Excerpts

Chapter 1 Trees in Mind: Simple Questions with Complicated Answers I never stopped thinking like a child," said Einstein. Neither should any of us. It's the way to get to the heart of things. Children ask ridiculously simple questions--like "Who made God?"--that have kept theologians busy for many a century. In such a vein we might innocently inquire, "And what, pray, are trees, that anyone should presume to write a book about them?" And "Why do plants grow into trees?" And "How many kinds are there?" Childish stuff, but it will serve to mark out the ground. What Is a Tree? A tree is a big plant with a stick up the middle. Everybody knows that. But that statement as it stands requires what modern philosophers would call a little "deconstruction." What, for a start, is meant by "big"? It's a relative term, of course, although if we choose we can put a figure on it--say, a minimum height of five or six meters. There is a case for doing this: if you are a forester, or are running a nursery, you need some guidelines. But guidelines are not definitions. They are ways of helping practical people do practical things. They do not--and are not intended to--capture what Aristotle would have called the essence of nature. For many trees grow big when conditions are favorable, and stay small when they are not. An oak is a noble tree in a forest or a park, but an acorn that falls in a fissure in some Scottish crag may spend a couple of centuries in bonsai mode, never more than a twisted stick. Yet it may turn out acorns that, if they should be carried to some fertile field, could again produce magnificence. Is the twisted stick less of an oak because it fell on stony ground? And if it remains an oak, is it not still a tree? Then again--a different kind of case--the world's many kinds of birches form the genus Betula. None are as huge as an oak may often be, but most are perfectly respectable trees. Yet there is one, Betula nana, that is adapted to the tundra of the north of Scotland and mainland Europe and is very small indeed. Do we say that all birches are trees except for the tough little Betula nana? Or do we say it's a dwarf tree? What of the stick that runs up the middle, the "trunk," which holds the "crown" of the tree aloft? Should there be just one, a solitary pillar, or are several allowed? Many a gardener and forester has insisted that plants with a lot of supporting sticks should be called shrubs. Again, for practical purposes such distinctions can be useful. If Alice's Queen of Hearts had instructed her long-suffering gardeners to plant her an arboretum and they'd come up with a shrubbery, their heads would surely have come off. But wild nature is not so easily pinned down. In the Cerrado of Brazil--the vast, dry forest, about the size of France, in the middle of the country to the south and east of Amazonia's rain forest--there are trees that form bona fide, big, one-trunked trees when they grow along the banks of the occasional rivers but become multistemmed, short shrubs where it's drier. The shrub is not merely stunted, like the oak in the rock. It is a discrete life-form. Many organisms exhibit what biologists call "polymorphism," meaning "many forms." Many kinds of fish, for example, have dwarf forms and full-size forms; some butterflies and snails are highly variable. Here we see a polymorphic tree--one form for the forest, another for the open ground. Then again, many big trees, including some cedars, many a mulberry, and the beautiful blue-flowered jacaranda, may grow from ground level with several solid trunks of equal magnitude. Each may be as big as a respectable oak. Are they trees or big shrubs? The family of the heathers, Ericaceae, also includes the rhododendrons from the Himalayas and the madrone trees of the United States, with their beautiful flaky, yellow, pink, and gray trunks (which add yet more color to the already wondrous hills of California). Rhododendrons tend to have many stems, while madrones are commonly content with one. But the rhododendrons can be just as big and solidly wooden as the madrones. In nature, in short, trees and shrubs are not distinct. Why should they be? Nature was not designed to make life easy for biologists. Must the central stick be of wood? That, after all, is what we generally mean by "stick." How, then, should we categorize banana plants? In general shape they resemble palm trees, with a thick central stem and a whorl of huge leaves at the top. But the stem of the banana plant is not made of wood. Its stem is formed largely from the stalks of the leaves, and its strength comes from fibers that are not bound together, as in pines or oaks or eucalypts, to form true timber; its hardness is reinforced, as in a cabbage stalk, by the pressure of water in the stem. So botanically the banana plant is a giant herb. But it looks like a tree and