The Tree (35 page)

The role of auxins in prompting trees to drop their fruits and shed their leaves in autumn (“abscission”) is still uncertain. Levels of a certain auxin drop as the leaves fall, but correlation is not cause. Yet the addition of this auxin can prevent leaf fall; and so it is applied commercially to stop the leaves and berries falling from decorative holly, and to keep oranges firmly on their branches until the pickers are ready. On the other hand, large amounts of another auxin can
promote
fruit drop—and so it is sometimes deployed to thin crops of apples and olives, enabling the remaining fruit to grow bigger.

Finally, another auxin has been modified to make weed killers, basically by promoting overrapid growth. This has great commercial value in agriculture and is not all bad: chemical control can be relatively benign for wildlife, if used selectively and decorously. But an auxin has also been modified to make the infamous Agent Orange, which was used to defoliate trees in Vietnam and thus (or so it was intended) to reveal the Vietcong. The policy was horribly destructive of wildlife as well as of people and was of very limited military use, not least because the Vietnamese army dug themselves in and lived largely underground. Agent Orange also contained dioxin as a contaminant, which causes horrible blistering of the head and body, and is probably carcinogenic. Thus the findings of science may be corrupted. Manufacture of Agent Orange is now banned in the United States.

The gibberellins, also discovered in the 1920s, also promote growth as auxins do—but again, they do many other things as well. In particular, they are highly concentrated in immature seeds and help break their dormancy. Gibberellins too, like auxins, can cause fruits to flesh out even in the absence of seeds. So they are used to produce seedless apples, mandarins, almonds, and peaches, as well as currants, cucumbers, eggplants, and grapes.

Soon after the gibberellins were discovered, abscisic acid (ABA) came to light. In contrast to auxins and gibberellins, its prime role is to suppress cell division and expansion—in other words, to suppress tissue growth. It also serves in seeds to suppress premature germination—germination before conditions are favorable. But despite its name, abscisic acid seems to play very little part in the shedding (abscission) of leaves or fruit.

The fourth of the major plant hormones, the cytokinins, have the opposite effect of ABA: they prompt cells to divide. The cytokinins were first discovered in 1941. They turned up first in coconut milk, but they are now known to occur in all plants. In some contexts cytokinins act in opposition to auxins and override them; so horticulturalists may use cytokinins to make side buds sprout, even when the apical bud is still in place.

The fifth of the basic hormones is ethylene, which affects many different aspects of a plant’s life, from the ripening of fruit to the falling of leaves and a great deal more besides. Ethylene is a chemically simple gas, and it may seem an odd choice for a hormone, for gases are wayward clouds of molecules that seem too unruly for precision work. But then, one of the surprises of recent years has been that the physiology of animals—not least of human beings—is profoundly influenced by nitric oxide, which is even simpler than ethylene. Like ethylene, nitric oxide seems to be involved in just about every system that has been looked at. It is the key even to Viagra—which operates by prompting release of nitric oxide, which, in turn, in this context, acts as a muscle relaxant and so allows engorgement. Ethylene does not act simply as a hormone within any one plant. It may also travel from plant to plant and so acts as a pheromone: an airborne hormone that acts on creatures other than the one that produces it. Whether nitric oxide also behaves as a pheromone, as ethylene does, is a most intriguing question.

Ethylene’s role as a hormone was first revealed in the 1880s, when the trees that lined the streets in many a city began to lose their leaves. German scientists were the first to find the reason: the streetlights were run on gas, and some of it escaped unburned. In 1901 a scientist named D. Neljubov showed that none of the several components of the gas had any defoliating effect, except ethylene. Ethylene was active at astonishingly low concentrations: 0.06 parts per million (or 6 parts per 100 million). On the other hand, the light from streetlights can prompt city trees to retain their leaves, so that the same tree may lose its leaves on the shaded side, but keep them far longer on the side nearer the lamp. City trees in the days of gaslights must have been horribly confused.

Ethylene soon proved to have other effects too. It causes fruit to drop, as well as leaves: and, like auxins, ethylene is used to thin commercial crops of plums and peaches, and to loosen cherries, blackberries, grapes, and blueberries in preparation for mechanical harvesting. Ethylene also promotes ripening, and herein lies another bizarre tale. In the early 1900s growers ripened fruit and walnuts in sheds that they warmed with kerosene stoves. But the more go-ahead growers switched to electricity. This was altogether more satisfactory: cleaner, more reliable, more
modern.
The only trouble was that the fruit no longer ripened. It wasn’t the warmth that did the ripening. It was the ethylene, leaking from the smelly and despised old heaters.

