The Tree
Page 30
In the late 1930s it became clear that plants do not measure the length of the day but of the night. If the light is turned on even briefly during the night—a minute from a 25-watt bulb would do—short-day plants such as strawberry will not flower. Contrariwise, a long-day plant that flowers in sixteen hours of light and eight hours of dark will also flower with eight hours of light and sixteen hours of dark—if the darkness is interrupted by a brief light. In truth, long-day plants should be called short-night plants; and short-day plants are really long-night plants.
In the next few years the underlying mechanism became clear—and again it is remarkably simple. Inevitably it depends on a pigment—for pigments by definition are chemical agents that absorb and reflect light, and so mediate a plant’s (or an animal’s) responses to it. In this case the pigment is phytochrome. Phytochrome exists in two forms, to either suppress or promote flowering; and light flips them from one form to the other.2 Again, these insights have been put to commercial use. Growers of chrysanthemums used to keep the lights in the greenhouse on at night to delay flowering until Christmas—until, in the 1930s, they saw that a brief burst of light at night would produce the same effect, and much more cheaply. Contrariwise, appropriate flashes will bring long-day plants rapidly into bloom, by artificially shortening the nights.
All these mechanisms are evolved—they have been shaped by the experiences of past generations. They can succeed, and serve the plant well, only if present and future conditions are like those of the past. If conditions change slowly over time, then any lineage of creatures, animals or plants, can adapt to the change. But if conditions change rapidly, then creatures that have evolved their survival strategies in earlier and different times find themselves caught out.
Human beings are changing the world profoundly and—by biological standards—with extreme rapidity. In particular, we are altering the climate. Present-day pines and oaks and birches in northern latitudes are adapted to the idea that long days are warm and short days are cold. Everything they do—germination, dormancy, the shedding of leaves (in the deciduous types), the production of flowers and cones—is geared to this assumption. If long days turn out to be cooler than expected, or significantly hotter, drier, or wetter, and if the cold days are not particularly cold, the whole life cycle can be thrown out of kilter. The confusions of urban trees, when light and temperature are out of sync, are just a warning of what may happen to all the world’s forests when the interplay of light, warmth, and moisture is altered on the global scale. If plants are seriously incommoded—and this applies to both wild trees or farm crops—everything else must suffer too. Of all the threats to the present world, this is the one that matters most. Yet, as discussed further in Chapter 14, the effects of climate change on plants are extraordinarily difficult to predict. The insights of modern science are wonderful, but absolute knowledge is a logical impossibility. In the end, we are just going to have to wait and see.
This, then, in broad-brush terms, is how plants keep themselves alive. But as living creatures they need to carry out two more tasks. They need to reproduce; and they need to get along with their fellow creatures, of their own and other species. How they do this is discussed in the next two chapters.
12
Which Trees Live Where, and Why
SIMILAR PLACES ALL THE WORLD over pose similar kinds of problems—of light, dark, heat, cold, flood, drought, altitude, toxicity—and all the many varied trees that live and evolve in any one place tend to come up with the same kinds of solutions. Thus the Douglas firs, pines, and spruces of the extreme north and the rimus and kahikateas of New Zealand’s south are all tall and steeplelike, to catch the light that comes at them from the side; while the cedars and umbrella pines of the Middle East and Mediterranean have flat tops, aimed at the sunlight beamed from overhead. The trees of tropical rain forests grow straight up through the crowd, while those of the Brazilian Cerrado, the African savannah, or the Australian bush spread themselves like cats. So it is that all the world’s forests conform to a score or so of different ecotypes—variations on a theme of boreal, temperate, or tropical; wet forest (rain forest) or distinctly dry; seasonal or aseasonal—where seasonal means winter-summer, or wet season–dry season. Within this general framework are a series of specialisms. There are forests that follow rivers (“riverine,” sometimes known as “gallery” forests). Those in mountains are called “montane.” At moderate heights they are “alpine”; but in some wet, warm places, as in much of Southeast Asia, the trees become lost in mist toward the tops and so become “cloud forest.” Some forests have their feet in water: swamp forests, with willows, alders, swamp cypresses, and the rest; and mangrove forests, at the edge of tropical, shallow seas.
Yet no two forests are alike. They are like art galleries: they all have pictures, but they don’t have the same pictures. The forests of Southeast Asia are rich in dipterocarps. Eucalypts are virtually confined to Australia—or would be, were it not for human beings, who have planted them virtually everywhere. Africa and Australia both have acacias in their wide open spaces—but they are different acacias. America, China, and Europe all have plenty of oaks—but each has its own selection. Oaks and willows in general (with very few exceptions) are confined to the Northern Hemisphere. Southern beeches (Nothofagus) are indeed inveterately southern. Araucarias too, at least in these modern times, belong exclusively to the south. Some species—and, in fact, some genera or even families—grow only on particular islands, to which they are then said to be “endemic.” New Caledonia has thirteen endemic species of Araucaria out of a world total of nineteen. Madagascar has six of the world’s eight species of baobab, and is the only place with the extraordinary trees of the Didiereaceae. Britain, on the other hand, has a miserable native list of only thirty-nine species, none of which are endemic. All of Britain’s natives occur elsewhere as well, mostly in much larger numbers than in the United Kingdom. Of course, lists of “British” trees may contain hundreds of species, many growing wild; but the vast majority are imported. The British are supremely acquisitive.
