by Colin Tudge
Once germinated, the young trees grow happily in sandy soil that is too dry for other species; for good measure, the young saplings can tolerate drought of a month or so, as well as sudden drops in temperature of the kind that for many trees are lethal. They grow swiftly when young—more than thirty-five centimeters (over a foot) in a year. This is a joke by the standards of tropical trees, some of which reach twenty meters in five years, but good for a land so niggardly in bright sunshine and general warmth. By their fourth or fifth year many of the young jack pines are producing their first cones—which by tree standards is markedly young. The Canadian ecologists Stan Rowe and George Scotter asked why they should be so precocious: why not focus their precious energy on more growth rather than on reproduction? Forest fires often leave a lot of fuel behind, and sometimes a second fire comes hard on the heels of the first. It seems a good idea to scatter a few seeds before the possible follow-up.
But it’s the cones and the seeds of the jack pine that are adapted most impressively and specifically to fire. The cones are hard as iron, their scales tightly bound together with what J. David Henry calls a “resinous glue.” Many creatures attack cones; but only the American red squirrel will take on the jack pine cone, and even the red squirrel much prefers the easier, fleshier meat of spruce cones. The cones may persist on the trees for many years, and the seeds within them remain viable: in one study more than half the seeds from cones that were more than twenty years old were able to germinate.
The cones do not open until there is a fire: it takes heat of 50°C (122°F) to melt the resin that locks the scales together. Then, they open like flowers. Thus the seeds are not released until fire has cleared the ground of organic matter and of rivals, and created exactly the conditions they need. The output is prodigious. After a fire in the taiga (the northernmost forest, which then gives way to tundra), the burned ground may be scattered with 12 million jack pine trees per acre.
But although the cone responds to fire, and only to fire, it is remarkably fire resistant. Thus, in the early 1960s, the biologist W. R. Beaufant found that the seeds inside would survive for thirty seconds even when the cone was exposed to 900°C (1,652°F)—the kind of temperature that potters use for firing. At a mere 700°C (1,292°F), the seeds were perfectly happy for at least three minutes. In short, it takes an awful lot of thermal energy to kill jack pine seeds when they are still in their cones. Trees seem to have evolved cork largely as an adaptation against fire; and jack pine cones contain cork too.
Yet there is more. For as J. David Henry has found, the cone does not respond simply to the presence of fire, like some crude unmonitored mechanical device. As it is heated, it releases resin from within. This oozes to the surface and “creates a gentle, lamplike flame around the cone,” which lasts for about a minute and a half. All in all, says Henry, “It seemed that, once ignited, the cone was programmed to provide a flame for the right amount of time to open the cone…. While a forest fire is needed to initiate this process, the cone itself is capable of providing the type and duration of flame it needs to open and disperse its seeds.” He then showed that, once open, the heated cone does not release its seeds until it has cooled down again—which in field conditions may take several days. So the initial opening is controlled; but when the cone is first open, the seeds are held back. They are not sent out like Daniel into the fiery furnace. Henry suggests a mechanism: perhaps the hairs that coat one side of the seed are sticky when hot, and hold the seed in, but lose their stickiness when cooled again. This is speculation, yet to be tested.
In any case, the adaptations are extraordinary. Jack pines belong among a fairly impressive short list of trees that not only resist fire but have become dependent upon it. They cannot reproduce without it. If there is no fire within a jack pine’s lifetime, it will die without issue. After a fire, jack pines may flourish and form a monoculture, for acre after acre. But without further fire, the jack pine forest fades away.
Yet there is one final twist. In practice not all of the jack pine cones need the fierce heat of a fire to open them. In the north, about one in ten open just in the warmth of the sun. In the Great Lake states to the south, where there are far fewer fires, most of the jack pine cones are able to open in the sun. Thus jack pines have a “mixed strategy”: the genes that make their cones so special are clearly of two kinds—some that gear the cone to fire and some that enable it to respond to sunshine. Geneticists call this a “balanced polymorphism.” Natural selection tips the balance toward the fire-dependent pines in the north and to the sunshine-dependent pines in the south. The jack pine isn’t simply the supreme specialist. To some extent, at least, it is the jack of all trades. Various other pines have comparable fire resistance and fire dependence, but none surpasses the scrappy jack pine in its adaptation.
