from ancestors that lived elsewhere. About 18,000 years ago, when the ice sheets of the most recent ice age had reached their maximum extent (figure 1.1), they covered nearly all of northern North America. The glaciated areas must have been like Greenland and Antarctica were until recently—barren expanses of ice, devoid of plant life. Then, as now, the ice was on the verge of disappearing. Conditions in the unglaciated regions near the margin of the ice sheets must have been bleak. The contrast with conditions now is worth contemplating. 1

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Figure 1.1. The area in North America covered by ice sheets at the end of the last ice age.

Once the climate began to warm up and the ice sheets started shrinking, newly exposed land became available for small plants, and hardy evergreen forests gradually invaded the area. Seeds were blown in from the south. At the time of maximum glaciation, evergreen forests stretched across the continent south of the ice margin; even the Great Plains were forested. East of the continental divide the most abundant trees were spruces and jack pines (the evidence that allows us to visualize the forests of the distant past is described in chapter 3). The forests south of the ice on the west coast had a richer mix of tree species and may have been much like they are now. The northward march of the forests into the newly ice-free land was inevitably slow. There was no soil to start with—only lifeless mixtures of boulders, gravel, sand, and clay, laid bare by the melting ice. The development of soil adequate for trees must have taken a considerable time. Different

origin of the evergreen forests

species of trees arrived to occupy their present geographic ranges at different times. Those that survived the ice age farther south than the spruces and jack pines had farther to come. And those that require shade had to wait until forests of sun-loving trees were casting enough shade for the shade-growers to invade. For example, hemlock is believed to have reached what is now northern Michigan about 3000 years later than white pine.1 The climate continued to change as it has done throughout time, independently of the current global warming (the topic of chapter 13). It was thought to have warmed fairly steadily between 18,000 and 10,000 years ago, except for a temporary cold interval around 12,000 years ago. In any case, warming resumed and temperatures reached a maximum about 8000 years ago, even before the ice sheets had completely melted. Because of the very long time required to melt huge masses of ice at the thenprevailing temperatures, probably around 2° Celsius higher than now (2010), two remnants of the original ice sheets persisted, one on each side of Hudson Bay, until 6500 years ago. They finally disappeared after the time of maximum warmth. Because of the slight downward trend of temperatures until very recently, some species of evergreen have lost ground and do not, nowadays, grow as far north as they did 8000 years ago.

The Advantages of Being Evergreen The second of the two questions posed at the beginning of this chapter was, why are the great majority of the trees in the northern forests evergreen conifers? Why conifers rather than broadleafs? To say that conifers are better adapted to the environment is no answer. The question then simply becomes, in what way are they better adapted? The answer cannot be simple, because conifers do better than broadleafs in two contrasting environments. They obviously thrive in the mild, moist climate of the Pacific coast, and they also prosper, if less luxuriantly, in the harsh cold, dry climate of the subarctic. Consider the Pacific coast rain forest first. The mild winters and abundant rain create an ideal climate for many trees, not only for conifers. Poplars, aspens, and alders (about which more in chapter 5) flourish in rain forest and grow much larger than do inland representatives of their species. But the magnificent conifers, the enormous trees that arouse the wonder of everyone who sees them, greatly outnumber the broadleafs.

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It is believed that it is their evergreenness that makes conifers superior to broadleafs in the Pacific Northwest. Because they have green leaves all through the year, conifers can perform photosynthesis whenever the weather is warm enough. They can profit from warm spells in late fall, early spring, and even in winter, when the broadleafs are leafless. Therefore, they are actively growing during a much larger fraction of the year than is possible for broadleafs. For rain forest, the most stressful time of the year comes during the summer drought. This is unusual, for everywhere else in our area winter is more stressful than summer. Water shortage affects both conifers and broadleafs, but the broadleafs are much less able to cope with this adversity because their big, thin leaves dry out more quickly than do the small, needlelike leaves of the conifers (more on this in chapter 4). Thus the annual drought comes at a time when the broadleafs are most vulnerable, the time when they are in leaf. In the north country, on the other hand, the ability to photosynthesize on any warm day of the year is no use to a tree growing where the only warm days are in summer. There must be some other explanation for the success of conifers in the vastly different conditions of the far north.

The Advantages of Long-Lived Leaves The success of evergreen conifers vis-à-vis broadleafs in the north probably has less to do with the effect of cold on the trees, and more to do with its effects on the soil. In a cold climate, dead vegetation takes a long time to decay because the bacteria that bring about decay act slowly in the cold. In a word, humus is slow to form. And if the ground is dry, dead vegetation simply dries out, without forming humus. Some dry soils are little more than dry sand, and others are merely thin, dusty coatings over hard bedrock. (For more on forest soil, see chapter 7.) The question that now arises is, why should conifers do better than broadleafs on poor soils? Once again, evergreen leaves probably confer an advantage. In this case the advantage is that evergreen leaves last for several years. Therefore an evergreen conifer, unlike a deciduous broadleaf, does not have to grow a new set of leaves every spring, and its demands for nourishment are correspondingly less. Moreover, evergreen leaves can begin photosynthesis earlier in the year than leaves on deciduous trees can, because the latter have to grow before they can function.

origin of the evergreen forests

The deciduous conifers in our area (tamarack and two other larch species, or collectively, “larches”) are an awkward exception to this generalization. How can they “afford” to grow new sets of leaves each year? A strong contributing cause is this: Larch leaves are small and widely spaced so that on a given tree, they shade one another to a lesser degree than do those on both evergreen trees and broadleafs. All larch leaves are more or less equally well illuminated and photosynthesize sugars with maximum efficiency.2 This accords with the subjective impression of lightness and brightness that larches give, and it is also an objective, scientifically measurable fact. The conclusion is that larches are as well adapted as evergreen conifers to the evergreen forest habitat; they will be treated as “honorary evergreens” throughout this book. Compared with broadleafs, conifers lead more frugal lives. Their life processes take place more slowly (details in chapter 6). In short, conifers live on a “waste-not-want-not” system; broadleafs, on an “easy-comeeasy-go” system. This enables conifers to “make do” with inferior soils.

Enduring the Cold If there is one adaptation that is more important than any other to a very far northern tree, it is cold hardiness. Trees and tall shrubs have to endure the lowest air temperatures that winter brings, temperatures that would kill a lightly clothed person in a very short time. Ground-creeping shrubs, and herbaceous plants that die back to ground level in winter are insulated, respectively, by the snow blanket that covers them and by the soil they are imbedded in. All the trees in our area, broadleafs as well as conifers, are hardy in the ordinary, gardener’s sense of the word. That is, they are unharmed by comparatively mild frosts, provided they have had time to become dormant before freezing weather sets in. But from a tree’s point of view, there are two sharply different intensities of frost, mild and severe, with the dividing line between them at a temperature of about – 40° Celsius (– 40° F). The trees in our area are adapted to escape frost injury in one of two quite different ways: some do it by supercooling; the rest, by extracellular freezing. Supercooling protects only at temperatures above – 40°, whereas extracellular freezing is effective at any naturally occurring temperature, however low. Therefore we have two kinds of trees, hardy and very hardy. The hardy trees are those that rely on supercooling to survive the winter;

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they cannot grow where minimum winter temperatures fall below the critical value. The very hardy trees are those capable of extracellular freezing. They can survive even in the neighborhood of Snag, in Yukon Territory south of the Arctic Circle, which holds the venerable record, set in 1947, of having once been the coldest spot in North America. The two frost-proofing methods used by trees work like this.3 In hardy trees, the liquids inside the cells become supercooled (that is, they don’t freeze) as the temperatures drop because there are no minute particles in the cells, or roughening on their inner walls, to act as nuclei around which ice crystals can begin to form. Below – 40°, they freeze anyway. In very hardy trees, the liquids inside the cells ooze out into the numerous spaces among the cells of the tissues at risk, and freeze in these spaces. This is extracellular freezing. The ice crystals, being outside the cells, do no harm. Only four conifers are capable of extracellular freezing: jack pine, tamarack, and white and black spruce. And only three broadleaf trees are equally hardy: paper birch, trembling aspen, and balsam poplar. These are the only trees that grow in the very northernmost forests. Notes 1. M. B. Davis, “Quaternary History and the Stability of Forest Communities,” in Forest Succession: Concepts and Applications, D. C. West, H. H. Shugart, and J. B. Botkin, eds. (New York: Springer, 1981). 2. S. T. Gower and J. H. Richards, “Larches: Deciduous Conifers in an Evergreen World,” in BioScience, vol. 40, 1990, pp. 818–26. 3. M. J. Burke et al., “Freezing and Injury in Plants,” in Annual Review of Plant Physiology, vol. 27, 1976, pp. 507–28.

Chapter 2

Identifying the Conifers

How Plants (Including Trees) Are Classified The trees considered in this book are the evergreen cone-bearing trees of once-glaciated North America, plus the larches (which aren’t evergreen) and the junipers and yews (whose cones resemble berries). The shrub junipers and yews (for not all junipers and yews are trees) are mentioned in passing. What unites these plants in the botanical sense is explained in chapters 3 and 4. That they form a cohesive group, resembling one another much more closely than any of them resembles a broadleaf tree, nobody would deny. The resemblances among some of them are very close, making it necessary to examine them carefully if they are to be correctly identified. First we consider the various groups of conifers and how they can be told apart, but to begin, a few paragraphs on the naming of plants will be helpful. The English names of the various groups of conifers are familiar to most people: the pines, the spruces, the firs, and so on. Each group is technically known as a “genus” (plural, “genera”), and belonging to each genus are one or more “species,” the members of the genus. (The fact that the word “species” is the same in the plural as in the singular causes frequent misunderstandings: in writing, relief is obtained by using the abbreviation “sp.” for the singular and “spp.” for the plural.) The scientific Latin name for every species of living organism is made up of two words. The first (always beginning with a capital letter) is the 7

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name of the genus. For example, all pines have the generic name Pinus; the specific names of two of the pines, lodgepole pine and red pine, are Pinus contorta and Pinus resinosa, respectively. When several species in the same genus are mentioned, the name of the genus is written in full only the first time; after that its initial suffices. Thus we would write “Pinus contorta and P. resinosa.” In formal scientific writing, the name (usually abbreviated) of the botanist who first discovered the species is added at the end of the two-part Latin name. Thus the full name of lodgepole pine is Pinus contorta Dougl. Here “Dougl.” is short for “David Douglas,” the Scottish explorer and plant hunter who collected in British Columbia and the western United States in the 1820s and 1830s. Plant names are always printed with the Latin part in italics, and the discoverer’s name, when it is given, in roman type. Sometimes two trees in different genera have the same specific name; for instance, the western larch is Larix occidentalis and the eastern white-cedar is Thuja occidentalis. Possession of the same species name does not mean that the two species are related any more than possession of the same first name means that two people are related. The species name merely singles out a particular member of a genus. Now we can proceed with the genera and species of conifers that grow in our area.

The Ten Genera Consider the ten genera of conifers in our area. They are the pines (Pinus), the larches (Larix), the spruces (Picea), the firs (Abies), the hemlocks (Tsuga), the Douglas-firs (Pseudotsuga), the thujas or “cedars” (Thuja), Nootka-cypress (Callitropsis, until recently Chamaecyparis), the junipers ( Juniperus), and the yews (Taxus). The reason for writing “cedars” with quotation marks will become clear later. In practice, it is better to drop the word and use “thuja” (pronounced “thuya”) as though it were English. For the moment, we consider only the characteristics of each genus that single it out from all the other genera, in a word, the diagnostic characters. These are the bare minimum of characters you must know in order to recognize a coniferous genus with certainty. There’s very little to memorize.

The Pines. In the pines, and only in the pines, the leaves are true “needles,” and in our area, they grow in bundles (fascicles) of at least two and at most five needles (figure 2.1).

identifying the conifers

Figure 2.1. Pine twigs.

The Larches. The next instantly recognizable genus is Larix, the larches (the most common species is also called “tamarack”). They are the only deciduous conifers in our area. In summer the leaves, which grow in tufts of up to 50 from dwarf, stubby twigs, are soft to the touch and apple-green. In the fall, they turn deep yellow or golden, and they are shed before winter. In its leafless winter state, a larch tree is still unmistakable and impossible to confuse with a broadleaf because a larch’s twigs and branches are covered with very distinctive knobbles (figure 2.2). These are the dwarf “spur” twigs that bear the leaves in summer. The Spruces. Most spruces are easy to recognize because their leaves are square in cross section like a wooden match; and like a match, a spruce needle will roll between your thumb and forefinger. This method of recognition works everywhere except on the west coast, where Sitka spruce (Picea sitchensis) grows; its needles, though four-cornered, are too flat to roll. Therefore on the west coast, it is necessary to use another characteristic of spruces. If you look at the dead branches, which are nearly always to be found low on a spruce tree, you will see that they are very rough, being covered with numerous small, hard “pegs.” These are persistent woody leaf stalks that remain on the twigs after the leaves have died and fallen (figure 2.3). Be sure that the twig you examine is not so old that the bark has peeled off. Not only the pegs but also the deep grooves on the twig are characteristic. You can recognize a spruce anywhere by its dead twigs. A character shared by the next four genera, hemlocks, firs, Douglas-firs, and yews, is their flat narrow leaves (often called needles in spite of their

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Figure 2.2. Larch twigs. (a) In winter; the “knobbles” are the short shoots that bear the numerous leaves in each cluster. (b) In summer.

Figure 2.3. Spruce twigs. (a) At small scale, showing rough “pegs.” (b) Close-up, showing angular leaves and details of the pegs.

identifying the conifers

flatness). In hemlocks, some firs, and yews, the leaves are two-ranked (that is, in two rows, one on each side of the twig). In spruces, Douglas-firs, and occasionally firs, they grow in all directions, making a leafy twig look like a bottle brush. The bottle-brush form is often partial and asymmetrical in twigs exposed to sunshine. Examples are shown in figure 2.4.

The Firs. The firs are quite easy to recognize (figure 2.5). They have two dependable diagnostic characters. First, the bark: except when it’s very old, the bark is conspicuously smooth and pocked with resin blisters from which sticky resin pours if you pierce the blister. The trunks of old trees, although fissured at the bottom, still resemble those of young trees near the top. Second, look at the base of a leafy twig where some of the leaves

Figure 2.4. End view of twigs of (a) Douglas-fir, (b) Sitka spruce (note the sharpness of the leaf tips), and (c) grand fir.

Figure 2.5. Fir. (a) Trunk, before old age, to show its smoothness and the abundant resin blisters. (b) Twig, with leaves and round, flat, leaf scars.

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have fallen: the leaf scars are circular and flat (sometimes even recessed), making the twig smooth to the touch, in contrast to the very rough twigs of spruces and the fairly rough ones of hemlocks.

The Hemlocks. A hemlock is most easily recognized by standing back and looking at the whole—if that’s possible. It is much more graceful, more lacey and feathery, than other conifers. The reason is that the leaves are small and flat, and the outer ends of the branches are slender, flexible, and drooping. If you stand under a tall hemlock, it often seems that the branches form a hollow dome above you. No other characters are the same in all three species of the genus, and they will be described below, where we consider the separate species. The Douglas-firs. First note the hyphen in the name. It’s there because a Douglas-fir is not a fir—that is, it is not a member of the genus Abies but is a member of its own genus, Pseudotsuga. A real fir, a balsam fir, for example, has no hyphen in its name. Douglas-firs grow only in the west, from the Rocky Mountains to the Pacific. Although we have not hitherto considered cones as a useful indicator of a tree’s genus (because they are often not present), it’s worth making an exception in the case of Douglas-firs because their cones are so distinctive and are nearly always present in abundance either on the tree itself or on the ground around it (just make sure the cones on the ground are indeed from the tree you are trying to identify!). Douglas-fir cones are on the tree for many months of the year; they are found from the uppermost down to the lowermost branches, whereas in trees of many other conifers, they are restricted to the uppermost branches. Figure 2.6 shows a Douglas-fir cone

Figure 2.6. Douglas-fir cone, and the back of a single scale with its tridentshaped bract.

identifying the conifers

and one of the detached tripartite bracts, which grow behind each cone scale and make identification of a Douglas-fir unquestionable.

The Thujas, Junipers, and Nootka-cypress. These three genera are clearly unlike those considered earlier, and they form a group whose salient character is the obvious difference between their leaves and those of other conifers. The leaves are small, overlapping scales, tightly pressed to the twigs like shingles on a roof (figure 2.7). They cannot be separated, and a twig tightly clothed in scale leaves is a single unit, conveniently called a “frond” or “branchlet.” The only exceptions to this generalization are two species of juniper: in one, a shrub, some of the leaves are needlelike, and in the other, also a shrub, all are needlelike. More details are given later. This is the place to explain why the word “cedars” appeared in quotation marks earlier. The North American trees called “cedars” are not cedars. True cedars such as cedar of Lebanon and deodar cedar are native to Eurasia and belong to the genus Cedrus. They are wholly unlike our two so-called cedars, which now have the official English names of “western redcedar” and “eastern white-cedar.” It is a mystery why one name is hyphenated but not the other. The muddle is compounded by the fact that one of the junipers (in the genus Juniperus) is called “eastern redcedar.” Then there’s the Nootka-cypress, which (note the hyphen) is not a true cypress. The member species of this trio of related genera are described later. The Yews. Only two yews grow in our area, and only one, found in the west, is a tree. Except when it has bright red arils (commonly called

Figure 2.7. Twigs with scale leaves. (a) Rocky Mountain juniper and (b) Redcedar.

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“berries” because that’s what they look like; more on them later), the tree is undistinguished, small, and dark, with thin purplish, peeling bark, and branches that often droop. Its leaves are mucronate, that is, with very short, sharp tips, best seen with a magnifier. More on our only yew tree with the species descriptions. What follows is a diagnostic key that summarizes the information needed to recognize each genus of conifer in our area. The key consists of a list of pairs of contradictory statements. The pairs are labeled 1, 2, 3, and so on, on the left. Start at the first pair and choose which of the two statements is correct. Then act on the instruction to the right of it, which will either be the name of your specimen’s genus or the number of the next pair of statements to choose between. One further point: The appearance of a tree’s bark is seldom used as an aid to identification because in many trees it’s too indefinite. All the truly distinctive barks are illustrated.

Key to the Genera of Conifers in Our Area 1a. 1b. 2a. 2b. 3a. 3b. 4a. 4b. 5a. 5b. 6a. 6b. 7a. 7b. 8a. 8b. 9a. 9b. 10a. 10b. 11a. 11b.

Leaves evergreen. ..................................................................... 2 Leaves not evergreen, apple-green in color. ........................ Larch Leaves not scalelike .................................................................. 3 Leaves scalelike. ..................................................................... 10 Leaves (needles) in fascicles (bundles) of two or more. ....... Pines Leaves growing singly .............................................................. 4 Trees ........................................................................................ 5 Shrubs ...................................................................................... 9 Leaves square in section (except Sitka spruce) from woody “leaf pegs” on twigs ............................................Spruces Leaves flat, not four-cornered ................................................... 6 Leaf scars flat, resin blisters on trunk ....................................Firs Not as above ............................................................................ 7 Cones with trident-shaped bracts .............................. Douglas-fir Not as above ............................................................................ 8 Trees with drooping or dangling tip ............................Hemlocks Leaves dark on top, paler beneath, no rows of white dots .. Yews Leaves spiky, sharp, and slightly curved ...........Common juniper Leaves dark on top, paler beneath, no rows of white dots .. Yews Branchlets bushy ............................................................. Juniper Branchlets forming flat sprays ................................................ 11 Bark brown, with long, vertical, fibrous ridges .................Thujas Bark pale, with short vertical, peeling scales ....... Nootka-cypress

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Making and using a key is fairly simple, but there are a few drawbacks. First, each description on the left must be true of all the species in the genus it leads to, which often means that a conspicuous characteristic of one of the species cannot be mentioned. Second, this key can be used in any season, which is why it makes no mention of cones (except for Douglas-fir, which nearly always provides cones). Third, a key should leave you in no doubt as to the correctness of your identification. But sometimes the description is unavoidably vague, for example, that for hemlocks. This makes it necessary to consult more complete descriptions, in this case by reading the species descriptions (given later) for all three hemlock species in our area. Fourth, always keep in mind that a key can be depended on only in the area specified in its title. If you try using it in another area, one with a different assemblage of plants, you may arrive at a wrong answer. Not every conifer you come across is easy to identify, even with a key. A tall tree in dense forest can present problems. Its lowest branches may be far overhead, making twigs, leaves, and cones unreachable. Young bark may not be visible, and old bark will often be overgrown with moss. And because it is surrounded by other trees, it may not have the shape characteristic of open-grown specimens. The difficulties become less as you grow familiar with the trees of a region, and familiarity is gained most rapidly if you first identify a number of “easy” specimens to gain a feel for each species.

The Thirty-Two Species Now we come to the identification of the species in each genus, for which it’s often necessary to consider the cones. All conifers bear two kinds of cones: seed-bearing, or female, cones, and pollen-bearing, or male, cones (the exception is the yews, in which seeds grow singly, in fleshy “berries,” rather than several in a cone). In pine, larch, spruce, fir, hemlock, Douglas-fir, thuja, and Nootka-cypress species, cones of both sexes are found on the same tree. In the junipers and yews, the sexes are separate; that is, a single tree (or shrub) is either male or female but not both. In all genera the pollen cones are small and short-lived. They are most noticeable in spring during the short period when they grow, expand, open,

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and shed their pollen. Soon afterward they wither and most of them drop off. For more on pollen cones, go to chapter 3, where the reproduction of conifers is considered. In this chapter all references to cones are to the seedbearing kind found on the trees or scattered on the forest floor. They will be referred to simply as “cones” in this chapter. Another fact about cones in general is that behind each of a cone’s woody scales, there is always a lesser “miniscale,” or bract, that sometimes protrudes noticeably and is useful in identifying a tree (see figure 2.6). The bracts are clearly separate from the scales in all genera except pines, in which they are fused together.

Pine Species. This genus has more species (nine in our area) than any other. The pines differ from all other genera (except Nootka-cypress and the junipers) in that their cones take two years to mature. So in most seasons you can find smaller, first-year cones and larger, mature, secondyear cones on a tree at the same time. The cones remain tightly closed until they are in their second year. Then (or in some species, several years later) the scales part and the winged seeds are shed (figure 2.8). The nine species of pines in our area belong to two distinct groups: hard pines and soft pines. They differ in several respects other than the hardness of their wood, and the differences are summarized in figure 2.9. Five of the pines (those with two or three needles per fascicle) are hard pines, and the other four (with five needles per fascicle) are soft pines. In all of them, the leaf fascicles are encased in scaly sheaths at first. These sheaths persist in the hard pines but are soon shed in the soft pines; see the

b

c

a Figure 2.8. Jack pine cones. (a) One year old. (b) Two years old or older. (c) Two winged seeds.

identifying the conifers

Figure 2.9. Distinguishing characters of (a) hard pines and (b) soft pines.

top pair of drawings in figure 2.9. The figure also shows how the shapes of the needles in cross section differ, and also the number of veins each needle has. Hard and soft pines are differentiated in the first pair of lines in the key below. After the key are a few more comments on the individual species.

Key to the Species of Pinus in Our Area 1a. 1b. 2a. 2b. 3a. 3b. 4a. 4b. 5a. 5b. 6a. 6b. 7a. 7b.

Two or three needles per fascicle (hard pines) ............................. 2 Five needles per fascicle (soft pines) ............................................ 6 Two needles per fascicle .............................................................. 3 Three needles per fascicle............................................................ 5 Western tree..................................Pinus contorta, Lodgepole pine Eastern tree................................................................................. 4 Needles at least 10 cm (4 in) long ................. P. resinosa, Red pine Needles not more than 6 cm long ..............P. banksiana, jack pine Western tree.....................................P. ponderosa, ponderosa pine Eastern tree..................................................... P. rigida, pitch pine Needles thick, stiff; mountain habitat ......................................... 7 Needles soft, slender; tall forest trees .......................................... 8 Cones over 7 cm (3 in) long, cylindrical....... P. flexilis, limber pine Cones less than 7 cm (3 in), egg-shaped ..........................................P. albicaulis,whitebark pine 8a. Eastern tree...................................... P. strobus, eastern white pine 8b. Western tree................................ P. monticola, western white pine

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Note that this key uses the range of a species (eastern or western) as a clue. This is not scientifically rigorous: there is always a chance of finding a species growing outside its previously known range, for how else could the limits of its known range at present have been discovered in the first place? However, to assume that your specimen is within its known range greatly reduces the number of details to be observed in identifying it. The two species of two-needle pines, namely, jack and lodgepole pines, are very similar and are closely related; together their geographical ranges extend almost across the continent. The ranges are contiguous and overlap in at least two places (figure 2.10); obvious hybrids can be found there. A note on the map: It does not show the entire ranges of the species, simply the boundary between their ranges; all that it tells you is that jack pine does not grow west of the boundary, nor lodgepole pine to the east of it. The two species have noticeably different cones (figure 2.11). Jack pine cones tend to be unsymmetrical and curved; they have tiny prickles (or, more often, none) on the back of each scale, and the cones point forward. Lodgepole cones are straighter, with big, sharp prickles, and they are usually bent back slightly.

Figure 2.10. The boundary separating the ranges of lodgepole pines and jack pines. Crosshatching shows areas where their ranges overlap and hybrids are found.

identifying the conifers

Figure 2.11. Cones of (a) jack pine and (b) lodgepole pine.

At first (as in the figure), the cones are closed. That is, the ripe seeds are held in by unopened cone scales, sometimes for more than 20 years. Only when the temperature is high do the cones open and allow the seeds to fall. The necessary heat comes most often from forest fires (in northernmost latitudes) or from hot sunshine (in more southern latitudes). The empty cones remain on the tree, so most trees are covered with a variety of cones—old cones, both open and closed ones, and one- or two-year-old cones containing seeds, as described earlier. An unfamiliar form of lodgepole pine, known as shore pine, grows along the Pacific coast in sandy soil close to the seashore. It is recognized as a subspecies of lodgepole and is named Pinus contorta contorta. It has at least one dense, smooth-topped crown, shaped like a mushroom cap. In figure 2.12 are drawings of the three pines considered so far, showing their relative heights (on average) and representative shapes. The third two-needle pine is red pine (P. resinosa), an eastern species. Its needles are much longer than jack pine needles (10 to 15 cm versus 4 to 6 cm, equivalently 4 to nearly 6 in). Its cones are quite ordinary, but when they fall they often leave a rosette of basal scales on the twig (figure 2.13). The bark of the tree is a rich red, and the crown a mass of glossy, dark green needles; the trunk provides good-quality structural timber. Because of all these virtues, the tree is often planted; it is notable for wind-firmness and flourishes on sandy, infertile soils — altogether a splendid tree.