So it is that plants control their form. Darwin wrote of the English wayside as a “tangled bank,” and in the jungle the tangle can be beyond unraveling. Yet each plant in the melee knows what it’s doing. Each contrives to position its leaves in the light, and send its roots to the ground (except for a few specialist epiphyte orchids, which send some of their roots upward), while the vines wrap around others for support. In short, all in the apparent havoc know exactly what they are doing. Each adjusts its shape to the conditions, and to the presence of the others; and all this achieved, it seems, through astonishingly simple chemistry. The workings of trees, like plants in general, are indeed wonderfully elegant.

But trees do not dwell only in the present. They remember the past, and they anticipate the future.

THE PAST AND THE FUTURE: MEMORY AND ANTICIPATION

How
trees remember, I do not know: I have not been able to find out. But they do. At least, what they do now may depend very much on what happened to them in the past. Thus if you shake a tree, it will subsequently grow thicker and sturdier. They “remember” that they were shaken in the past. Wind is the natural shaker, and plants grown outdoors grow thicker than those in greenhouses, even in the same amount of light; and so it is too that parkland oaks grown in splendid but breezy isolation are much more sturdy than those of the forest. Similarly, a larch tree remembers attacks by caterpillars. In the year after an assault it produces leaves that are shorter and stouter than usual. (Larches are among the few deciduous conifers.) Short, stout leaves do not photosynthesize as efficiently as the thinner, longer kind, but they are better at fending off pests. In subsequent years, however, the once-infested larch can and does revert to normal foliage. By then, the population of predatory moths would have plummeted, since the offspring of the original plague of caterpillars (which became moths) would have found nowhere to feed.

Most trees, like most plants of all kinds, are also aware of the passing seasons: what time of year it is and—crucially—what is soon to follow. Deciduous trees lose their leaves as winter approaches (or, in the seasonal tropics, as the dry season approaches) and enter a state of dormancy. This is not a simple shutting down. Dormancy takes weeks of preparation. Before trees shed their leaves they withdraw much of the nutrient that’s within them, including the protein of the chlorophyll, leaving some of the other pigments behind to provide at least some of the glorious autumn colors; and they stop up the vessel ends that service the leaves with cork, to conserve water.

How do the temperate trees of the north know that winter is approaching? How can they tell, when it is still high summer? There are many clues to season, including temperature and rainfall. But shifts in temperature and rain are capricious; they are not the kind of reliable signal to run your life by. Sometimes a winter may be warm—but frost is never far away. Some autumns and springs are freezing, some balmy. The one invariable, at any particular latitude on any particular date, is the length of the day. So at least in high latitudes, where day length varies enormously from season to season, plants in general take this as their principal guide to action—while allowing themselves to be fine-tuned by other cues, including temperature. So temperate trees invariably produce their leaves and/or flowers in the spring, marching to the rigid drum of solar astronomy; but they adjust their exact date of blossoming to the local weather. This phenomenon—judging time of year by length of day—is called “photoperiodism.” Most of the basic research on photoperiodism has been done on crop plants, which for the most part are herbs. But trees and herbs work in the same way. What applies to spinach and tobacco applies to trees too.

Knowledge of photoperiodism again dates from the 1920s, when agricultural scientists in America found that plants like tobacco, soybeans, spinach, and some wheat and potatoes would not flower if the days were shorter than a certain critical number of hours (often around twelve). But other plants would not flower if the days were too long: strawberries and chrysanthemums were among those that remained resolutely sterile if the days were longer than sixteen hours. There were some, though, that didn’t seem to mind the length of day. The three groups became known as “long-day,” “short-day,” and “day-neutral.” Long-day plants generally flower in high summer, and short-day plants in spring or autumn. As a further refinement, plants also seem “aware” that absolute day length has different significance at different latitudes. At very high latitudes, the longest days are twenty-four hours—the sun never sets—and a fourteen-hour day is of modest duration. But in the subtropics, fourteen hours is a long day—as long as any day gets. Sometimes the same species may grow both at high and at low latitudes, including, for example, the aspens of North America. Then the northern ones will treat a fourteen-hour day as short, and the more equatorial ones will treat a fourteen-hour day as long. Adaptation is all.

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

S
IMILAR 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.