Yet perhaps the tree that is most thoroughly adapted to the special conditions of the north—not the extreme north, but the central and northern coast of California—is the coastal redwood, Sequoia sempervirens.
Fire, Flood, and Mist: California’s Mighty Redwoods
The coastal redwoods inhabit—or, rather, create—temperate rain forests in a discontinuous belt, roughly nine miles wide, from Big Sur, south of San Francisco, north to the Oregon border. They are, of course, magnificent: the height of a cathedral spire, 60 to 70 meters. The tallest, known prosaically as “the Tallest Tree,” is in Redwood National Park and is 111 meters high. Its trunk is 3 meters in diameter—although this is quite slim by redwood standards. They often go up to 5 meters. Many live to one thousand years, and some reach more than two thousand.
All forests can be peaceful (you can sleep the sleep of the just in Amazonia without being carted off in pieces by voracious ants), but nothing compares with the tranquillity of a redwood forest. The columnar trunks reach far higher than in a tropical forest, where thirty meters is more standard. A tropical forest is a mass of small trees, battalions of poles, with just a few trunks of respectable garden size and only the occasional giant glimpsed through the gloom, all festooned with climbers and epiphytes. But the coastal redwoods for the most part are decorously spaced—except where they form little circles, so-called fairy rings. They are an army of giants, the biggest on earth. The ground beneath them is littered with the delicate, yew-like branchlets that the trees shed every few years, chestnut brown after death. The mosses and ferns, herbs and shrubs, dotted here and there, stay at the feet of the great trees. There is no importunate clambering. There are few birds. You rarely hear such silence as in a coastal redwood forest. The light is green. The sun shines through in sharp, bright shafts. On a warm, late afternoon the Pacific rain forest of coastal redwoods is perhaps the most serene of all earthly environments.
Coastal redwoods reroot themselves as the silt piles up around them.
But it’s not always like that.
The first problem is flood. Redwoods like moisture—a “mild maritime climate,” as it is sometimes described (though they don’t tend to like salt spray). Indeed, they go to great lengths to capture and condense the thick fogs of the cool Californian night and morning in their leaves. It falls as “fog drip,” and in the rainless summers it may add 30 centimeters (nearly a foot) of extra water. So they make their own climate, humid and shady.
But you can overdo the water. In winter, there may be 250 centimeters of rain. Storms are frequent; and with storms come flooding. In Humboldt County, redwood country, there were severe floods in 1955, 1964, 1974, and 1986. The floods of 1955 swept away sawmills, farms, and whole communities along the Eel, Klamath, and Van Duzen rivers. Buildings were buried deep under mud. More than five hundred redwoods were swept away along Bull Creek, a tributary of the Eel. Elsewhere the forest floor was buried under 1.3 meters of silt.
Radiocarbon dating showed that Bull Creek had often suffered such insults in the past. In fact, a study in 1968 cited by Verna R. Johnston showed that there have been fifteen major floods in the past one thousand years, and between them they have raised
the level of the whole surrounding area by more than 9 meters—the height of a three-story house.
In short, over time along these northern Californian coastal rivers, the banks and surrounding areas are eroded in some places and built up in others; this is the kind of pattern that is seen, for example, around the coast of Europe, as the North Sea picks up entire beaches from some places and dumps them somewhere else. The cartographers of eastern England have been particularly busy these past few centuries, and surely will be even busier as global warming strikes.
This is where the redwoods reveal their own set of tricks. Of course, if the ground is removed from around them altogether, they are swept away. But if they are merely buried, to a depth of a meter or so, they are untroubled. Most trees do succumb to such treatment. They are suffocated. But redwoods send up roots, vertically, from their buried lateral roots, into the silt above; and these verticals grow so quickly they sometimes come bursting through the surface.