California’s coastal redwoods get much of their water from mist.
So the first question is “Why?” We would expect each region to contain plants that are adapted to it—for if they were not, then they would soon be ousted by those that are. But why does each region have its own characteristic suite of native species? Why are some species (or genera or families) very widespread, while others are confined to single islands? Why are some islands rich in endemics (New Caledonia, Madagascar, Hawaii, the Canaries) while others (like Britain) have none?
There’s another kind of puzzle, too. Whatever group you look at—birds, butterflies, fish—you find there are many more species in the tropics than in the north or south; indeed, the farther you travel from the equator, the more the variety falls off. With trees the falling off is striking. The apparently endless boreal forest of Canada is dominated by only nine native species: a few conifers and the quaking aspen. The United States as a whole has around 620 native trees. India (much smaller than the United States) has around 4,500. In the Manu National Park of Peru, almost on the equator, twenty-one study plots with a total area of thirty-seven acres have yielded no fewer than 825 species of trees—about one-fifth the total inventory of all India, and considerably more than the United States and Canada combined. As we saw in Chapter 2, the Ducke Reserve of Amazonia has more than 1,000 different trees. Tropical America as a whole, from Brazil, Peru, and Equador up to Mexico, has tens of thousands of species. The true number can only be guessed. Why so many?
Both kinds of questions have been exercising biologists for several centuries (at least) and are still a hot topic: I attended the latest international conference on these matters at the Royal Society in London in March 2004. Hundreds of putative explanations are out there that between them encompass every aspect of the life sciences—and of the earth sciences, too. Some have to do with plant physiology, some with genetics, some with history, some
with evolutionary theory. All are pertinent; all, indeed, are interwoven. The following is a rough guide to the main threads.
WHY TREES LIVE WHERE THEY DO
Each lineage of trees began with a single tree: the first ever oak, the first ever kauri, and so on. So—to begin at the beginning—where did those “founders” arise? What is the “center of origin” of each species (or genus or family or order)?
It’s at least commonsensical (and we have to start somewhere) to guess that the founders arose in the places where their descendants now live in the greatest variety. Eucalypts are extraordinarily various in and almost exclusive to Australia, and there, surely, is their most likely origin. But life is not so simple. Oaks, for instance, span the Northern Hemisphere and are most various in both North America and China—which are divided by the Pacific if you go around one way, and by the rest of Eurasia and the Atlantic if you go around the other. Even if we assume that oaks arose in either North America or China, they must at some point have traveled to the other distant continent. But if they can make such a journey as that, might they not have begun in the middle, in Europe? Or could they have begun in some completely different place, where they no longer exist, such as Africa? Either way, it’s clear that the center of origin, even if we can work out where that was, does not by itself explain the present distribution. Clearly some trees in the past—perhaps most of them—began in one place and then dispersed to others. If they found their new locations congenial, they could then have formed entire new suites of species—so that these outposts then become secondary centers of diversification. Sometimes, too, the secondary outpost might be the kind of place that encourages the formation of new species. Thus there are many different pines in Mexico, but we need not assume that this is where they first arose. They are diverse because the first to arrive there found it congenial and the mountains provide many different niches where semi-isolated populations can each evolve along their own lines. Just to confuse the picture a little more, any particular lineage of trees might well be extinct in the place where it first arose. The places where particular trees now flourish may well be secondary outposts—or, indeed, outposts of outposts, or outposts of outposts of outposts.
“Diversity,” though (like all terms in biology), has various connotations. In this kind of context, it should not be measured purely in terms of number of species. We need to see how different the various species are, one from another—which is where molecular studies (of DNA) come into their own. Thus it may transpire that the twenty or so species of oaks, or pines, or whatever in place A all have very similar DNA. Place B may have only half a dozen species, yet the difference in their DNA may be profound.
It would be reasonable to conclude that the species in place A all arose from a single ancestor, who arrived there fairly recently, found the place agreeable, and diverged rapidly (and perhaps rehybridized, as outlined in Chapter 1). But the greater genetic diversity found in place B could be explained in two different ways. Perhaps all the trees did indeed arise in situ from a single founder, who arrived or originated in that place a great many years in the past, giving its descendants plenty of time to diverge. Or perhaps at least some of the very distinct trees originated in other places, and simply converged on the place that’s being studied. But then we can ask a further question. Is it possible to infer from their DNA which species in any particular family (or which family in any particular order) is the most primitive—this being the one that seems to have most in common with the original ancestor? Common sense suggests, then, that sites that have the greatest true diversity of species (the greatest variation in DNA) and/or include the species that are known to be the most primitive are at least reasonable candidates as the true center of origin. Of course, such a site could just be an ancient secondary center of diversity. The trees might be completely extinct in the place where the group truly arose.