competes with trees on their own terms, as a big plant seeking the light (although, like the trees of cacao and tea and coffee, the banana prefers a little shade). In fact, there are many lineages of trees--quite separate evolutionary lines that have nothing to do with one another except that they are all plants. Many plants, in many of those lineages, have independently essayed the form of the tree. Each achieves treedom in its own way. "Tree" is not a distinct category, like "dog" or "horse." It is just a way of being a plant. The different kinds have much in common, and it is good and necessary to have some feel for what is essential. But the essences of nature will not be pinned down easily. In the end, all definitions of nature are simply for convenience, helping us focus on the particular aspect that we happen to be thinking about at the time. There is no phenomenon in all of nature--whether it's as simple as "leg" or "stomach" or "leaf" or more obviously conceptual like "gene" or "species"--that does not take a variety of forms, and that cannot be looked at from an infinite number of angles; and each angle gives rise to its own definition. A horse cannot be encapsulated, as Charles Dickens's Thomas Gradgrind insisted in Hard Times, as "a graminivorous quadruped." There is more to horses than that. The way we define natural things influences the way we treat them--whether we speak of wildflowers or of weeds, of Mrs. Tittlemouse or of vermin. But in the end nature is as nature is, and we must just try with different degrees of feebleness, and for our own purposes, to make what sense of it we can. For the purposes of this book, the child's definition of "tree" will serve--albeit with slight elaboration: "A tree is a big plant with a stick up the middle--or could be, if it grew in the right circumstances; or is very closely related to other plants that are big and have a stick up the middle; or resembles a big plant with a stick up the middle." It is clumsy, but it will have to do. So to the next childish question. Why Be a Tree? A nonliving thing is passive. The atoms of which a stone is composed sit there for as long as it endures--until it is melted in some volcano, or dissolved by acid rain. But living things are restless, through and through. As soon as some living cell has constructed some protein, as part of its own fabric, it starts to dismantle it again. This constant self-renewal, powered by an endless intake of energy, is called metabolism. Metabolism--the basic business of staying alive--is half of what living things do. The other half is reproduction. It is not vital to reproduce in order to stay alive. Indeed, reproduction involves sacrifice; reproduction, as we will see later in this book, is often the last fling: many a tree dies after one bout of it. But it is essential nonetheless. At least, all creatures that do not reproduce die out. However successfully an organism may metabolize, sooner or later time and chance will finish it off. Everything dies. Only those that reproduce endure--or, at least, their offspring do. All individuals are part of lineages, offspring after offspring after offspring. But then, too, each creature finds itself in the company of other creatures, of its own kind and of different kinds. To some extent they are its rivals, to some extent it needs them--for food, shelter, mates, or whatever. Each successful creature, then--each one that survives at all, that is--must come to terms with the others around it. All of life's requirements--metabolism, reproduction, and the business of getting along with others--are difficult. Each creature must solve life's problems in its own way. There is no perfect, universal life strategy. Each has its own advantages and drawbacks. So it can pay a creature to be very small; or it can pay to be big. Each mode has its pros and cons. A plant that is big like a tree can stretch farther up into the sky, and so capture more of the sun's energy; and it can reach farther down into the earth, for water and minerals. This is the upside. But it takes a long time to achieve large size, and whether you are an oak tree or an elephant or a human being, the longer you take to develop the more likely you are to be killed before you reproduce. Being big is difficult, too. To hold a ton of leaves aloft in the sun and air requires enormous strength: specialist material like wood, and clever architecture. All trees have wood, by definition (apart from those granted honorary status, like bananas); but as we will see, wood is subtle stuff, requiring much chemistry and microgeometry. The many types of trees have essayed many architectural forms. Ginkgoes and conifers are built from repeats of a single simple module: a straight trunk up the middle with circles or spirals of branches at intervals. In others, like the elm, the lead shoot bends over