These rapid-growing verticals, however, are merely the emergency procedure, the frontline troops. Before long, new lateral roots grow from the buried trunk, just below the surface of the newly deposited silt; generally speaking, they are bigger and broader than the previous roots at the lower level. Thus an old redwood, one that’s survived many floods—and those a thousand years old or more must have survived more than a dozen—wind up with a multilayered root system, like an inverted pagoda: a fairly small set of roots deep down, then a bigger set higher up, and so on, each set corresponding to some earlier flood. The net result is a truly remarkable anchorage. In this way, they hold their otherwise precarious trunks steady for a thousand years. Incursions of silt kill most trees, but redwoods have made a virtue of it. Here, again, is nature’s opportunism.
Human beings are managing to queer the pitch, however. Forest clearance exposes the survivors to stronger winds, and may blow them over despite their anchorage. Roads alter the natural drainage from the hills, and so alter the pattern of flooding, leading to landslides, which nothing can resist. Added silt builds up in the middle of rivers, forcing the currents to the sides and undercutting the trees along the banks. More than a hundred ancient trees in California’s Avenue of the Giants were killed this way in 1986, when fifty-eight centimeters (nearly two feet) of rain fell in nine days. And although the redwoods have turned partial burial to their advantage, their roots are damaged if the soil above them is compacted—which it can be by the tractors that are used to extract logs, and by other traffic. In great gardens, as at Kew, soil that has been compacted by the feet of thousands of visitors coming to admire the trees is loosened by pumping in nitrogen gas under pressure. In some of New Zealand’s forests, visitors walk along catwalks a foot above the ground, with little bridges over exposed roots. Humanity needs wild trees. But sometimes we need to tend the wild as carefully as any garden. “Managed wilderness” may seem a paradox, an oxymoron; but it is a reality we have to come to terms with.
Redwoods are wonderfully fire resistant, too—and show yet more survival strategies. Their bark is a fireman’s dream: tough, fibrous, and up to a foot thick. Their timber retains a lot of water. They produce a pitch that is almost nonflammable. A severe fire will break through the bark, and thus allow access for subsequent fires. But the charred wounds on the boles of many old redwoods merely attest that they have endured.
Like aspens and jack pines, too, coastal redwoods have a wonderful ability to recover after fire. Almost uniquely among conifers, redwoods can sprout new stems and roots from trunks that have been burned, cut down, or blown over. From the earliest age—indeed, from the seedling stage—redwoods carry latent buds around the base and along the trunk. If they are damaged or irritated, these dormant buds develop into shoots or roots and then into whole new trees. When the parent tree is damaged the buds around the base sprout to form a fairy ring around it, of up to a dozen scions; and when the main tree dies they persist, in conspiratorial circles that indeed seem enchanted. Sometimes the process is repeated: the trees of the inner circle are damaged and more grow up around them. By such means redwoods may return even to areas that have been heavily logged.
The many thousands of cones on each redwood produce prodigious harvests of seeds. They do not germinate well, and redwood groves generally contain few seedlings. But, as with jack pines, the seeds fare best on bare, mineralized soil that has been stripped by fire or newly deposited as silt. Again, then, the redwood is equipped to return after fire—and for sexual reproduction, at least, may be said to be dependent on it. Again, the principle is common throughout nature: organisms often delay reproduction until conditions turn nasty and a new generation is needed to carry the day. Many organisms reproduce only in extremis.
Despite their solitary splendor in the western forests, coastal redwoods are not innately standoffish. They do not set out, for example, to poison their putative competitors, as some plants do. On the slopes, away from floods, they may mix well enough with Douglas fir, tan oak, grand fir, and California laurel. Tan oaks, too, sprout vigorously after fires, tolerate shade, and are in other ways resilient, with a bark rich in tannin that protects them against insects and fungi. But (like Douglas fir, grand fir, and California laurel) the tan oak succumbs to flooding and silting; so on the floodplains, the redwood reigns supreme.
Overall, the tropics show what nature can really do: how various it becomes when conditions are easy, or when the pot is stirred now and again, as by the drying of the ice ages, in a creative way. By contrast, in the high latitudes, which mainly means in the north, nature was repeatedly filleted by the ice ages, and what we see now are the few brave and specialist survivors.