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Figure 2.12. Representative shapes and relative heights of (a) jack pine, (b) lodgepole pine, and (c) shore pine.

Figure 2.13. Red pine: rosette of basal scales left by a fallen cone.

The two hard pines with three needles per fascicle are ponderosa pine (P. ponderosa), found on dry plateaus west of the Rocky Mountains, and pitch pine (P. rigida), with a limited range (in our area) on the Atlantic coast and a few sites in the St. Lawrence valley.

identifying the conifers

Ponderosa pine forms parklike forests at middle elevations and also grows, mixed with Douglas-firs, in the mountain forests. It reaches as far east as the Dakotas in the southern part of its range. It is tall, growing to 50 m (150 ft), and has long, dark, needles and big, prickly cones. Ponderosa’s cinnamon-colored bark flakes off the trunk in pieces shaped like those of a jigsaw puzzle, making conspicuous beds of flakes around the base of each tree (figure 2.14). Pitch pine’s most distinctive character is the clumps of needle fascicles growing directly on the trunk from imbedded buds (figure 2.15). The four soft pines, with five needles per fascicle, form two pairs. The two big ones are eastern white pine (P. strobus) and western white pine (P. monticola). Both are tall, forest trees with remarkably long cones

Figure 2.14. Ponderosa pine. (a) Cone and (b) fallen flakes of bark at the foot of the tree.

Figure 2.15. Pitch pine trunk, with tufts of green needles.

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Figure 2.16. Eastern white pine cones, first year and late second year.

(about 20 cm (5 in) in the eastern, and 30 (7.5 in) cm in the western white pine) often found on the ground beneath them. The striking whiteness of the tips of the scales catches attention in dim light (figure 2.16). The second pair, whitebark pine (P. albicaulis) and limber pine (P. flexilis), are known as “stone pines.” The names are confusing: both have whitish bark, and both are trees of the western mountains. Specimens above the tree line in the alpine tundra are usually stunted or dwarfed. Whitebark pine has the larger range; it often grows in exposed rocky sites near timberline in the Rockies and the Cascades south of 55° north latitude. Limber pine doesn’t grow so far north and is not found west of the Rockies, but it is common on the eastern foothills of the Rockies where dry stony ridges stretch out into the plains. The two species are hard to distinguish if they are without cones, but with them, it’s easy. The cones differ in size and shape (figure 2.17). Limber pine cones are much longer, and their cone scales spread open when the seeds are ripe; sometimes (not always) the tips of the scales are bent back (or reflexed ). Whitebark pine cones are shorter and egg-shaped or even spherical. The cone scales remain closed even when the seeds are ripe. They are liberated either when the cone rots and disintegrates or when it is torn to pieces by squirrels or birds searching for seeds to eat. (That is not all that happens; see chapter 10 for what comes next.)

Larch Species. Three species of larch grow in our area: eastern larch, or tamarack (Larix laricina), western larch (L. occidentalis), and subalpine larch (L. lyallii). Their most distinctive character is their deciduousness. A leafy summer twig and a leafless winter one were shown in figure 2.2.

identifying the conifers

Figure 2.17. Cones of (a) limber pine, with reflexed scales, and (b) whitebark pine.

Figure 2.18. Boundary between the ranges of tamarack and the western larches.

The needle clusters are anatomically different from pine-needle fascicles The clusters have an indefinite number of needles, anywhere from 10 to 60, of a variety of lengths, which sprout from dwarf branches. Needles also grow singly near the tips of vigorous shoots. The map in figure 2.18 shows the boundary between the ranges of the most common larch, tamarack, and the other two species, western larch and subalpine larch. Tamarack has the smallest cones, and the bracts behind the scales are short and hidden. In the other two species, the bracts are clearly visible, with long, protruding tips (figure 2.19).

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Figure 2.19. Cones of (a) tamarack and (b) western larch.

Western larch is a very tall tree that grows at low elevations, in deep, well-watered valleys among the western mountains. Subalpine larch grows at high elevations, at least 1500 m (4500 ft); it often marks the alpine tree line. Its young twigs are covered with a dense layer of white “wool” of tangled, matted hairs, best seen with a hand magnifier.

Spruce Species. Six spruce species grow in our area, enough to make a key, and also a map, useful aids to identification. Let’s start with the key. Keep in mind that the key is only useful in our area, namely northern North America, once ice covered.

Key to the Species of Picea in Our Area 1a. Leaves almost flat; near the Pacific coast ......................................Picea sitchensis, Sitka spruce 1b. Leaves square in cross section ..................................................... 2 2a. East of Lake Superior ................................................................. 3 2b. West of Lake Superior ................................................................ 5 3a. Twigs completely hairless .......................... P. glauca, white spruce 3b. Twigs fuzzy with short brown hairs (use a magnifier) ................. 4 4a. Cones long and narrow ................................ P. rubens, red spruce 4b. Cones short, almost spherical ................. P. mariana, black spruce 5a. Leaves short (about 1.2 cm or 0.48 in) ...... P. mariana, black spruce 5b. Leaves longer (about 3 cm or 1.2 in) .......................................... 6 6a. Leaves acid-tasting when chewed.............. P. pungens, blue spruce 6b. Leaves not acid-tasting ............................................................... 7 7a. Cone scales smooth-edged, scarcely longer than seed wings ......................................... P. glauca, white spruce 7b. Cone scales rough-edged, much longer than seed wings ................................P. engelmannii, Engelmann spruce

identifying the conifers

Figure 2.20. Ranges of different groups of spruces listed in the text. 1) white, black, and red spruces; 2) white and black only; 3) white and Engelmann only; 4) Engelmann only; 5) hybrids of white and Engelmann, also Blue Spruce; 6) Sitka spruce only; 7) Grassland, no spruces.

The map in figure 2.20 shows the regions in which species or groups of species grow. Here are the distinguishing characteristics between three pairs of similar species that are likely to be found together. • Red (Picea rubens) and black spruce (P. mariana) are easy to distinguish when cones are available: those of red spruce are long and pointed, whereas those of black spruce are egg-shaped to spherical. Spruces without cones are more likely to be red spruce (which drop their cones as soon as the seeds are mature) than black spruce (which retain their cones for years, releasing seeds a few at a time). Identification is difficult where both species grow together and often hybridize. • The distinction between white (P. glauca) and black spruce given in the key is the only reliable one. (It asked whether the twigs were hairless or fuzzy.) Some books show so-called typical shapes of their crowns, white spruce with a tapering conical crown, and black spruce with a “mop-head.” This is misleading; white spruces growing in waterlogged ground often have mop-heads, and black spruces often lack them. • Engelmann spruce (P. engelmannii) is closely related to white spruce and often hybridizes with it.1 Figure 2.21 shows, for these two species, a cone and one of its scales with seeds attached.

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Figure 2.21. Cone and a single scale showing the paired seeds of (a) white spruce and (b) Engelmann spruce.

Fir Species. There are only five fir species in our area, not enough to rate a key. Firs have very distinctive cones. They develop right at the top of the tree (which means they go unnoticed by people who don’t look up for them) and are unlike all other cones in two respects: they sit upright on the branch, and they don’t fall when the seeds are ripe. Instead, the scales drop one by one, leaving a cone’s axis upright and dead until it rots or is blown off, as shown in figure 2.22. Figure 2.22 shows another characteristic of fir cones, specifically those of balsam fir. In some, the bracts behind the scales are short and concealed (these cones are called “bractless”); in others they are long and protruding. Bracted cones are found only in the eastern part of balsam fir’s range and become more common the farther east you go: in Newfoundland, all fir cones are visibly bracted. Next, recognizing the five fir species: The most common, balsam fir (Abies balsamea), is the only eastern species, growing from the Continental Divide to Newfoundland. It grows with another fir, subalpine fir (A. lasiocarpa), and hybridizes with it, in a very small area centered on the Yellowhead Pass through the Rockies. Throughout the rest of its range, balsam fir is the only fir, so identification is certain. Of the four western species, noble fir (A. procera) has the smallest range in our area; it grows in the Cascades of Washington and Oregon. It is a tall tree with blue-green foliage and an open dome-shaped crown. Its enormous cones are sometimes 45 cm long (17.7 in).

identifying the conifers

Figure 2.22. Balsam fir. (a) Bractless cone. (b) Cone with bracts. (c) Cone axis after most of the scales have fallen.

Figure 2.23. Subalpine fir.

Subalpine fir grows nearly everywhere in the western mountains from Washington to Alaska. It is the smallest of the firs, and where it grows in the open, it is neatly spire-shaped (figure 2.23). It resembles its close relative, balsam fir, which also has a regular conical crown, but that of subalpine fir is much narrower and steeper. At slightly lower elevations on down to sea level is amabilis fir, sometimes called Pacific silver fir (A. amabilis). And at sea level is grand fir (A. grandis).

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Figure 2.24. Twigs of (a) grand fir and (b) amabilis fir.

These two low-elevation firs are most easily distinguished by noting the contrasting pattern of the leaves on their twigs. In grand fir, the leaves are strictly two-ranked; amabilis fir also has two-ranked leaves and, in addition, a row of smaller, forward-pointing leaves that cover the twig (figure 2.24).

Hemlock Species. Hemlocks as a genus are sometimes hard to recognize, but the separate species are easy to distinguish.2 There are only three, and one of these, eastern hemlock (Tsuga canadensis), grows only in the east, from the Great Lakes to the Atlantic coast. The other two are westerners: western hemlock (T. heterophylla) and mountain hemlock (T. mertensiana). The former grows along the Pacific coast and in valleys in the western mountain ranges; the latter grows only on mountains near the coast. These two western species are easy to distinguish. Western hemlock has small leaves of a variety of sizes on each twig (unlike any other conifer). The leaves of mountain hemlock, in contrast, tend to form neat, decorative, circular rosettes (figure 2.25). Douglas-fir Varieties. Douglas-fir is always treated as one species with two varieties (a “variety” is a lesser unit than a subspecies). They are Rocky Mountain Douglas–fir (Pseudotsuga menziesii var. glauca) and coast Douglas-fir (P. m. var. menziesii). Their geographic ranges are obvious from their names, and the most noticeable difference between them is in their cones (figure 2.26). The

identifying the conifers

Figure 2.25. Twigs of (a) western hemlock and (b) mountain hemlock.

Figure 2.26. Douglas-fir cones. (a) Interior variety and (b) coastal variety.

cones of the coastal variety are longer and sleeker than those of the Rocky Mountain (often called “interior”) variety.

Thuja Species, Nootka-cypress, and Juniperus Species. The two cedars in our area, eastern white-cedar (Thuja occidentalis) and western redcedar (T. plicata), cannot possibly be confused. They grow in widely separated regions. The eastern species grows only east of the prairies, and the redcedar only in the western mountains and along the Pacific coast. Their size difference is striking; see figure 2.27, which shows the two trees on the same scale. The scalelike leaves are similar (see figure 2.7 for those of redcedar), as are their small woody cones with only a few scales. Both species have symmetrical seeds with wings on both sides (figure 2.28), in contrast to most

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Figure 2.27. Two thujas on same scale. (a) Eastern white-cedar and (b) Western redcedar.

Figure 2.28. Western redcedar. (a) Twig with cones. (b) A single cone, enlarged. (c) Seeds; note double wings.

conifers in which each seed has a single wing (see, for example, figure 2.21 for those of spruces). Western redcedar is renowned for the immunity of its wood to decay. Nootka-cypress (Callitropsis nootkatensis) grows in the rain forests of the west coast and is very like redcedar in many respects. In the southern part of their shared range, Nootka-cypress usually grows at higher

identifying the conifers

elevations than redcedar, but farther north it is found down to sea level. The two species are easy to tell apart if cones can be found. A Nootka-cypress cone is a very small sphere surfaced with woody scales (figure 2.29); those lying on the ground are likely to be dismissed as deer droppings if they are noticed at all. Leaf color also separates the two trees: redcedar leaves are yellow-green; those of Nootka-cypress, a bluish green. And the two trees have very different bark (figure 2.30). Lastly, Nootka-cypress is not a true cypress (note the hyphen in its English name). There are three tree species of Juniperus, two western ones called junipers and an eastern one officially named “eastern redcedar.” Here it is called eastern juniper.

Figure 2.29. Nootka-cypress. (a) Branchlet and (b) cones.

Figure 2.30. The barks of (a) western redcedar and (b) Nootka-cypress.

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All have scalelike leaves that are very much like those of the thujas, but the cones are unlike any so far described. They are like berries, green at first and then dark blue with a white bloom. They are found only on female trees—in this genus, the sexes are separate. The “berries” are really cones with fleshy, joined scales; the bumps on the surface are the concealed tips of cone scales. Eastern juniper ( Juniperus virginiana) has stiff needles as well as scalelike leaves. Its range in our area is small: it grows only in the Ottawa valley and near the shores of the southern Great Lakes. The two western species (until recently treated as one) are Rocky Mountain juniper ( J. scopulorum) and seaside juniper ( J. maritima). The latter was discovered in 2007.3 The two species are indistinguishable without microscopic comparisons of their DNA. Rocky Mountain juniper was thought to grow in a few scattered sites in the Rockies, with an outlying site on the southeastern coast of Vancouver Island. Now it appears that these outliers are, in fact, different species, now called seaside juniper. In these junipers, all the leaves on young trees are needlelike, and those on older trees, scalelike, making it easy to mistake mature and immature trees for two separate species (figure 2.31).

Figure 2.31. Rocky Mountain juniper. (a) Scale leaves. (b) Spiky needle-leaves. (c) Twig of old tree. (d ) Twig of young tree.

identifying the conifers

Figure 2.32. Western yew twig, with “berries.”

Yew Species. The only yew in our area that is a tree is western yew (Taxus brevifolia), described earlier (in the genus descriptions). It grows In the Rockies and along the west coast. Its noteworthy characteristic is its cone, found only on female trees in spring. Yew “cones” are even more aberrant than junipers cones. They are soft, red, globular cups, each with a hard, spherical, green seed visible inside (figure 2.32). The red cup Is known as an aril and, improbable as it seems, is a modified cone scale. Careful dissection shows that it is a single, fleshy scale on a minute, dwarfed axis on which the other scales are atrophied and crowded just below the aril. Conifer Families We have now considered all the genera and species of conifer in our area. What about a coarser classification? The genera fall into distinct groups, or “families” (this is true for all plants), and it is worth recognizing them and naming them. The families to which “our” conifers belong are the Pinaceae, the Cupressaceae, and the Taxaceae (all plant family names have the unattractive ending “aceae,” normally pronounced “acey” to rhyme with “lacey”). Note that family names are always printed with a capital initial and in roman type. The largest family is the Pinaceae, or pine family, which includes the pines, larches, spruces, firs, hemlocks, and Douglas-firs. These are all conifers with needle leaves and woody seed cones that have scales arranged spirally around an axis (figure 2.33). Next comes the Cupressaceae, the cypress family, which includes thujas and junipers in our area (and true cypresses of the genus Cupressus in other parts of the world, which explains the family name). It includes all

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Figure 2.33. Seed-bearing cones of (a) hemlock (Pinaceae), (b) western redcedar (Cupressaceae), and (c) juniper (Cupressaceae).

the coniferous trees with scale leaves that form all or part of their foliage at maturity. The seed cones (figure 2.33) are either small woody cones, with few bracts arranged in opposite pairs (thujas), or blue “berries,” with three fleshy bracts fused together (junipers). The final family is the Taxaceae, the yew family, with only one member, the western yew, in our area. Its distinctive characters were described above. Notes 1. E. Daubenmire, “Taxonomic and Ecologic Relationships between Picea gauca and Picea engelmannii,” in Canadian Journal of Botany, vol. 52, 1974, pp. 1546–60. 2. J. L. Farrar, Trees of the Northern United States and Canada (Ames, Iowa: Blackwell, 1995). 3. R. P. Adams, “Juniperus maritima, the Seaside Juniper, a New Species from Puget Sound, North America,” in Phytologia, vol. 89, 2007, pp. 263–83.

Chapter 3

Reproduction of Conifers

Pollen Cones and Pollen An ovule is a seed before it is fertilized by pollen; only after fertilization does the ovule develop into a seed that matures until it is capable of germination. So far we have considered only seed cones. Now we come to pollen cones and pollen. Pollen cones (figures 3.1 and 3.2) are smaller than seed cones and do not persist for nearly as long. They shrivel and dry up as soon as the pollen has been shed in spring. Although most of the pollen cones soon fall off the trees, dried up, dark brown pollen cones can still be found on the twigs of pine trees right into late fall or winter; they fall off at a touch. In the great majority of conifer species, seed cones and pollen cones grow on the same tree. Yews and junipers, however, have separate female and male trees: the females bear only seed cones and the males, only pollen cones. Whatever the species, a pollen cone always consists of an axis with stamens all around it. The stamens are small, stalked, scalelike organs with pollen sacs attached to them. In species belonging to the pine family, stamens are always numerous and are arranged in a spiral around the axis. In the cypress and yew families, the pollen cones have far fewer stamens, seldom more than a dozen (figure 3.2). The stamens are umbrella-shaped, with the pollen sacs on the inwardfacing, protected surfaces of the umbrellas.

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Figure 3.1. Jack pine pollen cones.

Figure 3.2. Pollen cones of (a) Douglas-fir and (b) Rocky Mountain juniper. (c) Pollen grain of a red pine.

The pollen of conifers is familiar to anybody who has brushed against a cone-laden branch in spring, releasing dense clouds of yellow pollen. The quantity produced is enormous. It is borne on the wind to female cones (seed cones) awaiting fertilization. It blows everywhere and often collects as a yellow film on calm water surfaces—rain puddles, ponds, and the sheltered backwaters of lakes. Most people know pollen as “dust,” in airborne clouds or as waterborne films. The individual grains (see figure 3.2c for an example) are of much more than passing interest, however. A pollen specialist (a palynologist)

reproduction of conifers

can identify the genus, and sometimes the species, of tree that a pollen grain came from by examining its shape and texture.1 (Of course, if a pollen grain is collected for examination from a known tree, identification isn’t a problem.) Pines and spruces (prolific pollen producers) have comparatively large grains, each with a pair of bladders (floats) attached to it; figure 3.2c shows a grain of red pine pollen. These are the most easily identified conifer pollen grains. Those of larches, hemlocks, and Douglas-fir are bowl-shaped and much less distinctive. Unfortunately pollen grains are too small for their shapes to be visible with hand magnifiers. A large grain might be 50 µm across (that is 0.05 mm; 1 µm, a micrometer, is one-millionth of a meter, or 0.00004 in.). Palynology, the study of pollen deposits, is a science in its own right. It examines evidence on ancient climates and ancient vegetation. Pollen is produced in enormous quantities. That which settles on water eventually sinks to the bottom and becomes incorporated in the accumulating sediments. Sediment cores taken from these layers of mud contain pollen grains that were shed somewhere nearby, and many of the layers can be carbon-dated. The grains are almost indestructible. Long-buried ones are microfossils that persist unchanged, in recognizable form, for thousands of years. It is possible to infer from them how the vegetation of an area, and hence the local climate, have varied over past centuries and millennia.

Pollination For conifers, pollination is the transfer of pollen grains from a pollen cone to a seed cone, where they will fertilize the ovules. The process is called wind-pollination, and the lightweight grains are easily borne on the gentlest of breezes. Some of the grains come to rest in unpollinated seed cones. All the cones described in chapter 2 had already been pollinated. Before pollination they were tiny, with soft, open scales between which pollen grains could sift down and reach the ovules. These “conelets” are so small they are easy to miss. To find them, search carefully in early spring. In pines, look among the dense tufts of needles at the tip of a branch; small, new, bright red conelets can often be found. Two are shown in figure 3.3a; note that most of the needles surrounding them are not shown.

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Figure 3.3. Lodgepole pine seed cones. (a) Two conelets before pollination (many pairs of needles surrounding them are not shown). (b) Second year, mature cone not yet open.

After a pollen grain has come in contact with an ovule, it grows a pollen tube that passes through a small hole in the ovule’s integument, or outer skin. The tube carries a sperm cell right into the ovule itself, and fertilization is achieved. (Note: a pollen grain is not itself a sperm as is often believed: it contains sperm cells.) Now contrast this with the pollination of flowering plants. In plants with showy flowers, the pollen is carried from one flower to another by pollinators, mostly bees. In plants with inconspicuous flowers, such as those of grasses and many broadleaf trees, the pollen is wind-borne. In both cases, the pollen is trapped on a sticky surface in the receiving flower. The pollen grains put out pollen tubes, just as they do in conifers, but in flowering plants the tube has obstacles to overcome before it can enter an ovule. Flowering plants’ ovules are contained within a closed carpel, a “vessel” that holds one or more ovules. Therefore, a pollen tube has to penetrate a barrier to reach and fertilize an ovule in a flower. Figure 3.4 shows simplified diagrams of what happens in a conifer and a broadleaf. This contrast between conifers and flowering plants is the defining difference between the two groups of plants, and explains their scientific names, based entirely on Greek. The conifers are in the Gymnosperma, from gymnos, “naked,” and sperma, “seed.” The flowering plants are the Angiosperma, from angeion, a “vessel” or closed container. In English, they are gymnosperms and angiosperms. The differences between the two are considered further in chapter 6.

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Figure 3.4. Sections of seeds. (a) Conifer (gymnosperm). (b) Broadleaf (angiosperm). The integument of the ovule is stippled. (The diagrams are greatly enlarged and simplified.)

A Contrast between Seed Cones and Pollen Cones An interesting point to notice about conifer cones is that pollen cones and seed cones are not equivalent in the anatomical sense. The giveaway is the arrangement of the bracts in the two kinds of cones. Bracts are small, flat modified leaves, and in a seed cone there is a bract behind every scale. As mentioned in chapter 2, in some species the bracts are longer than the scales and are easily seen without breaking the cone apart. In our area the long-bracted species are western larch (see figure 2.19b), alpine larch, noble fir, bracted balsam fir (see figure 2.22b), and Douglas-fir (see figure 2.6). In most other species the bracts are hidden; two examples are shown in figure 3.5. In the cones of thuja and Nootkacypress each bract has become fused with its scale and can be seen only with a microscope, but a bract is there. No matter what the species, in seed cones each scale is always backed by a bract. In pollen cones, on the other hand, there is a bract at the base of each whole cone, not at the base of each stamen (figure 3.6). This leads to the conclusion that a single scale in a seed cone is equivalent to a whole pollen cone: each is backed by one bract. Or, what comes to the same thing, a single seed cone is equivalent, anatomically speaking, to a whole cluster of pollen cones.

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Figure 3.5. The backs of seed-cone scales with inconspicuous bracts. (a) Eastern white pine; scale and bract are fused together. (b) Western hemlock.

Figure 3.6. Cluster of pollen cones of eastern white pine. Note the bract behind each cone.

Vegetative Reproduction Sexual reproduction is a chancy affair. Pollination may fail, or seeds may not germinate. As a backup, many species of plants reproduce asexually, and the most common way of doing this is by vegetative reproduction— that is, part of a living plant starts actively dividing and growing new tissue, which becomes detached and grows into a new plant. This form of reproduction has the advantage of dependability, but the drawback is it offers no possibility of genetic variants being produced. Vegetative reproduction is common in flowering plants, particularly in nonwoody plants— think of buttercups, irises, couch grass, and many, many more, and of any plant from which a gardener can successfully take a cutting and grow a new plant from it. Among flowering trees (broadleafs), trembling aspen (Populus tremuloides) is the best-known vegetative reproducer. Its spreading roots send up sprouts that grow into independent trees, forming a genetically identical clone of trees that together may cover many hectares.

reproduction of conifers

Figure 3.7. A naturally layered clone of subalpine fir: (a) in winter and (b) in summer.

Few conifers reproduce vegetatively, but here are examples of some that can. Subalpine fir and black spruce develop small clones by layering. This is a natural occurrence of a process gardeners often use to grow new shrubs such as forsythia. Consider a subalpine fir in alpine tundra: a heavy winter snowpack weighs its branches down until many touch the ground, where their lower sides are abraded by sharp rocks. Roots grow from the wounds, and side branches grow up to make new trees above the new roots. The result is a neat, circular patch of trees with the largest, ancestral tree at the center (figure 3.7). Such clones lose their distinctiveness with age, as competition with their neighbors kills many of the young trees. Two eastern species that spread by layering are balsam fir and eastern white-cedar. A redcedar that has been blown over by wind without disturbing some of its roots will reproduce vegetatively if the soil below the fallen trunk is constantly wet. Roots begin growing on the underside of the downed trunk directly below the healthiest branches, and eventually, when most of the rotting wood has slowly crumbled, a straight row of new trees is left

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Figure 3.8. Branches of a fallen western redcedar growing up into new trees.

in its place (figure 3.8). Eastern white-cedar often reproduces in the same manner.

Nootka-cypress can be multiplied by taking stem cuttings from the tips of branches. I know of no cases where this has happened naturally. Lastly, pitch pine is capable, in a sense, of vegetative reproduction, although “resurrection” might be a better term. Injured or burned trees sometimes recover when dormant buds under the bark come to life after the injury and produce new shoots. The buds sometimes lose their dormancy needlessly, or so it appears, and grow tufts of needles on an apparently healthy tree (see figure 2.15). Note 1. I. J. Bassett, C. W. Crampton, and J. A. Parmelee, An Atlas of Airborne Pollen Grains and Fungus Spores of Canada (Ottawa: Research Branch, Canada Department of Agriculture, Monograph No. 18, 1978).

