Extract from The Secret Life of Trees by Colin Tudge
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 bonsaid 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 kind of birch form the genus Betula. None are as huge as an oak may sometimes 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 plants - 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.