The Tree (47 page)

Flavones impinge directly upon our senses, did we but know it. Among them are anthocyanins, the material that, in a myriad slightly different chemical forms, provides plants with a broad palate of pigments, all red but ranging from gentle tints through the brightest scarlet to maroons and purples. Flowers and fruits are often red, of course—but so too, surprisingly often, are leaves. In particular, the young shoots of tropical trees are often red—as are, in the autumn, the dying leaves of many a temperate tree, perhaps most famously the maples that are the pièce de résistance in the glorious autumn colors of New England, one of the greatest natural shows on earth.

Anthocyanin is not produced gratuitously, it seems. It is a secondary metabolite to be sure, but it is no mere accident. Chemically, it is expensive to make. So why is it produced in young tree shoots (especially in the tropics) and in dying leaves (especially in temperate countries)? What do the two have in common?

New work based both in the United States and in New Zealand shows beyond reasonable doubt that anthocyanin is protective. In particular it is an antioxidant. Creatures like us and trees need a constant supply of oxygen, of course, to stay alive. We need to burn sugars to provide ourselves with energy. But oxygen does such a good job on sugars because it is so lively, and we admit it to our bodies at our peril. Left unchaperoned, oxygen gives rise to a whole range of “free radicals” that destroy our flesh and corrupt our DNA—and do the same in trees. Any creature that aspires to live where there is oxygen (anywhere, in fact, that is not some murky swamp) must equip itself with antioxidants, to keep oxygen (or, rather, the radicals it gives rise to) under wraps. In people, these antioxidants include many a vitamin, including C and E. In plants, they include some enzymes and the agents known as phenolics. And they include anthocyanin.

Trees, like all creatures, are most vulnerable to attack by oxygen radicals when under stress. Young leaves, before they have all their chemical and physical defenses in place, are tender. High in tropical trees, exposed to the fiercest sun and sometimes short of water, young leaves are extremely vulnerable. Extra anthocyanin protects them.

But why should dying leaves produce such expensive stuff? Why should they be protected when their number is already up? Precisely because the state of dormancy that temperate trees enter in the autumn is
not
a simple shutting down. Before they pack up for the winter, deciduous trees withdraw as much nutrient as possible from their leaves. Chlorophyll, the principal protein, is broken down, the nitrogen it contains carefully drawn back into the body of the tree, to nourish next year’s growth. But the breaking down is stressful. It opens the leaf to attack by oxygen radicals before the work is complete. So (modern theory has it) trees such as maples produce anthocyanins to keep the oxygen in check, and allow the withdrawal of chlorophyll to proceed in good order.

We might ask, of course, why
all
deciduous plants don’t do as the maple does, why most of them are
not
bright red in autumn. The general evolutionary answer would be that nothing is for nothing. Anthocyanin helps the maple to sort out its problems—but it is expensive. Other trees may simply find that it is more trouble to produce anthocyanins in the autumn than to endure a little damage. Or they may simply not have got around to producing anthocyanins in autumn, for not every species does everything that is theoretically possible.

The general notion that anthocyanin is actively protective fits neatly with a whole range of otherwise bizarre observations from other trees (and other plants) in many different circumstances. Thus it is that when insects bite the horopito tree,
Pseudowintera colorata,
of New Zealand the wound turns red: an outpouring of protective anthocyanin. It also fits the common observation that New England’s autumn colors are most spectacular when the weather is cold or has been so. It’s then, after all, that the trees are under greatest stress. Horopito is a relative of Winter’s bark, in the Winteraceae.

Finally, though, we should note in passing that not
all
autumn color is due to anthocyanin, or indeed to active production of extra pigments. The yellows and browns of autumn leaves seem simply to represent pigments, notably yellow carotene, that are left behind after the tree has stripped its leaves of chlorophyll. They really do reflect innate inefficiency. But we should be grateful for this too. The reds are beautiful; but they are the more beautiful because there are browns and yellows and every other shade alongside them.

There are many more twists as the game of chemistry unfolds. An array of plants has been shown to produce various terpenes only
after
insects have begun to feed on them: another economy. These terpenes discourage the invading pests from laying their eggs. But in addition they also
attract
the natural enemies of the pests, so that they fly in from far and wide to see the pests off: swarms of parasitic wasps or ladybugs summoned to feast on aphids. So far such effects have been shown in maize, cotton, and wild tobacco. I know of no specific examples in trees—but, again, it would be very surprising indeed if they were not to be found. Most herbaceous plants have trees that are close relatives and, beyond doubt, it’s just a question of finding the time and resources to look. Again there are commercial possibilities. Chemicals that summon help from insects that kill pests could surely be developed to protect crops.