Chapter 4

The Life and Growth of a Conifer

Wood Conifers and broadleafs differ greatly in their internal organs, most noticeably their wood. In all trees, the trunk and branches contain tubes to carry liquids up and down the tree, from roots to leaves and back. The tubes leading upward, plus (in some trees) strengthening fibers, constitute the wood, technically, the xylem. The tubes leading downward, and carrying dissolved sugars from the leaves to the rest of the tree, constitute the phloem. Unlike xylem, there is no popular name for phloem. It will become clear later why xylem (that is, wood) and phloem, in spite of their being equally necessary for the growth of a tree, should rank so differently in public awareness. Everyone is familiar with wood, but only botanists and naturalists have ever heard of phloem. The primary reason for the contrast is that wood has commercial value and phloem has none. Consider the wood first. The structure of wood differs greatly between conifers and broadleafs. Wood contains the tubes that carry water and dissolved nutrients (sap) up the tree. In conifer wood there are no continuous “tubes”; there are merely chains of long, narrow cells, known as tracheids, linked to each other through small, porous membranes. In contrast, broadleaf wood contains continuous tubes known as vessels to carry the sap. A vessel consists of a chain of cylindrical cells linked end to end; when first formed, these cells have thin end walls that form partitions, but the 43

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end walls soon dissolve, leaving the vessels unobstructed, so sap can flow easily through them. Water travels upward in the tracheids only in daylight. Notwithstanding popular belief, the flow is not by capillary action, which is the way water rises a short way up a narrow glass tube, or in an absorbent sponge. It isn’t nearly strong enough to lift water right to the top of a tree. What happens is this: The cells in a germinating seed contain water that has been sucked a very short distance up, by capillary action, through the minute root hairs. After that, the water continues to flow upward through each of the growing, lengthening cells (tracheids) as an unbroken column of sap in a closed pipe. Each sap column is kept intact as the tree grows, by intermolecular attraction. This accounts for both “transpiration pull” and “cohesion,” two terms often used without definition. They mean what they say, and the scientific explanations are in the realm of molecular physics. Breakage of a column (cavitation) is rare —much rarer in conifers than in broadleafs because tracheids are narrower than vessels. Also, the enormous number of sap columns in a tree trunk ensures that breakage of a few of them does no harm. Figure 4.1 shows the contrast between tracheids and vessels. Most tracheids have diameters of less than one-twentieth of a millimeter while those of vessels are sometimes 20 times as great. Besides being exceedingly narrow, a tracheid is tapered at both ends. In a chain of connected tracheids, each one overlaps those above and below it so that the tapered ends are pressed against each other. Thus every chain of tracheids touches the chains on each side of it, and the side-by-side tracheids are linked to each other through small porous membranes through which sap can flow. The tracheids of conifers are far less efficient than the vessels in broadleafs. The rate of flow of sap in conifers is about half a meter (1.5 ft) per hour on average, a snail’s pace compared with that in some oaks in which it can be twenty times as great. Also, tracheids are less specialized than vessels in that they perform two distinct functions. Not only do they carry the sap, but also they give wood its mechanical strength. In the more highly evolved broadleafs, there is a division of labor among the cells in the wood. The two functions, sap conduction and mechanical support, are performed by two different kinds of cells: The vessels serve only as sap conductors; they haven’t the strength and rigidity to support a

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Figure 4.1. Diagrams of (a) tracheids and (b) vessels, before and after disintegration of the cross walls.

tree. Strength is provided by wood fibers—long, narrow, thick-walled cells whose only function is to provide mechanical support. The importance of tracheids to coniferous trees should now be obvious. Coniferous wood consists almost entirely of tracheids. They are popularly known as “wood fibers,” but in botany-speak only the strength-giving cells in broadleaf wood have that name. An individual tracheid is too small to examine with a hand magnifier, but conifer wood has many interesting characteristics that can be looked at without a microscope, and they are the topic of the next section.

Cut Stumps and Whole Trees The cut stump of a conifer, with its concentric annual rings from which its age can be counted, is a familiar sight. The rings are formed as the tree grows. Outside the wood is the cambium, a one-cell thick, cylindrical “sleeve” encasing the wood. Apart from the reproductive system it is the most important tissue in the tree, being the site of all the outward (radial) growth of the tree. Throughout the growing season the cambium cells divide repeatedly. They split in half lengthwise; at each division a wall forms, dividing what was previously one cell into two “daughter” cells, one toward the outside

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of the trunk, the other toward the inside. One of these daughters, most often the outer one but occasionally the inner one, remains a cambium cell itself, taking the place of its “mother” cell. What happens to the other daughter depends on whether it is the inner or the outer of the two daughter calls. If it is the outer one, it becomes a phloem cell (described later). If it is the inner one, it goes to form new wood. In doing so, it divides lengthwise a few more times, and the resulting long narrow cells become tracheids. The tracheids are the wood: their continual formation increases the diameter of the trunk year after year. For this to happen without the cambium being ruptured, the cambium itself must increase in circumference to match the growth of the wood. It does this by cell divisions that produce side-by-side daughter cells as needed. In this way, it remains as a sleeve that encloses the continually expanding wood inside it. New tracheids are formed throughout the growing season. Those formed in spring are comparatively large in diameter and thin-walled; they form “spring wood,” also called “early wood.” Those formed later in the season, as fall approaches and growth slows, have smaller cavities and much thicker walls. They form “summer wood,” also called “late wood.” Because it is formed of thick-walled tracheids, summer wood appears darker than spring wood, hence, the annual rings. The rings are much more distinct in conifers than in many broadleafs. In broadleaf wood, there are fibers as well as vessels, and in many species, large and small vessels are mixed together haphazardly so that annual rings are barely detectable; this applies to maples, birches, willows, and cherries, among others. Figure 4.2 shows a section of the stump of a seven-year-old pine, as well as an enlargement of one part of it showing more details. One of the details is a ray. A ray is a vertical sheet of tissue, only one cell thick, extending from the phloem (out of sight at the top of the picture). The rays allow liquids to move horizontally; they provide channels through which some sap moves from wood to phloem, and some carbohydrate “food” (that is, dissolved sugars, also called “sap”!) from phloem to wood, without which the respective tissues would dehydrate or starve. The rays also store starch (combined sugar molecules) for future use. Figure 4.3 shows how tracheids and rays appear in a tangential section. Rays are bigger and easier to see in broadleafs than in conifers. But if you examine the section of a conifer stem (or branch) that has been cleanly cut and then sanded, the rays can usually be seen as thin, light-colored

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Figure 4.2. (a) Cross section of a seven-year-old pine stem. (b) Micrograph showing a cross section of the tracheids and a ray.

Figure 4.3. (a) Tangential section of a pine trunk. (b) Micrograph showing the tracheids and rays.

lines radiating outward. Ray cells have weaker walls than tracheids have. When a cut stump eventually dries out, it is the walls of the ray cells that rupture under tension, producing the characteristic radiating cracks in an old stump. Apart from tracheids and rays, the only other structures to be found in wood are resin ducts, hollow channels lined with small living cells that secrete resin. Resin is an antiseptic that prevents microbes from getting

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into wounds in a tree and repels insects that might otherwise introduce pathogenic microbes. Resin is found in the wood of pine, spruce, larch, and Douglas-fir, but not in the wood of other genera. They can evidently get by without it. Wood is not all the same from a tree’s center to the cambium. The wood at the center, the heartwood, is dead, and in some species it has a darker color than the rest of the wood because the dead cells are clogged with chemicals such as resin, tannin, and gum; these inhibit microbes and make the wood resistant to decay. Outside the heartwood comes the sapwood, with living tracheids: the innermost ones simply store water, while the outermost, next to the cambium, conduct sap up the tree. In some species there is no visible contrast between heartwood and sapwood, but in all species heartwood adds to the mechanical strength of a trunk. However, a tree can survive without it. The living part of a tree is a tall, hollow, tapering cone, perched like a dunce’s cap on a conical core of solid, dead heartwood. If the heartwood decays and crumbles away, a living, hollow tree remains and continues to grow (figure 4.4). Cut stumps and also cut branches reveal another interesting difference between conifers and broadleafs. When a growing column of wood (trunk or branch) leans away from the vertical, the lopsidedness causes growth

Figure 4.4. (a) A living hollow tree; only the heartwood has rotted. (b) The layers of a tree.

the life and growth of a conifer

Figure 4.5. Reaction wood. (a) Conifer (compression wood). (b) Broadleaf (tension wood).

of what is called reaction wood, marked by wider than normal annual rings. In conifers the reaction wood forms on the lower side, where the stress acting on the wood tends to compress it: it is therefore called compression wood. In broadleafs, reaction wood forms on the upper side, where the wood is under tension: it is therefore called tension wood. The contrast is best seen with broadleafs having distinct annual rings (figure 4.5). Another property of wood that can be seen without a microscope is its spiral grain, which is present to some extent in nearly all conifers. A straightgrained tree, with its chains of tracheids arranged vertically, is a rarity. Spiral grain is difficult to detect in living trees because the grain of the wood is concealed by the bark. It becomes very noticeable in burned-over forest where the trees have been killed but not consumed, and the charred bark has fallen off, leaving the wood exposed. Figure 4.6 shows a dead lodgepole pine that had been killed in a fire in the Rocky Mountains 12 years previously. In some species, the effect of the wood’s spiral grain is that it ensures that sap rising in the tracheids is shared equally among the branches of the tree, which may radiate in all directions, even if the root system happens to be asymmetrical. Trees in which this happens have been tested experimentally by injecting a colored dye into the tracheids and observing its flow.1 Likewise, the food-carrying tubes of the phloem (described later) are spiral, to ensure that food flowing down from the leaves is shared equally among the roots even if the foliage happens to be asymmetrical. This is the main advantage of spiral grain. A minor one is that spirally grained wood is more apt to bend than to break in strong winds.

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Figure 4.6. Trunk of a lodgepole pine with right-spiral grain. The bark is gone because of a fire.

Outside the Cambium A couple of pages back, figure 4.2a showed the cross section of a tree in its entirety. The cambium, one-cell thick and too small to show, separates the wood (mostly white in the picture) on the inside from several other tissues, including the phloem, perhaps some cork, and the bark, all of them in the narrow outer zone (shown in black). The upward transport of sap (mostly soil water) happens in the wood, and the downward transport of food happens in the phloem. The apparent difference in size is astonishing. Three factors account for this. First, only a thin outer layer of the wood is engaged in sap transport at any one time. As described in the previous section, the inner wood is dead heartwood, and the inner sapwood is inactive: its cells (tracheids) are used for storage. So the sap-conducting part of the wood is not as large as it appears. Second, most of the tubes comprising the phloem live for less than a year. They are stretched to destruction by the expanding wood that they surround. Third, the volume of liquid moving up a tree greatly exceeds the volume moving down. The upward traveling liquid is mostly water, and only a small fraction of it becomes part of the growing tree; the rest is continually evaporated from the leaves in a process analogous to sweating. A tree needs a lot of water in addition to the amount it uses for building new tissue. Sap is only a dilute solution of the minerals the tree must have; getting

the life and growth of a conifer

Figure 4.7. Douglas-fir. (a) Trunk, showing the black, charred ridges left by a fire some years ago and the unburned bark that has since grown between them. (b) Part of a cross section of a trunk.

enough of them takes large quantities of water. A tree also needs to sweat, to keep its temperature from rising dangerously in hot weather. Lastly cells full of water are rigid compared with empty, flaccid cells: think of a balloon, blown up and then burst. Figure 4.7b shows part of a large tree’s cross section. The inner part of the section (on the right) shows the outer four annual rings of the xylem. The first layer outside the invisibly thin cambium is the phloem; outside that is the cork. The pattern is seldom as clear as in the drawing: the model was a large, recently cut Douglas-fir, a species having especially thick cork. The bark of a similar Douglas-fir that survived a fire and is still alive and standing is shown in figure 4.7a. The ridges of the bark that were blackened by fire are separated by normal, undamaged bark (light in color) that has grown since the fire, in the furrows between the burned ridges. The different tissues outside the cambium are clear in figure 4.7b. More often what you see is a mishmash of indistinguishable cells, crushed and torn apart by the force of the expanding column of wood inside the cambium, forming what is popularly called “the bark” even though the inner part of it is living. (Often the word “bark” is used to mean only the dead bark.) The smooth bark of firs is unusual. Their cork forms a continuous sleeve that expands in time with the interior tissues, instead of becoming

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fragmented. This is not altogether a blessing, as it deprives a fir of an expendable outer layer, making it vulnerable to short-duration fires. The phloem is all the tubes that carry food down from the leaves to the rest of the tree. The tubes are columns of sieve cells, connected by pores, and they live for only a year or less because of the stretching they are subject to. This explains why phloem has no annual rings. The phloem is close to the outer surface of a tree, and the tree’s life depends on it. That is why girdling a tree is fatal to it. The “plumbing” in conifers and broadleafs is similar in principle but markedly different in details that need not concern us. To sum up concisely: Conifers have tracheids going up and sieve cells going down. Broadleafs have vessels going up and sieve tubes going down. Cork is seldom noticeable in cut stumps, but it is wonderful material. It is waterproof, so it protects a tree from drought. It is a good insulator, against freezing in winter and baking in summer and also against lowintensity forest fires. Its resilience, impossible to imitate synthetically (think of modern, synthetic wine-bottle stoppers), protects a tree from impacts with other trees and falling branches in a storm. If there are wounds, dead cork acts as a barrier against microbes because it is uneatable for them. The pale arcs in figure 4.7b are short sections of cork cambium. The word “cambium” by itself always refers to the cylindrical sheet of cells that divide to produce tracheids and sieve cells; its more general meaning is “a sheet of cells that divides to give daughter cells on both sides,” and this is what cork cambium does. It exists in small patches that create only cork cells. As already described, the cells formed on the outside of the main cambium contribute negligibly, if at all, to a tree’s girth. What about vertical growth? How does a tree grow taller? It does so entirely by growing precisely at the top of the trunk and the tips of the branches. These so-called growing points consist of small domes of actively dividing green cells, the only cells in the tree to divide horizontally into upper and lower daughter cells. The lower ones become differentiated as they are left behind, and become new cells of the tissues already described. The uppermost cells keep on dividing thus increasing the height of the tree until it is fully grown. Then the divisions come to a stop. All appreciable growth is by these cell divisions at the tips of trunk and branches.

the life and growth of a conifer

Leaves A conifer’s leaves (indeed, any plant’s leaves) are its “food factory.” It is in the leaves that photosynthesis takes place, the creation of sugars from water and carbon dioxide in a reaction powered by sunlight and involving the green chemical chlorophyll.2 Therefore a tree can grow only when it has an abundance of leaves, and there are usually many million on a fullgrown conifer. Of the two ingredients needed for the manufacture of sugar, one, carbon dioxide, comes from the air; the other, water, comes up from the soil through the tracheids of roots, trunks, branches, and twigs. For the ingredients to react, they have to come together, and air, containing its minute proportion of carbon dioxide (about 375 parts per million at present), has to get inside the leaves. This it does through tiny pores called stomata (singular, stoma) in the leaf surfaces. The stomata provide the only route by which air and, with it, carbon dioxide, can reach the internal spaces in the leaves. There it can be absorbed, through their thin walls, into the cells containing chlorophyll. If it were not for the stomata, the air would be unable to get inside because leaves have an impermeable “skin,” or cuticle. Many needle-leaved conifers also have a layer of wax outside the cuticle. The wax is the cause of the bluish bloom on leaves such as those of the white pines and the spruces, and it makes them unwettable. Stomata also allow oxygen and water vapor, the by-products of the photosynthetic reactions, to escape. If they didn’t, we should all quickly suffocate. A tree takes in far more water than it needs for building new tissue. The soil water is an exceedingly dilute solution of the nutrients the tree needs, so the tree must exhale quantities of excess water vapor, via the stomata, to get rid of it. The process is called transpiration. The quantity of water transpired is prodigious. For example, one hectare of Douglas-fir forest can transpire 50 tons of water on a single day in summer, a weight much greater than that of the tissue gained over the same area, as the result of photosynthesis, in a whole growing season. While photosynthesis is going on, the same trees are gaining weight at a rate of about 20 to 25 tons per year.3 (This is fresh weight, so more than half of it is water.) No wonder the volume of wood (which carries water upward) is so much greater than the volume of phloem (which carries the sugar solution made by photosynthesis downward).

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Figure 4.8. (a) Lower side of a fir leaf. (b) Part of it magnified to show the bands of stomata. (c) An open and a closed stoma, with guard cells distended and collapsed, respectively.

The stomata are small oval pores less than one-twentieth of a millimeter long, and they are very numerous. They can be seen with a strong hand magnifier as tiny white dots ranged in rows along the leaves. The white stripes on the undersides of the leaves of hemlocks and firs, for example, are bands of closely packed stomata (figure 4.8). Each stoma is sunk in a pit below the general leaf surface, and the opaque white material that makes the visible dot is wax that almost (but not quite) blocks the pores. A stoma is a gap between two guard cells. It can open and close. When the guard cells are replete with water and fully distended, they curve and gape apart so that the stoma opens and air can enter. When they are not distended, they straighten, and the stoma closes (figure 4.9). Usually the stomata are open only during daylight when photosynthesis is going on, and a continuous supply of carbon dioxide is required. The striking difference between conifer leaves and the broad, thin leaves of most broadleafs is explained by the fact that conifer leaves in our area (except those of the larches) are evergreen. Because the leaves remain on the trees all through the winter, when temperatures are often too low for the roots to function, they are adapted to conserve water. The water in a tree must be conserved at a time when it cannot be replenished. The low surface-to-volume ratio of conifer leaves helps. Even so, the total area of leaf surface in an entire conifer tree is comparable to that of a broadleaf tree of the same size because conifer leaves are so numerous. Transpiration

the life and growth of a conifer

Figure 4.9. Side view of a stoma (diagrammatic): (a) open and (b) closed.

is slowed, and water conserved, by the thick waterproof wax cuticles of the leaves and by the sunken stomata; broadleafs’ stomata, in contrast, are flush with a leaf’s surface. These adaptations also give strength to conifer leaves, which are built to last; most last between 3 and 10 years, but in some firs for as much as 20 years. In winter, conifers can photosynthesize at temperatures down to –7° Celsius (13° Fahrenheit) and possibly even lower.

Roots When a conifer seed germinates, the embryonic roots immediately begin to grow and spread, absorbing soil water from all the soil within reach. The water supply is sufficient for a seedling, but the growing tree soon needs more water than its roots can absorb. The problem is solved when the probing roots encounter the underground parts of a fungus such as a mushroom or a bolete. A digression on fungi is necessary here. A mushroom, for example, is only a temporary, above-ground outgrowth from a subterranean fungus whose permanent “body” lives entirely underground in the form of a maze of fine filaments, or hyphae (singular, hypha). A hypha is a long, growing and branching tube, partitioned into cells. A single hypha is visible to the naked eye only in exceptional circumstances, but hyphae often unite to form easy-to-see white strands, found on the undersides of logs

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and fallen branches. Hyphae tangled together to make a white “felt” are common too. Hyphae in the mass form mycelium (plural mycelia). These unsubstantial components are the vegetative (nonreproductive) parts of individual fungi. It is impossible to distinguish between one individual and another when digging in forest soil. There is no way of knowing whether two apparently separate pieces of mycelium are linked by invisible hyphae. However, the underground mycelia send up distinctly separate “fruiting bodies” such as mushrooms to grow above ground and to liberate spores and scatter them in the open air. Drawing a parallel between fungi and green plants should clarify this. A mushroom growing up from a buried mycelium is analogous to a flower growing up from a green plant. A mushroom produces and scatters spores much as a flower produces and scatters seeds. Both spores and seeds are the products of sexual reproduction. A mushroom turns to mush and disintegrates after its spores are shed; a flower withers and dies after its seeds are gone. The mycelium then keeps on growing and so does the green plant (unless it’s an annual). When a young, lengthening conifer root makes contact with a fungal mycelium, they grow together and their cells become connected in a liaison so close that a new organ, belonging to both tree and fungus, is formed. This is a fungus root, or mycorrhiza (plural mycorrhizae). The relationship between the two components is mutualistic. Each of them benefits its partner. The fungus member of a mycorrhiza gains sugars (nourishment) from the conifer that it could not make for itself; it lacks chlorophyll and so cannot photosynthesize. The conifer gains what amounts to a tremendous increase in the absorbing surface of its roots. Roots without mycorrhizae can absorb water only through their short root hairs. Roots with mycorrhizae have access to a far greater volume of soil because the fungal hyphae keep on growing and branching until they have reached every pore in a very large soil volume. Once a tree has formed mycorrhizae, it usually stops growing root hairs: they are no longer needed. Figure 4.10 shows a few of the mushrooms (and a bolete) that are known to form mycorrhizae with conifers; there are many more. The two unrelated groups of organisms, conifers in the plant kingdom and mushrooms in the fungus kingdom, are so dependent on each other that the survival of both depends on their mutualism. There is more on mycorrhizae in chapter 7.

the life and growth of a conifer

Figure 4.10. A few of the fungi that form mycorrhizae. (a) Fly agaric. (b) Peppery milkcap. (c) Edible bolete. (d) Chanterelle. (e) Clubfoot clytocybe. (f) Stinking russula.

Figure 4.11. A hemlock stump, (a) just after the tree was felled, then (b) after a ring of outer tissue has grown, and finally, (c) after a complete cap of bark covers it.

Besides being linked to fungi, tree roots also become linked to each other. Neighboring trees become naturally grafted when their roots chance to touch and press against each other. In forests that have been partly logged in the past, look for cut stumps that haven’t rotted. Any tree that has been felled is fatally wounded, but sometimes it goes on living long enough to grow a callus of scar tissue over the wound (figure 4.11), just as it grows a callus over a nonfatal wound like that caused by the breaking off of a branch. The scar

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tissue sometimes makes a protective cap completely covering the wound. How can this happen when photosynthesis has become impossible? The answer is, it comes from nearby trees whose roots are grafted to those of the stump. A single large tree can provide enough food to keep many stumps alive, but they will all die quickly if their provider tree is felled.4 Finally, roots have another indispensable function. They anchor a tree and hold it upright (unless the wind is too strong). Notes 1. H. Kubler, “Function of Spiral Grain in Trees,” in Trees, vol. 5, 1991, pp. 125–35. 2. The chemical formula is 6CO2 + 6H2O + light → C6H12O6 + 6O2. Starches are chemically united groups of sugar molecules. Carbohydrates are all compounds of carbon, hydrogen, and oxygen (and nothing else). 3. E. J. Mullins and T. S. McKnight, eds., Canadian Woods: Their Properties and Uses, 3rd ed. (Toronto: University of Toronto Press, 1981). 4. F. H. Bormann, “Root Grafting and Non-competitive Relations between Trees,” in Tree Growth, T. T. Kozlowski, ed. (New York: Ronald Press, 1962).

Chapter 5

Broadleafs Growing among the Conifers

Broadleafs in a Harsh Climate Although evergreens dominate the northern forests, a few broadleafs are found with them, and they are the subject of this chapter. The trees and shrubs considered here are those living in the harsh climate of the true north, not those in the mixed forests of the St. Lawrence valley and southern Great Lakes where climate and soils are hospitable to many broadleafs. The broadleafs described here play an important part in the coniferous forests. They belong to four genera: the willows (Salix), the poplars (Populus), the birches (Betula), and the alders (Alnus). Shrubs as well as trees of these genera are common in the evergreen forests. All have catkins (figure 5.1). A catkin consists of densely packed rows of flowers growing along a short stalk that may be upright or dangling. The individual flowers are tiny and devoid of petals. There are female catkins made up of female, seed-bearing flowers, and male catkins made up of male, pollen-producing flowers. The female catkins of alder differ from all the others in that they resemble a tiny cone: they are so conelike that they are often called “cones.” These four catkin-bearing genera belong to two families. The willows and poplars are in the willow family, Salicaceae, in which each plant (tree or shrub) bears either male or female catkins (not both), and in which the tiny seeds bear long fluffy hairs (the “cotton” of cottonwood, a poplar subspecies) so that even a gentle wind will carry them far from their 59

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Figure 5.1. Twigs with catkins. (a) Birch. (b) Alder. (c) Poplar. (d) Aspen.

parents. The birches and alders belong to the birch family, Betulaceae, in which each plant bears catkins of both sexes, and in which the seeds are winged. The flowers of these four broadleafs and the cones of conifers are adapted to life in high latitudes where wind-pollination is more likely to succeed than insect-pollination. The female flowers (in broadleafs) and the cones (in conifers) must be pollinated early in the year if their seeds are to ripen in the short growing season, and there is always the risk that the weather will be too cool for bees to be on the wing when they’re needed. Catkins, like conifers’ pollen cones, are adapted to wind-pollination, and the poplars,

broadleafs growing among the conifers

birches, and alders are wholly dependent on wind to pollinate them. But many of the willows rely on wind-pollination only in a cold spring. When spring is warm, pollen-covered pussy willows (which are the male catkins of some species of willow) produce nectar that attracts bees to act as pollinators. (This presumes that bees are sufficiently numerous. At the time of writing, bee populations are declining.) Of these four genera of conifer “companions,” the willows are probably the least important for their effects on the conifers. They are companions only in the sense that they grow, in suitable habitats, throughout the north country. The great majority of willows are shrubs rather than trees, and it’s rare for large numbers of them to occupy ground where conifers would otherwise grow. Many species grow only where water is nearby, along the shores of lakes, rivers, and streams; others, such as net-veined willow (Salix reticulata), are prostrate shrubs, equally at home in arctic tundra and boreal forest. The willow genus is so enormous (more than 80 species in our area), and the species so hard to identify, that to investigate them is a job for specialists. No more will be said of them here. What follows concentrates on broadleafs that have an appreciable effect on the welfare of conifers. Luckily, they are easy to identify.

