by Colin Tudge
Acacias show a similar picture. Acacias are those lovely, lonely, sprawling trees of tropical grasslands worldwide that provide such essential shade and fodder for giraffes, camels, gazelles, and the domestic cattle and goats of nomadic pastoralists. Acacia is a huge and messy genus with 1,300 species – which should probably be further subdivided, perhaps into five or more different genera. Be that as it may, the basic haploid number of the whole group is 13, so the default diploid number is 26, but there are polyploids with up to 208 chromosomes – sixteen times the haploid number. In some of these, it’s clear that the ancestor simply doubled (and then sometimes redoubled) its chromosomes. Others clearly arose as polyploid hybrids.
In birches, the haploid number is 14, so the diploid number is 28 – but some species have up to 112 chromosomes, which means they are octoploid: and there are some aneuploid hybrids in cultivation. In northern Europe, the silver birch, Betula pendula, and the downy birch, B. pubescens, can look very similar, and some have suggested they are the same species. But silver birch is a diploid with 28 chromosomes and downy birch is a tetraploid with 56. Downy birch presumably arose from silver birch, but now, following polyploidy, it is very clearly a separate species. Alders, too, show much the same kind of thing. Clearly the variety depends in part on past hybridization of what had been separate species.
How many more species of trees will turn out to be hybrids or polyploids, or fertile polyploid hybrids? Another century or so of serious study will throw a great deal more light. Science takes time.
There is one final set of complications. If different populations of trees become isolated one from another, then eventually they may evolve into separate species. But in the shorter term, the separated populations may remain similar enough to breed easily together – that is, they are still the same species – and yet become genetically distinct to some extent, and may look different. Then biologists say that the two populations are different ‘races’ or ‘varieties’ of the same species; and if the variety is really distinct, they may call it a ‘subspecies’. Varieties of plants that arise through informal selection on traditional farms are called ‘landraces’. Domestic varieties of plants that have been produced through formal breeding programmes are called ‘cultivars’ (and domestic races of animals are called ‘breeds’).
Sometimes, both in the wild and in domestication, ‘variety’ simply means a subset of the species. Among domestic crops, the different varieties of runner beans are of this kind: subsets of the runner bean species as a whole, but breeding sexually (by seed), and genetically still diverse. But many plants also reproduce vegetatively as well, by means of bulbs or tubers – or, as with many trees, by suckers from the stem or roots. A tree produced vegetatively in the wild may remain attached to its parent so that parent and offspring together form an entire copse (as in English elms, or groves of giant redwoods). Indeed, the parent tree and the offspring that grow from its suckers may cover many hectares, as in the aspens of Canada. Growers and foresters often reproduce their favoured trees by cuttings, which of course they separate from the parent. Whether they are separated from the parent or remain attached, all the offspring that are produced vegetatively are genetically identical with each other, and with their parent (who of course is a single parent). All the offspring are then said to be ‘clones’ of each other, and of the parent: and the whole genetically identical group is collectively called a ‘clone’.
Thus among apples, all the Cox’s orange pippins there have ever been are a clone: cuttings of cuttings of cuttings that were taken from the first ever Cox’s orange pippin tree that was produced (from a tree grown from a pip) in the nineteenth century. Cox’s orange pippin is only one of many hundreds of apple varieties, each with its own special character – Egremont Russet; Bramley; Beauty of Bath; Worcester; Discovery; and so on and so on. Each of those varieties is simply a clone. All belong to the single species, Malus domestica.
So how do we answer the very simple question, ‘How many kinds of tree are there?’ Well, in the wild (as in cultivation) you may find that what you construe to be different ‘kinds’ are indeed different species; or they may be different varieties of the same species; or they may be hybrids of other pairs of species – hybrids that in the fullness of time may be perfectly capable of hybridizing again with some other, apparently quite separate, species. But then again, you may find two patches of aspen (or elm, or willow) that look quite different – and then find that each patch is simply a clone; and that the two clones are really from the same species and might even have arisen from seeds produced by the same parents. And if you ask a grower or a forester how many kinds of tree there are, they may well suggest that the number is virtually infinite – since they regard each of their cultivars as distinct, and know that there could be as many different kinds as breeders care to produce.
So let us be more specific and ask with what surely is irreducible simplicity, ‘How many species of trees are there?’ At this point the biologists must surely stop prevaricating and provide a clear answer. But the only honest answer is: ‘Nobody knows.’
STILL COUNTING
In truth we can never know for sure how many species of tree there are. As J. S. Mill pointed out in the nineteenth century, it is impossible to know, in science, whether you know everything there is to know. However much you know, you can never be sure that nothing has escaped you. With trees, there are many good reasons to think that a great deal has escaped us. Every so often some highly conspicuous tree turns up which either has never been seen before, or is known only from fossils, and has long been presumed extinct. Two classic examples are discussed in Chapter 4: Metasequoia, the dawn redwood and Wollemia, regrettably dubbed the Wollemi pine.