Exactly how plants know that they are being attacked, and so tell their genes to produce more insect repellent, is not known in detail. It is clear, though, that one essential ingredient in the chain of chemical communication is salicylic acid, which is widespread among plants (willow bark is particularly rich in it: this is the source of aspirin). One modified form of salicylic acid is methyl salicylate, which is volatile: produced in one plant it can float off in the atmosphere and so affect another. Thus it may serve as a “pheromone,” an airborne chemical signal. This means that a tree that is being attacked can not only induce defenses in other parts of the same tree but can also warn other trees in the vicinity that trouble is afoot. So it is that when elephants in Africa feed from the mopane trees of Africa (
Colophospermum mopane
) they take just a few leaves before moving on to the next tree. Furthermore (so it is claimed) the elephants move
upwind
to a new tree. Evidently the mopane increases its output of tannins as the elephants browse, so the leaves become less and less palatable. Evidently, too, they emit organic materials (the identity of which I do not claim to know: perhaps the tannins themselves) that act as pheromones, and so warn other mopanes downwind that an attack is imminent and they too should produce more tannins. Some acacias are said to behave in the same way, in response to giraffes. We cannot hear the trees calling each to each, as T. S. Eliot claimed to hear the mermaids. But the air is abuzz with their conversations nonetheless, conducted in vaporous chemistry, and the ground too, via the bush telegraph of their roots.

Although it is clear that parasites are damaging, and although trees and their various tormentors go to such extraordinary lengths to overcome one another, we still must ask whether the attacks that we perceive to be so heinous are always as destructive as they seem. Thus, many trees—including those of the Rosaceae and perhaps most dramatically linden trees—are attacked every year by millions of aphids. The trees crawl with them. Linden leaves become sticky with honeydew—the surplus sugary sap that the aphids excrete. The stickiness attracts disfiguring sooty black fungi. Aphids also carry viruses. It seems downhill all the way. But perhaps, says Oxford ecology professor Martin Speight, the honeydew that falls is good for the soil. Leguminous trees such as acacias, as well as nonlegumes such as alder, harbor nitrogen-fixing bacteria in their roots. Many other plants, including many grasses, exude organic compounds from their roots that feed colonies of bacteria that remain free in the soil but fix nitrogen nonetheless. Perhaps the honeydew dripping from the leaves of linden trees has just this effect: creating an environment in the soil beneath where nitrogen-fixing bacteria can thrive. Perhaps, taken all in all, the overall effect of the aphids is neutral—or even beneficial. Other insects, too, may help trees along by their nitrogenous excretions. Termites have nitrogen-fixing bacteria in their guts. Perhaps they bring extra nitrogen to tropical forest trees that are often starved of it, and so on the whole their attacks may be helpful.

Trees that seek to be pollinated by insects must sacrifice nectar, pollen, fats, or parts of the flower itself, all of which are metabolically expensive. Perhaps in the same spirit trees may sacrifice a proportion of their leaves—even half of them in a season—for the sake of soil fertility. Many thrive on marginal soils, after all, and perhaps they would not if it were not for pests, which perhaps are not so pestilential after all. Foresters hate the parasites that reduce growth rate. But does it matter to the tree? Trees, we may assume, are not proud. Does a mahogany
mind
if it develops a bushy top, instead of a fine, straight bole? So long as it is still able to reproduce, why should it? Gardeners of a certain kind typically zap everything that moves with the vilest toxins they can find, and then spend their evenings cutting back the growth they perceive to be superfluous. Why not let the pests do the pruning? Organic gardeners who eschew all vileness often have the best gardens.

However—and it’s a big “however”—the whole equation changes if the conditions change. Pests or diseases that can be shrugged off as part of life’s rich pattern in favorable circumstances, and may even bring net benefit, may be seriously bad news if the tree is under additional stress. For example, and notably, pests are kept at bay to some extent by the simple physical pressure of the sap. In drought, this pressure is reduced. Thus (says Professor Speight) bark beetles commonly make little headway into eucalypts—except in times of drought. Drought, and consequent stress, happen often enough to keep the beetles in business. Contrariwise, being saturated through and through—“waterlogged”—is bad too.

Here is where our own restless and meddlesome species,
Homo sapiens,
plays many a part. We have carted entire battalions of pests around the world to fresh woods and pastures new, to where the native plants have had no time to adapt. “Exotic” pests (and weeds) are often the worst. If it is true (as it seems to be) that the enormous heterogeneity of tropical forest is in part a response to disease—no tree can afford to be too near another of the same kind—then it follows as night follows day that monocultural plantations of tropical trees will be especially vulnerable (as is true of all monocultures). I do not know if mahoganies in plantations suffer more from shoot-borer moth than in the wild, but it seems likely, and we know that many a mahogany grows straight and tall in the natural forest, towering over its neighbors, without any help from pesticide.

More broadly, we are changing the climate. General warming is already enabling many an animal pest to move away from the equator, into the realms of temperate trees that have had no time to adapt to them and cannot, of course, run away. It is impossible to predict in detail how warming the whole globe will affect the climate of any particular spot, but we can be sure there will be plenty more drought, and plenty more waterlogging. Parasites in general adapt to changing conditions many times more quickly than trees can. Some insects have a generation time measurable in days. In some bacteria it is minutes. Either may go through a thousand and more generations while a tree is still feeling its way. Trees are opportunists, but they are slow opportunists. It is the stock in trade of parasites to seize new opportunities very rapidly indeed. The armed but often amicable truce that has evolved over many a millennium between trees and their hordes of parasites could be horribly thrown off balance over the next few decades. There are a few things we can do—some specific controls, some replanting of more resistant species, and every possible attempt to minimize warming. But for the most part, we will just have to wait and see.