Poplars Members of the poplar family—that is, the trees known as poplars, cottonwoods, and aspens—are fast-growing, hardy trees. Life is usually rather short for each individual tree, but they propagate so rapidly that there’s little risk of their numbers decreasing as long as the environment they need does not diminish. Two poplar species, balsam poplar and trembling aspen, are adapted to endure the intensely cold climates of northern Canada and Alaska (see chapter 1) and can therefore grow in the company of similarly adapted conifers. They are found either as tracts of pure poplar or aspen forest or mingled with the conifers, almost everywhere in our area. The two species are easy to recognize. Trembling aspen, often called quaking aspen, is famous for its trembling leaves. The reason they tremble, even in the gentlest breeze, is that they have flat petioles (leafstalks). If you look at an aspen leaf with its blade lying flat on your hand, you see the strapshaped petiole edge-on. The blade of the leaf is almost circular (figure 5.2). Trembling aspen bark is greenish white, smooth, and powdery. If you rub

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Figure 5.2. Trembling aspen, bark and leaf.

it, it whitens your skin much as a piece of white chalk would. The bark does not lose its characteristic smoothness until near the end of the tree’s short life. Trembling aspen seldom lives for more than 100 years. Balsam poplar has larger, more tapered leaves; the petioles are not flat, so the leaves don’t tremble. It has deeply furrowed bark once it is fully grown (figure 5.3), and like trembling aspen, its life span is short. Westerners are familiar with an almost identical west coast tree that used to be known as black cottonwood. It grows to tremendous size in the luxuriant Pacific rain forest. It hybridizes so readily with balsam poplar that it is now treated as a geographically separate subspecies of balsam poplar, Populus balsamifera ssp. trichocarpa. Trembling aspen and balsam poplar resemble each other in that neither can live with their crowns shaded. To live, their leaves must receive full daylight. They also resemble each other in having shallow, but very widespreading root systems; the importance of this will become clear later. The two species require rather different environments. Balsam poplar must have a moist soil and therefore does best on low-lying ground, especially valley bottoms. Trembling aspen can get by with much less water and does fairly well even on dry hillsides. Between them, the two species can take possession of almost any suitable tract of country provided their seeds arrive there at an appropriate time. This is the crux of the problem. A suitable tract is one open to the sky and devoid of competing vegetation. The circumstances in which a new

broadleafs growing among the conifers

Figure 5.3. Balsam poplar, mature bark, and leaf.

stand of poplars can get started are uncommon. The open, unoccupied ground they must have is most often the result of a forest fire, perhaps in a spruce or jack pine forest. The fire must happen in a place where fresh poplar seeds are available to seed the newly burned area. It must happen at a time when no competing plants have had a chance to establish themselves before the poplar seeds arrive. And there must be enough water available in the very short period that the seeds are viable (less than a month) for them to germinate. The water can come as rain or, for balsam poplars, from the waterlogged ground of river floodplains and sand bars. Only on the uncommon occasions when all these special conditions are met will poplars invade a new area successfully. The opportunity comes much more rarely for trembling aspens, which must rely on well-timed rain for their water supply. What makes the co-occurrence of all these required conditions so unlikely is that the seeds of these species are viable for a mere three weeks after they are shed, an unusually short interval. The seeds of most plants remain viable for at least a year and many for several (up to 100 or more) years. Once poplars have invaded, they are tenacious. The reason is that they spread not only by seed but also by sending up new shoots (root suckers) from their widely spreading roots. Trembling aspens, especially, depend on root suckers to perpetuate themselves because their seeds so seldom have a chance to germinate. A single “parent” tree (a genet) may have a root

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system 50 m (164 ft) across, and the whole large area becomes populated with root suckers that quickly grow up to new trees (ramets). Genetically speaking, the ramets are merely parts of the genet. A whole grove of ramets will grow extensive root systems that will produce more ramets, and so on indefinitely. Only a catastrophe such as a disease or a very hot fire will halt the process. In this way, extensive clones of trembling aspens are formed, each many hectares (or acres) in area, and all genetically identical (this defines a clone). Poplars have effects on conifers, and vice versa. The seedlings of shadetolerant conifers, particularly spruces, benefit during their early years by having a cover of poplars to “protect” them. A forest in which young conifers are growing up among taller, over-shadowing poplars is a common sight in the north country. The poplars serve as nurse trees for the conifer seedlings and saplings. In spring and summer they provide a moist, cool environment for the young conifers. In fall they supply a mulch of fallen leaves that conserves soil moisture. And in winter the poplar trunks act as snow fences, causing snow drifts to accumulate among them; the deep snow engulfs the young conifers—small ones are often completely buried—and insulates them from the cold. As a final benefit, the poplars die young and so do not compete for soil nutrients with the rapidly growing conifers once their usefulness as nurse trees has ended. Although poplars benefit conifers, they receive no payback. But when fire kills the conifers, poplars can invade again. One other Populus species should be mentioned. It is the largetooth aspen (Populus grandidentata), which is very much like the trembling aspen except that its leaves are almost twice as large and have toothed margins. Its geographic range is not nearly as big as that of trembling aspen; it is an eastern tree that grows too far south to need to be “very hardy” in the sense defined in chapter 1.

Birches Paper birch (Betula papyrifera) is the most familiar and the most eyecatching broadleaf to grow in evergreen forests. It is intolerant of shade, fast-growing, and short-lived, so it serves as a nurse tree in the same way that aspens and poplars do. Paper birch (also called canoe birch) grows nearly everywhere in our area. Two other species of birch have roughly the same range, except that

broadleafs growing among the conifers

they do not grow east of Manitoba. Both are very hardy and grow among conifers as far north as the Mackenzie River delta. They are Alaska birch and water birch. Alaska birch (B. neoalaskana) resembles paper birch in having fairly similar, peeling bark, but the bark is orange to cream-colored rather than pure white and doesn’t peel so easily. It is a much shorter tree than paper birch, and typically grows in wet soil or bogs, often with black spruce. Water birch (B. occidentalis) is the smallest of the trio and is more often a shrub than a tree. Its leaves are like those of other birches, but its bark is dark bronze-colored, with numerous thin, horizontal, whitish lines closely scattered all over it. These three birches can act as nurse trees just as poplars do, but only paper birch is tall enough to be useful for long.

Alders The alders are the fourth group of broadleafs to share the evergreen forests with conifers. Two species, green alder (Alnus viridis) and speckled alder (A. incana), are very hardy northerners, but they are not restricted to the north. Green alder is found throughout our area; speckled alder has much the same range except that it stops some distance short of the Pacific coast. These two species often grow no larger than shrubs. A third alder (figure 5.4), found only in the Pacific coast rain forest from Alaska to California, also deserves mention here although it isn’t very hardy. It is red alder (A. rubra), a medium-sized tree that has a most beneficial effect on the conifers of the coastal rain forest where it is common. Like birches and poplars, it grows rapidly and dies young, which makes it a good nurse tree for conifers. In addition, along with all other alders, it is a nitrogen-fixer. All plants require nitrogen, but they cannot absorb and use pure, gaseous nitrogen straight from the air. Instead, they must acquire it through their roots from dissolved nitrogenous compounds such as nitrates. They do this with the help of certain species of soil bacteria that fix nitrogen,(that is, combine with it chemically). These bacteria are particularly abundant in the soil surrounding alder roots because the roots secrete a substance that stimulates the bacteria to multiply. Some bacteria penetrate the root cells and cause swellings—nodules—to develop. The nodules are bright orange and easy to see if you dig up a shallow root-tip (figure 5.4). When

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Figure 5.4. Red alder. (a) Bark; it is covered with patches of different species of lichen. (b) Leaf. (c) Rootlet bearing clusters of bright orange nodules.

the alders die and decay, their nitrogen compounds enrich the soil and become fertilizer for the conifers. (The process is like that in legumes such as peas, beans, and alfalfa, which use different species of bacteria and form scattered, small, white root-nodules, not nearly as easy to see as those on alders.)

Chapter 6

Two Kinds of Trees: Conifers and Broadleafs

Introduction We have now taken a detailed look at the dominant trees in the northern evergreen forests—conifers—and a cursory look at the few deciduous trees that grow there with them—broadleafs. It is time to consider the contrast between the two kinds of tree. Not all conifers are evergreen (think of the larches), and not all broadleafs are deciduous (think holly). The differences between them are profound, so profound that it is ludicrous, nowadays, to lump them collectively as “trees.” Using one word for a combination of two such entirely different entities is inappropriate in the light of modern scientific knowledge.

The Ancestry of “Trees” It may surprise many naturalists to know that conifers and broadleafs are less closely related to each other than are mammals and birds. The first conifers appeared about 300 million years ago, and the first flowering plants, about 140 million years ago1 (in what follows, the word “about” applies to every number). So flowering plants first appeared 160 million years after conifers. The first birds appeared 250 million years ago, and early mammals, that is, hair-growing animals that suckle their young like the primitive duck-billed platypus, 180 million years ago. So mammals came 70 million years after birds. 67

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If it’s legitimate to measure genetic divergence (or evolutionary change) by the time it takes to happen, the flowering plants differ from conifers more than mammals differ from birds. Botanists rest their case. Since angiosperms (including broadleafs) and gymnosperms (including conifers) became separated, both groups have continued to evolve and have left fossils, some of species still living and others of species now extinct. Recall that the angiosperms include all flowering plants, using the words in their botanical, not horticultural, sense: grasses and sedges, and broad-leaved trees with scarcely noticeable flowers belong to the angiosperms. The gymnosperms include, besides the conifers, two other groups, both semitropical, that reproduce in the same way. First is the exotic maidenhair tree (Ginkgo biloba), which is often grown as an ornamental and is the only surviving member of a larger group whose other members are now all extinct. Second is the cycads with several species; a typical cycad looks like a dumpy palm tree with big cones, and is often grown as a hothouse ornamental. It seems likely that conifers and their relatives are on their way to extinction. Their numbers and variety appear to have been dwindling for millions of years. The world is poorer in conifer species than it was millions of years ago, and as their variety gradually diminishes, flowering plants seem to be diversifying. This is the reason for believing that flowering plants have evolved more than conifers. They have been able to adapt faster to ever-changing conditions. Or, what comes to the same thing, nature is selecting them.

The Basic Difference between Conifers and Broadleafs The greatest contrast between conifers and broadleafs is in their reproductive systems. One anatomical difference was described in chapter 3: the ovule in a cone is naked in the sense that the pollen tube has direct access to it. The ovule in a flower is inside a carpel, a soft outer covering that the pollen tube has to penetrate to reach the integument and ovule. But that is only half the story, the less interesting half. The other half is that whereas gymnosperms need only one sperm cell to fertilize an ovule, angiosperms need two. Figure 3.4 shows this most surprising difference, as well as that between the layers of tissue the pollen tube must penetrate, as already described.

two kinds of trees

Skip the next three paragraphs if you don’t want the technical details. First, a reminder on the chromosomes of sex cells: In flowering plants, as in humans and other mammals, every cell in the body except for the sex cells contains a duplicate set of chromosomes; such a cell is called a diploid cell. The sex cells, on the other hand, both male and female, each contain only a single set of chromosomes and is called a haploid cell. Therefore, when a sperm fertilizes (unites with) an egg, the result is a diploid cell, the zygote, which is the first cell of an embryo. The zygote divides and redivides repeatedly, eventually becoming a ripe seed, a baby, a puppy, or whatever the case may be. Now refer to figure 3.4. Figure 3.4a shows the course of events in a conifer ovule: it has one sperm for one egg. The sperm reaches the egg, they unite to form a zygote, and that’s that, just as in mammals. Figure 3.4b shows the course of events in a broadleaf flower. The ovule contains two cells to be fertilized: a haploid egg cell and a diploid endosperm cell. The pollen tube delivers two haploid sperms from the pollen grain. They are not the same size. The larger of them unites with the egg to form a diploid zygote, which becomes an embryo. The smaller sperm unites with the diploid endosperm cell, as it is called, to form a triploid cell, with three sets of chromosomes. The triploid cell divides and redivides to form nutrient endosperm tissue, consisting of triploid cells, inside the seed; it nourishes the growing embryo somewhat as a mammalian placenta nourishes a fetus. The whole process is called double fertilization. In short, it takes two sperms to make a “baby,” which seems a sufficient reason for recognizing conifers and broadleafs as only very distantly related. The two groups should obviously not be lumped together merely as “trees.” There are other notable differences between gymnosperms and angiosperms, and we consider them next.

Gymnosperms Are Woody This is one of the differences that is easily observed. Nearly all conifers are trees; only a very few are shrubs, and none are herbaceous plants, in a word “herbs” (in the botanical, not the culinary sense)—that is, soft, nonwoody plants that die back in the fall. Only a small fraction of the angiosperms are trees (the broadleafs), and a horticulturist or nursery gardener would never think of some trees as “flowering”— the flowers of such trees as oaks and elms simply go unnoticed. Nonbotanists divide seed plants

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into trees and non-trees, whereas botanists divide them into gymnosperms and angiosperms, and the two divisions don’t correspond. In fact they are like the two lines below; but note that the comparison is between conifers, which have only two subsets (trees and shrubs), and flowering plants, which have three subsets (trees, shrubs, and herbs); the very few gymnosperm shrubs have been disregarded in the diagram. Trees Conifers

Non-trees Flowering plants

The Speed of Living One of the most striking differences between conifers and broadleafs is that conifers live their lives much more slowly: individual trees live longer, and after their deaths they take longer to decay. In a nutshell, conifers have longer life spans and longer “decay spans” than broadleafs. For example, here are the results of some observations made in British Columbia, based on 13 species of conifers and 8 of broadleafs.2 The average life span was 700 years in the conifers and only 210 years in the broadleafs. Likewise with the decay spans, the averages were 170 years in the conifers and 35 years in the broadleafs. (The times for Nootka-cypress were so extreme they were not included in the averaging. For this species the expected life span is almost 2000 years, and the decay span 600 years.) All the numbers are approximate. Another process that is much slower in conifers than in broadleafs is the development of a newly fertilized egg in an ovule to a mature seed ready to germinate. Consider yet another process: recall from chapter 4 that sap rises at only about 0.5 m (1.6 ft) per hour in a conifer’s tracheids and at several meters per hour in a broadleaf’s vessels. The speeds at which sugar solutions descend in the phloem differ correspondingly. One would expect a consequence of this difference in the efficiency of the “plumbing” to be that conifers would grow more slowly than broadleafs. But in fact, there is no consistent difference. The reason is that the cross section of all of the conducting tubes conveying liquids up and down is not consistently different between conifers and broadleafs. Envisage the wood: The two kinds of wood have a different mix of cells. About 90 to 95 percent of conifer wood consists of conducting tracheids, and only 5 to 10 percent is made of nonconducting cells

two kinds of trees

that give the tree some mechanical support. About half of broadleaf wood consists of vessels, and the other half, of strong fibers, which are not part of the plumbing. This difference between the two kinds of trees cancels out, so far as its effect on speed of flow of the sap is concerned.3

The Architecture of Trees Conifers and broadleafs are obviously different in shape. In conifers, the trunk grows straight up without dividing (unless the growing point at its tip has been destroyed during growth, by a porcupine, for example). In most of the pine family, the branches grow out from the trunk like the spokes of a wheel, more or less horizontally, and each tier of branches is younger and therefore has a smaller radius than the one below it. As a result, the tree is usually conical in outline, provided it isn’t crowded by closely spaced neighbors. In that case it is conical only at the top and columnar below. Figure 6.1 shows some typical outlines.

Figure 6.1. Silhouettes of some familiar trees as they grow in a forest. (a) White spruce. (b) Eastern white pine. (c) Western hemlock. (d ) Amabilis fir. (e) Eastern white-cedar.

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It is often supposed that all conifers are completely conical. And, further, that this is an adaptation to ensure that the tree can shed snow easily and thus be saved from branch-breaking overloads. This opinion is held by those who try to find plausible explanations for everything in nature in terms of the natural selection of beneficial modifications. It is far more likely that the conical tops of conifer trees happen automatically, simply from their manner of growth. In the 300 million years during which conifers have been evolving, the climates in which they grew had snow in winter for only a very small fraction of the time. Broadleafs are sometimes shaped like conifers, but very often their trunks divide, their branches grow to be almost as thick as the trunk divisions, and the crown of the tree becomes rounded. The difference is pronounced in such broadleafs, but not in all.

Vegetative Reproduction A difference already noted at the end of chapter 3 is the rarity of vegetative reproduction in conifers compared with its commonness in broadleafs.

The Aroma of Conifers A notable contrast between conifers and broadleafs is detectable with your nose: many conifers have a distinctive turpentine-like odor, that of the volatile terpenoids dissolved in their resin. These chemicals have been found in 168-million-year-old (middle Jurassic) fossil gymnosperm wood, that is, from a time before angiosperms had evolved.4 The characteristic smell of a pine wood, for example, is a reminder of the great antiquity of the trees. The chemicals are useful to the trees, and their presence is a result of natural selection: they are a self-defense mechanism. They are released when a tree is wounded by insect attack, and they repel, or poison, the attackers.5 Notes 1. For plants, P. R. Bell and A. R. Hemsley, Green Plants: Their Origin and Diversity, 2nd ed. (Cambridge: Cambridge University Press, 2000). For animals, R. Dawkins, The Ancestor’s Tale (London: Phoenix, 2004).

two kinds of trees 2. M. Fenger et al., Wild Life and Trees in British Columbia (British Columbia: Ministry of Forests and Range and Lone Pine Publishers, 2006). 3. J. Whitfield, In the Beat of a Heart (Washington, D.C.: John Henry Press, 2006). 4. O. A. Marynowski et al., “Biomolecules Preserved in ca 168 Million-Year Old Fossil Conifer Wood,” in Naturwissenschaften, vol. 94, 2006, pp. 228–36. 5. J. Bormann et al., “Terpenoid Defenses in Conifers: cDNA Cloning, Characterization, and Functional Expression of Wound-Inducible (E)-α–Bisabolene Synthesis from Grand Fir (Abies grandis),” in Proceedings of the National Academy of Sciences USA, vol. 95, 1998, pp. 6756–61.

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Chapter 7

Life on the Forest Floor

The Soil The forest floor consists chiefly of soil, which makes it a good place to start. By tradition, northern forest soil is described as “poor,” which it obviously isn’t for conifers. It simply means poor for agriculture. One of the most distinctive soils is podsol, the typical soil in conifer forests. (For brevity, let’s ignore the fact that its modern name is “spodosol.”) This soil is easily recognizable if you expose it by digging a hole, or even more easily by seeing it exposed in an eroding stream bank. It has a thin, dark top layer of packed needle-leaf litter gradually decaying into humus (newly decomposed material with organic ingredients only). Immediately below that is a layer, often thick, of almost pure white sand. The white sand is sterile, but comparatively nourishing red or brown sandy soil lies below it. Sometimes, however, this “good” layer is useless to trees because the mineral-bearing water draining slowly down into it cements the soil into a hardpan impenetrable to tree roots. The soil is cold, acidic, and sometimes wet. It is too acidic for many flowering plants. Among the few that are at home growing in it are members of the heath family (Ericaceae). Most of them are shrubs, sub-shrubs, and woody ground-creepers, for example, kinnikinnick (Arctostaphylos uva-ursi), Labrador tea (Rhododendron (was Ledum) groenlandicum), blueberries and huckleberries (Vaccinium spp.), cranberries (Oxycoccus spp.), salal and creeping snowberry (Gaultheria spp.), swamp laurel or 74

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lambkill (Kalmia spp.), and leatherleaf (Chamaedaphne calyculata). Of these, some prefer open forests on dry, often sandy ground; others grow in patches of wetland in the forest. All are adapted to acid soil of low fertility, whether wet or dry.

Forest Flowers A group of flowering plants that grow only in conifer forest are members of the family Monotropaceae. They are non-green, lack chlorophyll, and so cannot photosynthesize. Therefore, they must obtain their sugars readymade. Examples of them, shown in figure 7.1, are candystick (Allotropa virgata), pinesap (Monotropa hypopithys), Indian-pipe (Monotropa uniflora), and gnome-plant (Hemitomes congestum). These plants used to be known as saprophytes, meaning they obtained their sugars from dead organic matter, but it is now known that, in fact,

Figure 7.1. (a) Candystick. (b) Pinesap. (c) Indian-pipe. (d) Gnome-plant.

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no plant can do this. They are parasites on fungi that in turn are parasites on conifers, which makes them epiparasites on conifers.1 The fungi concerned are the mycorrhizal fungi described in chapter 4, where they were not called parasites because the attachment between fungus and conifer is mutualistic. One could say, though, that each was parasitic on the other. (The terminology that has developed is confusing.) Hence, we are dealing with a three-part linkage (figure 7.2) in which a conifer, a fungus, and a parasitic plant (in this case, Indian-pipe) are all connected. It’s possible that the pinesap contributes a growth stimulant to the fungus; even if it does, its contribution to the threesome is probably negligible. The epiparasitic

Figure 7.2. The linkages among a conifer and its mycorrhizal fungus, plus a mycoheterotrophic plant, in this case Indian-pipe.

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plant can also be described as myco-heterotrophic. A heterotrophic plant is one that cannot feed itself (in contrast to an autotrophic plant that photosynthesizes its own sugars). The prefix myco is from the Greek mukes, a mushroom. The plants already listed are not the only myco-heterotrophs. Some orchids, too, such as the various coralroots (Corallorhiza spp.), behave in the same way. Another group of flowering plants that grow among conifers belong to the wintergreen family, Pyrolaceae. These green plants are believed, on strong evidence, to be ancestral to the non-green plants just considered. Examples are pipsissewa (Chimaphila umbellata), single delight (Moneses uniflora), and one-sided wintergreen (Orthilia secunda), shown in figure 7.3. To summarize, the most common flowers on the coniferous forest floor belong to three families: one, of plants that prefer acid soil; a second, of epiparasitic plants; and a third, of ancestors of the epiparasites. A plant in one of these categories truly belongs in the forest. Others are “intruders” that have chanced to find hospitable spots.

Figure 7.3. (a) Pipsissewa. (b) Single delight. (c) One-sided wintergreen.

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The Floor of the Boreal Forest The soil in coniferous forest is typically cold, as noted earlier. In the boreal forest of the far north, it is cold enough to freeze, and the result is permafrost. Near its southern limits, permafrost forms only in occasional patches, and its upper surface (the permafrost table) is more than a meter below ground level. Going north, permafrost becomes progressively more extensive and thicker, with the permafrost table approaching ground level until it is close to it. The unfrozen layer at the surface in summer is the active layer. The thickness of the frozen layer ranges from zero at its southern boundary to about 600 m (1970 ft) in the High Arctic. With increasing cold, plant remains decompose ever more slowly, causing the soil water to become more and more acidic. A ground cover of sphagnum moss (numerous species of the genus Sphagnum) develops on the soil above the permafrost table. Generation upon generation of moss piles up, compressing previous years’ accumulations below it. The microbes that bring about decay are sluggish in the cold and gradually rot the dead moss, which becomes peat. Peat is a good insulator, causing the soil to cool and the permafrost table to rise; the result of this negative feedback is deadlock—in other words, equilibrium—and the permafrost table stays put (more or less). The waterlogged moss-covered ground is muskeg. Conifers, mostly scattered tamarack and clumps (clones) of black spruce, grow in it. Going north, the trees become smaller and sparser and finally, at the tree line, disappear altogether. Figure 7.4 shows the geographical pattern of permafrost country and muskeg. As the map shows, in the eastern boreal forest, muskeg is plentiful south of the permafrost zone. It develops where water is prevented from draining away by an impervious layer under the soil. The layer need not be frozen ground; it could be hardpan or clay. Regardless of what prevents drainage, however, muskeg is always cold, wet, and acidic. Flowering bog plants, as well as the conifers just mentioned, also grow in muskeg, for example, Labrador tea, bog laurel, leather-leaf, and sundew (Drosera spp.). Around the margins of muskegs, where the ground level is slightly higher and the drainage better, black spruce and tamarack are replaced by white spruce, sometimes with an admixture of balsam poplar. The ground cover is still moss, but the Sphagnum mosses are replaced by feather moss

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Figure 7.4. Map showing muskeg lands (stippled). The lines show the boundaries of permafrost country: the dash-dot line is its southern limit; the dashed line is, simultaneously, the northern limit of trees and the line north of which the permafrost is continuous.

(Hylocomium splendens) and big red-stem moss (Pleurozium schreberi) (figure 7.5). Where forested land becomes waterlogged, perhaps from a change in subsurface water flow, it sometimes becomes too soft to support the trees growing on it. The trees begin to lean in all directions, and the result is a drunken forest (figure 7.6). But the ground isn’t wet everywhere. A component of the northern boreal forest (the taiga) that is most attractive scenically is the “lichen woodland,” which grows on land drier than that occupied by the two varieties of spruce-moss forests. In lichen woodland, the trees, chiefly white spruces, are spaced widely enough to grow moderately tall, and the ground cover is an unbroken expanse of pale gray-green reindeer moss (Cladina rangiferina), an unexpected color for a forest floor.

Valuable Dead Wood and Debris From the ecological point of view, dead trees have value as well as living ones. They are the source of woody debris, everything from whole, fallen dead trees to twigs and small fragments of wood. Logs and tree branches, in particular, are valuable on account of the habitats they provide for miscellaneous vertebrate animals, from fish to snowshoe hares.

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Figure 7.5. (a) Sphagnum moss. (b) Feather moss. (c) Big red-stem moss.

Figure 7.6. Drunken forest.

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The ecological habitats in rivers and streams flowing through evergreen forests are well supplied with fallen wood: recall that conifer wood decays much more slowly than broadleaf wood, so the quantity available is much greater in coniferous forest than among broadleafs. Large woody debris (“LWD” to stream ecologists) is greatly valued for the fish habitat it provides in forest streams. LWD is defined as any chunk of wood, ranging from a whole trunk and its root wad down to branch fragments more than 10 cm (0.4 in) in diameter and 2 m (6.6 ft) in length. These large chunks create places where the current slows, and pools form, where fish can rest and hide concealed in the shade cast by the chunk itself. Crevices in the dead wood contain a variety of invertebrates, chiefly immature insects, which the fish eat. Piles of LWD often slow a stream enough to reduce its erosive power appreciably, thereby stabilizing the flow. The merits of woody debris are now realized as never before, and it is allowed to remain where it has fallen even in the “managed” forests of parks, where it used to be cleared and removed as a form of tidying up. Dead wood is also ecologically important on the forest floor away from streams. Small animals live there, and all animals, without exception, need places to rest, hide, and give birth. Fallen wood provides a variety of usable sites. The commonest vertebrate animals living on the forest floor are salamanders (only in the south), and also shrews in wet sites and deer mice in dry sites throughout our whole area. They are all worth watching for. The commonest salamander is the eastern red-backed salamander (Plethodon cinereus). It is a terrestrial salamander, which is to say it spends its whole life on land and has no aquatic, tadpole stage. It lays its eggs in cavities in waterlogged wood, and the hatchlings are like miniature adults. It takes sharp eyes to spot them. The same description covers its relative in the Pacific rain forest, namely, the western red-backed salamander (P. vehiculum). The species name vehiculum here means “conveyance”: during courtship the male carries the female around on his back. The smallest mammals, shrews and deer mice, are described with other mammals in chapter 10.

