The Tree (50 page)

Even in the absence of any specific policy, however, human beings have an immense influence on the likelihood and frequency of fires. Thus fires in forests tend to begin with the leaf litter, and in the savannah, they commonly begin with the grasses. Over the past few decades Brazilians and North Americans have introduced several grasses from Africa, which they feel make better fodder for cattle. Some of these grasses have crept into the Cerrado, the dry forest, primarily along the disturbed ground along the verges of the roads that now crisscross the country. From there they spread into the surrounding land. These particular grasses, as it happens, burn more slowly than the native grasses; and this prolonged burning is far more damaging to other vegetation than the quick, albeit hotter flames generated by the native grasses.

That is the first problem. Then, through the 1980s, the fire brigade of Brasilia, which is in the Cerrado, decided to show that it could suppress fires altogether. For fourteen years it succeeded. Then came the general election—won by the present president, Luiz Inácio Lula da Silva, known as Lula. Everyone was given the day off to vote. Including the fire brigade.

While the firefighters were away, the Cerrado caught fire. Since the Cerrado has been catching fire for as long as it has existed (at least since the last ice age, ten thousand years ago, and probably far longer), the trees are adapted to it. But two things had changed. First, the grasses were now slow-burning. Second, thanks to the heroic efforts of the fire brigade, there was a huge backlog of litter and dead grass. So the fire raged as never before. The trees of the gallery forest, which runs along the rivers, are not fire-adapted, but on the whole they don’t need to be: their innate wetness keeps normal fires at bay. But this was a superfire, and it swept through the gallery forest as well. The result was, of course, devastating. Eucalypts, imported some decades ago from Australia, are now spreading happily. (Though, thankfully, not exclusively. The natives are fighting back too.)

For this kind of reason—accidents, changes in the vegetation, and policy that perhaps is misguided—fires these days are often bigger than ever before. North America, Australia, and southern Europe have seen some horrendous blazes in recent decades. The fire in Indonesia in 1987 in a tropical rain forest that should be free of fires left a haze that hung around for months, as if in the wake of a volcano or a nuclear explosion. Fires need plenty of air—and very big fires, like nuclear explosions, increase their own supply: the rising heat creates an updraft that drags in air from all around, a veritable wind, and so produces yet another positive feedback loop: heat of a degree that makes nonsense of all adaptations, reducing everything remotely flammable to ashes.

Global warming, brought about by the greenhouse effect, will make fires worse in several ways. First, general warming, sometimes accompanied by drying, obviously increases the risk; and more frequent tropical storms, with lightning, will provide the trigger. Second, more photosynthesis means more leaf litter, which means more tinder to set the forest ablaze.

One solution might be to remove the leaf litter, but this would be immensely difficult on all but the smallest scale and could in the end be damaging in other ways. The leaf litter provides the soil organisms with their supply of carbon; and these organisms include the nitrogen-fixing bacteria, which are such a vital source of fertility. On the small scale, I have seen the effects of the removal of leaf litter in Brazil, where some coffee farmers, obsessed with hygiene, remove the dead leaves from the ground—and so lose their fertility. Cacao is particularly damaged. The flies that pollinate its flowers breed in the leaf litter. Remove it, and there will be no fruit.

All in all, then, fire is a perennial problem, and global warming is making it worse. If and when trees burn, the carbon trapped within them is released: all the good work of their growth is undone. The ground that’s left behind them is bare. The organisms in the soil are constantly releasing carbon dioxide as they respire: they are releasing carbon from the organic material in the forest floor. When the trees are gone there is nothing to absorb this carbon dioxide, and the organisms of the soil add yet more to the climate’s woes. In any case, sometimes fire enters the soil itself and burns the organic material directly, sometimes persisting underground for months. It seems most unlikely, but it happens nonetheless.

