The Tree (39 page)

Why should this be so? The explanation was provided most ingeniously by Maureen Raymo, from the Massachusetts Institute of Technology, in the 1980s; she linked the cooling to continental drift. For, as we have seen, about sixty million years ago the tectonic plate that bore the vast island landmass that is now India began to crunch into the south of Asia, and puckered the land in front of it to produce the Tibetan plateau, with the Kunlun Shan and the Himalayas to the north and south. The plateau and the mountain ranges form, as Raymo puts it, “a giant boulder.” The wind sweeps over the Pacific to the south and east, picking up water along the way; and as it hits the high land at the top of India it rises, cools, and releases its water, which falls in the rainstorms known as the monsoons, and runs away in eight great rivers that include the Ganges, Brahmaputra, Indus, Yangtze, and Mekong. The mass of water feeds both the forests and the farms of Asia (although the land left in the rain shadow includes the Gobi, the Mediterranean, and the Sahara).

But the water that falls in the monsoon rains is not pure. Rainwater always contains gases dissolved from the atmosphere, and among the most soluble is carbon dioxide. Thus rain—all rain, everywhere—is a weak solution of carbon dioxide, otherwise known as carbonic acid. As the carbonic acid falls on to the Himalayas and the Tibetan plateau, it reacts with calcium and magnesium in the rock to form a weak solution of bicarbonates. This is washed away in the rivers and into the ocean, where the bicarbonate salts become incorporated with the ocean bed and eventually (thanks to plate tectonics) are thrust down into the magma beneath. The net result is that carbon dioxide is steadily leached out of the atmosphere—and has been leached, storm by storm, for the past forty million years. The dwindling carbon dioxide causes what is sometimes known as an “icebox effect”: the opposite of a greenhouse effect. This may sound fantastical, but sober calculations based on elementary chemistry and the topography of the mountains suggest that it is all eminently plausible.

The Inconstant Sun

There is one more variable. In the early twentieth century the Yugoslav mathematician Milutin Milankovitch sought to find a relationship between the fluctuations in climate and the changes in the earth’s orbit as it circles the sun. In general the earth’s orbit is almost circular—but at times it becomes more elliptical. Ellipses are like flattened circles, pointed at both ends; and when the earth is at the points, it is farther from the sun than it ever is when the orbit is more circular. When the earth is more distant, said Milankovitch, the amount of sunlight striking it can go down by 30 percent, and the climate then is obviously cooler. The orbit passes from near circular to more elliptical in cycles, known as Milankovitch cycles, that last around 100,000 years each. So, said Milankovitch, we can expect periods of relative cold to alternate with periods of relative warmth every 100,000 years.

When the general level of carbon dioxide in the atmosphere is high, the periodic cooling caused by the Milankovitch effect may not be too disturbing. But when atmospheric carbon dioxide is low, the extra cooling can change the world radically. By about two million years ago (thanks to the continued depletion of atmospheric carbon dioxide by the rocks of the Tibetan plateau), the earth was on the point of freezing over. In practice, over the past two million years the earth
has
frozen over, at least partially, every 100,000 years or so. These periodic freezings were the ice ages. On the northern continents, glaciers and mountains of ice in places reached depths of several miles, extending through Europe to southern Britain and France and through North America to what is now New York and beyond. Ice sheets centered on Antarctica encased much of the Southern Ocean and encroached at least into the south of Australia. During the ice ages, the world was cooler, or course. But it was also much drier, as much of the world’s fresh water was locked into ice, and the cold oceans evaporated less freely. The creatures that lived around the poles (particularly in the north) and around the equator were both affected by the combined effects of cooling and drying. But the biodiversity of the north and in the tropics was affected in totally opposite directions.

