The Tree (46 page)

In general, then, trees, like all living creatures, have a mixed relationship with their own kind and with all other creatures: part war, part peace, and part uneasy truce. This is true even of the creatures that eat them and cause diseases.

LIFE’S TORMENTS—AND AUTUMN COLORS

All trees, like all plants, are beleaguered from the time they are seeds to the time they are returned to the earth by predators and parasites. Predators in this context means big herbivorous animals, from cattle and squirrels to leaf monkeys; and parasites are loosely defined here to include the viruses, bacteria, and fungi that are commonly known to cause disease, and all the animals such as worms, insects, and mites that burrow into them, and indeed all the insects and other creatures commonly classed as pests. Old-fashioned accounts of ecology tended to pass over the parasites as if they are mere accidents. Yet they are major drivers in all of nature and may determine the shape and direction of an entire ecosystem. We have seen the role of nematodes in the relationship between figs and fig wasps. More profoundly, the need to avoid parasites may largely explain the huge variety of trees in tropical forests: no tree can afford to be too close to another of the same kind, for fear of infection. More cogently still, it may be that if there were no parasites, there would be no sex, and the transformation of all life would then be absolute. It isn’t simply that creatures would live their lives very differently. Without sex to mix the genes, creatures like us (and oak trees and mushrooms) would not have evolved at all. It seems, indeed, that we are as we are, and trees are as they are,
because
our respective ancestors had to cope with disease.

In truth, parasites and other pests do a great deal of damage, and trees seem particularly vulnerable because they must stay in the same place for so long—unlike annual plants, which, metaphorically speaking, are here today and gone tomorrow. Most tree diseases pass most of us by most of the time, but in some cases the disappearance of a species of tree is remarkable. In Britain, for example, everyone became aware of Dutch elm disease, caused by fungi of the genus
Ophiostoma
and carried by various bark beetles. Elms had been one of Britain’s most characteristic trees: the ones most likely to persist in hedgerows, where traditional farmers were happy to retain them for shade and as a future source of timber—a casual exercise in agroforestry. Elms feature strongly in the landscape paintings of Constable, from Suffolk, and they also grew so rampantly in the west country that they were known as the “Wiltshire weed.” But within about a decade, between the 1970s and 1980s, English elms above the size of a small shrub were all but eliminated—one of the most dramatic extinctions in historical times.

Of course, all trees suffer from pests and diseases to some extent. An oak tree typically loses about half of its leaves each year to insects. Caterpillars sometimes take virtually all of the first crop of young leaves in spring, whereupon the oak may respond with a second flush in May and June, known as “Lammas growth.” (Although Lammas, meaning “loaf mass,” is an ancient British Celtic festival—long since Christianized—that celebrates the pending harvest and falls on August 1. Hmm.) Periodically we read of threats of various kinds to oaks or chestnuts in Europe and the United States from fungi or viruses or whatever, until it seems we will soon be lucky to have any traditional species at all.

The world’s two most valued tropical hardwoods, teak and mahogany, both have dedicated pests that beleaguer them in the wild and hugely affect their economy in plantations. Teak suffers primarily from the defoliator moth,
Hyblaea puera,
whose caterpillars may strip the leaves completely almost every year, soon after they emerge. This leaves the trees gaunt and skeletal—teak trees are often a sad sight—and also means they take much longer to reach harvestable size. Thus traditional plantations in India typically raised teak on an eighty-year cycle. Modern selection and cultivation has brought this down to thirty years. But in Brazil, where the defoliator moth mercifully remains absent (the trees left it behind in their native Asia), the cycle of harvest is down to eighteen years (or so Brazilian foresters are hoping). New research in India on biological control promises to deal with the moth at last, but we have yet to see whether it works. Mahogany is plagued in particular by caterpillars of shoot-borer moths, which burrow into the growing tip and destroy it. The tree does not die, but instead of growing straight and true as a prestige timber tree should, it sends out a mass of branches below the ravaged tip, like a bush. The reasons are as described in Chapter 11: the growing bud normally sends out a hormone (an auxin) to suppress such unruly behavior. With the source of the hormone gone, the lesser buds beneath are given free rein. Many other valuable trees worldwide (the cinnamon plantations on Madagascar come to mind) have their own particular murrains that are of huge economic importance.