Open Water If you compare a geographic map showing the range of the beaver2 (Castor canadensis) with a map of “our” area (see figure 1.1), it’s obvious that the two overlap almost everywhere. The conclusion follows at once that

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Figure 7.7. Beaver.

beavers live close to conifers almost everywhere, and knowing that they eat wood suggests that they rely on conifer wood for food. But this is not so: beavers use conifers only for emergency rations and occasionally for dam and lodge building when a conifer is within easy reach. Their favorite foods are the phloem, leaves, twigs, and buds of those extra-hardy broadleafs (described in chapter 5) that, by surviving the coldest winters in North America, are true members of the northern forests. Beavers establish themselves wherever ponds and lakes collect on low-lying ground among stands of the broadleafs (figure 7.7). They then use the trunks and branches of the trees to create their own habitat, by building lodges and dams. In addition to lodges, they dig burrows to live in, in the banks of ponds held back by the dams. They are inhabitants of the northern evergreen world just as much as the forest animals described in chapter 10. Among the birds found close to lakes and ponds in the evergreen forest in season are hole-nesting ducks. Four species are particularly noteworthy for the tremendous contrast between their overwintering territory and their breeding territory, namely, buffleheads, American goldeneyes, Barrow’s goldeneyes (only in the west), and common mergansers. Large numbers (not all) of these birds spend their winters diving for fish in the storm-exposed, coastal salt waters of the nearest ocean (Atlantic or Pacific) and their summers nesting in cavities in forest trees. They seem indifferent to whether the trees they use are conifers or broadleafs, and living or dead. Because ducks cannot excavate their own nest holes, they must find ready-made ones. For the smallest ducks, buffleheads, abandoned holes of large woodpeckers or flickers are suitable. The larger ducks use holes made by injuries to a tree, such as the socket left where a branch has fallen

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or the hollow at the top of a snag (a standing dead tree). In any case, compared with such migratory birds as warblers and shore birds, these ducks experience a truly remarkable change in scenery while going from one to the other of their seasonal habitats. Notes 1. M. L. Bidartondo, “The Evolutionary Ecology of Myco-heterotrophy,” in New Phytologist, vol. 167, 2005, pp. 335–52. 2. A. W. F. Banfield, The Mammals of Canada (Toronto: University of Toronto Press, 1981).

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Chapter 8

Parasites on the Conifers

The Value of Rot and Decay Indisputably beneficial fungi, as described in chapters 4 and 7, are those that unite with tree roots to form mycorrhizae. Now we come to wooddestroying fungi, those that cause wood to rot, decompose, or decay— all three words mean the same. These fungi are discussed in innumerable books devoted to tree “diseases,” describing the damage they do to timber. However, they are vitally necessary in the maintenance of living, growing forests. Without them, dead woody debris, fallen trees, logs, branches, and twigs would accumulate on the forest floor year after year without end. Just visualize it—only then can the immense importance of decomposers be appreciated. They are true recyclers (as is fire; see chapter 11).

Decay Fungi The decay fungi are an entirely different set of fungus species than those forming mycorrhizae. No species is occasionally mycorrhizal and occasionally a destroyer. The fruiting body of most decay fungi is known as a conk or shelf fungus (sometimes, a bracket fungus). A conk grows on the surface of its tree host, and its hyphae grow directly into the tree’s tissues without touching the soil. Three types of conk are illustrated in figure 8.1. Their shape depends on where on a tree they grow. The majority project out like a 84

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Figure 8.1. Three decay fungi on conifers. (a) Red-belted polypore. (b) Hemlock varnish conk. (c) Velvet-top fungus (or dye polypore).

shelf from a tree’s trunk, and their hyphae grow into the trunk. Some resemble short-stemmed mushrooms and grow upright from a horizontal surface like that of a cut stump or exposed root. A few others grow as complete discs apparently on the forest floor, but their hyphae grow down into the shallow roots of the host tree. The appearance of wood decayed by these fungi depends on which of the tree’s chemical constituents the fungus consumes. The two most common constituents of wood are cellulose, forming about 50 percent of its mass, and lignin, about 30 percent. Their colors are white and rusty redbrown, respectively. Therefore, a fungus that consumes cellulose leaves leftovers of red-brown rotted wood that is mostly lignin. Conversely, if the lignin is consumed, a white rot is left. Red-brown rots are the more common kind in conifers. Most of the decay fungi are polypores (family Polyporaceae), whose spores are liberated through numerous fine pores on the lower surface of the cap or shelf. Of the fungi shown in figure 8.1, red-belted (also known as red-banded) polypore (Fomitopsis pinicola) is both common and widespread. It attacks the majority of conifers. The conk is hard, woody, and perennial, but it grows a fresh pore surface every year. The conk is either flat or hoof-shaped, and colored in concentric zones: black on the inner part and creamy white

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at the outer edge, which is smoothly rounded. The red “belt” is one of the blended zones between the color extremes. The fungus attacks dead trees, producing reddish brown, rotted wood that is divided first into neat cubes, which later crumble. This type of rot is caused by so many decay fungi, it is of little help in recognizing the fungus species that made it. A common conk in eastern forests is hemlock varnish conk (Ganoderma tsugae) (figure 8.1b). It attacks several conifers, most often eastern hemlock. When it grows as a semicircular shelf, it attaches itself to its host by a short thick stem. The conks are annual, and when fully grown in summer, they have glossy, red tops with a pale rim; they look as though they had been lacquered. Velvet-top fungus, also known as dye polypore (Polyporus schweinitzii), is another widespread and easily recognized decay fungus. Its disc is usually all brown and depressed at the center; it is covered with velvety short hairs. Sometimes it grows as a shelf near the bottom of a trunk, and sometimes (figure 8.1c) as flat discs growing up from a buried root into which its hyphae penetrate. It can spread from root to root in an infected stand. Many rot fungi attack the roots of conifers. A common one in the Pacific rain forest is laminated root rot (Phellinus weirii), found on many species, especially Douglas-fir and western redcedar. It spreads from tree to neighboring tree through root grafts, infecting extensive clumps of trees all at the same time; this gradually weakens the roots, making them less wind-firm. A winter storm can then blow all the linked trees down together, leaving a big opening in the forest. Clumps of blowdowns are easily recognized by their exposed roots, which are mere stumps of roots and smooth because all of the small rootlets have rotted away. This fungus has no conks, the sporeproducing surface being merely a brownish crust on the underside of a fallen trunk. If the trunks of nearby trees appear blackened at ground level, that is a strong clue that they are infected too. As the rot penetrates up a tree, often for some distance above ground level, wind may snap the tree at that level, leaving a tall, spiky stump clearly showing the laminations implied by the rot’s name, which result from gaps opening between the tree’s annual rings.

Rusts Another group of fungi, distinct from mycorrhizal fungi and decay fungi, is the rust fungi, or simply, the “rusts.” They damage coniferous trees seriously and seem to have no redeeming ecological functions. Most are so small that the symptoms they cause are far more noticeable than

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the rusts themselves. With a few exceptions, they require two host plants: they grow alternately on a conifer host and then a flowering-plant host. And their life cycles are astonishingly complicated. How natural selection can have led to the success of such apparently needless complications is a mystery. Here is an example, to illustrate. After that, we set details aside and deal with what’s visible. The example is white pine blister rust (Cronartium ribicola), which kills or damages the five-needle pines. Its two hosts are first, five-needle pines, and second, shrubs of the genus Ribes, that is, all the wild or domesticated gooseberries and currants collectively called “ribes.” The rust spends generations, or sequences of generations, successively on one host and then the other, spreading by spores. The spores are of four distinct kinds —hence, the complexity. (Skip this paragraph unless you are really interested; the information comes from the Canadian Forest Service’s web page.) White pine blister rust has four types of spores. The spores from the blisters (which develop on the conifer’s bark) are aeciospores. Some of them land on ribes, called the telial host. There, they germinate on the leaves and penetrate the host through stomatal pores. Then they grow into little yellow pustules on the ribes leaves, which produce three kinds of spores. Two kinds, teliospores and basidiospores, infect white pines; the third kind, uredinospores, infects other ribes. This allows several generations of the rust to succeed each other on ribes plants, but a generation on white pines must be preceded and followed by a generation on ribes; it cannot directly infect another pine tree. Now for what can be observed in the field: Consider white pine blister rust first. From the commercial forestry point of view, it is, at times, the most damaging rust in our area. When an appropriate spore settles on the trunk or branch of a white pine tree and begins to grow, the trunk or branch swells and becomes discolored. The rust continues to grow for several years before it’s ready to produce spores. When it is ready, numerous white blisters break out on the bark of the infected tree, and spores are formed inside the blisters. As they ripen, they become bright orange, swell, and rupture the white blister, enabling them to scatter. A close look at the open blisters oozing brilliant orange spore masses is worthwhile when the opportunity arises. The spores infect the leaf of a nearby ribes shrub (possible shrubs can always be found somewhere nearby). A patch on the ribes leaf surface becomes rough and rust-colored, and produces three kinds of spores, two kinds capable of infecting a pine and the third kind capable of infecting other ribes. And so the cycle continues. See figure 8.2.

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Figure 8.2. (a) White pine blister rust on a pine branch. (b) Ribes leaf. (c) Staghorn white pine with the top killed by white pine blister rust.

Attacks by this rust are usually lethal for a young tree: it will probably be girdled and killed as the growing rust consumes its host’s tissues. In older, taller trees, an infection somewhere high up on the trunk may girdle it there and kill only the part of the tree above the girdle (figure 8.2c). Such trees are called staghorns. Other species of Cronartium, all called blister rusts, attack other conifers, and other shrubs or herbs, in similar fashion. A common rust-fungus genus, Chrysomyxa, causes its conifer host to grow witch’s brooms (rust fungi are not the only cause of witch’s brooms, as we shall see later). A witch’s broom is a densely bushy mass of proliferating branches that is obviously pathological. Sometimes a broom’s leaves are green but paler than the leaves on healthy branches of the same tree. Chrysomyxa causes brooms with yellow leaves; often many of a broom’s branches are dead with withered leaves, and only the living ones show yellow. For example, spruce broom rust (Chrysomyxa arctostaphyli) causes very dense yellow brooms on spruces. Its alternate host is

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Figure 8.3. (a) Yellow witch’s broom caused by spruce broom rust. The black twigs are dead. (b) Twig of the alternate host, kinnikinnick.

the ground-creeping shrub kinnikinnick. An infected kinnikinnick leaf has purple-brown leaf spots (figure 8.3). A common rust genus that affects the incomes of Christmas tree growers and some fruit growers is Pucciniastrum. Its conifer hosts are firs (Abies spp.), and the alternate hosts are blueberries and huckleberries (collectively, Vaccinium spp.). When the affected conifers are small balsam firs in a plantation of potential Christmas trees, the grower may lose his crop. And if the alternate host is any of the several species of Vaccinium grown commercially, then the fruit grower loses. Figure 8.4 shows affected hosts of both kinds: Pucciniastrum goeppertianum growing on evergreen huckleberry (Vaccinium ovatum) causes the huckleberry twigs to thicken and become pale brown and spongy; an infected shrub can be spotted from some distance. Pucciniastrum vaccinii on its conifer host produces small white pustules on the lower sides of hemlock leaves. The final rust to mention is a gall-forming rust, one of the very few rusts that does not alternate between two unrelated hosts. It confines itself to the two- and three-needle pines and is very destructive. In addition to killing trees, it stunts and deforms many others. Western gall rust (Endocronartium harknessii) causes large numbers of conspicuous galls to develop on the trunk and branches of an infected pine;1 there may be scores of them on a single tree (figure 8.5). There are often witch’s brooms as well, just above the galls. The bark on each big spherical gall flakes off, exposing the smooth wood beneath, except where collars of bark remain at the top and bottom of the gall.

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Figure 8.4. (a) Evergreen huckleberry infected with the rust Pucciniastrum goeppertianum. (b) Eastern hemlock leaves from below, with pustules produced by the rust P. vaccinii.

Figure 8.5. Gall caused by western gall rust on lodgepole pine.

Dwarf Mistletoe Fungi are not the only parasites on conifers. There is also a genus of parasitic flowering plants that attack them, the dwarf mistletoes.2 A dwarf mistletoe is utterly unlike the familiar Christmas mistletoe, although both kinds belong to the same family of plants, the Loranthaceae. The dwarf mistletoes, as their name implies, are exceedingly small plants, easily overlooked unless deliberately sought for. The place to start

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searching is on a tree with one or more witch’s brooms, as dwarf mistletoe is the most frequent cause of brooms. The leaves on the brooms match those on the rest of the tree, unlike those of the yellow, rust-caused brooms described earlier. A luxuriant green witch’s broom may give the impression that the tree it grows on is flourishing, but that is not the case. Or the leaves on a broomed tree (both on the broom itself and on the rest of the tree) may be shorter, and paler in color, than those on an unparasitized tree. Therefore, as it has less chlorophyll, the affected tree must be growing more slowly than its unparasitized neighbor. Trees with witch’s brooms die prematurely, and in infected forests, dead, leafless trees, with dead, leafless witch’s brooms on their trunks and branches, are common. In many forest areas, especially in the west, dwarf mistletoe is more injurious than all the decay fungi put together. Dwarf mistletoes belong to the genus Arceuthobium. They are found growing out of the bark on the trunk and branches of an infected tree in little tufts; the trunk or branch is often (not always) swollen. The dwarf mistletoe plants are yellowish green to brownish green: they have some chlorophyll and photosynthesize weakly, but they cannot survive without sugars from their host trees. Sometimes the plants are bushy, as in figure 8.6, and sometimes they are single-stemmed. The sole eastern species is Arceuthobium pusillum, common on the eastern spruces. Among the western species are Arceuthobium americanum, the largest, which grows on lodgepole pine (this is the one shown in figure 8.6); A. douglasii, which grows on Douglas-fir; and A. campylopodium, which grows on a large number of hosts: firs, spruces, pines, hemlocks, and junipers.

Figure 8.6. Dwarf mistletoe (Arceuthobium americanum) on lodgepole pine.

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Figure 8.7. Dwarf mistletoe. (a) Male plant. (b) Female plant.

The visible part of a dwarf mistletoe is only part of the plant. Growing from it, into the tissues of its host tree, are long strands of tissue that do what roots do in a self-supporting plant. The purpose of the visible tufts, growing into the open air, is to produce pollen and seeds. A dwarf mistletoe plant is either a male whose flowers produce pollen but lack seeds, or a female plant whose flowers bear seeds but produce no pollen. Figure 8.7 shows part of a plant of each sex. The mechanism that discharges the ripe seeds is a masterpiece of evolution. Each fruit (the oval objects on the plant in figure 8.7b) contains one seed. As the fruit ripens, the stalk supporting it lengthens and curves over, until it is pointing downward. Then, when the fully ripe seed falls, its outer skin contracts suddenly and the seed within is shot upward in the same way that a piece of wet soap will shoot upward if you suddenly squeeze it. The flying dwarf mistletoe seed can reach a speed of 80 km (43 miles) per hour and can land more than 10 m (32 ft) from its starting point, hopefully on another potential host. Notes 1. Y. Hiratsuka and J. M. Powell, Pine Stem Rusts of Canada (Ottawa: Canadian Forestry Service Forestry Technical Report No. 4, 1970). 2. J. A. Baranyay, Lodgepole Pine Mistletoes in Alberta (Ottawa: Canadian Forestry Service Publication No. 1286, 1970).

Chapter 9

Insects and Conifers

Insects as Feeders Coniferous trees are home to a variety of different insects. Their presence adds to the biodiversity of evergreen forest ecosystems, and for that we can be thankful. At the same time, a conifer forest in which nearly all the trees are home to millions of a particular species of bark beetle is a doomed forest that will soon disappear. Such a species is defined, by some people, as a “pest.” That is a judgment, not a definition, but for brevity the quotation marks around “pest” will be omitted in what follows. This chapter is primarily concerned with pest insects because they are common—if they weren’t, they wouldn’t be pests—and therefore much more likely to be encountered than rare species. Their lifestyles and behaviors are also much better known, because it has been financially worthwhile for forest researchers to concentrate on them. Most of these insects belong to one of four orders: Coleoptera (beetles), Lepidoptera (moths and butterflies), Hymenoptera (sawflies), and Homoptera (bugs). Note that “bugs” here does not mean all creepy crawlies and unidentified microbes; it means members of the insect order Homoptera, which is described later. The most important part of a pest insect from a forester’s point of view is, not surprisingly, its mouthparts, defined in terms of how they work. Thus beetles, moths, and sawflies all have chewing mouthparts, whereas bugs have piercing and sucking mouthparts. As for the insects that chew, 93

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some chew harder than others: it takes strength and effort to eat wood, and so this activity is confined to beetles and their larvae (grubs) for the most part. Some tiny beetles are (at present) the most destructive of all forest insects. The leaves of conifers are also fairly hard to bite into, with their thick, wax-coated epidermises: the insects that feed on them are primarily caterpillars and sawflies. The much weaker mouthparts of bugs can only penetrate a tree’s tissues where they are young and tender.

Beetles Beetles form the largest of all insect orders: about 40 percent of all the world’s insects are beetles. At present, the most serious forest pests in northern North America are bark beetles, which have destroyed immense tracts of forest in the west, including Alaska, where various species attack all the pines, all the spruces, Douglas-fir, and western hemlock. In the east, other species attack tamarack and white spruce. Bark beetles are remarkably small, about the size of a grain of rice. Figure 9.1c shows a bark beetle beside two other common beetles that are described later.1 Where they are at work, signs of them are everywhere. The immature grubs tunnel into the soft, rich phloem of many conifers, leaving patterns of grooves, known as galleries, exposed when the bark of a dying tree flakes off. The galleries can be seen on the inner surface of fallen bark

Figure 9.1. Common forest beetles. (a) White-spotted sawyer beetle (male). (b) Golden buprestid beetle. (c) Mountain pine bark beetle. (All on the same scale.)

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and the outer surface of the uncovered wood. Some species (not all) make galleries with a very regular herringbone pattern (figure 9.2a). Other species appear to favor no particular pattern. Experts can often tell from the galleries which species of beetle made them. The galleries are started by the adult beetles, which have tunneled into the tree from the outside. The females lay their eggs at intervals, and when the eggs hatch, the newborn grubs start eating (and tunneling) on their own, at right angles to their mother’s gallery in the case of species that make herringbone patterns. The grubs grow as they progress, so their tunnels widen. At the tip, the grub pupates, and when the adult emerges, it tunnels off in a different direction not in the plane of the pattern (figure 9.2). Bark beetles are divided into so-called primary and secondary attackers. Primary attackers (mostly in the genus Dendroctonus) choose healthy, undamaged trees, whereas secondary attackers (in the genera Ips and Scolytus) choose trees already weakened by primary attackers. Dendroctonus beetle populations are exploding throughout our area at present (2010). The outbreak is worst in the west. Mountain pine bark beetle (Dendroctonus ponderosae), one of the most destructive, attacks lodgepole, ponderosa, western white, and whitebark pines. Lodgepole

Figure 9.2. (a) Bark beetle’s gallery. (b) Buprestid beetle’s galleries. The grubs that bored them are shown below. (a and b are not on the same scale.)

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pine bark beetle (D. murrayanae) specializes in lodgepoles but is expected to spread eastward and infect jack pine too, throughout the whole of the boreal forest. The needles on trees killed by these beetles turn a bright rusty red: flying over many parts of Washington, British Columbia, and Alaska, you can see huge tracts of forest red and dead—it is a spectacular natural disaster. The epidemic began in the early 1990s. The beetle population keeps growing, probably because global warming allows it to survive in warmer modern winters instead of being killed by the cold (more on this in chapter 13). An infested tree can be detected before its leaves change color: its bark is peppered with small, black “shot-holes” where adult beetles have entered or emerged; reddish brown bark dust from the chewing is sprinkled on the bark and the ground below; and the trunk is dotted with big pale yellow dots marking pitch tubes through which resin dribbles out (“pitch” means pale yellow resin). The resin drowns and carries away some of the beetles. A heavy flow of resin is a tree’s only defense against the beetles, and it works only when the attackers are few. While the beetles are few, a tree stays healthy enough to produce enough resin to overwhelm all the beetles infesting it. This victory for the tree is called a pitch-out. To overcome the tree’s defenses, the beetles attack en masse; by doing so, they manage to kill a tree so quickly that it has no time to produce resin in useful amounts. To do this, the first attackers must signal other members of their species to join them, which they do by secreting sex attractants (pheromones). A bark beetle’s efforts to consume wood (which is hard work) are usually aided by a fungus, blue stain fungus, that attacks and softens wood. In return, the beetles spread the fungus by carrying its spores from tree to tree. The arrangement is mutualistic, like that between a tree and its mycorrhizal fungi. It provides yet another case of mutual interaction between conifers and fungi. Blue stain fungus can nearly always be found when you search the stumps or stacked logs of felled lodgepole pines. Consider next some larger beetles, the metallic wood-boring beetles, also called flathead borers or buprestids, which are well known for their dazzling colors: green, blue, bronze, copper, and golden. Even the black ones have a metallic sheen. Figure 9.1b shows an example. Their legless grubs are flattened, so their tunnels are wide (see figure. 9.2b). The “flat head” is actually a segment behind the head; the head itself is much smaller. Some species specialize in dead or dying trees; others attack healthy ones.

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An example is the golden buprestid (Buprestis aurulenta), a brilliant copper-colored beetle that bores into living pines, firs, Douglas-fir, spruces, and other conifers in the western mountains. Its pest rating is high. Another buprestid, the charcoal beetle (Melanophila consputa), is attracted by the smoke of forest fires. The beetles lay their eggs on the scorched wood of both conifers and broadleaf trees as soon as a fire has cooled sufficiently. It is reported that “the beetles often gather around the firefighters, biting them and trying desperately to lay their eggs on the smoldering trees,”2 a splendid example of determined motherhood. Our largest beetles are the long-horned beetles. They are instantly recognizable as longhorns. Several of them are also called sawyer beetles, for example, the white-spotted sawyer beetle (Monochamus scutellatus) whose grubs feed on pine wood. The adult beetle is at least 25 mm (1 in) long (see figure 9.1a). The male’s antennae are about as long as the body, but the female’s are only half as long. Sawyer beetles attack a variety of conifers and specialize in recently cut wood. Eggs are laid in crevices in the bark of a fresh-cut log, and when the grubs hatch, they tunnel into the phloem first, where the going is easiest and the food richest. As they grow larger and stronger, they chew into the xylem, making tunnels wide enough to accommodate a pencil. On a calm, quiet summer day, you can hear the rhythmical sawing sound of the grubs’ powerful jaws.

Caterpillars and Pseudocaterpillars Everybody is familiar with caterpillars, the larvae of moths and butterflies. Many are coniferous forest pests. Less well known are pseudocaterpillars, the larvae of sawflies. They belong to the order Hymenoptera, whose more familiar members are bees, wasps, and ants. Sawfly larvae look so much like caterpillars (figure 9.3) that it is hard to tell them apart. The clearest difference is that true caterpillars have at most five pairs of prolegs, whereas pseudocaterpillars have six pairs or more. Prolegs are abdominal legs that differ from regular (thoracic) legs in that they are short, stubby, and unjointed. But they’re helpful in enabling their owners to cling to twigs and leaves. All the caterpillars that damage conifers do so by eating the foliage: their mouthparts are strong enough to cope with the epidermis of a conifer leaf.

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Figure 9.3. (a) Caterpillar, order Lepidoptera. (b) Pseudocaterpillar, order Hymenoptera. (See text.)

A serious insect pest of conifers in the east—in the Maritime Provinces of Canada and the New England region of the United States—is spruce budworm (Choristoneura fumiferana). Its caterpillars defoliate balsam fir, black spruce, red spruce, white spruce, eastern hemlock, jack pine, and tamarack. Its abundance is believed to fluctuate over an approximately 35-year cycle. It was exceedingly abundant in the late 1970s and has been comparatively sparse since the mid-1980s. Its next “high” may soon be upon us. An adult spruce budworm (not a “worm,” of course) is a small, inconspicuous gray-brown moth which flutters around in huge numbers in midsummer when an outbreak is in progress. Besides leaves, the caterpillars also eat buds, young twigs, and young cones: they eat everything edible for them in whole forests of fir and spruce. When you walk through an infested forest, feeding caterpillars rain down on you steadily, and those that miss leave sticky silk threads hanging in the air to be walked into. Fullgrown caterpillars pupate into easily recognizable chrysalids (figure 9.4). Some closely related budworms are found farther west, where they infest jack pine and western spruces. In the same family as the spruce budworm is the larch case-bearer (Coleophora laricella). It attacks all the larches, damaging but seldom killing them. A heavily infested tree is an arresting sight. Most of its leaves are hollow and colorless for half their length, and many have disembodied hollow leaf tips sticking out at right angles from them. Figure 9.5 shows what happens. A caterpillar sucks the inner tissue from a leaf tip (figure 9.5a); it may depart, leaving the leaf colorless and with a neat hole in it (figure 9.5b) or it may make a case for itself from the first leaf and “wear” the case while it attacks another leaf (figure 9.5c).

insects and conifers

Figure 9.4. Spruce budworm. (a) Two pupae (chrysalids) on a twig, and a tent of silk threads at the tip made by a budworm intending to pupate there. (b) Adult spruce budworm moth.

Figure 9.5. Larch case-bearer at work and the result. (See details in text.)

Next we come to the “loopers” or inchworms, those endearing caterpillars that progress by alternately straightening and looping their bodies as though measuring their journeys in body lengths. The looping requires them to use their thoracic legs and their prolegs alternately, so the two kinds of legs have to be equally efficient. A good example is the hemlock looper (Lambdina fiscillaria). It defoliates hemlocks, firs, and spruces in both eastern and western forests. In the

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Figure 9.6. Hemlock looper caterpillars, defoliating a hemlock twig.

west, it also attacks Douglas-fir, the western larches, and western redcedar. Walking through a forest densely populated with loopers is unpleasant if hordes of them are ready to pupate. Like spruce budworms, they drop from the trees, leaving sticky silken threads that make a clinging cobweb to walk through. When scared, a hemlock looper often “vanishes” by straightening out nearly upright on its prolegs to resemble a twig (figure 9.6). The adults are inconspicuous moths, active mostly at night. The last group of caterpillars to consider is the tussock moths. Their eye-catching caterpillars are decorated with a variety of tussocks (tufts) of white, black, or colored bristles, arranged in distinctive patterns in each of the several species. The bristles on some species cause intense irritation in susceptible people. Their bristles float on the wind: indeed whole caterpillars are often wind-borne like dandelion seeds, with the bristles acting as parachutes. It’s unusual for larvae to be the dispersing age-group of an insect species, but in tussock moths it compensates for the winglessness of the adult females of many species. The fir tussock moth is an example (figure 9.7). Its scientific name deserves attention: Orgyia pseudotsugata. The orgies aren’t limited to Douglas-fir: in spite of the specific name, it attacks true firs (Abies) as

insects and conifers

Figure 9.7. Fir tussock moth caterpillar.

well, mainly in the west. The females appear wingless (atrophied wings are present but they look like hairy legs). They lay their eggs on the remains of the chrysalis from which each of them has recently emerged, and die immediately afterward. The caterpillar is about 30 mm (1.2 in) long, with long, black tussocks fore and aft, and short, tan ones along the body; it looks like an animated toothbrush.