Yet fire is not the only threat from global warming. There will also be more storms—as Hurricane Ivan, most unseasonably, proved in the Caribbean and the southern United States in the late summer of 2004. Again, forests in general are adapted to occasional, partial wipeouts; the pioneer trees in particular (like
Cecropia
and mahogany) depend on them to provide occasional glimpses of the sky. But again, frequency and intensity are all. Huge storms—in northern climates as well as in the tropics—lead to total wipeout, with masses of trees left to rot and hundreds of square miles of ground respiring away their stored carbon.

Global warming, too, cannot come about smoothly. From the outset, scientists have predicted that the general increase in global temperature will, for a time, perhaps a very long time, lead to sudden outbursts of weather that in the historical records at least are quite unprecedented—extremely hot or extremely cold, or wet or dry, or simply out of season. Again the Cerrado around Brasilia provides a small but cogent example. Thus, on 3 September 2004, at the start of what should have been the rainy season, it did indeed rain, more or less on schedule, for an hour and a half. The plants sent out their green shoots. Flowers appeared out of nowhere, as desert flowers so miraculously do. But there was no follow-up. That was all the rain there was. The new shoots and flowers were burned up in the sun. Such scenes must be occurring a thousand times around the globe—but this particular occasion was recorded by Dr. Stephen Harris, from Oxford. Here is the tropical equivalent of the false spring followed by a late frost, so often experienced in the fickle climates of temperate lands and which all gardeners fear.

Plants will be fooled, too, on the global scale, just as we have seen that city trees may be deceived when the streetlights come on. Trees in high latitudes are geared to alternate patterns of long days and short days, accompanied by fairly predictable swings of temperature: warm when the days are long, cold when they are short. Global warming changes the rules. Soon, the short days of northern winters and spring will be warm too. In general, the effect should be less damaging than it would be the other way around: if trees prepared to produce their buds in answer to the lengthening days of spring, only to encounter sharp late frosts. Even so, it does not augur well. Trees in all latitudes are finely adapted to the kind of climate that prevailed during the past few thousand years. A sudden change—and the threatened changes are very sudden by biological standards—will take the rug from beneath their feet.

Finally, it seems very likely that the insects and other pests that now find life difficult in northern climes will find it progressively easier. Animals that can move quickly and easily do not need to adapt genetically. They simply pull up stakes and move on. Again, there are plenty of examples, both in agriculture and forestry, of an apparent migration northward. We will just have to wait and see what damage is wrought by this.

In short, global warming needs to be taken very seriously; and although the matter is not open and shut, the sum of evidence, plus common sense and basic biological theory, suggests that the more forest we retain, and the more new forest we plant, the better. Europe in general is planting more trees, after hugely reducing its cover in the “enlightened” eighteenth century and the zealously industrializing nineteenth century. But other countries, anxious to join the party of neoliberal economics, are still reducing their forests in the interests of what they see, and are encouraged to see, as modernity. Though Brazil now has an enlightened president in Lula, it too has mooted a scheme to reduce the forest of Amazonia, the biggest and most important tropical forest on earth, by about 50 percent over the next few decades. Brazil, of course, has to get its own economy straight. But the world as a whole has to help the Brazilians get straight without felling their trees. Brazil is a long way from where most of the rest of the world lives, and only a minority are lucky enough to visit it. But we all need its forest.