So far, this book has dwelt mainly on the basic cast list: who’s who, where they all are, and why. Ecology begins when we ask how all the different players interact. That is the subject of the next chapter.
13
The Social Life of Trees: War or Peace?
“NATURE RED IN TOOTH AND CLAW,” wrote Alfred, Lord Tennyson in the 1830s. “No man is an island,” said John Donne, about two hundred years earlier. Both are right. War and peace are threads that run through all of life, from the cradle to the grave, from the seed to the compost heap. Everything we do reflects the tension between conflict and cooperation.
The tension is evident in reproduction. No individual creature needs to reproduce. Many a bachelor uncle and aunt live perfectly happy lives without offspring. But if any lineage of creatures abandons reproduction altogether, it dies out. Yet reproduction is costly. It takes energy to produce gametes. It takes even more to produce viable, fertilized eggs or live young. Many creatures die after a single bout of reproduction, including most annual plants and even some mighty trees, like the big bamboos and some of the greatest palms. Even a few eudicot trees follow such a strategy. Over eighty years or so in the forests of Panama, Tachigalia versicolor, of the Fabaceae family, grows into a tree imposing enough for any royal park—then blossoms, sets seed, and dies. I have stood at the feet of one already dead. If parent organisms do not die (as most do not), then their offspring may become their assistants, as in many bird families, and in our own societies when they are working well. But children also become rivals. Contrariwise, the greatest threat to the offspring often comes from its parents—and so as we have seen, tropical trees that aspire to grow too close to the mother tree may perish from its diseases. The offspring of Tachigalia, presumably, are better off as orphans. The theme of parent-offspring rivalry runs through all of literature, beginning with the Bible and the Greeks (and doubtless through preliterate cultures too)—and the greatest themes of literature are those of nature. Reproduction is necessary, to be sure. But it takes a heavy toll.
Figs—all 750 species of them—need their own wasps to pollinate them.
Sex adds another layer of complication. Sexual reproduction is less efficient than the many asexual strategies seen in nature. It takes two parents to produce even a single offspring by sexual means, whereas cloning requires only one.
Yet most creatures practice sex—including all the trees that I know about, even though many of them also reproduce asexually. In truth, sex has nothing directly to do with reproduction. Natural selection has favored sex for another reason: because it mixes genes from different organisms, and so produces variation. Offspring produced sexually are all genetically unique, different from either of their parents and from their siblings. This offers long-term advantage, since variation is a key ingredient of evolutionary change. But natural selection does not look to the future. Sex must bring short-term advantage—for if it did not, sexual beings would lose out to asexual ones, who in principle can reproduce twice as quickly. What is the short-term advantage? There are two main hypotheses: one (from the American biologist George Williams) says that if all the offspring are different, this increases the chances that one, at least, will find itself in favorable conditions and live to pass on its parents’ genes. The other (from England’s Bill Hamilton) says that short-term variation keeps parasites on the back foot. If all members of a population are the same, then any parasite that can attack one of them will be able to defeat all of them. If they are all different, then each individual poses new problems. There is direct evidence to support this idea. Certainly, modern crops grown as genetically identical monocultures are especially vulnerable to parasites.
For sex to work, each creature must find a mate. For animals this can be dangerous—many a lion and stag has died from battles with rivals; many a male spider has died in his chosen one’s jaws. Trees, rooted to the spot, must find a way to transport pollen from male flower to female. The female flowers (or the female parts of hermaphroditic flowers) must in turn be able to capture pollen. Yet they must not be too promiscuous and allow themselves to be entered by pollen of the wrong type. In general, they reject pollen from other species. Many trees, including domestic plums and apples, also reject pollen from trees that are too genetically similar to themselves: by such self-incompatibility they avoid the genetic perils of incest. Growers of plums and apples must grow varieties side by side that complement each other. Again, the tensions between males and females are a principal theme of literature, and always have been, running as strongly through the Bible and the Koran as anywhere else.