Sawflies As noted earlier, sawfly larvae are so similar to caterpillars, they are called pseudocaterpillars. The adults are entirely different. They are in the order Hymenoptera, and, like wasps, have a sharp ovipositor (egglaying organ) at the tip of the abdomen. The ovipositor consists of two saw-toothed blades, hence, the name “sawfly” (not to be confused with a sawyer beetle). Unlike wasps, the adults lack wasp waists (figure 9.8) and they don’t sting. The jack pine sawfly (Neodiprion pratti) is a good example of the group. The larvae are highly gregarious: masses of them packed side by side can be found on the leaf tufts of infested pines. Their reproductive arrangements are like those of other Hymenoptera: the female stores the male’s sperm and can lay either fertilized eggs, which produce females, or unfertilized eggs, which produce males. She usually produces far more females than males, so males are rarely seen. The males are smaller than females and have showy, feathery antennae, in contrast to females’ plain ones. And they are fast fliers as they are never weighed down with eggs.

Bugs “Bugs” in its non-slang sense means insects in the order Homoptera. These insects have mouthparts that stab and suck, with stylets like those of mosquitoes, whereas the other insects considered so far have mouthparts

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Figure 9.8. Jack pine sawfly. (a) Larva or pseudocaterpillar (note the number of pairs of prolegs). (b) Adult female (about one-half the length of the white-spotted sawyer beetle in figure 9.1). (c) Row of eggs aligned in a slit in a pine needle.

that bite and chew. This means that bugs can penetrate a tree’s tissues only where they are young and tender. The stylets contain two parallel channels: one for sucking up food, the other for injecting saliva. Bugs differ from beetles, moths, and sawflies in much more than the structure of their mouthparts, however. The insects discussed in previous sections have a life cycle that goes through a complete four-stage metamorphosis. Each individual insect is successively an embryo in an egg, a larva, a pupa, and an adult. In contrast, bugs go through incomplete metamorphosis with only three (or two) stages: egg, nymph, and adult in most of them (but only two stages in the few that bear live young). Only about 15 percent of all insects have incomplete metamorphosis. Besides bugs they include crickets, grasshoppers, dragonflies, and mayflies. In these insects, the nymph stage doesn’t contrast as strongly with the adult as the larval stage does in the majority of insects. A newborn nymph looks more like a miniature adult of its species, complete with eyes, jaws, legs, and wing-buds, the precursors of functional wings. A nymph grows, molting at intervals, so that a series of roomier skins encase its growing body. Its wings, in species that have them, start as small stubs (wing-buds), which grow until the final molt, when the adult emerges with full-sized wings that enable it to fly.

insects and conifers

Now for the bugs of the coniferous forest: The adelgids are particularly noteworthy for the damage they do. They draw attention because of the big galls some of them make. Most have complicated lifestyles—they switch from a primary host tree to a secondary host tree of another species, spending from one to several generations on each. Adelgids used to be called “aphids,” to which they are closely related. For example, the balsam woolly “aphid,” as it used to be called, is actually an adelgid. The difference between the two groups is that aphids bear live young whereas adelgids lay eggs. The balsam woolly adelgid (Adelges piceae), not a gall-maker, is a serious pest that came to North America, from Europe, in about 1900. It has a simple (for adelgids) life cycle: it sticks to one host, balsam fir, and all of its members are believed to be females. They are wingless. Males, if there are any, did not immigrate with them, so all reproduction has been parthenogenetic. A balsam woolly adelgid is a tiny, wingless, black bug about the size and shape of a pin head. She secretes filaments of white wax that become tangled into a protective “woolly” cover for her (figure 9.9a). The white dots sometimes seen sprinkled all over the trunk and branches of a balsam fir tree are, in fact, woolly adelgids. They infest subalpine fir as well as balsam fir. The female lays her microscopically small eggs beneath her. When they hatch, each newly emerged nymph, known as a crawler, sets off in search of a good spot to attach herself; or she may be blown by the wind or carried on a bird. Once she has found a suitable spot, she molts for the first time and then inserts her stylet into the cells of the fir. Firs have thin bark, and the nymph’s

Figure 9.9. Balsam woolly aphid. (a) Adult, against a dark background to show the white wax. (b) Nymph with long stylet.

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stylet is long and slender; it can reach soft tissues through exceedingly narrow cracks in the bark. The nymph starts sucking and goes on sucking in the same place for the rest of her life, injecting a toxin into the tree as she does so. Without ever shifting, she goes through successive molts to adulthood, lays her eggs, and dies. A barnacle’s life is active and busy by comparison. The visible effects on the firs is the yellowing of young leaves and swellings on the branches, which become gnarled in time. In a heavily infested tree the leaves then turn rusty red and the tree dies. Another adelgid with a similar life cycle is the hemlock woolly adelgid (A. tsugae). It is abundant on the Atlantic coast where it damages, and slowly kills, huge numbers of eastern hemlocks. Its range in the east extends northeast as far as Maine, so it is found only in the extreme southeastern part of our area, but it is advancing northward, and global warming may hasten its spread. It is also found in the west, where it lives on western hemlock and mountain hemlock, but these hemlocks are much more resistant to its attacks and suffer little damage. Two common adelgids cause big, conspicuous galls to appear on their host trees. They are the eastern spruce gall adelgid (Adelges abietis) and, in the west, Cooley’s spruce gall adelgid (A. cooleyi). Cooley’s is the more common, more noticeable one. It looks much like the balsam woolly adelgid but spends some of its generations on Douglas-fir, its alternate host, where it doesn’t cause a gall. On spruce trees, it does. When a batch of newly hatched crawlers crawl out from under their mother, they travel to the leafy tip of the twig or branchlet they’re on, and there small groups of them choose their own leaf and insert their stylets into the base of it. As the nymphs suck sap, the leaf tissue swells until all of the closely spaced swellings coalesce into one big gall containing about 200 growing nymphs. The gall looks like a small pineapple, bristly with spruce leaves at first, but one by one the leaves fall off. The nymphs inside grow to adulthood and emerge from the gall, leaving it peppered with exit holes. The gall dries, turns brown, becomes curved, and resembles a deformed cone (figure 9.10). A heavily infested tree is instantly recognizable by the tightly curled tips of many of its twigs. The most insignificant bugs in a conifer forest are scale insects, appearing as tiny white dots on infested conifer leaves. It has been said of them that “no one on seeing these creatures for the first time would guess that they were insects. In fact, it is doubtful if, on cursory examination, a person unfamiliar with them would even regard them as living organisms.”3

insects and conifers

Figure 9.10. Cooley’s spruce galls.

This very fact makes them worthy quarry for a naturalist. The one you’re most likely to come across is the pine needle scale (Phenacaspis pinifoliae). When a female crawler creeps out from under her mother’s scale, she searches for a place to settle just as other bugs do. Having found a good site, she molts, losing her eyes, legs, and antennae in the process. She is small and flat and not clearly segmented. All the same, she constructs a scale for herself, and then remains concealed under it, but not attached to it, for the rest of her life, with her stylet permanently inserted into the plant tissue she feeds on. She repairs her scale from the skins cast at successive molts as she grows, gluing them on with wax that she secretes. She reproduces under her shell, either oviparously (eggs) or viviparously (living young), and either sexually or parthenogenetically. A male crawler grows for a longer time than a female, and when he reaches adulthood at the last molt, he emerges as a complete, fragile, tiny winged insect 2 or 3 mm long (barely 0.1 in). He has eyes and antennae but no mouthparts. He does not eat. As soon as he has mated, he dies. No wonder the females can go unrecognized as “living organisms.”

Parasitoids The only “good” insects in a conifer forest are those that benefit mankind by killing the “bad” ones, that is, the “pest” insects. The most interesting ones are parasitoids, insects that parasitize other insects and kill them in so doing. The difference between a parasite and a parasitoid is that the former has a host and the latter a victim.

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Figure 9.11. Two ichneumons.

The parasitoids concerned are ichneumons, sometimes called ichneumon flies. They belong to the order Hymenoptera. They look like slender wasps with exaggeratedly long, thin “waists.” In flight, their legs dangle down like undercarriages. Some of them are spectacular: the biggest species are over 12 cm (4.7 in) long including the ovipositor, which is sometimes longer than the body. Figure 9.11 shows two comparatively small species. Usually, a female ichneumon stabs her host—a caterpillar or a beetle grub—with her ovipositor and lays an egg in its body. When the egg hatches, the ichneumon larva eats the host larva from the inside. The parasitoid grows within the host while the host’s flesh steadily disappears. A grisly detail: the parasitoid is reputed to leave its host’s vital organs until last, so that the host doesn’t become carrion while the parasitoid is still feeding. The meal ends when the parasitoid is ready to pupate and no victim’s flesh remains.

Ants and Others Lastly, we look at two other animals of the forests that are so common and so inoffensive that they seldom attract attention. They are ants and woodlice (also known as sowbugs), called “others” in the section heading because they are not insects—they are, surprisingly, crustaceans. But first, the ants: As a group, they are believed, by some entomologists, to be the most abundant macroscopic land animals on earth. They belong to the order Hymenoptera and, like bees, are social insects living in colonies, whose members are queens, sterile workers, and males. A widespread, easily recognized group of ants common in coniferous forest are the mound-building ants of the genus Formica. They are sometimes called “thatching ants” because they thatch their mounds with conifer needles. Their mounds are particularly noticeable in early spring,

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when the ants’ winter dormancy ends. As temperatures rise, a mound that seemed dead in winter comes alive gradually: at first, a few sluggish ants will be noticed crawling slowly over their mound, and then, after a few warm spring days, they become progressively more numerous and more active, until the whole mound is covered by a continuous, seething layer of them. They feed on living and dead insects, recycling dead animal matter and destroying plenty of living pests. Also, some of them “farm” aphids and “milk” them for the honeydew the aphids produce, sometimes leading to the argument that aphids are pests and, therefore, that ants are pests too. Thus, from the human point of view ants are both good and bad (you can guess which side insecticide manufacturers are on). Woodlice are as common and as easily recognizable as ants. Both belong to the phylum Arthropoda, but to different subdivisions within the phylum. Ants, like all other insects considered so far, belong to the class Insecta. Woodlice, in contrast, belong to the subphylum Crustacea; they are the only common and easily recognizable terrestrial crustaceans. They undoubtedly descended from aquatic ancestors: they absorb oxygen through gills and so must live in damp places to survive. Their flat, oval, segmented bodies are supported on seven pairs of short legs. From the human point of view they are admirable animals: they are detritivores, subsisting entirely on dead vegetation of all kinds. Those in coniferous forest are, like ants, important recyclers in their ecosystem, and they don’t deserve to be ignored. Notes 1. J. B. Milton and K. B. Sturgeon, eds., Bark Beetles in North American Conifers (Austin: University of Texas Press, 1982). 2. L. A. Swan and C. S. Papp, The Common Insects of North America (New York: Harper and Row, 1972), p. 391. 3. S. A. Graham, Forest Entomology Third edition (New York: McGraw-Hill Book, 1952), p. 234.

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Chapter 10

Some Mammals and Birds of the Forest

Food and Shelter All warm-blooded animals need food and shelter, and some find what they need, in whole or in part, in coniferous forests. What have the forests to offer? The most dependable food supplies are seeds and browse for the herbivores. For the carnivores, there are worms, sowbugs, insects, spiders, any vertebrate animals that they can catch and kill, and accessible birds’ eggs. Shelter is needed for four purposes: for breeding, for sleeping and resting, for shelter from bad weather, and for concealment. Evergreen trees, both live and dead, provide for these needs. The variety of small animals that can use suitable “wildlife trees” is remarkable. Efforts to conserve such trees are well worthwhile.1 The animals we consider in this chapter are not in a conifer forest by chance but because it supplies their needs. We have space to consider only a few of the species that make their homes in the forest. Some are secretive, others are unconcerned by human presence, but all are worth watching for and observing closely.

Seldom Seen Mammals Let’s start with the smallest land mammals, shrews and mice. Figure 10.1 shows two species found everywhere in our area. They are animals with 108

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Figure 10.1. (a) Masked shrew (Sorex cinereus). (b) Deer mouse (Peromyscus maniculatus).

very small individual territories, about 400 m2 (about 4300 sq ft) for shrews and a hectare (2.5 acres) for a deer mouse. This means that individual members of these species that start life in a forest will very probably spend their whole lives there. Shrews are tiny, furtive, and seldom seen. They belong, with moles, to the order Insectivora and are voracious carnivores. Their chief food is insects, but they also eat slugs, centipedes, sowbugs, and bigger items such as salamanders and baby mice. They must have moist habitats. They are active in all seasons, scurrying through tunnels in the duff (decaying or dried-out vegetation) in summer and under the snow in winter. They are too small to hibernate: they race through life to meet their immediate food needs and have no time to collect lasting reserves. They are believed to use tunnels made by mice for their food forays and as resting spots. The most common shrew is the masked shrew. It’s mask is imperceptible. The most common mouse is the deer mouse, a typical mouse with big ears, long tail, and white feet and underparts. Like shrews, mice are active all year. They often gather in close-packed groups to conserve heat in very cold weather. At other times, a deer mouse makes a warm, grass-lined nest in which to rest and bear young. The nest is often hidden under a pile

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of woody debris; it may also be up a tree rather than on the forest floor. Somewhere near its nest, the mouse has a seed cache in which to store food for winter. Undoubtedly, there are vastly more homeless humans in the world than there are homeless mice. Perhaps the most interesting of the seldom seen mammals are bats. Our area is home to many species, and a large proportion of them roost in coniferous forests at least part of the time. Bats don’t nest in the sense that birds nest, but they do need a variety of roosts for different purposes and often use cavities in coniferous trees. They need hibernation roosts (hibernacula) for winter. These roosts must be cool but above the freezing point all through the winter, and the humidity must be high, otherwise the bats’ wings would dry and the membranes would lose their suppleness. Because they hunt at night, they need daytime resting roosts where they can keep warm or cool according to the weather, and be sheltered from storms and concealed from predators. To meet these requirements, bats move from one roost to another to suit the circumstances. Lastly, they need maternity roosts (or nurseries) where they can bear their young (“pups”) and care for them for the first day or two. Bats spend their active hours in flight, hunting. They spend their resting hours hanging upside down, sleeping. They spend their hibernating hours hanging upside down, torpid. And the females spend the final hours of their pregnancies hanging by their thumbs to give birth, in a warm, well-protected maternity roost. Pups are born singly: the newborn drops into a “bag,” consisting of the mother’s tail (interfemoral) membrane (figure 10.2), which joins tail to leg on each side. The pup is naked at birth— hence, the need for a warm roost. As soon as it is born, it scrambles up through its mother’s thick fur to one of her nipples, starts to suckle, and hangs on with its claws until it has grown fur of its own. Roosts for all a bat’s varied needs can be found in secondary holes in coniferous and other trees, and elsewhere, in caves, in abandoned buildings, and even in occupied buildings that are dependably warm. Many bats, especially the little brown bat, have become urbanized; they roost in farm buildings and hunt their insect prey at night where the insects are attracted in greatest number—around street lights. (The specific name lucifugus, meaning “fugitive from light,” must have been chosen before some bats acquired this rewarding habit.) Other little brown bats have maintained their status as wildlife, and some of them roost in conifers. Another widespread species that seems particularly

some mammals and birds of the forest

Figure 10.2. Little brown bat (Myotis lucifugus). Note the tail membrane.

attracted to conifers is the silver-haired bat (Lasionycteris noctivagans), which lives in appropriate habitats all across North America between the latitudes of Lake Winnipeg in the north and Cape Hatteras in the south. But it is not common anywhere. Its roosting places in coniferous trees include abandoned woodpecker holes, deep cracks in big trees, and the gaps that form beneath overhanging bark partly detached from its tree. Ancient oldgrowth forests, with many old and dead trees, provide numerous roosts.

Squirrels and Their Relatives The smallest forest mammals big enough and “public” enough to be familiar to everybody are red squirrels. They are rodents, and conifer seeds are their chief food, but in season they eat birds’ eggs and nestlings as well; they also collect mushrooms, expose them to the sun to dry, and then cache them. Their careful preparations for winter are interesting to watch. In summer they collect huge numbers of closed, seed-filled cones of pines and spruce, which they get by foraging through the treetops, nipping off conebearing twigs, and letting them drop to the forest floor. At intervals they come down and gather the fallen cones into caches. A cache is usually well hidden in a hollow tree or a hole in the ground, but occasionally you come across an eye-catching heap of cones ready to be cached (figure 10.3).

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Figure 10.3. Red squirrel (Tamiasciurus hudsonicus) and a pile of cones about to be cached.

As winter progresses and a squirrel uses up its cone supply, the cache becomes a midden, a mound of cone scales and stripped cone cores, which it is pointless to hide. This makes middens easy to find. Big ones may represent several years’ work by one squirrel. A red squirrel makes a nest of grass for itself hidden among evergreen branches: a witch’s broom provides an exceptionally good site. Foresters have mixed opinions about the activities of red squirrels. While it’s true they eat an enormous quantity of seeds, they also ensure that seeds are widely scattered, in caches they started and never finished. Another squirrel with much the same diet as the red squirrel is the northern flying squirrel. Being nocturnal, flying squirrels are seldom seen; the best time to look for them is in the first couple of hours after sunset. Their diet is the same as the red squirrels’. They nest in tree hollows.2 Neither red nor flying squirrels hibernate, so it is not surprising that their survival depends on well-stocked food caches. But caches are also made by some hibernators, for example, chipmunks and ground squirrels. Two species, one in each group, are never found far from conifers: they are yellow pine chipmunk and the golden-mantled ground squirrel. Both live in the western mountains, usually in open woodlands of Douglas-fir and ponderosa pine (otherwise known as “yellow pine,” hence, the name of the chipmunk). The two species are rather similar in appearance (figure 10.4), and someone seeing a golden-mantled ground squirrel for the first time

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Figure 10.4. (a) Yellow pine chipmunk (Eutamias amoenus). (b) Golden-mantled ground squirrel (Spermophilus lateralis).

often mistakes it for a chipmunk. The ground squirrel has no stripes on its face and is larger than a chipmunk. However, the most noticeable difference between them is their behavior: The chipmunk is forever dashing hither and thither energetically and seems never to need a rest; it usually runs with its tail held vertically upright. The golden-mantled ground squirrel is comparatively stolid; it usually holds its tail at an oblique angle rather than upright.

A Rodent and a Lagomorph The largest rodent to be encountered in conifer forests is the porcupine. Porcupines are entirely vegetarian, and they find plenty of succulent greenery to eat in summer. In winter, they must resort to stripping bark to get to the most nutritious tissue below it, the phloem. Their scraping leaves large, irregularly shaped patches of gnawed “wood,” visible from a distance (figure 10.5a). The grooves left by a porcupine’s teeth make it obvious that the barkless patches are the work of an animal, and if they are so high on the tree that the animal concerned must have been a tree climber, they can only be the work of a porcupine. Similar scars at a low level are more likely to have been made by porcupines than by anything else, if they are on a conifer. Porcupines also browse on twigs and buds within reach from the ground. Often, they nip the main growing point, the terminal bud, of a seedling

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Figure 10.5. (a) Porcupine gnawing bark. (b) Forked tree injured as a seedling by a porcupine.

or small sapling. This kills the leader and causes two (sometimes more) side branches to grow upward and thicken, creating a forked tree (figure 10.5b). This does no harm to the living tree, but it destroys the tree’s usefulness to a forester, as do the bark-stripping scars already described. Ergo, porcupines are “pests.” A porcupine makes its den in any suitable cavity on the forest floor, such as a hollow log or the hole left by a fallen tree which is often sheltered by the root wad. The snowshoe hare is another well-known animal of the northern forests (figure 10.6). It is not a rodent, but a lagomorph (order Lagomorpha). Hares are easiest to find in winter when their huge, snowshoe feet make easily recognizable tracks. Their white winter fur fails to conceal them against a snowy background: the twitching of their black-tipped ears gives them away. In winter they browse on conifer twigs; their sharp teeth leave smooth cuts, as if made by shears. The height of these scars shows how thick the snow was when a hare was eating. They also chew the bark of young trees, sometimes girdling and killing them. Snowshoe hares are famous for the way their populations fluctuate. In some years they are scarce, and in other years populations of them explode. Their population size in a patch of forest goes through a cycle, usually lasting between 8 and 11 years. When they are abundant, they eat so much that they kill off the plants that would otherwise provide fodder for young hares. The result is a population crash as numbers of them starve.

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Figure 10.6. Snowshoe hare (Lepus americanus).

Then, while hares are scarce, the vegetation recovers, and the hare population grows, until such time as it becomes overabundant and crashes again. The cycle repeats indefinitely. At the top of the cycle, hares can damage forests of young trees severely. A case is known in which a 10-hectare (25 acre) tract of forest containing more than a million young jack pines was home to about 1000 snowshoe hares. The hares stripped off the bark and branches of all but 40 of the pines.3 This was exceptional, however, and hares are not usually treated as “pests.”

Big Herbivores Among other herbivores that use the conifer forests in winter for browse and shelter are members of the deer family (Cervidae), namely, moose, elk (wapiti), caribou, white-tailed deer, mule deer, and, on the west coast, black-tailed deer. The most northern of these, the caribou (Rangifer tarandus), merits particular attention because many populations of them are dwindling fast and are at imminent risk of extinction, and also because their digestive systems are unique. Taxonomists have divided the species into a confusing array of subspecies.* Here, the important division to notice is into migratory and nonmigratory caribou. The migratory kind, collectively, are barren-ground caribou, which migrate between calving grounds in the treeless tundra in summer and the sheltering habitat of taiga and forest in winter (figure 10.7). * I have avoided the Latin names as they are a matter of debate and intolerably long. The whole species, which includes Eurasian reindeer, is Rangifer tarandus.

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Figure 10.7. Barren-ground caribou, which overwinters in the northern forests. (From A Naturalist’s Guide to the Arctic by E. C. Pielou, University of Chicago Press, 1994, with permission.)

They are seldom regarded as “forest animals” but they should be: that’s where they spend half their time. The nonmigratory caribou are woodland caribou, a subset of which are separately identified as mountain caribou. The woodland caribou do not assemble into vast herds that migrate to the tundra to bear their young. They remain scattered south of the tree line. The anatomy and behavior of caribou are adapted to the available food, which varies with the seasons, and the chosen habitat. The digestive systems of caribou produce the enzyme lichenase, which enables them to digest lichen, a feat impossible for other deer species. In the winter, their richest food supplies are “hair lichens,” which hang in festoons from conifers’ trunks and branches. It takes several decades for hair lichens to grow; therefore, undisturbed old-growth forests provide the richest supplies. Ground lichens such as so-called reindeer moss (species of Cladonia and Cladina) are buried under snow in winter, but while the snow is not compacted, the caribou can get at the lichens by “cratering”

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Figure 10.8. American marten (Martes americana).

(shoveling the snow away with their big, sharp-edged hooves). In summer, snow is not a problem, and other, richer foods besides lichens are accessible too: grasses, sedges, and leafy shrubs such as willow. The comings and goings of caribou are therefore governed by their winter food: by its digestibility (ensured by lichenase) and its accessibility (aided by big, sharp hooves). Natural selection ensures survival of the animals best adapted to the local habitats with their local snow regimes.

Carnivores Next, we come to the top of the food web. There are altogether eight mammalian predators at the topmost level of the forest food web, a surprisingly large number. Besides wolves, they include foxes, lynx, and five members of the weasel family: wolverines (the largest and rarest) and, successively smaller, fishers, martens (figure 10.8), ermines, and least weasels. Least weasels are tiny, rarely more than 20 cm (8 in) long, and are said to be the world’s smallest carnivores (this excludes shrews, which occasionally eat baby mice). A least weasel’s diet consists entirely of mice and voles. To see one carrying the body of a mouse as large as itself by the scruff of the neck is a memorable sight. Probably the best known of the carnivores is the lynx (figure 10.9). Snowshoe hares are their principal prey, so lynx populations cycle to match

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Figure 10.9. Lynx (Lynx canadensis).

the hare cycle in their area, but with a lag of about two years. When hares are few, lynx get by, poorly, on smaller prey—mice, squirrels, deer fawns, grouse chicks, and the like. The supply is inadequate for a big lynx population: some starve and others move away. Everyone knows what a lynx looks like, but they are seldom seen, being wary, silent, and nocturnal. Their paw prints are large and show no claw points: like other cats, lynx retract their claws most of the time.

Big Omnivores The largest animals in the evergreen forests are two bear species, the grizzly (brown) bear (Ursus arctos) and the American black bear (Ursus americanus), which is occasionally cinnamon-colored. Both eat a wide variety of plants and animals, including fish. It has been found that in the course of their fish-eating, bears inadvertently fertilize the forest where they go fishing. It happens in the Pacific coast rain forest, in the belt of land on each side of salmon spawning streams.4 When Pacific salmon (Oncorhynchus species) swim upstream to spawn, over a two-month period in the fall, bears gather on the stream banks to catch and eat them. Because the salmon inevitably die a day or

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two after spawning, and the majority are taken by bears after they have spawned, the salmon population is scarcely depleted by the bears. After catching a spawned-out, moribund salmon, a bear normally carries it some distance from the stream bank before eating it; it may go as far as 100 m (nearly 400 ft) inland from the bank, probably to avoid interference from other bears. A bear is a messy eater; it leaves leftovers—the viscera, testes, and some meat clinging to the bones—lying on the ground. The leftovers also include each salmon’s hard lower jawbone. By collecting and counting these, researchers have been able to discover how many salmon were taken from a stream they were investigating. In one investigation, the bears were estimated to be getting, on average, a catch of 13 salmon per day each. The decaying scraps and the bear’s feces contribute almost as much nitrogen to the forest floor, in a strip 200 m wide (about 800 ft) adjacent to the river’s banks, as would chemical fertilizer spread by a forestry company. The trees grow large, and it is known from chemical isotope measurements that the trees are nourished by marine-derived nitrogen (MDN). The trees gain, the bears gain, and the salmon don’t lose because they were within hours of death anyway. A more benign symbiotic triangle is hard to imagine.

Birds Only a few birds are “conifer specialists” in the sense that their needs can be met only by coniferous trees. It is interesting to observe them and their behavior. First for browsers: Two birds habitually browse on the leaves, buds, and twigs of conifers, the spruce grouse (figure 10.10), found throughout our area, and the dusky grouse, found only in the west. Although they are ground-nesters, they spend much time in winter perched high in coniferous trees, where their browse is not buried under the snow. The conifer-seed eaters are the red crossbills, white-winged crossbills, pine grosbeaks (figure 10.11), and pine siskins, whose bills are clearly adapted for eating seeds. Crossbills use their bills to force cone scales apart, and then reach the seeds with their tongues. A crossbill moves around in a tree like a parrot, holding a twig with its bill while it shifts its feet, then gripping the twig with its toes while it shifts its bill. These birds nest in conifers too. Woodpeckers are common in conifer forests and, with their strong bills and muscular necks, are capable of digging into tree trunks to make their

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Figure 10.10. Spruce grouse (Dendragapus canadensis).