Why the Tropics Gained and the North Missed Out

In the north, the cumulative effect of advancing and retreating ice, in the roughly 100,000-year cycles, was devastating. In North America, Greenland, Iceland, and Eurasia, as the ice encroached from the north it simply obliterated all the creatures in its path that could not move away and caused all the ones that could move to migrate to the south. Reindeer and lynx, archetypal Arctic creatures, came down into southern Europe. Some species of trees were wiped out. Others scattered at least some of their seeds ahead of the ice, and so their descendants lived on, but farther and farther south. At the end of each ice age, as the glaciers retreated, the remaining creatures could extend north again. The plants generally did so in a fairly orderly sequence. Often the land had been scoured of soil, and the vascular plants had to wait for the hardiest pioneers, like moss, to build it up again. The trees followed in their wake, with the hardiest first: birches, aspens, and pines, then oaks, beeches, and chestnuts. Records of ancient pollen in the north show this sequence, or “succession,” over and over again. Oaks and ashes were typically among the later species to be installed and often became dominant, as in Britain. When any particular group of plants becomes dominant, they and their undergrowth are commonly said to form the “climax vegetation.” In more recent years, however, the classic concept of “climax” has been challenged somewhat. Everything is transient, seen on the geological timescale. Oak forest may be ancient, but it is merely the latest installment in a sequence that’s continuously unfolding.

Clearly, the advancing and retreating ice in the north reduced diversity. As the ice came south, it caused extinctions. The trees that followed it north again were a selection of those that had survived the southern putsch: only the tough could undertake the northward journey at all, and the race was to the swift. Whatever diverse creatures occupied the northern lands before each ice age were weeded out. In the past two million years the weeding has been repeated at least a dozen times.

In tropical latitudes, by contrast, the cooling was less than devastating. Even in an ice age, the equator was still warm. But tropical creatures were affected by the drying caused by the general cooling and the entrapment of water in polar ice. The vast equatorial forests, which in interglacial times were continuous within each continent, became patchy. Patchiness provides precisely the conditions required to produce more species, as different populations become reproductively isolated one from another and each evolves along its own lines. Then, when each ice age ended, the patches expanded again, and the newly evolved species from each different place were brought into contact again—to form astonishingly diverse assemblages of the kind we see now in the Amazon. That, at least, is the theory, and it is highly plausible. We see the same phenomenon among fishes—particularly the cichlid fishes of the great African lakes. In glacial times the lakes were reduced to a series of big ponds, each developing its own cichlid variants, which then reconverged to form the great inland water masses of today. Various human incursions have caused extinctions, but until recent years Lake Victoria had at least three hundred species of cichlids and Lake Malawi had more than five hundred.

In short, both the northern and the equatorial forests felt the effects of the ice ages. But as the ice came and went, the northern forests lost more and more species, while the tropical forests became more and more diverse.

The ice ages, too, had one more dramatic effect. They caused forests to disappear in some places but allowed them to flourish in others. Thus if we could look at the world’s forests throughout geological time we would see them racing over the surface of the globe. The rain forest of Queensland seems to have been there forever. But, like the Great Barrier Reef just offshore to the northeast, it has been there only since the last ice age ended, less than ten thousand years ago. The people of ten thousand years ago were modern, like us. Some people were building cities—Jericho is about that age. Many had long been navigators. Farming was beginning on a settled scale. Doubtless they had priests and paid taxes. At least some of the stories in the Bible and the memories of the ancient Egyptians and of present-day Australian Aborigines extend back that far.

I
T SEEMS TO ME
that all of the ideas outlined above to explain the diversity of tropical forests and the impoverishment of temperate forests could apply at any one time. Each kind of influence could build on the others. Two ideas seem most cogent, however, at least to me. The first is the impoverishment of northern faunas and floras, and the diversification of tropical creatures, by the fluctuations of the ice ages. The second is the grand if simple idea of Dobzhansky’s: that in the tropics the main pressures are biological, while in temperate and boreal lands the stresses are mainly physical. In the tropics the critical pressure comes from parasites, which make it advantageous for any one species to be rare and for the individuals to be widely spaced. In the north, the physical pressures are of various kinds, and only the toughest or the most adept survive. The aspens, jack pines, and coastal redwoods of North America make the point beautifully.

