The Secret Life of Trees Read online

  Thus life is perforce competitive: hordes of creatures of thousands of different kinds are all after the same things, and most live directly at the expense of others. But it is also, just as inescapably, cooperative. Trees are good competitors. But they are also among the world’s most exemplary cooperators, forming a host of mutualistic relationships for one purpose or another with an enormous variety of different creatures, from the bacteria and fungi that help them to feed to the many, many different kinds of animal that help them with different stages of their reproduction. Trees do not seem to be aware, as dogs and monkeys are aware. They do not have brains. But they are sentient in their way – they gauge what’s going on as much as they need to, and they conduct their affairs as adroitly as any military strategist. Why be ‘aware’ when you can simulate all that awareness brings? They surely don’t think, as animals do. But they orchestrate their fellow creatures nonetheless. A forest is a forest because it has trees in it, not because it may have sloths and toucans or squirrels or chimpanzees. The trees are the prime players and the animals are the dependants.

  The human debt to trees is absolute. Modern evolutionary theory has it that we owe our brains – our art, our inventiveness, and presumably much of our deviousness – to our sexuality. We dance and paint and joke and tell stories to impress potential mates – or such at least was the crude beginning of our wits, on which we have built. But pigs and squirrels and elephants are clever too. They also must attract mates. So why have pigs produced no concert pianists, or professors of jurisprudence? Another ingredient is needed – one suggested a long time ago by more conservative biologists. Our brains and our dexterity evolved together: they are an exercise in co-evolution. Pigs are clever, but their hands are hoofs: nothing there with which to express their dreams and insights. We, by contrast, can translate our thoughts into action: our artefacts (as Robert Pirsig put the matter in Zen and the Art of Motorcycle Maintenance) are ideas in space. Brains are expensive organs (they require a huge amount of energy) and unless they produced some immediate pay-off, natural selection would select against them. But because we have hands (at the end of long, strong, extremely mobile arms), brains do provide pay-offs, manifest not least in a thousand kinds of tools with which to effect further manipulations. Hands provided the encouragement, the selective pressure, to make our brains even brainier; and the growing brains in turn encouraged more dexterity. But the only reason we have such dextrous hands and whirling arms is that our ancestors had spent 80 million years or so (so some zoologists calculate) in the trees. Arboreal life requires dexterity and hand–eye coordination. Squirrels almost became intellectuals, but not quite. Monkeys and apes came closer – but they stayed up in the trees, where they are obliged to squander their fabulous skills just on getting around. Our ancestors, somewhere in Africa, came to the ground when the climate dried up and the trees retreated. They learned to walk on two legs (which no other primate or any other mammal of any kind has learned to do convincingly) and freed their versatile hands and arms for other purposes. Were it not for that pedigree we would remain as intellectually frustrated as elephants and dolphins sometimes seem to be.

  Archaeologists speak of the Stone Ages, and the Bronze Age and the Iron Age and the Steam Age, and now we have the age of the internal combustion engine and nuclear power and space and IT. But every age has been a Wood Age – ours at least as much as any in the past; and perhaps, in the decades to come, even more so. Ice-Age Russians made houses from the bones of mammoths, the Inuit use ice, and the people of the Bronze-Age Orkneys built remarkable villages, with restaurants and mausoleums, from slabs of rock. But great architecture demands wood. The ruins that survive from classical times are all of stone but that’s only because wood rots. Architecture in stone and bricks evolved from timber architecture, and needs wooden-handled tools and wooden scaffold for its construction — and timber roofs and rafters. Wood, in this energy-conscious age, may well begin to replace steel, or much of it, in the grandest buildings.

  Wood was the first serious fuel, too – and human beings clearly learnt the use of fire at least 500,000 years ago, long before we were as big-brained as we are now. No fuel: no smelting – so no Bronze Age or Iron Age or modern machines. No wood: no ships. No ships: no ocean travel – no human beings in Australia, New Zealand or any other island that could not be reached simply by hitching a lift on floating vegetation (as many a beast is thought to have done, from rats to monkeys and tortoises). No ocean travel: no empires: no modern politics. A woodless world would have had advantages. But we could also say no wood: no civilization.

  Yet timber is not the end of it. Trees are the source of drugs, unguents, incense, and poisons for tipping arrows, stunning fish and killing pests; of resins, varnishes, and industrial oils, glues and dyes and paints; of gums of many kinds including chewing gum; of a host of fibres for the rigging and hawsers of great ships (whether made of wood or not) and for the stuffing of cushions – and of course, perhaps above all these days, for paper. All that, plus a thousand (at least) kinds of fruits and nuts and – in traditional agrarian societies – a surprising amount of fodder for animals, including cattle and sheep, which most of us assume live primarily on grass. As a final bonus, the wooden husks of many a tree fruit make instant household pots and drums and ornaments.

  In short, without trees our species would not have come into being at all; and if trees had disappeared after we had hit the ground we would still be scrabbling like baboons (assuming the baboons allowed us to live at all).

  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 bo tree (in 528 BC, tradition has it), but to the birth of humanity itself.

  But 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 onwards, 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’. But it was 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. But most of this book 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; 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 each other, 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 own 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 to 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. But 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.

  I

  What Is a Tree?

  1

  Trees in Mind: Simple Questions with Complicated Answers

  Round-leaved and altogether beautiful: the Judas tree

  ‘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 metres. 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 to 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 favourable, 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’d mode, never more than a twisted stick. Yet it may turn out acorns which, 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? Or then again – a different kind of case – the world’s many kinds of birch 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’, that 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 rainforest – there are trees that form bona fide, big, one-trunked trees when they grow along the banks of the occasional rivers, but become multi-stemmed, 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, or 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 beautiful flaky, yellow/pink/grey-trunked madrone trees of the United States (which add yet more colour 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 of wood. Its stem is formed largely from the stalks of the leaves, and its strength comes from fibres which 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 cocoa 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 each other except that they are all plants. Many plants, in many of those lineages, have independently essayed the form of the tree. Each achieves freedom 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 to 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 which 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’ 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 wild flowers 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.