But there is also a practical reason for ignorance. Most kinds of tree, like at least 90 per cent of organisms of all kinds, live in tropical forests, and tropical forests are very difficult to study – largely because there are so many trees in the way. It requires hundreds of personyears, and heroic years at that, to list the species even in relatively small areas of tropical forest; and despite the best efforts of legal and illegal loggers, the tropical forest that remains to us is still mercifully vast – so that all of Switzerland, for example, could easily be lost in Amazonia. (Amazonia is the forest that surrounds the Amazon River: it occupies the western half of Brazil, and extends into Peru, Colombia, Bolivia and Venezuela. With a total area of more than 4 million square kilometres, it is about a hundred times bigger than Switzerland, which is a mere 41,000 square km. Amazonia is also about sixteen times bigger than the United Kingdom, which is around 240,000 square km.)
So it is that from the sixteenth century onwards a succession of naturalists-cum-conquistadors, administrators, soldiers, traders and priests, became obsessed with the flora and fauna of tropical America, and set out to identify, describe and collect what was there. Dedicated research expeditions were mounted from the eighteenth century, driven by scholarship and supported by empire and commerce – not least in search of new and valuable crops, of which rubber became the jewel. Greatest of all the explorers, so many believe, was the German Alexander von Humboldt who, together with the French physician and amateur botanist Aimé Bonpland, travelled 10,000 kilometres in South America between 1799 and 1804, on foot and by canoe. They collected 12,000 specimens of plants, including 3,000 new species, and hence doubled the number known from the western hemisphere. On their return they published the thirty volumes of Voyage aux régions équinoxiales at von Humboldt’s expense (it cost him his entire fortune), of which von Humboldt wrote twenty-nine volumes and Bonpland contributed just one, although von Humboldt insisted that they share the authorship of the whole. The book was first published in English between 1814 and 1829 in five volumes as Narrative of Travels to the Equinoctial Regions of the New Continent during the years 1799–1804. The great revolutionary Venezuelan general Simon Bolivar (1783–1830) commented that ‘Baron Humboldt did more for the Americas than all the conquistadores’.
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The young Charles Darwin loved von Humboldt’s writings, and carried the Narrative with him on his journey on the Beagle in the 1830s which changed his own life and went on to change the world. The Narrative also lured Alfred Russel Wallace to the Amazon, to which he set sail in 1848 with Henry Walter Bates, an inspired amateur collector of beetles. Wallace stayed for four years before malaria and gut trouble forced him to return to England – although he set off to the Malay archipelago a couple of years later, in 1854, and stayed for eight years. Bates stayed in the Amazon for eleven years and among other things described a form of mimicry in which innocuous and tasty butterflies are protected by their wondrous resemblance to other butterflies that are noxious and toxic. He also collected an estimated 14,712 species from Amazonia, including 14,000 insects, and 8,000 of his creatures were new to science.
The Yorkshireman Richard Spruce (with whom Wallace corresponded from Malaysia) stayed in South America even longer than Bates – for fifteen years – and gathered more than 30,000 specimens from 7,000 species. Spruce, Wallace, Bates, von Humboldt, Bonpland and many more were iron men, obsessively collecting, bottling, pickling, pinning, pressing and drying for year after year, always recruiting the help of the local people who were and are naturalists par excellence because their lives depend on knowing the creatures around them. Yet I believe that Spruce spoke for all of them when one day on the Amazon, aboard the steamer Monarca, he wrote: ‘There goes a new Dipteryx, there goes a new Qualea – there goes a new “Lord knows what.”’ All that effort over many years provided but a glimpse of what was out there.
Now, of course, the solo naturalists, the upper-middle-class (though far from rich) von Humboldt, the upper-middle-class (and significantly rich) Darwin, and the self-made artisan collector-naturalists like Wallace, Bates and Spruce, have been replaced by teams of scientists from the world’s great universities and government institutions, relentlessly quartering the Amazon and everywhere else and systematically recording all there is. Now, we might suppose, all is more or less sewn up. In truth, a century and a half after Spruce, his lamentation seems almost as cogent as ever. We have very little idea indeed what’s out there. Estimates even of the total number of species in the world as a whole differ by an order of magnitude, from a possible 4 or 5 million to 30 million or more (though neither figure includes bacteria). Most biologists opt for a compromise of around 5 to 8 million. After several hundred years of conscientious natural history and a century of formal science the task even of listing all there is seems hardly to have begun. Nature is very big, and very various indeed.
Thus it is impossible to count all the different species of tree – or to be sure that they have all been counted. But biologists can at least guess.1 Extrapolating from what is known, they estimate that there are around 350,000 species of land plants in general. At least 300,000 of them are flowering plants. Around one fifth of these are trees. There are also some non-flowering trees, among which the conifers are by far the most important; but there are only about 600 different species of conifer, so they don’t much affect the overall statistics. So there are probably around 60,000 species of trees in the world, plus quite a few thousand hybrids; although any of the species or hybrids might be further subdivided into an indefinite number of wild races or cultivars. Sixty thousand seems a good working number.