Trees, like all living creatures, contrive in various ways to make life difficult for their parasites. Commonly, tree leaves are low in nutrients: the parasite has to work prodigiously hard simply to get enough to eat. All trees present physical barriers to would-be predators and parasites, including thick, waxy cuticles on their leaves that inhibit the entry of fungi or bacteria, while deciduous trees plug the scars left by their falling leaves with cork, like Elastoplast. Finally, trees are fabulous chemists. In addition to the proteins, fats, carbohydrates, and other materials they need to synthesize for the everyday tasks of staying alive, they also turn out a huge range of recondite molecules known as secondary metabolites. Clearly these are not essential for day-to-day living. Some trees produce some kinds of secondary metabolites, and some produce other kinds, and some seem to produce very little at all. In times gone past botanists wrote them off as waste or by-products: things the tree produced apparently through carelessness. That is how plants might have produced them first of all, in the deep evolutionary past. Now it is clear that secondary metabolites play many vital roles in the life of the plant—and paramount among them is the repulsion or destruction of would-be predators and pests.

But although pests and predators clearly do cause huge problems, the relationship between trees and their tormentors is not a simple battle. The subtleties are far from understood—the research is difficult, and most studies so far have focused on the pests of herbaceous crop plants, which are easier to work with than trees and offer quicker financial returns. But already we can see that between trees and their parasites there is the same counterpoise of antagonism and collaboration—war, peace, and uneasy truce—that we find in all ecology. Over time we can discern coevolution, as each player in each relationship adapts more and more minutely to the other. When the relationship is antagonistic, this coevolution becomes an arms race, with predators or parasites and prey each upping the ante as the centuries pass. When it is cooperative, the relationship tends to become more intricate with time, until the various players become totally interdependent. The little that is so far known about trees and their parasites already reveals relationships of endless subtlety.

Upping the ante is the first sign of an arms race. So it is that many trees have spines and prickles. But spines and prickles (like cuticles and corky plugs for leaf scars, and all the secondary metabolites) require a lot of energy to produce. So we find that in various ways, trees contrive to be minimalists. Thus, as we noted earlier, many species of palms that live in continental forests where predators abound are spiked as fiercely as the walls of a medieval prison, while related types, on islands free from abuse, are spikeless. So we find too that the leaves of holly are spiny on the lower branches, where they might be browsed by deer and cattle, but tend to be spineless higher up. In general, a plant that can do without spikes and such adornments has energy to spare for other things, like rapid growth—and in a competitive world, other things being equal, it doesn’t pay to waste energy on things that are not necessary.

In their secondary metabolites, too, we see on the one hand a continuous upping of the ante—the trees becoming more toxic, the predators and parasites evolving new ways to cope—but also the constant need, on both sides, to economize.

Among the commonest of the secondary metabolites—very evident in oaks, for example—are the tannins. Tannins bind with the proteins of animals and in various ways disrupt their feeding—and are used for tanning leather, making it tougher and more waterproof, which is where they got their name. Heartwood rich in tannins is evidently less prone to rot than wood without tannins—although, of course, old oaks tend eventually to be hollow; and in truth (for nothing is simple) trees that are only partly hollowed (but not so much that they fall apart) may be
stronger
than those that are still solid, just as an iron pipe may be stronger than a solid rod. Cattle, deer, and apes are among the creatures known to be put off their feed by too many tannins, but rodents and rabbits have joined the arms race and have adapted to them. They produce an amino acid (proline) in their saliva that binds with tannins and blocks their activity. Other mammals are attracted to the astringency of tannins—and so it is that human beings like tea and tannin-rich red wines. But then, for mammals at least, tannins are not all bad. Evidently they block the chemical signals that cause blood vessels to contract. Red wine is known to protect against heart disease—and this may be in part because the tannins help dilate the coronary blood vessels that feed the wall of the heart. Tea, a cardiologist assures me, has the same effect: pleasing news indeed.