Figure 10.11. (a) Pine grosbeak (Pinicola enucleator). (b) White-winged crossbill (Loxia leucoptera).

nesting holes. Such birds are called primary cavity nesters. Most of them nest in conifers or broadleafs indifferently, but one that specializes in conifers is the northern black-backed woodpecker (figure 10.12a). Secondary cavity nesters are birds that nest in old woodpecker holes, or other holes not made by themselves. There are plenty to be found because woodpeckers do not reuse their holes but excavate new ones each year. Also, they make roosting holes in addition to nesting holes. A secondary cavity nester that doesn’t use woodpecker holes is the brown creeper (figure 10.12b). Creepers build their well-hidden nests in the crevices behind loose bark on large, living or dead, conifers. They search the crevices, on their home tree and nearby trees, for many species of insect

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Figure 10.12. (a) Northern black-backed woodpecker (Picoides arcticus). (b) Brown creeper (Certhia americana).

prey, including bark beetles, spruce budworm, sawfly larvae, and scale insects. This amounts to an appreciable “ecological service” for the trees. Two other birds that consume plenty of insects are the red-breasted nuthatch and the mountain chickadee, a westerner (figure 10.13). Both can be either primary or secondary hole nesters, and both are voracious seed eaters. But their feeding techniques are quite different, seen best when you can watch a mixed flock “cleaning up” all of the available insects in the crown of a conifer. Nuthatches climb down a tree searching for insects in every crack in the bark. Chickadees are more lively; they forage among the flexible tips of the ultimate twigs and often hang upside down. Two of the most interesting birds that depend on conifer seeds for most of their food live in the western mountains. They are Clark’s nutcracker and Steller’s jay (figure 10.14). The nutcracker is dove-gray with black and white wings and tail; the jay is a dark blue bird, the color blending into a black crest on the head. They eat the seeds of the two stone pines, whitebark pine and limber pine, which grow near and above the tree line. Recall (chapter 2) that the seeds from these two pines are the only ones that are effectively wingless. These two birds disperse the seeds.5

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Figure 10.13. (a) Red-breasted nuthatch (Sitta canadensis). (b) Mountain chickadee (Parus gambeli).

Figure 10.14. (a) Steller’s Jay (Cyanocitta stelleri). (b) Clark’s nutcracker (Nucifraga columbiana).

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The nutcracker is especially adapted to transport seeds; indeed, the nutcrackers and the pines are mutualistic. The nutcracker has an expandable pouch under its tongue in which it can store more than 150 stone pine seeds. In summer, the birds use their bills to chisel open the closed cones of whitebark pine and extract the seeds. Before tucking each seed into its pouch, a bird will inspect it to ensure that it’s edible, that is, contains a viable embryo. This ensures that only viable, nourishing seeds are collected. The nutcracker then carries its load of seeds some distance (up to 45 km [28 miles]) before caching them in an open wind-swept area of the alpine tundra, where snow will not lie for long. This makes the caches easily accessible as food sources in winter. Several groups of birds may cache close together at a shared site. And, as always happens, not all the stored seeds are eaten. So when, on an exposed, windswept mountain slope, you find a big grove of these pines, it is probably the earlier site of some nutcrackers’ communal caching ground. Steller’s jays make caches too, but they are less specialized. They collect limber pine seeds fallen from open cones and carry only a few at a time, as they have no pouches. And instead of making communal caches, the jays are loners and drive away other jays from their chosen sites. Another jay of the coniferous forest is very common—for many people, too common. It is the well-known gray jay or whiskeyjack that awaits hikers and campers in areas where they are likely to be found. The gray jay, an omnivorous scrounger, accepts camp scraps or hikers’ sandwiches if they are offered, and helps itself to what is not offered. In uninhabited regions, gray jays divide their attention between insects and seeds. Several species of owl spend much or all of their time in evergreen forests. All are nocturnal raptors (birds that grasp their prey with their talons and tear them apart with their hooked bills). Most are secondary cavity nesters. Examples are the barred owl (its call, “Who cooks for you? Who cooks for you all?” never fails to draw attention), great gray owl, eastern and western screech owls, northern hawk owl, and boreal owl (figure 10.15), and, throughout the southern part of our area, the northern saw-whet owl. Deer mice and small birds are their usual prey. Two more birds that nest among conifers may come as a surprise—both are sea birds. The first is Bonaparte’s gull, a smallish gull that lives (except in the breeding season) on Pacific and Atlantic beaches. But to breed, it goes to the northern forests everywhere west of James Bay and nests in a coniferous tree.

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Figure 10.15. Boreal owl (Aegolius funereus).

The most surprising tree nester is the marbled murrelet, a robin-sized sea bird of the Pacific Ocean that spends its life at sea except in the breeding season, when it comes ashore to nest in the rain forest. Its nesting arrangements were discovered as recently as 1974: the nest is made in the deep layers of moss on the wide, horizontal branches of giant rain forest trees such as western redcedar and Sitka spruce. Notes 1. M. Fenger et al., Wildlife and Trees in British Columbia (Vancouver: Lone Pine Publishing, 2006). 2. Island Protection Society, Islands at the Edge (Seattle: University of Washington Press, 1984). 3. W. Rowan, “Reflections on the Biology of Annual Cycles” in Journal of Wildlife Management, vol. 18, 1954, pp. 52–60. 4. T. E. Reimchen, “Some Ecological and Evolutionary Aspects of Bear-Salmon Interactions in Coastal British Columbia,” in Canadian Journal of Zoology, vol. 78, 2000, pp. 448–57. 5. S. V. Wall and R. P. Balda, “Remembrance of Seeds Stashed,” in Journal of Wildlife Management, vol. 92, no. 9 (September), 1983, pp. 61–64.

Chapter 11

Natural and Unnatural Interference

Thus far we have considered only the interactions between coniferous trees and other living things. Now we come to their interactions with the nonliving: fires, the weather, and machinery. (Humans without machinery are almost harmless. With it, they are the most serious forest pests on the planet.)

Fire The effect of fire on individual trees is injury or death; its effect on forests is to bring about forest renewal. Fire, like decay, disposes of dead vegetation. Without fires, as noted in chapter 8, the forest floor would become an impenetrable tangle of fallen trees, broken branches, and withered and rotting vegetation of all kinds. Dead trees and their parts contain all the mineral nutrients they obtained from the soil while they were growing, and the nutrients remain locked up inside them until the dead remains decay or burn. In cool northern latitudes, decay happens too slowly to keep up with the continual supply of dead material, hence, the value of fires to keep the forests as a whole growing vigorously. Fires cause the sudden appearance of open, sunlit, ash-covered ground where before it was deeply shaded and the surface soil was moist and organic. Therefore, the tree species that invade and succeed have different requirements than those that were burned. This is the chief form of forest succession throughout the forests in our area, and it is discussed further later. First, let’s consider individual fires. 125

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There are three kinds of fires: surface fires, crown fires, and underground fires. Surface fires burn everything flammable lying on the forest floor but do not seriously injure full-grown trees of fire-resistant species, those with thick layers of cork under the bark such as red pine in the east and ponderosa pine and Douglas-fir in the west. The open ponderosa parklands in arid parts of the west owe their existence to frequent surface fires (figure 11.1). The fires consume fallen dead branches and kill young seedling pines, but the older pines are uninjured and have enough space to grow tall and stately in the absence of competing saplings. Most pines are fairly fireproof. Trees with less cork, such as firs and spruces, are killed by comparatively mild fires. The most impressive fires are crown fires. Some fires burn only through the trees’ crowns, as in figure 11.2. Other fires are combinations of surface and crown fires. Everything above ground burns, and if quantities of fuel (dead trees, branches, snags, and logs) have accumulated since the last fire, the result is a devastating holocaust in which winds generated by the fire itself can become strong enough to snap the trunks of large trees. The third kind of fire is the ground fire. Ground fires happen only where there is an accumulation of peat. All the action is subterranean, and the only evidence that a fire is in progress is occasional wisps of smoke emerging from small holes in the ground and the pleasant, if disturbing, smell of smoldering peat. A ground fire that was started by campers but not completely doused can spread a long way under the surface without changing the appearance of the ground above. When this happens, a normal-looking forest floor may be only a thin roof over large, blackened caverns where the peat has been burned away. Ground fires are also started by surface fires. They can continue undetected for months, smoldering slowly and invisibly below innocent carpets of needles. Nowadays it is realized that it is not necessarily wise to fight all fires. A mild fire that is no threat to camps, cottages, and towns, and that is burning up a modest amount of accumulated fuel, is best left to burn itself out. If it is needlessly extinguished, the next time fire strikes there may be so much fuel that the fire becomes dangerously uncontrollable. Fire is a natural event. In wilderness country, fires started by lightning have recurred more or less regularly ever since the land became forested. Fire tends to recur in regular cycles, whose periods depend to a high degree on the climate and the species of trees forming the forest (these two factors are obviously closely related). The average cycle length for many forests is

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Figure 11.1. Surface fire.

Figure 11.2. Crown fire.

100 to 200 years; it is much shorter, about 25 years, where the climate is dry and forests of two-needle pine (jack in the east, lodgepole in the west) grow on thin, sandy or rocky soils. It is long, more than 400 years, in the

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Pacific coastal rain forest, and longer still in muskegs and wet peat bogs in the far north. A fire does not necessarily clear away all the living trees and accumulated fuel in one fell swoop. Everyone is familiar with tracts of burned forest with the dead snags still standing. In wet climates large chunks of wood may persist through several fire cycles, with only a fraction of their mass burning away in any one fire. For some forests, fire is clearly a necessary part of their life cycles. For example, as noted earlier, parklands of ponderosa pine cannot remain as they are indefinitely without regular fires. If fires are prevented, in time such parklands are replaced by dense forests of Douglas-fir. The fact that fires are crucially important in maintaining particular kinds of forest indicates that some tree species have evolved not merely to survive fire, but actually to promote it.1 In other words, trees that thrive only in open, sunlit places actually require frequent fires to persist. A forest of such trees is called a fire-dependent forest. Trees that survive a fire are seldom wholly unscathed. Sometimes the effect is only a superficial charring that does no harm to the tree (see the scorched Douglas-fir in figure 4.7). Hotter fires make deep scars. Old scars become healed in time and overgrown by new annual layers of wood. The fire history of a felled tree can often be observed by counting the number of annual rings from scar to cambium for each scar observable in the stump. In living trees the old scars are concealed; the only way to inspect them is to take a core of wood with a corer. Corers of several sizes are needed: short, thin ones for small trees, and long, thicker ones for big trees. Cores shouldn’t be collected except for worthwhile research, as coring a tree makes it susceptible to infection. This brings us to the topic of recognizing the various scars found on living trees and identifying their causes. Most often a fire scar is caused by a single, intense lightning strike, which affects only the tree it strikes momentarily and leaves neighboring trees unscathed. Such scars are usually “church-door” scars, as shown in figure 11.3. Its noticeable characteristic is that it reaches to the ground, unlike, say, a scar left by a bear sharpening its claws. A bear scar cannot extend higher than a bear’s reach above the ground; also protruding branchlets may be unaffected, and there are often tattered strips of torn bark at the top of a bear scar. Another kind of scar is the “scrape scar” caused when a tree is scraped by a falling neighbor, which leaves snapped and splintered branches as well as

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Figure 11.3. (a) Church-door scar caused by fire. (b) Bear scar.

scraped trunks, sometimes on both trees. It’s always worthwhile to identify (if you can) the cause of a scar. Bark beetles and porcupines make recognizable scars, as described in chapters 9 and 10. Naturalists who concentrate only on near-perfect specimens of the organisms they encounter are missing a lot.

Forest Succession The process in which one forest is replaced, in time, by another is ecological succession. Succession by a forest exactly like the one that has just burned is possible. A lodgepole pine forest often succeeds itself: a new forest of lodgepole saplings grows up among the blackened snags of their fire-killed parents, as in figure 11.4. The parent population may have been a “dog-hair” stand. This is what happens if densely seeded, burned-over land produces a dense young forest of trees all of the same size. None can get ahead enough to crowd out its neighbors, so competition, as a cause of self-thinning, fails. A case is known in which a 50-year-old dog-hair forest contained 25,000 trees per hectare (2.5 acres), or about as many bodies per unit area as in a closely packed mob of people. The spindly trees were so dense it was impossible to walk among them.

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Figure 11.4. Young lodgepole pines among their burned parents.

Fortunately this seldom happens. More often the trees in a pioneer stand are not so evenly matched and the stand self-thins. This leaves space for other trees to invade. Some tree species are light-demanding (or sun-loving). This is the case with larches and most pines, especially jack and lodgepole pines as just described. Other species are shade-tolerant. Eastern and western hemlocks are very shade tolerant; thujas, firs, and spruces slightly less so. Eastern white pine and Douglas-fir are intermediate. However, shade and the lack of it are not the only factors controlling succession. The successors to the pioneers may differ in the speed at which they grow, in the wetness of the soil they prefer, in the availability of various nutrients, and in whether or not the forest floor is covered with moss. Different factors are paramount for different species. And to be a successor, a species has to be present in the vicinity and produce adequate seeds at the time seeding becomes necessary. Because of all this, succession is often impossible to predict. One can only wait and see, and explain what happened by hindsight.

Snow and Wind Now for other inorganic agents that affect conifers. Ordinary snow is no threat to an evergreen; it may only break a few branches. But an avalanche

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Figure 11.5. Avalanche paths.

in the mountains can take out hundreds of trees in seconds, leaving an avalanche path. All mountain travelers are familiar with avalanche paths, or slide scars as they are also called—long, light green swaths down a mountainside with dark conifer forests to left and right (figure 11.5). The slide scar is often overgrown with young broadleaf trees and shrubs, such as aspens, birches, alders, willows, honeysuckles, and many more. (The alders, either mountain alder or speckled alder, are lumped as “slide alders.”) Conifer seedlings are absent except perhaps at the bottom where the slope levels off and the avalanches have slowed and lost their force. The deciduous broadleaf trees and shrubs can recover if their tops are snapped off every few years; provided the roots are undamaged, new shoots can grow up from the broken stumps. After a slide, the path can quickly regain its light green deciduous cover. But conifers cannot recover: if a trunk is snapped off, the tree dies. The few small conifers growing at the bottom of a slide where the slope levels off have grown since the last unusually big avalanche: more frequent smaller avalanches don’t reach them. Wind damage is less serious but much more widespread. Two examples are shown in figure 11.6. The dumbbell tree is shaped by a strong wind

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Figure 11.6. (a) Dumbbell tree. (b) Flag tree.

blowing sharp snow crystals against the lower part of the tree, just above the snow on the ground, which protects the lowermost branches. Directly above them, the low branches are ice-blasted to destruction. The flag tree on the right is a common sight wherever there are strong prevailing winds. One of the most remarkable effects of wind and ice on conifers was first discovered and described in 1976.2 The effect is on whole forests rather than on individual trees. It is known in Japan and North America, in the latter only in the extreme southeastern part of our area, in pure balsam fir forests. The phenomenon is at its best on Mount Katahdin in Maine. Seen from the air, across a valley to the opposite mountainside, is a series of whitish stripes, forming more-or-less parallel lines in the dark green forest at intervals of about 100 m (300 ft) or a bit more. The stripes are belts of dead and dying trees. Research into the cause showed that there is an interesting pattern of trees of different ages and conditions, as can be seen in figure 11.7b, which illustrates how the forest would appear from the side, looking at a slice of the forest at right angles to the stripes. Note the sequence of trees as you go left from the standing dead trees on the right (only one is shown in the drawing). The dead trees themselves are at the downwind edge of a stripe; immediately to the left of it are fallen dead

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Figure 11.7. Regeneration waves (a) from the air and (b) from the side (diagrammatic).

trees with tiny fir seedlings growing among them. Continuing leftward (upwind), you find progressively older trees, ending with fully mature living trees, after which the cycle starts again with standing dead trees which look white from a distance. Observations of these stripes over a period of time showed that the stripes are moving, like the crests of breaking waves but at the exceedingly slow speed of about 1.5 m (4.5 ft) per year, in the direction of the prevailing wind. The discoverer’s (Douglas Sprugel’s) explanation of what is happening in these “regeneration waves” is as follows. Balsam firs are comparatively short-lived trees, and by the time they begin to show signs of old age, at about 70 years, they are very susceptible to damage and death. The most common cause, in the northern Adirondacks, is the cold weather in winter, when rime coats the trees and winds

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are strong. Rime is ice that forms when supercooled liquid water droplets freeze directly onto a surface, in this case the tree’s leaves and branches, making a thick, heavy coating. (It contrasts with hoar frost which is merely a lightweight coating of feathery ice crystals with air trapped among them.) In a forest with stripes, the heaviest coatings of rime form on the most exposed trees, which are those just to leeward of a belt of low seedlings and saplings. Because they are unsheltered, these trees are also the ones that sway most in the wind, causing their ice-laden branches to snap off. The damaged trees die, and in the strip of ground left empty, seedling firs establish themselves when spring comes. In the following winter, the newly unprotected strip of mature trees becomes covered with rime ice in its turn, and dies; another strip of seedlings grows to replace it, while the preceding year’s strip, now a year older and taller, is left behind. In this way, a wave of newly dead trees advances into the prevailing wind, year by year. They are called “regeneration waves” or (in Wikipedia) “fir waves.” Balsam fir and red spruce often grow together, but because these two species have different life expectancies, regeneration waves cannot form in them. They can appear only if all the trees become decrepit and prone to damage from rime at the same age, and this happens only in a one-species forest. The life expectancies of balsam fir and red spruce are 80 years and 300 years, respectively. The short-lived firs grow faster than young spruces and overcome them in the competition for space while they are young. If the forest were to grow old, the longer-lived spruces would have time to dominate the firs, but the cycle forced by rime ice and wind seldom gives them that time. Before long, the spruces die off and a pure fir forest remains.

Air Pollution and Acid Rain The term “air pollution” is best confined to toxic pollution by poisonous gases. It is sometimes used, confusingly, to include the nontoxic greenhouse gases that are causing global warming (see chapter 13). It is often unclear whether use of the term arises from ignorance or a deliberate plan to obfuscate people. In any case, the chief greenhouse gases, carbon dioxide and water vapor, are necessities of life, quite the opposite of toxic gases, which are what concerns us here. Toxic gases (as well as unwanted carbon dioxide) are released into the atmosphere by industries burning fossil fuel, especially coal. The chief toxic

natural and unnatural interference

gases are oxides of sulfur and nitrogen. As air pollutants, they affect forests by dissolving in falling rain and acidifying it. This kills much of the aquatic life in lakes and ponds and affects the water from which the trees obtain mineral nutrients. Conifers are more vulnerable than broadleafs. Their growth is slowed, their reproduction reduced, and their death rate rises. Affected trees are also thought to become more susceptible to pests and diseases. The effects of toxic air pollution are much more localized than the effects of greenhouse gases. The latter diffuse into the atmosphere and spread worldwide, whereas the rapidly soluble toxic gases are quickly washed out of the air by the rain they turn to acid, and affect the ground and freshwater below over a comparatively small area. Remedial action taken nowadays has largely been successful, and the only region in our area where the forests are still badly damaged is in the southeast. The forests on the Adirondack Mountains are the most seriously affected.3

Logging This is the chief “unnatural interference” promised in the chapter heading. The interaction is not with living things but with machinery. An undisturbed forest provides many products besides wood. It also provides intangible “ecological services,” especially the following: It regulates the carbon cycle, hence, the amount of carbon dioxide in the air. It regulates the water cycle, protecting the soil from erosion and supporting the aquatic life in rivers and streams. And it conserves biodiversity. The tremendous monetary value of these services has been comprehended only recently, and even now (2010) not by everybody. Estimating their dollar value is difficult, and the need for them is steadily increasing as the world’s natural forests shrink. The carbon cycle is globally important. It consists in the take-up of carbon, in the form of carbon dioxide (CO2), by photosynthesizing plants growing new tissue. Then comes the excretion of CO2, by all living things (including plants) by respiration. Finally, everything that burns or rots liberates CO2. Trees, by acting as a carbon sink, slow the cycle. The quantity of CO2 in the atmosphere remains constant only when these processes are in balance, and upsetting the balance, by killing vast quantities of vegetation in the process of logging, is one of the causes of the presently increasing concentration of CO2 in the atmosphere, and hence, of global warming. More on this topic in chapter 13.

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The water cycle is well known in outline.4 Briefly, it goes like this: ocean water and fresh waters evaporate, the vapor condenses to form clouds, and the clouds return the water, as rain or snow, back to the surface again. A local disturbance of the cycle can have serious local effects. Clear-cutting a forested area is one such disturbance. Recall from chapter 4 that a single hectare (2.5 acres) of full-grown forest can transpire 50 tons of water on one summer day. That water has been delayed on its journey from land to clouds inside the trees, and if the trees are felled, it stays on the ground. If the ground is level the water remains there as pools, or it creates a swamp—to the astonishment of people unaware of transpiration. If the ground slopes, the surplus water flows away. If it flows on the surface, it erodes rivulets and channels and the banks of the stream or river they flow into. If it flows some distance underground, it reappears and causes flooding and erosion in adjacent lowlands. Erosion carries away valuable soil nutrients, and deposits unwanted sediments on the beds of streams, rivers, and lakes, killing much of the freshwater benthos (invertebrates living at the ground-water interface) and the bottom-feeding fish that eat them; it also smothers the spawning gravels of salmon and trout. Untouched forest slows and stabilizes the water cycle. The water cycle in a logged forest is disrupted by logging roads and logging trucks, especially in hilly terrain. Temporary roads, hurriedly built for extra-heavy trucks, lead to numerous landslides that often dam streams. Where the roads cross streams, the streams are often forced into misaligned, “hanging” culverts, impassable barriers to migrating fish. A visitor to recently clear-cut forests in the mountains of the west can easily see the landslides. They cause quantities of forest soil to disappear from the slopes and to reappear as dams in the valleys below, which derange the drainage patterns and cause erosion and its consequences. A damaged or destroyed forest leads to severely altered terrain. Logging trucks driven over unbridged streams tend to have the same effects on a smaller scale. Clear-cutting inevitably leads to loss of biodiversity. That is, it brings about the permanent loss of many uncommon genotypes. It kills almost all the small animals that live in closed forest; their habitat is destroyed and similar habitats are too far away for them to retreat to. The number of species of animals at risk is far greater than most people realize. For example, there are three species of deer mice and eight species of shrews in our area; only one of each was described in chapter 10. The commonest deer

natural and unnatural interference

mouse (Peromyscus maniculatus) has no fewer than 31 distinguishable subspecies in our area, many of which are confined to very small areas, as a map of their distribution shows.5 Likewise the commonest shrew (Sorex cinereus) has six subspecies, each of whose range is only a fraction of the range of the species as a whole. Dozens of similar examples could be given. All demonstrate that wide-ranging species normally consist of numerous, small, genetically distinct, local populations; each of them is separately at risk of going extinct, and such extinctions go unrecognized by observers who make no allowance for intraspecies diversity. We are losing genetic diversity much faster than is generally realized. Besides clear-cuts, “managed” forests also lessen biodiversity. Taken together, all forest animals require a wide variety of habitats. Managed forests are deliberately simplified to make work easier. Management leads to fewer habitats and, therefore, fewer species. It also changes the fire cycle: fires are sometimes extinguished, while other fires are deliberately started. And management alters the natural water cycle. Altering a naturally evolved system almost always leads to a reduction in the value of the natural system’s ecological services. Clear-cutting has caused, and will continue to cause, the disappearance of many organisms that make their homes in “old” trees. “Old” in this context means “of an age to cut down.” For many forestry companies this age was, and in many cases continues to be, 75 years. Obviously, even if a stand is allowed to regenerate naturally after logging, it will never develop to the old-growth stage if growth is always halted at 75 years. The original biodiversity of the stand, what it was before the first “harvest” was taken, will never return if climate change prevents reinvasion by organisms that used to live there. Possibly many lichens adapted to growing on the barks of old trees in our forests have been driven to extinction already.

Notes 1. R. W. Mutch, “Wildland Fires and Ecosystems—A Hypothesis,” in Ecology, vol. 51, 1970, pp. 1046–51. 2. D. G. Sprugel, “Dynamic Structure of Wave Regenerated Abies balsamea Forests in the North-eastern United States,” in Journal of Ecology, vol. 64, 1976, pp. 390–93. 3. J. C. Jenkins et al., Acid Rain in the Adirondacks: An Environmental History (Ithaca: Cornell University Press, 2007). 4. E. C. Pielou, Fresh Water (Chicago: University of Chicago Press, 1998). 5. A. W. F. Banfield, The Mammals of Canada (Toronto: University of Toronto Press, 1981), p. 168.

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Chapter 12

The Big Picture

Introduction Now that we have considered in detail the conifers and the ecosystems they create, it is time to see how this can be summarized by mapping the geographic extent of the ecosystems, and coincidentally the areas of dominance of one or more identifying conifer species. The result is a mosaic of “regions.” As ecologists well know, when you try to define boundaries between ecosystems, either in words or graphically on a map, a large number of rather subjective choices have to be made. Usually, there are no clear, natural boundaries, and the ones drawn on a map are unavoidably arbitrary. Maps are useful, but it pays to be aware of their limitations.

