The Tree (2 page)

Perhaps this is why we feel so drawn to trees. Groves of redwoods and beeches are often compared to the naves of great cathedrals: the silence; the green, filtered, numinous light. A single banyan, each with its multitude of trunks, is like a temple or a mosque—a living colonnade. But the metaphor should be the other way around. The cathedrals and mosques emulate the trees. The trees are innately holy. Christians with their one omnipotent God may take exception to such pagan musing; but the totaras and the kauris were sacred to the Maoris, and the banyan and the bodhi and the star-flowered temple trees (and many, many others) to Hindus and Buddhists, and the roots of this reverence, one feels, run back not simply to the enlightenment of Buddha as he sat beneath a peepul, or bo, tree (in 528
B.C.,
tradition has it) but to the birth of humanity itself.

Yet Christianity did give rise to modern science. The roots of science run far back in time and from all directions—from the Babylonians, the Greeks, many great Arab scholars in what Europeans call the Middle Ages, the Indians, the Chinese, the Jews, and the much underappreciated natural history of all hunter-gatherers and subsistence farmers everywhere. But it was the Christians from the thirteenth century onward, with an obvious climax in the seventeenth, who gave us science in a recognizably modern form. The birth of modern science is often portrayed by secular philosophers as the “triumph” of “rationality” over religious “superstition.” It was, however, much more subtle and interesting than that. The great founders of modern thinking—Galileo, Newton, Leibniz, Descartes, Robert Boyle, the naturalist John Ray—were all devout. For them (as Newton put the matter) science was the proper use of the God-given intellect, the better to appreciate the works of God. Pythagoras, five centuries before Christ, saw science (as he then construed it) as a divine pursuit. Galileo, Newton, Ray, and the rest saw their researches as a form of reverence.

This book is written in that same spirit. Of course, I don’t claim to walk on the same plane as Pythagoras and Galileo, but I don’t think it’s too pretentious to aspire at least to drink at the same spring. This book is mainly about the science of trees—what modern research is telling us about them. The last chapter is about the uses we make of them, and what they do for us, and why for reasons that are purely material they must be conserved: our survival depends on them. Most of this book, however, is not about their usefulness but about what they
are:
how they came into being, what kinds there are and where they live and why, and how they live, competing and cooperating. The revelations build by the week: how they may live and grow huge on what seems like nothing at all; how they draw prodigious quantities of water from the ground, send it up into the atmosphere, and then (so some have claimed) may call it in again, by releasing organic compounds that seed fresh clouds; how they speak to one another, warning others downwind that elephants or giraffes are on the prowl; how they mimic the pheromones of predatory insects, to summon them to feed upon the insects that are eating their leaves. Every week the insights grow more fantastical—trees seem less and less like monuments and more and more like the world’s appointed governors, ultimately controlling all life on land (and in the oceans too, vicariously), but also the key to its survival.

So this book presents science not as it is often presented, as a tribute to human cleverness and power, but truly in a spirit of reverence. I like the idea (I have found that some people don’t, but I do) that each of us might aspire to be a connoisseur of nature, and connoisseurship implies a combination of knowledge on the one hand and love on the other, each enhancing the other. Conservation—of all living creatures, including trees—has little chance of long-term success without understanding, which depends in large measure on excellent science. But conservation cannot even get on the agenda unless people care. Caring is an emotional response, to which science has often been presented as the antithesis. In truth, science cannot be done properly without a cool head. Yet when the science is done, its primary role (to reverse an adage of Marx’s) is not to change the world but to enhance appreciation. That is the purpose of this book. Science in the service of appreciation, and appreciation in the service of reverence, which, in the face of wonders that are not of our making, is our only proper response.

A
UTHOR’S
N
OTE

The following abbreviations have been used throughout the text:

“Judd” refers to Walter S. Judd, Christopher S. Campbell, Elizabeth A. Kellogg, Peter F. Stevens, and Michael J. Donoghue, (editors),
Plant Systematics,
2nd ed. (Sunderland, Mass.: Sinauer Associates, 2002).

“Heywood” refers to V. H. Heywood, ed.,
Flowering Plants of the World
(Oxford: Oxford University Press, 1978).

Round-leaved and altogether beautiful: the Judas tree.

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.

Thus the advantages of treedom are both manifold and manifest. Big plants can metabolize more effectively because they command so much earth and sky; and they can produce literally tons of seeds, to be scattered far and wide. Small wonder that a third of all land is covered in forest. But being big is complicated—all that chemistry and architecture—and it is risky, because all the time a tree is growing, time and chance and other creatures are working on its downfall. So it is that many other plants, such as mosses and liverworts, never acquired the means to be big at all; but they have still made a very good living during the past 400 million years, just by sticking to damp and easy places. Then again, trees cannot grow where it’s too dry or the soil is too thin, and so they leave scope for many smaller plants that can. So the world’s grasslands are vast too, like the savannahs of the dry tropics, the prairies of temperate North America, the pampas of subtropical South America, and the steppes of Asia. These grasslands at best have scattered trees, though they grade into open woodland—many small trees but with big, mainly grassy spaces in between, as in the dry, tropical Cerrado of Brazil. Furthermore, trees are classic keystone species: simply by existing and doing their thing, they create niches where other creatures can live. Hence forests create endless scope for small, quick-growing plants—herbs and ramblers—to occupy the ground in between the trees; and a vast variety of plants of all kinds (mosses, liverworts, ferns, and many kinds of flowering plants, including many relatives of the arum lily and of the pineapple, some cacti, and most of the orchids) grow on the trees themselves, as epiphytes. Overall, too, there is more room for small plants than for big ones. Whole, viable populations of small plants may need only a few square yards, while a population of wild trees that is numerous enough to endure will generally need many acres. So although there are tremendous theoretical advantages in being a tree, the species of trees are outnumbered by nontree plants by about five to one. The nontrees live in places where trees cannot—and in the niches created by trees.