and the next shoot in line takes over the lead, until it too bends away and the one below that takes over. In yet others (particularly some tropical trees), the branches that grow upward from the horizontal branches repeat the form of the whole tree--it's as if a new, miniature forest grew aloft, from the horizontal branches of the giants below. And still others, like oaks or chestnuts, are more free-flowing. There are many basic designs. The point is, though, that such design is necessary. Being big requires a lot of engineering as well as a lot of chemistry, and it takes a long time to put into place. But the bigger trees grow, the more they are vulnerable to wind--and tropical storms regularly cut swaths as big as Los Angeles through the world's rain forests. For the purposes of reproduction, most creatures pursue one of two main strategies. Some, known as K-strategists, produce just a few offspring at a time, which in general are large at the time of their birth to give them a good chance in life; after they are born, typically, the parents take good care of them. K-strategists tend to be long-lived and reproduce several times in their life, often at long intervals. Orangutans, elephants, eagles, and indeed human beings are classic K-strategists. Other creatures, known as r-strategists, produce an enormous number of offspring. Inevitably, each individual offspring is small, and so has little chance of survival. But there is safety in numbers. Codfish are noted r-strategists. They produce up to two million eggs at a time. The newly hatched fish live for a while as plankton, floating fairly helplessly--and most perish: they just get eaten. But so long as each pair of codfish manage to produce just two surviving offspring in the course of their lives, the lineage of cod will carry on. Despite the enormous prodigality of their reproductive strategy, its fantastic wastefulness, codfish are immensely successful--or at least they were until North Sea fishermen became too technically proficient, or too "competitive," and disastrously reduced their numbers. Cod live a long time. But many r-strategists, like flies, run through their entire life cycle in a few weeks: birth, growth, reproduction, death. Thus populations of flies may rise and fall from near zero to plague proportions in what seems like no time at all. Trees seem to get the best of both worlds. Many--most--produce huge numbers of seeds and may do so repeatedly. A mature oak or beech may produce many millions of seeds in a good year (good seed years are known as "mast" years), and although they won't do this every year, they may well have scores or even hundreds of prolific years in the course of their lives. They are r-strategists indeed, in a good year at least as prolific as codfish. Yet many trees--including oaks--produce seeds that are large and that do not need to germinate immediately: each has a very good chance of survival. To this extent they are K-strategists too. To combine the advantages of the K- and r-strategies an organism must be truly mighty. Yet there is a downside too: most trees must grow for several years, and many must endure for several decades, before they can reproduce at all; and all that time they are vulnerable. We don't think of trees as r-strategists, because they are so big and long-lived. Their populations do not boom and bust like those of flies. They cannot, we imagine, leap to take advantage of newly created environments as a fly or a mouse may do. Yet once we venture beyond our own puny timescale and take the long view, we see that they can and do do this. Thus when the last ice age ended in the Northern Hemisphere, around ten thousand years ago, the forests of birches and alders that had been whiling away the time farther south were able virtually to race toward the North Pole in the wake of the retreating glaciers; and they will surely resume their advance as global warming reduces the polar ice still further. By the same token, the huge tropical rain forest of Queensland, in the Southern Hemisphere, has not been there forever, as it may seem. Like the Great Barrier Reef, which stands just off the Queensland coast and is as long from end to end as Great Britain, the rain forest of Australia grew up only after the last ice age and is a mere ten thousand years old. Macbeth was shocked to see the Great Wood of Birnam shift a few miles across the moor to the Hill of Dunsinane. But if we could take a time-lapse view of all the world this past few million or tens of millions of years, as cold has followed warm has followed cold, we would see vast and apparently immovable forests flitting over the surface of the globe like the shadows of clouds. Excerpted from The Tree: A Natural History of What Trees Are, How They Live, and Why They Matter by Colin Tudge All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.