Forest Regions A map of the forest regions is a necessity for any traveling naturalist faced with these questions: What are the predominant trees hereabouts, and why? The map in figure 12.1, and the tree lists that follow, are about as compact a way of presenting the information as is attainable. The letters used to label the regions are as follows: “T” and “B” represent the taiga and the boreal forest, respectively (some mappers combine them, treating the taiga as part of the boreal forest). “R” is the Pacific coast rain forest. “M” is the mountainous western region; it will be divided into subregions in the account below because it has three dissimilar 138

the big picture

Figure 12.1. Map of the forest regions in the once-glaciated area. The regions, labeled by letters, are defined and briefly described in the text. (Map based on one in Trees of the Northern United States and Canada, by J. L. Farrar [Markham, Ontario: Fitzhenry and Whiteside, 1995].)

subregions too small to differentiate on a map of this scale. “G” and “A” represent the Great Lakes and St. Lawrence region and the Acadian region, respectively. “P” represents areas too dry for conifers. It supports prairie grasslands with a northern strip of trembling aspen and balsam poplar forest. Note that the boundaries of the zones are shown as stippled bands in the map, to emphasize that they are mostly broad and indefinite on the ground. Even as shown here, some are not fuzzy enough: where there is no abrupt change in climate and topography, an abrupt boundary is obviously inappropriate. Below are species lists of the principal conifers in the regions. For compactness some lists relate to pairs of regions, with species found in only one member of the pair differentiated with asterisks. The dominant trees in an area (those both large and abundant) are shown in capital letters. Two species (balsam fir and grand fir) are underlined to show that they grow only near the southern boundary of the region concerned. The subdivision of region M is as follows: “Ms” is land at high elevations on the several mountain ranges—the Rockies, Purcells, Selkirks, and Monashees on the east and, separated from them by a gap, the Coast Range and the Cascades on the west. “Mm” is the dry plateau country

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between the two sets of mountain ranges, in the rain shadow of the coastal mountains; locally it’s called “the dry interior.” “Mc” is the so-called Columbian subregion or, more descriptively, the interior rain forest. It is on the lower slopes of the eastern mountains, far enough from the coastal mountains to escape their rain shadow effect. Now for the lists:

REGIONS B AND T. (An asterisk marks trees never present in the tundra.) tamarack, white spruce, black spruce, balsam fir,* jack pine* REGIONS G AND A. eastern white pine, red pine, balsam fir, red spruce, white spruce, eastern hemlock, tamarack, jack pine, pitch pine, white spruce, black spruce, eastern juniper REGIONS R AND Mc. (An asterisk marks species only in the Pacific rain forest.) western redcedar, sitka spruce,* douglas-fir, western hemlock, western white pine, lodgepole pine, shore pine,* amabilis fir,* grand fir, mountain hemlock, western yew, Nootka-cypress, Rocky Mountain juniper REGION Ms. (An asterisk marks species only in the Rocky Mountains.) lodgepole pine, subalpine fir, engelmann spruce, douglasfir, whitebark pine, limber pine,* western larch, subalpine larch,* Rocky Mountain juniper REGION Mm. ponderosa pine, lodgepole pine, Douglas-fir In all the regions except the Great Lakes, the St. Lawrence, and the Acadian, conifers dominate. The forests of the latter two regions vary between coniferous and mixed (that is, with various broadleafs as well as conifers). Even a fuzzy map exaggerates the distinctness of the forest pattern. When you fly over it, one conifer species looks much like another (unless it’s larch or evergreens killed by bark beetles). The clearest patterns to be seen from the air are the irregular patchwork of conifer stands of different ages left by scattered fires, as well as patches of mixed-age forest, possibly representing a forest that has gone through all stages of its succession. If so, it can reasonably be called “mature,” but it is unlikely to be stable, that is, in a state of equilibrium. Stability implies that nothing ever changes:

the big picture

equilibrium equals stasis. Such a state is impossible given climate fluctuation, and storms, fires, insect infestations, decay, rust fungi, and so forth. This is why the hundred-year-old notion of a climax forest is out-of-date. The term is seldom used now by professional ecologists. Only occasionally would the view from a plane window reveal a boundary between regions, except where the boundary coincides with the Continental Divide (between regions B and M). Flying from boreal forest into taiga and beyond reveals slow, continuous change, which is fascinating to watch. The tall trees become progressively shorter until the ground cover is a forest of “little sticks,” in local parlance. Then the dwarf trees become sparser and sparser, finally petering out in the barren grounds of the tundra. Along the way, tracts of muskeg steadily become bigger and more numerous. In the two southeastern regions, G and A, the human population is much denser than in the northern regions, and much of the forest is managed, with the results described in chapter 11. It also means that open ground is often planted with trees, so that postfire pioneers are not needed for reforestation. Two natural pioneers do grow in the area, however. They are red pine and pitch pine. Managed forests are also common in the southern, populous parts of regions R and M. In R, the Pacific coast rain forest, tracts of mature oldgrowth forest, containing the last remaining specimens of rain forest giants, are at risk of destruction. Region M, the mountainous territory inland of the rain forest is currently being deforested by the pine bark beetle infestation, which is killing huge expanses of lodgepole pine forest.

What Controls Which Species Grow Where? The trees found at any one site depend mainly on the climate, the terrain and soil, the location from which their ancestors immigrated, and the number of years since the site was last burned. In mountainous country, elevation is another important factor. It is worthwhile comparing the map in figure 12.1 with a climatic map showing isotherms (lines of equal temperature). The manner in which the long northern boundaries of B and T, trending from west-northwest to east-southeast, parallel the isotherms is striking. It shows how temperature governs the ranges of tree species. In those two regions, both the conifers listed earlier, white and black spruce, and the four “very hardy” broadleafs

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(canoe birch, speckled alder, balsam poplar, and trembling aspen) grow. The ranges of white and black spruce and the four broadleafs spread across the whole continent, from western Alaska to eastern Labrador. Nearly all of the other conifers can be described as either eastern or western but not both, and maps of the southern parts of their ranges show that this division into eastern and western species applies to all of North America. This shows that as the ice sheets of the last ice age melted along their southern front, trees migrated into land north of their starting points, retaining their easternness or westernness: they did not mingle. The one clear-cut boundary line is the Continental Divide. Presumably the topography—the height of land—formed an unbreachable barrier for the trees on either side. The barrier evidently prevented the species on one side from competing with, and possibly excluding, closely related species on the other side. Our area’s conifer diversity has benefited from this. The same is true for broadleafs.

Chapter 13

Global Warming and the Forests

Introduction The fact that the climate is warming because of the buildup of greenhouse gases in the atmosphere is generally accepted. To begin, note that what is happening is called both “climate change” and “global warming.” Take your choice. The merit of the term “climate change” is that it allows for gaps in the warming; it doesn’t sound like nonsense when an unusually cold spell strikes in a limited area. It also emphasizes that rising temperatures in most of the world are not the only change: precipitation and winds are changing too. In any case, there’s no need to use one term consistently; rather, it is reasonable to use whichever is appropriate to the context. Let’s start with a brief outline of the elementary physics of what’s involved, and how rising temperatures affect precipitation and soil, especially permafrost.

The Physics of Climate Change The increasing use of fossil fuels by the world’s growing population is adding to the natural concentration of carbon dioxide (CO2) in the atmosphere. Water vapor, CO2, and methane (CH4), in that order, are the three strongest natural greenhouse gases (GHGs), which are opaque to outgoing heat rays (infrared rays) from the surface of the sun-warmed earth. Water 143

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vapor (hereafter simply called “vapor”) is the most important ingredient, but it is so commonplace, and present in such ever-varying amounts, that it is often left out of theoretical calculations. Acting together, the gases trap much of the sun’s heat that would otherwise be radiated back to the sky. Over the past 250 years, the concentration of CO2 has been rising rapidly, causing the current greenhouse effect: the average temperature is increasing over the whole earth. Note the word “average.” The occurrence of cold spells here and there, even long ones, does not negate the fact that, averaged over the whole world, the temperature is indisputably rising. Note also that we are not concerned here with temperature trends in the more distant past. Two other GHGs are important; both are in the stratosphere. They are natural ozone, and chlorofluorocarbons (CFCs), industrial chemicals that came into use in the 1930s. Chlorofluorocarbons were found to be destroying ozone in the stratospheric ozone layer that shields the surface life below from the sun’s damaging ultraviolet radiation, so their use was discontinued, worldwide. Even so, the amount still persisting will go on destroying ozone for years. Today, temperatures are rising faster in the Arctic than in the Tropics. Researchers1 predict that the concentration of CO2 is unlikely to become stabilized at less than 650 parts per million by volume (ppmv). The level was about 280 ppmv in the preindustrial era, before 1750. An increase as great as predicted would (or will?) lead to a worldwide average temperature rise of about 4° Celsius (7.2° Fahrenheit), which, it is said, would be “catastrophic.” Possibly, a positive feedback (intensifying the warming) will progress, for a while, like this: WARMTH

→ MORE VAPOR → INCREASED GREENHOUSE EFFECT → WARMER.

But eventually there would surely be a counteracting negative feedback like this: WARMTH

→

MORE VAPOR

→

MORE CLOUDS

→

SUNLIGHT REFLECTED

→

COOLER.

Moreover, as warming peat decomposes, quantities of methane, which is a much stronger GHG than CO2 (on a per volume basis), are liberated. The result is additional positive feedback: WARMTH

→ PEAT DECAYS → METHANE EMITTED → WARMER.

global warming and the forests

Global warming theory is bedeviled by positive feedbacks that boost it and negative ones that counteract it.

How Will Climate Change Affect the Forests? Three of the possible effects of climatic “warming,” in the narrow sense of the word, were just listed. But “warming” has a wider connotation. It also implies an increase in the total energy received from the sun. Indeed, warmth, another word for heat, is one of energy’s many forms. Some of this energy manifests itself as mechanical energy—in a word, the winds become stronger.2 The strongest, most damaging storm winds to affect North American forests blow in latitudes south of our area or near its southern limits, but as the climate warms, they can be expected to shift northward. Downbursts of wind will probably be the most serious. A downburst consists of a radiating system of horizontal straight winds that spread out from the point where the dense central column of downward-moving air in a thunderstorm hits the ground. A famous recent downburst struck in 1999 in the Boundary Waters Canoe Area of Minnesota; this “Big Blowdown,” as it is called, with winds in excess of 150 km (93 miles) per hour, destroyed 1500 km2 (580 square miles) of forest. After such blowdowns, patches of forest are left bare, unshaded and unprotected from subsequent winds, causing abrupt habitat change over large areas. Pioneer tree species then invade, and with them, wind-borne insect pests and pathogenic fungus spores, especially of rusts. These consequences of climate change are beginning to be felt in our area, and the possibilities are too numerous for predictions to be worth attempting. Increasing solar energy is also causing increased precipitation on a worldwide scale. But whether local climates will become wetter or drier is anybody’s guess. Let’s return to the effect of warmth in the ordinary sense on tree growth. Throughout our area, living trees show signs of becoming more luxuriant, or anyway bushier, as the climate warms. Clear evidence of this phenomenon is appearing in the taiga-tundra ecotone (the fuzzy boundary between two regions). Pairs of photos, taken in 1949–50 and in 2000, respectively, show that the gaps in black spruce stands have filled in, probably owing to vegetative spread, a common form of reproduction in black spruce. Also, alder, dwarf birch, and willow shrubs are multiplying and advancing in Canada and on the Alaska

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North Slope.3 These shifts have been in progress for as long as 100 years, however, and may be evidence of continuing recovery of the tundra from the Little Ice Age that lasted from about 1350 to 1870. To detect a northward migration of a single species of forest conifer, or of a particular group of conifer species, would be difficult; they wouldn’t stand out in air photos in the way that islands of black spruce stand out against a pale tundra background, for example. Such migrations are probably not happening in any case. Trees find it far more difficult to invade the space occupied by other trees than they do to invade gappy vegetation with patches of open ground. The established trees differ negligibly, if at all, in their adaptedness to the environment, and their seeds vastly outnumber those of the invaders. Migration because of shifting isotherms will probably happen only when tracts of forest are burned or wind thrown, making big gaps available. More on this in the next section. Drought caused by climate change appears to be thinning the mature forests in the southwestern part of our area. Studies were made on 76 undisturbed coniferous forest plots in western Canada and the United States to discover how climate change is affecting them.4 The plots chosen supported apparently healthy forests at least 200 years old; the average age of the plots was about 450 years, and some were more than a thousand years old. At the outset the number of living trees in all plots combined was 58,736, with a wide range of ages and mostly belonging to three genera: pines, hemlocks, and spruces. By the end of the study, 11,095 trees had died, a number far in excess of predictions calculated from estimated death rates. The death rate increased in 87 percent of the plots. The researchers concluded that summer droughts resulting from less precipitation are the cause of the increased death rate, and it is intensified by a positive feedback: temperatures are rising at about 0.3° to 0.4° Celsius (0.5° to 0.7° Fahrenheit) per decade, so the annual snowmelt comes progressively earlier every spring, the meltwater quickly drains away, and the regular summer drought starts earlier. Many trees die as a result. Thinning of these old forests is not the only outcome; because of varying susceptibilities of different tree species to the stress of drought, the relative proportions of the species—the forest composition—must change. In time they will become entirely different forests. It doesn’t necessarily follow, however, that the old forests will change from carbon sinks to carbon sources, as was once thought. Young forests are carbon sinks (they sequester carbon) because growing trees absorb

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more CO2 for photosynthesis than they liberate in respiration. (If they didn’t, they wouldn’t be growing.) In contrast, the old trees in old forests are no longer growing. Also, where there are old trees there are likely to be dead trees plus fallen or broken branches, all decaying and liberating CO2. This seems to imply that an old forest must be a carbon source. But the argument overlooks the fact that when a big old tree dies, a big opening is made in the canopy. Light reaches the forest floor, and seeds of many kinds can germinate, leading to a succession of herbs, shrubs, saplings, and young trees, all of which can photosynthesize vigorously. This has led to the conclusion, supported by observations, that old-growth forests are usually carbon sinks.5

Fire and the Forests Rising temperatures in high northern latitudes are lengthening the fire season in the evergreen forests and increasing the fire risk. Big forest fires can have a pronounced effect on the climate. The immediate effect is a sudden warming, first, because of the release of large amounts of CO2 from the burning wood, and second, because the charred, black surface of the burned area has a much lower albedo (reflectivity) than the living forest had, so that a greater proportion of the incoming solar radiation is absorbed by the ground rather than being reflected back to the sky. The effect is reversed by snow cover on the ground in the following and succeeding winters. A layer of clean snow has a very high albedo. Numerically, the albedo of a surface is simply the proportion of the incoming radiation that it reflects; being a ratio, it has no units. For example, the albedo of soot-blackened ground may be as low as 0.05; the same ground, under fresh snow, may have an albedo as high as 0.90. To summarize: Just after a fire: FIRE

→

BLACK GROUND

→

LOW ALBEDO

→

MUCH ENERGY ABSORBED

→

WARMING.

The following winter: FIRE

→ OPEN SNOWFIELD → HIGH ALBEDO → LITTLE ENERGY ABSORBED →

COOLING.

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The return of the summer albedo to normal after a fire takes many years. At first grasses and herbs spread over the black postfire embers, increasing the albedo. After that, tree regeneration begins so that, in time, saplings grow to become full-grown trees and the albedo falls again to its prefire value. Winter albedo changes gradually, too. It is at a maximum when a thick bed of pure snow covers the burn, and then becomes less and less as shrubs, saplings, and trees grow taller and taller. Finally (again assuming no accidents), the original forest is restored, and the winter albedo is the same as it was before the fire. The conclusion of this argument comes as a surprise. It is that the longterm effect of a fire will be a decrease in greenhouse warming because the increased albedo caused by the fire trumps the increase in GHGs produced by the fire. A large fire in interior Alaska in 1999 appears to have behaved in this way. So the earlier opinion that forest fires would automatically accelerate climate warming has been shown to be incorrect, at least in this example.

Insects, Lightning, Wind, and Snow (Again) Not surprisingly a warmer climate with warmer winters is having a marked effect on forest insects.6 The most spectacular example of this is the population explosion of pine bark beetles throughout the west. It is at its worst in British Columbia where, in 2007, 13.5 million hectares (33 million acres) were infested. That’s the area of a square tract of land with sides 367 km (230 miles) long. The area infested is still (in 2010) expanding. As mentioned, global warming is the cause of the population explosion. The overwintering beetles used to be killed off almost completely because winter temperatures stayed below −40° (Celsius and Fahrenheit) for a week or more (at least a few survived, or the population would have disappeared), but now that such low temperatures no longer last for such long periods, the beetle population is growing exponentially. The forests are mostly huge expanses of lodgepole pine. A way of slowing the spread of the beetles has been suggested, namely, by quickly replanting recently burned land with young trees of a variety of species. This would make the forest more resistant to insect attack. Insect-killed trees bring us back to the topic of fires again. The high flammability of dead, tinder-dry trees is obvious. At the same time, the risk of their being set ablaze is increasing. Because of global warming, and therefore increasing energy in the atmosphere, lightning storms are

global warming and the forests

becoming more frequent and more severe. Winds are becoming stronger: they blow trees over, and they carry burning embers and pest insects into adjacent forests. Fires affect the hydrology of forested land in the same way that clearcutting does (see chapter 11). Dead trees don’t absorb water and transpire it into the air. This leaves all the rainwater that doesn’t evaporate quickly still in the soil, where it causes increased runoff, both above and below the surface. Extra runoff then erodes valuable soil. Lastly, consider the combined effects of wind and snow. Where climate has warmed and precipitation has increased, heavy loads of wet snow will accumulate on branches. When ordinary winds are replaced by gales, huge numbers of snow-laden trees are likely to be blown down.

The Value (If Any) of Predictions Many attempts have been made to predict the effects of global warming on forest growth, tree migrations, forest insects and diseases, forest fires, and forest hydrology, and the interactions of all these variables. Mathematical models have been devised to make the predictions quantitative. All climate models, and there are many, make predictions based on “reasonable” assumptions. But they cannot handle complete surprises. A nasty surprise will probably happen when one of the many factors affecting climate unexpectedly crosses a threshold that sets off an uncontrollable, runaway feedback. Consider methane. A vast quantity of it is trapped on and under the sea bed and below the permafrost, which acts as a lid. It is trapped in the form of methane clathrate (or hydrate), which is a frozen mixture of methane molecules and water molecules arrayed in a crystalline spatial pattern. It is stable (frozen and solid) only as long as the temperature remains low enough and the pressure high enough. As global warming melts the permafrost and warms the oceans, the clathrates lose their stability and turn frothy: methane gas gushes out. It is quite possible that a threshold exists at which the speed of the feedback will increase abruptly and enormously. But what is the threshold, and when should we expect to reach it? Other nasty surprises will be genuine surprises, wholly unforeseen, and impossible to incorporate in predictive models. In any case predictions are not actions. And the dire effects that are predicted may take a very long time to wear off. What should be done

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to ameliorate the hardships and damage that severe climate warming will cause? Surely, we should first preserve the free “ecological services” that the natural world supplies. In this context, evergreen forests do the following: • They act as massive, irreplaceable carbon sinks, even the old forests. • They purify water and air. • They store large volumes of water and liberate it slowly, thus conserving water and preventing erosion. • They prevent wind damage in an increasingly stormy world. • They conserve biodiversity.

Most of these services are hard to quantify because they are so thinly spread. They consist of a very large number of individually small miniservices, which, as professional ecologists have shown, add up to a highly significant total.7 Therefore, one of the most important steps North American governments can take to ameliorate climate change is to reduce the destruction of the northern evergreen forests, restore deforested land, and increase the area left undisturbed. Eighteen thousand years ago the area discussed in this book was covered nearly everywhere by a layer of ice that was kilometers thick. Now, when climate change threatens everyone’s welfare, the forests in the same area form one of the largest carbon sinks on earth. Surely our species has intelligence enough to preserve what protects us? But whether the actions required to preserve ecosystem services will come soon enough to ward off catastrophe, only time will tell. Those who take action, and everybody else, must wait and see. Notes 1. K. Anderson and A. Bows, “Reframing the Climate Change Challenge in Light of Post2000 Emission Trends,” in Philosophical Transactions of the Royal Society A, doi:10.1098/ rsta.2008.0138, 2008. 2. C. J. Peterson, “Catastrophic Wind Damage to North American Forests and the Potential Impact of Climate Change,” in The Science of the Total Environment, vol. 262, no. 3, 2000, pp. 287–311. 3. K. Tape, M. Sturm, and C. Racine, “The Evidence for Shrub Expansion in Northern Alaska and the Pan-Arctic,” in Global Change Biology, vol. 12, 2006, pp. 686–702. 4. S. Luyssaert et al., “Old-Growth Forests as Global Carbon Sinks,” in Nature, vol. 455, 2008, pp. 213–15.

global warming and the forests 5. J. T. Randerson et al., “The Impact of Boreal Forest Fire on Climate Warming,” in Science, vol. 314, 2006, pp. 1130–32. 6. W. A. Kurz et al., “Mountain Pine Beetle and Forest Carbon Feedback to Climate Change,” in Nature, vol. 452, 2008, pp. 987–90. 7. G. C. Daily and K. Ellison, The New Economy of Nature: The Quest to Make Conservation Profitable (Washington, D.C.: Island Press, 2003).

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Index

Abies, 8 Abies amabilis, 27–28 balsamea, 27 grandis, 27–28 lasiocarpa, 26 –27 Adelgid, 103 –104 Alder, 3, 59, 65– 66 Alnus rubra, 65 Angiosperm, 38, 39, 68 Ants, 106 –107 Aphid, 103, 107 Arceuthobium, 9 Aspen, 6, 32, 40, 60 – 62 Avalanche, 130

Caches, 111–112, 123 Callitropsis nootkatensis, 8, 30 Cambium, 45– 46, 48, 50 –52 Caribou, 115–116 Catkin, 59 Chickadee, 121 Chipmunk, 112 Chlorophyll, 53 Climate change. See Global warming Clone, 40, 64 Cones, 15, 19 Cork, 51, 52 Cottonwood, 59, 60 – 62 Crossbill, 119 –120

Bark, 11, 14, 15, 20, 21, 31, 42, 49, 50 –52, 57 Bark lice. See Adelgid Bears, 118, 128 Beavers, 82 Beetles, 93, 94 – 97, 102, 141 Betula, 59 “Big Blowdown,” 145 Birch, 6, 59, 60, 65– 66 Blue stain fungus, 96 Bracts, 12, 16, 26, 39 – 40 Broadleafs, 2, 4, 6, 7, 38, 43, 45, 46, 49, 54, 69, 131 Budworm, 98– 99 Bugs. See Homoptera

Decay, 4, 30, 48, 70, 74, 78, 84 – 90 Douglas-fir, 8, 11, 12, 13, 29, 36, 37, 39, 48, 51, 53 Downburst, 145 Drought, 4, 52, 146 Extracellular freezing, 5, 6 Fibers, 45, 71 Fir, 11 Amabilis, 28, 72 Balsam, 26 Grand, 28 Noble, 26 Subalpine, 28

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154

index Fire, 125–129, 147–148, 149 Fungus, 55–57 Fungus, Velvet Top. See Polypore, Dye Galleries, beetle, 94 – 95 Galls, 89, 104 Global warming, 143 –149 Grosbeak, 119 Ground squirrels, 112, 113 Grouse, 119, 120 Gymnosperms, 38, 68, 69, 70 Hardiness, cold, 5 Hare, Snowshoe, 114 –115, 118 Heartwood. See Sapwood Hemlock, 3, 10, 11, 12, 34, 37, 40, 54, 72, 131 Eastern, 28 Mountain, 28 Western, 28 Homoptera, 93 Hypha, 55 Ice, 6 Ice age, 1–3 Ichneumon, 106 Indian-pipe, 75 Juniper, 7, 8, 13, 16, 29, 31 Eastern, 32 Rocky Mountain, 32 Seaside, 32 Juniperus. See Juniper Juniperus maritimus, 32 scopulorum, 32 virginianum, 32 Landslides, 136 Larch, 5, 9, 22–24 subalpine, 22 western, 22 Larch casebearer, 98– 99 Larix laricina, 22–24 lyalli, 22 occidentalis, 22 Layering, 41 Leaves, 53 –55 Lichen, 79, 116 –117, 137 Lynx, 117 Marten, 117 Mineral nutrients, 51, 74, 125, 135

Mistletoe, dwarf, 90 – 91 Mouthparts (insects’), 93 – 94, 98, 101 Mushrooms, 56 –57, 112 Mycelium, 56 Mycorrhiza, 56 –57, 76, 84 Nodules, root, 66 Nitrogen fixer, 65 Nootka-cypress, 13, 19, 30 –31 Nurse log, 64, 65 Nutcracker, 121 Nuthatch, 121 Ovules, 35, 38–39, 68–70 Owls, 123 –124 Palynology, 37 Parasites insects, 93 –105 on trees, 76 –77, 84 – 90 Peat, 78, 126, 128, 144 Pheromone, 96 Photosynthesis, 53 Picea, 2, 3 Picea engelmanni, 25 glauca, 6, 25 mariana, 25 pungens, 24 rubens, 25 sitchensis, 29 Pine, 8, 16, 17 eastern white, 3, 21, 40, 87 jack, 2, 3, 18, 20 limber, 22 lodgepole, 18, 19, 20, 49, 50 ponderosa, 20 –21 red, 19 western white, 21 whitebark, 22 Pinus, 8, 16, 17 Pinus albicaulis, 22 banksiana, 2, 6, 18, 49 contorta, 18, 19, 20, 49, 50 flexilis, 22 monticola, 21, 87 ponderosa, 20 –21 resinosa, 19 rigida, 20 –21, 40 strobus, 3, 21, 40 Pinesap, 75 Pitch, 96 Pitch tube, 96

index Pollen, 15, 35–37, 68– 69 Pollination, 35–38 Polypore, Dye, 86 Poplar, Balsam, 6, 62 Population cycles, 98, 114, 117, 133 Populus, 40, 62, 64 Porcupine, 71, 113 Pseudotsuga menziesii. See Douglas-fir

red, 25 Sitka, 9 white, 6, 25 Squirrel, 22, 111–112 Stamens, 35 Stomata, 53 –55 Succession, 125, 129 –130, 140, 147 Supercooling, 5, 6, 134

Rain forest, 3, 4, 31, 62, 65, 81, 86, 118, 124, 141 Redcedar, western, 13, 29, 31, 34, 41 Regeneration wave, 133 Resin. See Pitch Resin duct, 47 Ribes, 87 Roots, 55–58 Root grafting, 58, 86 Rots, 85 Rusts, 86 – 90

Tamarack, 5, 9, 78 Taxus brevifolia, 33 “Thuja,” 8 Thuja, 8 Thuja occidentalis, 13, 29 plicata, 13, 29, 31, 34, 41 Tolerance, shade, 130 Tracheids, 43 – 44 Transpiration, 44, 53, 136 Tsuga canadensis, 28 heterophylla, 28 mertensiana, 28

Salmon, 118–119 Sap, 43, 46, 48, 49 Sapwood, 48, 50 Sawfly, 101 Scale insects, 104 –105 Shrew, 108 Sieve cells, 52 Snow, 5, 41, 64, 72, 130, 132, 147, 149 Solar energy, 145 Spiral grain, 49 –50 Spruce, 2, 3, 9 black, 6, 25 blue, 24 Engelmann, 25

Vessels, 43 White-cedar, eastern, 13, 29 Wilderness, 126 Wildlife trees, 108 Willow, 59, 61 Wind, 126, 130, 131, 133 Wood, 43 –50 reaction, 49 spring and summer, 46 Woodpecker, 82, 111, 119 –120 Yew, 7, 9, 13, 15, 33

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