The Tree (4 page)

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, or seed) in the nineteenth century. Cox’s Orange Pippin is only one of many hundreds of apple varieties, each with its own special character: Golden Delicious, Bramley, Jonagold, Arkansas Black, Discovery, and so on and so on. Each of those varieties is simply a clone. All belong to a single species,
Malus domestica.

So how do we answer the very simple question “How many kinds of trees 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. Then again, you may find two patches of aspens (or elms, or willows) that look quite different—and then learn 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 trees there are, he or she may well suggest that the number is virtually infinite, since growers 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 John Stuart 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 that 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 5: metasequoia, the dawn redwood, and
Wollemia nobilis,
regrettably dubbed the Wollemi pine.

But there is also a practical reason for ignorance. Most kinds of trees, like at least 90 percent 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 person-years, 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 Ecuador. With a total area of more than 1.6 million square miles, it is about a hundred times bigger than Switzerland, which is a mere 16,000 square miles. Amazonia is also about sixteen times bigger than the United Kingdom, which is around 94,000 square miles.)

So it is that from the sixteenth century onward 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 on, driven by scholarship and supported by empire and commerce—not least in search of new and valuable crops, of which rubber became the jewel. The 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, traveled six thousand miles 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 Simón Bolívar (1783–1830) commented that “Baron Humboldt did more for the Americas than all the conquistadores.”

The young Charles Darwin loved von Humboldt’s writings and in the 1830s carried the
Narrative
with him on his journey on the
Beagle
that 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 remained 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; 8,000 of his creatures were new to science.

The British explorer 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, one day on the Amazon, aboard the steamer
Monarca,
when 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 four or five million to thirty million or more (though neither figure includes bacteria). Most biologists opt for a compromise of around five to eight 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 trees—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 nonflowering trees, among which the conifers are by far the most important; but there are only about 600 different species of conifers, so they don’t much affect the overall statistics. Thus 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, 60,000 seems a good working number.

Most of those species are in the tropics. Britain may seem to have hundreds of different species of trees, 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 United States and Canada just exceeds six hundred. Yet the New World tropics (the “neotropics”), stretching south from the Mexican border as far as Chile and Argentina, contain tens of thousands of species, sometimes with hundreds of different species per acre. Why the tropics are so much more various is discussed in Chapter 12.

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 plants, 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

W
E 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. If we seek to exploit trees, or to conserve them, or simply to admire and appreciate them as they so richly deserve, we need first and foremost to know who’s who. So first we must try to identify and describe what species are out there. So far biologists have listed nearly two million—perhaps one in four of the total. Then we must ascribe a name to each, partly as an aide-mémoire, but mainly so as to communicate our findings with others. Third, 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 endeavor 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.

How can we make sense of so much diversity?

WHO’S WHO?

Identification is the beginning of all natural history. Nature appears to us as the grandest conceivable theater, 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 identification 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 forests, flowers, which are the principal guide to botanical identification, are usually absent. In seasonal rain forests (with a distinct wet and dry season), many trees gear their flowering to the rains, so flowering is to some extent predictable. But much rain forest (as in much of Amazonia) is nonseasonal, 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 one another. But what those signals are is unknown, at least to us. To the human observer, the flowering seems random. In any case, in a tropical forest (at least in a “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 meters 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. Rain-forest trees all face the same kinds of conditions, and have adapted in the same general kinds of way. Rain forests 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, 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 rain forests 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 gray, dappled with lichen and moss.

In a temperate forest you can be fairly sure that any one tree is the same species as the one next to it—or, at least, it will be one of a cast list that is unlikely to exceed more than half a dozen (oak with ash in much of Britain; lodgepole pine with aspen in the northernmost reaches of North America; alder, Scotch pine, and spruce in the Baltic; and so on). But in Amazonia in particular you can be fairly sure that any one tree is
not
the same species as the one next to it. Often there is a third of a mile between any two trees of the same species, and there can be up to 120 different species of tree in any one acre. So the task, often, is to identify an individual tree that may be not much thicker than your arm from the appearance of its bark, out of a total cast of several hundred (or thousand) possibilities—which may well include some that haven’t previously been described, so that there is nothing to refer back to.

In practice, there are three main routes to identification, whether of trees or of any living creature, and in practice botanists and foresters use them all together. The first is to make use of botanical “keys.” These are lists of characteristic features with a series of decision points, which you work through like a flow chart. For example: “Do the flowers of this particular plant have four petals or five?” If four, you move to question X, and go on from there; if five, you go on to Y, and so on. Such keys first became popular in the eighteenth century. The great French biologist Jean-Baptiste Lamarck, best remembered for his pre-Darwinian theory of evolution, was particularly good at devising them. They are very much in the spirit of the Enlightenment.

The second, ultramodern way to identify a tree—or any living creature—is to sample its DNA. Modern scientists working in tropical forests commonly take a bore of cells (a biopsy) from the cambium under the bark, which is the most reliably accessible living tissue in a tree. They then send the sample back to the laboratory (although field tests of DNA are becoming available).

Ultimately, both of these standard methods—the diagnostic keys and the DNA printouts—rely on some kind of central reference point of information. The principal kind of reference point is the herbarium.