A TALE OF THREE NORTHERNERS

Canada is the world’s second-biggest country, at nearly four million square miles, and more than a third of it is boreal forest. Yet this huge northern forest is dominated by nine species of trees. There are six conifers: jack and lodgepole pines, black and white spruces, balsam fir, and the larch known as tamarack
(Larix laricina).
The three broadleaves are aspen, balsam poplar, and paper birch, which sometimes form pure stands and sometimes are mixed in with the conifers. There is more diversity to the south of the boreal forest (including more broadleaves). The beauty is haunting, but life is hard. Only a few species can cope with the northern winters.

Odd though it may seem, a crucial feature of this coldest of all lands is fire. Those species that are especially equipped to cope with it can steal a very large advantage over those that are less adept. One adept is the aspen, sometimes known as the trembling or quaking aspen,
Populus tremuloides.

Tremulous but Tough: The Quaking Aspen

You would not immediately suspect, if you confronted an aspen in an urban park, that it is among nature’s most resourceful trees. It has a languid air, with wanly fluttering leaves on long, flat stalks, which in autumn turn a melancholic yellow. Its trunks, at least when young, are ghostly smooth and greenish-white (though deeply grooved near the base when older). Yet for all its bloodless foppishness the quaking aspen has the widest distribution of any North American tree, and in large stretches of the far north it is the dominant and at times the only species. Why is this so?

Because of a series of neat tricks, is the answer. The aspen’s whirling leaves resist the wind by not resisting: they ride the blows, go with the flow. It’s wise of the tree to lose them in the winter, when they can do no good. What really counts in the north, though—what ensures that aspens may dominate for acre after acre, when trees that are more obviously shaped for the boreal stresses have long since fled—is their ability to bounce back after fire.

The aspen has long lateral roots, which, at intervals, send up suckers that grow into entire new trees. Many other broad-leaved trees produce suckers, including the paper birch, another pale and ghostly denizen of the boreal forest. But only the aspen spreads itself so wide: an Alaskan ecologist, Leslie Viereck, has found such suckers more than eighty meters from the parent trunk. Fire inevitably strikes the boreal forest sooner or later, and if it occurs in spring or summer it will kill the aspens along with everything else, because it burns the organic matter within the ground, including the aspen’s trailing roots. But if fire strikes in winter when the ground is frozen, or in spring when it is still wet, the roots survive. Then the suckers rapidly grow up to form new trees—rapidly because they already have a vast, established root system to draw on. Thus the mixed forest, probably mostly conifers with a few aspens, may be replaced in months by a grove of aspens.

The overall form of the aspen—lateral roots with suckers—is reminiscent of the calamite trees, primeval relatives of the modern horsetails
(Equisetum),
with their long trailing rhizomes (which are underground roots swollen to form storage organs). All the aspen trees in the grove that springs up around the parent, together with the parent itself if it survives, form a clone: a related group of genetically identical individuals. They all come into flower together, and all shed their leaves together. Since they also remain physically joined (by the lateral roots), some biologists have suggested that the entire grove should be regarded as a single organism; and since it may be hundreds of meters across in all directions, such a grove would rank as one of the largest organisms on earth. It’s been suggested, too, that some groves date back to the end of the last ice age, since they would have been among the first on the scene after the ice melted. Thus, at around ten thousand years, an aspen clone would also be among the oldest organisms on earth.

This is an intriguing thought, worth musing over, but perhaps not to be taken too seriously. The trees that look like individuals in a grove really are individuals, for all intents and purposes. If some accident or outward-bound gardener were to sever the lateral roots that join the trees to their neighbors, each one would happily live alone—just as young strawberries do when they are cut loose from their runners. What the aspens really illustrate is the power of asexual (or vegetative) reproduction. Most vertebrate animals, including human beings, and many plants cannot pass their genes on from generation to generation except through the medium of sex. But many organisms can bypass sex, at least some of the time. They simply generate copies of themselves. Human beings can do this only in Greek mythology. But as the aspens demonstrate, asexual cloning can be a useful trick.