This is where fossils come to our aid. In more and more sites all around the world, palaeontologists are now finding fossils of truly astounding quality that reveal the structure of ancient plants in the most minute detail. Pollen is particularly informative. It is highly characteristic, and often allows identification at the level of the genus. It is also very enduring, often to be found in the deepest mud beneath lakes, or in rock that derived from the mud of lakes that are now long gone. Pollen is to palaeobotanists what teeth are to scholars of ancient mammals. Fossil and subfossil pollen sometimes provides a continuous record of ancient floras over tens of millions of years.
Fossils can be deceptive, however. Fossilization is a rare event. The oldest fossils known of any particular group do not necessarily represent the very first of that group. Indeed—given that all groups are rare in their early stages—the oldest known fossils are most unlikely to represent the first that actually existed. Neither do the latest ones known necessarily represent the most recent. The most recent fossils of Metasequoia and Wollemia are both millions of years old; yet both these trees have proved to be alive and well, in China and Australia, respectively.
But fossils do give us some certainties. If a fossil of a particular tree turns up in a rock that’s 100 million years old, we know that that tree did indeed live in that place, and that its species was there at least 100 million years ago. Thus we know for sure that the family Araucariaceae, now confined to the south, did once live in the Northern Hemisphere. Contrariwise, the absence of southern beech fossils in the Northern Hemisphere does not prove that the southern beeches never came north of the equator. As the adage has it, “Absence of evidence is not evidence of absence.” Even so, the fact that thousands of trawls from hundreds of sites over many decades have failed to produce any southern beech pollen in the north is at least a strong suggestion that they have always been out-and-out southerners. Southern beeches, presumably, really did arise in the Southern Hemisphere.
But there is another huge complication. Conifers as a whole first arose several hundred million years ago, and some modern conifer families are well over 100 million years old. Flowering plants as a group are much younger; still, many families of them are tens of millions of years old. Since the time when many plant families began, much of the land on which they stand has moved dramatically.
AND YET THEY MOVED
The first suggestion that the continents are moving around the globe came from the German geologist Alfred Wegener, who in 1915 coined the expression that translates as “continental drift.” Wegener found that if you cut up a map of the world and shuffle the bits around, the existing continents and the big islands, particularly of Australia, Africa, South America, and Antarctica (and Madagascar and New Zealand), fit together like a toddler’s jigsaw. In the north, the eastern coasts of North America, plus Greenland and Iceland, when shoved sideways, abut neatly with the western coast of Europe. The Atlantic is shaped like a snake. The coincidences just seemed too great. Surely, he said, the different continents and islands must once have been joined together, then split and drifted apart. At first, many scientists were thrilled with Wegener’s idea. Then they decided that it was impossible (meaning that they could not think of a mechanism), and most declared that it was obviously ludicrous. By the time of his death, in 1930, he was more or less outcast. Only a few brave hearts supported him.
But the brave hearts turned out to be right. The continents have moved and, measurably, are still moving. The mechanism that drives them began to become apparent after the 1950s. The center of the earth is hot, as had been known for some decades. Indeed, it is so hot that the entire interior swirls with convection currents, like a simmering kettle. The interior rock is the magma that flows out when volcanoes erupt. The continents are made of lighter rock and float on the restless magma like froth on a slow-moving river.
The continents move slowly—only a few centimeters a year—but they have had plenty of time, and over the past few billions of years their peregrinations have been prodigious. Five hundred million years ago (in Cambrian times) the present landmasses were scatter
ed islands. Places now in the tropics have at times been near the poles, and vice versa. Places that are now in the heart of continents have been islands, and present-day islands have been part of mighty continents. Most of what is now North America was an island that straddled the equator. Siberia was a subtropical island in the Southern Hemisphere. And so on.
All this was before any plants had come onto land, and long before there were trees. But by about 265 million years ago (the mid-Permian), when already there were plenty of trees, all the islands had massed together to form one great landmass known as Pangaea. By about 200 million years ago (in the early Jurassic), when the conifers and cycads were in their pomp and the flowering plants were yet to come on the scene, Pangaea began to split more or less in half to form two great “supercontinents”: Laurasia to the north, and Gondwana to the south.
Ever since—through all the time that the flowering plants have been evolving—those two great supercontinents have been breaking up. Laurasia has split to form present-day North America, Greenland, Europe, and most of Asia. Gondwana has fragmented to become present-day Antarctica (a huge continent), South America, Africa, Arabia, Madagascar, India, and Australia, plus a fairly long catalog of today’s islands, including New Zealand and New Caledonia.
The details of this redistribution are roughly outlined in the figure on pages 286–87. Two features in particular are outstanding. Note, first, that South America was an island for tens of millions of years, until it finally made contact with North America, via what is now called Panama, around three million years ago—which in geological time is very recent. India, too, was an island that drifted north for tens of millions of years, until it finally crunched into the south of Asia. The crunch took place about sixty million years ago (not long after the dinosaurs disappeared)—and the impact, slow and inexorable as it was, caused the rise of the Himalayas. (The Himalayas would still be rising were it not for the erosion that tends to keep them at the same height.) Australia is still an island, drifting north. Opinion is divided on whether it will miss Asia altogether, crunch into China, or obliterate Japan.