Most of those species are in the tropics. Britain may seem to have hundreds of different species of tree, but most of them have been imported by human beings. Only thirty-nine are believed to be true natives (and one of them, the common juniper, may in fact have been brought in by ancient people). The vast boreal forests of northern Canada are dominated by only nine tree species – the quaking aspen, and a handful of conifers. The total of trees that are native to the US and Canada just about exceeds 600. Yet the New World tropics (the ‘neotropics’), stretching south from the Mexico border as far as Chile and Argentina, contain tens of thousands of species, sometimes with hundreds of different species per hectare. Why the tropics are so much more various is discussed in Chapter 11.
Meanwhile, two more immediate questions present themselves. First, how on earth can anyone – the most astute of hunters and gatherers, or the most learned of professors – keep tabs on 350,000 or so species of plant, including around 60,000 trees? How can we begin to comprehend so many? Secondly, how did the enormous complexity that is entailed in being a tree come about? These matters are addressed in the next two chapters.
2
Keeping Track
How can we make sense of so much diversity?
We share this world with millions of other species, and engage directly with many thousands of them – for food, shelter, medicines, aesthetic pleasure, and sometimes just because we need to stay out of their way. At least at a few stages removed, all of them affect us so some extent, and we in turn affect them. For the purposes of both exploitation and conservation, we need to know who’s who. So first we must try to identify and describe what species are out there – and so far biologists have listed nearly 2 million; perhaps one in four of the total. Then we must ascribe names to each, partly as an aide-mémoire, but mainly so as to communicate our findings with others. Thirdly, we must classify: place the creatures we have identified into groups, and then nest those groups in larger groups, and so on. Without classification, naming becomes ad hoc, and we could not hope to keep track of more than a few hundred different kinds, and probably a lot fewer.
The reasons for all this endeavour are not purely practical. Science is an aesthetic and spiritual pursuit. The more that is revealed, the more wondrous nature becomes. The more we know about living creatures, the more deeply we can engage with them. This is the appetite, as Hamlet said, that grows from what it feeds on.
But the problems of identification, naming and classification are many and diverse. This after all was the first task that God gave to Adam (Genesis 2: 19), and although a lot of Adam’s descendants have been hard at it ever since, there’s still an awfully long way to go.
WHO’S WHO?
Identification is the beginning of all natural history. Nature appears to us as the grandest conceivable theatre, endlessly unfolding. There can be no understanding at all until we have at least some inkling of the cast. We must be able, again to quote Hamlet, to tell a hawk from a handsaw.
But identification can be difficult for all kinds of reasons – even of trees, which are so big and conspicuous, and which do not run away. We have already seen the practical problem posed by some willows: that both leaves and flowers may be needed for identification, but the two may not be present at the same time. Yet whatever problems may confront us in temperate climes, we can be sure that the tropics will pose far worse. In tropical forest, flowers, which are the principal guide to botanical identification, are usually absent. In seasonal rainforest (with a distinct wet and dry season), many trees gear their flowering to the rains, so flowering is to some extent predictable. But much rainforest (as in much of Amazonia) is non-seasonal, and trees may flower at any time. To be sure, different trees of the same species generally flower simultaneously, for if they did not, they could not pollinate each other. So they must be responding to signals from the environment at large, or else (or in addition) they must be communicating with each other. But what those signals are is unknown, at least to us. To the human observer, the flowering seems random. In any case, in tropical forest (at least in ‘secondary forest’, which is forest that is regrowing after previous harvesting or clearance) the trees grow close as a football crowd, and most are remarkably thin, like poles, and grow straight up and disappear into the gloom, twenty metres overhead. Even if there are flowers, you won’t necessarily see them.
The leaves may not be too helpful either, at least when viewed from the ground. Rainforest trees all face the same kinds of conditions, and have adapted in the same general kinds of way. Rainforests are wet by definition. But in some there is a dry season, and even when there isn’t it doesn’t rain all the time. The
forest floor may be moist right enough, but the topmost leaves of the canopy are far above it, and are exposed to the fiercest sun. I have spent time on towers in quite a few rainforests and remember in particular in Queensland, near Cairns, how lush and green it all was on top – but also how uncompromisingly desert-like it felt. So the uppermost leaves must resist desiccation. Yet from time to time, and in due season every day, they must also endure tremendous downpours. Leaves that can cope with such contrasts tend to be thick and leathery (to resist drought), oval in shape, and have a projection at the end like a gargoyle, known as a ‘drip tip’, to shoot off surplus rain. Many hundreds of trees from dozens of only distantly related families have leaves of this general type. But even if you can see the leaves, it is hard to be certain if they belong to the tree you are interested in or to the one next to it, or to some epiphyte or liana slung over its branches. Often, in short, you have nothing to look at but bark. The trunks of tropical trees are sometimes highly characteristic, deeply furrowed or twisted like macramé, but in most species the bark is simply smooth and grey, dappled with lichen and moss.