A
HERBARIUM IS
a collection of plants or bits of plants, and sometimes fungi, kept in a preserved state. Flowers and leaves predominate—dried and pressed between sheets of absorbent paper, then mounted on good-quality paper. Seeds can commonly just be kept cool and dry; some will remain not only intact but viable (able to germinate) for decades or even centuries. Wood and bark, too, last forever if clean, cool, and dry. A few technical refinements may be needed, including the odd fungicide and insect repellent (herbaria typically smell of mothballs), but in essence the methods are childishly simple—and, indeed, flower pressing has been a favorite childhood pursuit ever since the first known herbarium was founded in Italy in the sixteenth century. Typically, these days, all the specimens are stored in steel cupboards as in a vast locker room: an austere environment.

Despite the innate simplicity, the world’s herbaria collectively are among the world’s most valuable possessions, albeit largely out of the public eye: a summary of botanical knowledge, from all parts of the world, stretching back several hundred years—for there are still at least twenty herbaria in Europe that date from the sixteenth century. Some herbaria contain vast, general, global collections, such as the one at the Royal Botanic Gardens, Kew, in west London; Kew is one of the world’s oldest botanic gardens dedicated to science, as opposed simply to collecting plants. But all, including Kew, specialize to some extent, either on particular regions or on particular groups of plants. Thus, within the United States, Yale’s herbarium in the Peabody Museum holds about 350,000 specimens, mainly from the States. The Plant Resources Center at the University of Texas at Austin has more than a million specimens—about a quarter of which come from Texas itself, and largely focused on the daisy family, Asteraceae. Harvard holds 5 million specimens of plants and fungi—including 33,000 specimens of wood. Most herbaria contain several or many specimens of the same species, and most contain many species that are also held in other herbaria. This is important, both to show the range of variation within any one species and for security: if any one herbarium was destroyed (flood, war, fire, whatever) most of the species it contains will probably be represented somewhere else. Absolutely irreplaceable, however, are the “type specimens”: examples of the first ever examples of a particular species, as their discoverer first found and described them. Most of the major herbaria have their share of type specimens. Alas, too, such has been the rate of extinction this past few centuries that many type specimens are of species that no longer exist.

Keeping track of all the specimens within any one herbarium is a vital and major task—as great and important as collecting them in the first place. Keeping track is a large part of the curator’s job. An even greater challenge is to coordinate all the information contained in all the world’s many hundreds of herbaria. This is made easier in this age of computers and the Internet, but it still requires hundreds more person-years to complete—and indeed the task can never be finished, since many of the world’s herbaria are still expanding, and all the active ones are constantly being revised. Cross-comparison and constant revision often reveal that the same plant has been given several different names in different herbaria, or that many different plants have sometimes been bundled together under the same name. So even if the herbaria did not expand further, there are many years of work to be done just to sort out what’s there already. But such glitches are inevitable in a global endeavor of this magnitude. Between them the world’s herbaria are a truly fabulous source of knowledge; and they demonstrate that despite all appearances, human beings, when they put their minds to it, can work together wonderfully and achieve what no one person or society could create on their own.

T
HERE IS ALSO
a third way to identify plants—and this is simply to
know.
This is how we recognize friends and family. Recognition is unconscious; and the unconscious does not need to check out the short list of diagnostic features that the key maker or the forensic image makers must refer to but takes into account a dozen (or a hundred?) secondary cues—quirks of speech, expression, and so on. Many people grew up in tropical forests and some of them recognize trees from the feel of their leaves, their scent, or, indeed, the texture of their bark, as readily as any of us recognize our cousins and our aunts. In Brazil these indigenous experts are called
mateiros.
Mike Hopkins of EMBRAPA (Brazil’s research center for forestry and agriculture) told me that he has sometimes known
mateiros
to disagree with specialist botanists. Typically, the botanist says that two similar trees are the same, while the
mateiro
says they are different. Resolution becomes possible when DNA from the cambium is analyzed or the trees bear flowers and fruit. In such disputes, says Dr. Hopkins, “I have never known the
mateiros
to be wrong.”

The
mateiros
are wonderful, but they have their limitations. A
mateiro
who is supremely at home in one place may be less at ease in another, which may well have a different complement of trees. Then again, the subtleties of the subconscious are not easily conveyed. It’s important that everyone who visits the forest with a serious purpose—scientist, environmentalist, forester—should be able to identify what they see. Serious travelers may also want to identify each species accurately, especially if they truly aspire to be connoisseurs. But in a tropical rain forest, where there are so many different types, most of us are content to identify roughly at the level of the family—as in “related to mahogany” or “probably one of the Lauraceae.” For professional foresters, however, harvesting from the wild, correct identification of particular species is the difference between riches and failure. It is vital for the trees, too. If the wrong ones are cut down by mistake then entire species may be driven to extinction. To convey the information formally and reliably, we have to go back to basic diagnostic features that can easily be described: whether the leaves are alternate or opposite, the texture of the bark, and so on. In short, identification needs formal field guides and/or keys, and herbaria in scholarly centers where difficult material can be assessed definitively, and preferably a lab for molecular biology
—and
the
mateiros.
Sometimes all are available. Sometimes not.