The Tree (40 page)

Two further morals emerge from the tale of the aspen. First, appearance in nature can be highly deceptive. The aspen doesn’t look tough, yet it may flourish where other trees, more obviously equipped for action, with needle leaves and pointed crowns that shed the snow, succumb. But then, many of nature’s most successful creatures seem to go out of their way to look bizarre and delicate. Sometimes the bizarreness is bravado, intended to attract mates, as with the plumes of the peacock and the bird of paradise. Sometimes flamboyance is a warning, as with poisonous lizards and caterpillars. But sometimes, when we look at an animal or a plant, we just get the physics wrong. Moths and butterflies look absurdly frail, fair game for anyone, but they are ubiquitous, and in their season in the Amazon the yellow butterflies come at you in blizzards. Nature moves in mysterious ways, which itself is a moral for all who would presume to take nature in hand.

Second, many creatures survive when you might not at first sight expect them to, because of just one particular trick. The analogy is with an old and arthritic fencing master who wraps up the young athletic tyro with disdainful ease just because he knows a few wrinkles that the novice does not. The aspen thrives where others fail because of its suckers. That might not seem like much, but it works.

But the trick works only if the forest fire spares the lateral roots. If fire breaks out in summer, when the soil is dry and unfrozen, the roots are cooked and the aspen succumbs. Then the jack pine,
Pinus banksiana,
comes to the fore: a quite different tree with a quite different survival strategy.

A Doughty Fighter: The Fire-Dependent Jack Pine

J. David Henry, in
Canada’s Boreal Forest,
calls the jack pine “a scrappy tree”; in former times (when the timber of nobler trees like the white pine,
Pinus strobus,
was more available), it was often treated as a weed. Now, however, with the alternatives diminished, it is widely used for fence posts, telegraph poles, and miners’ pit props, as well as for Christmas trees; and together with the black spruce, says Henry, it “is a mainstay of the pulp and paper industry.”

The jack pine has several qualities that seem to equip it for a life with fire. As in many conifers, the branches nearer the ground tend to die off as the tree grows taller, not least because they find themselves without light. In the jack pine, these dead branches simply fall off: the tree is said to be “self-pruning.” If the dead branches were allowed to persist, they would provide a “ladder” for the fire from the ground to the top. The physics of fire is in many ways counterintuitive. Crucially, a hot fire that burns itself out quickly can be less damaging than one that’s somewhat cooler but lasts longer. Jack pine needles are high in resin and often low in water, especially in the droughts of spring and summer, when fires are likely, and so they burn hot but quick. On much the same principle, the jack pine’s bark is flaky. It picks up surface fires but then burns swiftly and does little harm. The stringy bark of the eucalypts in Australia, hanging loosely from the iron-smooth bole beneath, is protective in much the same way. In both cases the discarded bark acts as a decoy, like a hamper thrown from a troika to divert the chasing wolves.

On the other hand, when jack pine bark accumulates on the ground (as it does if there is a long interval between fires), surface fires—particularly in spring and summer—can be very fierce. Then most trees of all kinds are killed. But the jack pine is typically the first to spring back. For a very hot fire in the summer burns both the leaf litter on the surface and the organic material in the soil itself, leaving a bare, mineral soil behind. Jack pines germinate well in such soil—and, indeed, are inhibited by leaf litter: organic matter isn’t always everybody’s friend. They like bright sunlight, too, and appreciate the open space.

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.