Insects in general are put off by tannins—but, as part of the arms race, some have evolved ways of coping. Leaves tend to focus first on growth, and only then have energy to spare to create physical defenses and secondary metabolites; so pests such as the moths whose caterpillars feed on oaks commonly focus on the youngest leaves. Deciduous trees, in turn, seek to outwit the moths by producing their springtime leaves with tremendous speed. Thus the buds of oaks seem to unfold before your eyes. Still, though, the moths are liable to win because they have already laid their eggs on the oak’s buds. The caterpillars emerge just before the leaves, and so are lying in wait. How the eggs
know
when exactly to hatch is unknown. Do they simply respond to the same climatic signals as the oak buds do? Or do they pick up some chemical signal from the oak itself?

Many trees and other plants produce secondary metabolites known as “terpenes” that are specifically insecticidal. Among the best known terpenes are the pyrethroids, which human beings have extracted in particular from African daisies of the genus
Chrysanthemum
(but not the same species as the “chrysanthemums” of the florist) and have adapted as commercial insecticides. Pines, firs, and many other conifers harbor similar agents in their resin ducts. Of course, it is expensive for the tree to produce such chemical agents. But the conifers economize by not producing more than they really need: at least, when they are attacked by bark beetles they produce more terpenes in response.

The terpenes also include the “limonoids” found in citrus fruits: the skins of oranges and lemons also repel insect predators. The most powerful insect repellent known is also a limonoid; this is “azadirachtin,” produced by the all-purpose medicinal neem tree,
Azadirachta indica.
Azadirachtin will repel insects in astonishingly low concentrations (fifty parts per billion) and also has other toxic effects—yet it has no toxicity to speak of in mammals. So, like the pyrethroids, it is favored as a commercial insecticide (and modern politics being what it is, the neem tree is now the cause of international disputes).

Each of the chemical agents cited so far tends to stay within the plant that produces it. But many others are volatile, meaning that they rapidly evaporate and float off in the wind. Some of these volatiles, particularly those known as “essential fatty acids,” are highly scented. Hence the fragrance of sage, mint, basil, and other such relatives of teak. Hence, too, the powerful medicinal tang of the eucalyptus. Chemical repellents that are not volatile have no effect until the predator has taken its first bite—but the volatile ones warn insects and other creatures to stay away
before
they attack: a more sophisticated measure altogether. Still, as with tannin, these essential oils often prove agreeable in small quantities, and human beings, ever opportunistic, extract the oils of eucalypts (and many other plants) for perfumes and medicines—simply by boiling them up and then distilling the oil.

Then again, a few specialist animals, equally opportunistic, have developed ways of coping with such repellents. Thus the essential oils of eucalypts are toxic to most animals (in their raw state), but koalas are equipped with a huge extension of the gut (the cecum) that is packed with symbiotic bacteria, and these detoxify all the noxious compounds in eucalypt leaves. Very few other creatures can cope with eucalypts, which is one reason why the trees are so successful. Because koalas can cope they have the entire run of Australia’s host of eucalypts (600 species or so) almost (though not quite) to themselves. Indeed, koalas will eat very little else, and usually nothing else at all,
except
eucalypts—and different populations of koalas typically confine themselves to just one or a few eucalypt species and reject others. Very few mammals are even close to so specialized. Pandas munch almost exclusively on bamboo, but they will eat many other things besides if given half a chance, from omelettes to roast pork. But creatures that specialize in eating toxic tree leaves pay a price. Brains are particularly susceptible to toxins. Koalas, like the leaf-eating monkeys and the peculiar leaf-eating Amazonian birds known as hoatzins, have smaller brains than their more omnivorous relatives. Even the best friends of the koala have little praise for its intellect.

Legumes (Fabaceae) are among the most accomplished chemists of all. Many produce soap-like “saponins” that interfere with animal digestion. Many produce “flavones,” some of which are strong insecticides—and, again, have been exploited commercially. Legumes often produce agents that limit the estrogen hormones of mammals, so that sheep grazing on legume-rich pastures often become infertile. I know of no direct evidence of such activity in leguminous trees, but it would be very surprising if there were none. Many leguminous trees secrete similar flavones into the soil—not to kill insects but to help establish good relations with the nitrogen-fixing bacteria that they seek to entice into their roots. Complex chemicals in general are versatile. Any one molecule, with or without further chemical adjustment, might kill one group of creatures and attract others.