The Tree

by Colin Tudge

  WHY SOME FIGS HAVE BIG FRUITS DESPITE EVERYTHING: THE MYSTERY SOLVED

  The syconia of figs qualify as bona fide fruits when the seeds within are mature and ready for scattering. Many animals come to prey upon them. Many, like monkeys, may do some dispersing at the same time but on the whole are destructive. The figs’ main allies at this stage of its life, dispersing its seeds without taking more than their fair share, are various birds and fruit-eating bats.

  Each species of fig produces either red fruit, to attract birds, or green fruit, to attract bats. Fruit-eating birds hunt by day and rely on vision, and bright red does the trick. Among the birds, mannikins are the main specialist fruit eaters, while tanagers, tyrant flycatchers, and woodpeckers are opportunist feeders, taking what’s on offer but happy to eat other things as well. Bats hunt by night, and for them plain green will do. In fact, fruit-eating bats are less active on nights of the full moon, when there is more light, than on dark nights. On moonlit nights they are picked off by owls: a gothic encounter indeed. The birds that dispense figs are all of roughly the same size and so, accordingly, are the red fruits that have evolved to attract them. The fruit-eating bats are of various sizes. So, correspondingly, are the fruits they eat. Bats, unlike birds, carry fruit away from the tree to eat it at leisure elsewhere, in some more private roost. Perhaps this too is a defense against predators. Otherwise owls (or civets or leopards) might simply hang around the fruit trees in wait.

  Here, at last—or so it seems—comes the answer to the puzzle: why figs bother to produce big fruits, which seem to cause them so much trouble. Big bats fly farther than small bats. So, on average, big fruits are dispersed more widely than small fruits. Dispersal, in tropical forests, needs to be as wide as possible. Big fruits seem to lose out at every turn, but in the end, they are worth it.

  This, at least in outline, is the fig story so far. Several morals are attached to it. The point that Allen Herre emphasizes is how hard it is, in biology, to relate theory to what happens in the field. The basic interplay of figs and wasps is complicated enough; but then the complications multiply and multiply again as you stir in the cryptic wasps that look like pollinators but in fact are parasites, and the nematodes, whose virulence depends on the number of foundresses, and so on. However much we know about nature, we can never know enough. Science is wonderful—the studies that have led to the present understanding of fig wasps are breathtaking: a brilliant amalgam of natural history, persistence, imagination, and intellect—and yet the more that’s known, the more it seems there is to know.

  Already it is clear, though, that if we do anything to interrupt the lives of the wasps—are too free with insecticide, for instance—then we will kill off the figs, or at least ensure that the present generation is the last. The fruits of figs are essential provender not only for bats and birds but for a host of other creatures too. In Panama, figs of various kinds are in fruit all through the year, while most other fruits are far more seasonal. There are times when figs are all there is. Take away the figs, and half the fauna could be in serious trouble. The whole ecosystem balances on a pinpoint—and we could tip it into oblivion without thinking; or, indeed, we could let it slip through our fingers even if we were trying very hard to save it. On the other hand, precarious though it seems, figs and wasps have maintained their relationship in one form or another without interruption for more than forty million years. There is robustness in the system. If only we can work with it, it might pull through yet.

  Finally, given that so many trees rely so heavily on animal pollinators and dispersers, we might ask what happens to them if their allies disappear. One highly intriguing answer comes from the island of Mauritius, in the Indian Ocean.

  THE DODO AND THE TAMBALACOQUE: A SAD TALE WITH A FAIRLY HAPPY ENDING

  On the island of Mauritius lives Calvaria major, known as the tambalacoque, one of the vast tropical family Sapotaceae. But in 1977 Dr. Stanley Temple of the University of Wisconsin reported in the journal Science that the only tambalacoque trees left on Mauritius were all over three hundred years old.2 There were no young ones. Since the tambalacoque lives exclusively on Mauritius, these ancients were the only ones left in the world. Yet the tambalacoque used to be common—common enough to be used for lumber. The remaining trees were fertile and were clearly pollinated, for each year they produced plenty of seeds. So what was going wrong?

  A little background is called for. Mauritius is an island—one of the Mascarene Islands—deep in the Indian Ocean, a few hundred miles to the east of Madagascar. As we have already seen (not least among the Didiereaceae and the baobab trees of Madagascar itself) creatures on islands tend to evolve in strange directions; and among the strangest creatures on Mauritius was the dodo. Biologically speaking, the dodo was a flightless pigeon—but huge: as big as a modern farmyard turkey, with a great hooked beak, a bewildered expression, and tiny wings and downy chick-like feathers: a singularly daft-looking animal.

  So long as Mauritius remained undiscovered and uninhabited, the dodo thrived. But in the fifteenth century, at the start of the great age of discovery, the first European sailors arrived. Passing ships then stopped regularly at Mauritius for fresh water and fresh meat—not least from the giant Mauritius tortoises. The sailors also ate dodoes, though they have left mixed reports of them: some said they were very tasty, and others complained that they were tough and greasy. Perhaps the time of year mattered. Perhaps the dodoes were succulent after the rainy season when they were well fed and stringy after the lean dry season. In any case, by 1681—barely two hundred years since the first European ships made landing—the dodoes had been exterminated. Indeed, they have become the world’s favorite symbol of extinction: “dead as a dodo.” To be sure, it probably wasn’t the sailors that finished them off. European rats, which ate their eggs, and other creatures that the ships brought with them, including monkeys, probably did the trick.

  When the dodo went extinct, the tambalacoques stopped breeding—or so Dr. Temple suggested. For tambalacoques produce very big seeds, about two inches across, surrounded by an enormously hard and woody husk up to nearly a sixteenth of an inch thick: too thick, apparently, to allow the young seedling to emerge unless the walls are first weakened. The dodo, said Dr. Temple, did the necessary weakening.

  The dodoes ate the fruit of the tambalacoque. They digested the pulpy exterior, and the big wooden pip passed to the gizzard—the extension of the gut that birds pack with stones and use to crush seeds. Tambalacoques evolved seed coats that were thicker and thicker, in response to the dodo gizzard’s enormous crushing strength. Eventually they became so thick that the seeds could not germinate at all unless they had first been eaten by a dodo. Of course, the seeds would fail if they were crushed in its gizzard. But if the pips were merely abraded, or “scarified,” they would germinate much better than if not; indeed, they needed the scarification. Here, then, was another example of coevolution. Dr. Temple tested his theory by feeding tambalacoque seeds to turkeys, which are not related to dodoes but are ecologically equivalent. Wild turkeys eat hickory nuts. The turkeys crushed some of the tambalacoque seeds—seven out of seventeen—but after six days or so they gave up on the other ten, and either coughed them up or passed them through their guts. Those ten seeds, duly abraded, did germinate.

  As we have already seen, many other seeds benefit from some pretreatment from animals in one way or another. In India, teak seeds are sometimes prepared for sowing by laying them out on the forest floor where the termites can get to them. They germinate better after the insects have nibbled some of the seed walls away (though it’s important not to leave the seeds out too long). On the whole, the tale of the dodo and the tambalacoque has all the elements of a classic.

  The story has only one shortcoming. It does not seem to be true. Areas of forest in Mauritius have now been fenced, and cleared of the pigs, deer, and monkeys that meddlesome Europeans have introduced to the island over the past three centuries—and lo, in the cleared areas, you
ng tambalacoques have been springing forth. Evidently, it wasn’t the presence of dodoes they required but an absence of imported herbivores. This is excellent news for the tambalacoque. But it is a pity, indeed, to kill off such an excellent story.

  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.

  I
n 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?