WATER AND SOIL: THE PARTICULARITIES OF RAIN AND FLOOD

Trees shift a prodigious quantity of water—from the soil up into the leaves, out through the stomata, and away into the air. The water, drawn, as we have seen, in long, thin threads up the xylem, generally flows at less than 6 meters per hour but can sometimes reach 40 meters an hour: enough to reach the top of the tallest tree in two hours. A big tree can transpire 500 liters in a day. A hectare (2.47 acres) of wood or of plantation with 100 well-grown trees (planted 10 meters apart—a modest stocking rate) pushes out 50,000 liters, or 50 cubic meters: enough to fill, say, a hotel swimming pool. One square kilometer of such woodland (100 hectares, or 247 acres) would send out 5,000 cubic meters—enough to fill two Olympic-sized swimming pools (for Olympic pools must measure 50 meters by 25 meters by 2 meters). This is
per day.
The catchment area of a river that feeds into a village may cover scores or hundreds of square kilometers. Vast amounts of water are thus sent up into the atmosphere that otherwise would add to the groundwater and run away into the rivers. Thus the danger that the rivers will overflow their banks is reduced. The water that is sent up into the sky forms clouds and will fall again some other day, or in some other place; but so long as the downfall is spread out over time and space, the ground should not be overwhelmed.

Trees also reroute the rain as it falls. Many epiphytes, perched high in tropical trees with no roots to the ground, go to great lengths to trap whatever water they can: bromeliads in particular trap the falling rain in their pineapple-like whorls of leaves (and mosquitoes and tree frogs may breed in the pools that they create: aquatic ecosystems in miniature, high above the forest floor). But the tropical trees themselves commonly contrive to jettison surplus water, their leaves fitted with drip tips. Yet on all trees, a fair proportion of any one shower is caught in the leaves, and since the leaves are hung out high above the ground like washing on a balcony, the water evaporates again before it reaches the forest floor. It is returned to the atmosphere whence it came—to fall again as rain somewhere else, or in the same place on some other day. Several smaller showers, spaced out, are easier to cope with than one downpour. Thus the weather is ameliorated. Forest floors tend to be permeable, too: less trampled than open grassland, and penetrated by many a root. So the water that does reach the ground is more likely to sink in, and less likely to run away, than on pastureland. Once the water is in the soil, as we have seen, it is summarily sucked up again and shot back into the atmosphere.

All in all, then, a mature forest is wondrously drying. Where dryness is a problem, you can have too many trees, as many a householder has discovered as the desiccated clay under their home contracts and the foundations crack. Eucalypts are famously desiccating, with long taproots reaching down to the groundwater, transpiring long after other trees have given up. They may create drought around them and kill the surrounding trees if planted in the wrong places, as I have seen them doing in the dry Cerrado forest around Brasilia. But if you live at the foot of a mountain, on the rainy side, the more trees there are on the slopes above, the better. Of course, if the rain is prodigious and persistent, as it may be in some monsoons, then the forest and the ground it stands in can be saturated like anything else, and the surplus will run off just as it would run from bare ground. But trees prevent many a smaller flood; and even when the forest is overwhelmed and sheds its surplus water it should hang on to its soil.

In August 2004 London suffered a flash flood that discharged nearly 800,000 gallons of sewage into the Thames. This is small beer compared to many a flood of late, but what’s significant are the suggested remedies: water butts to slow the flow of water from roofs into the streets and drains; porous pavements to allow the ground beneath to take up some of the surplus. These measures do not, of course, reduce the total mass of water. But they slow it down. They dole it out more slowly into the streets, drains, and rivers, which are then not so likely to be overwhelmed. All this would involve extensive civil engineering, simple in principle but extremely expensive. Yet this is the service that trees provide in every river catchment throughout the world
for free.
In principle, all we have to do is leave them alone. Of course, too, the water that finally does flow from a forest is generally clean. When bare hillside is flooded it’s mud that flows off, and that is more damaging and dangerous by far.

On the other hand, nothing is ever quite straightforward. In general, trees ameliorate the ill effects of too much rain: of course they do. In general, too, their roots help hold the soil in place. How could it not be so? But this does not mean that any old tree or group of trees, planted any old how, will always be helpful. Teak, for example, typically has huge leaves, as big as dinner plates. Each one can catch a prodigious quantity of water—which then flows from the tips in commensurately enormous drops. These fall from on high like glass marbles, and on impervious soils may batter the surface into a glassy crust—which increases runoff and eventually, if there is any slope at all, may simply slide away. If the soil is more permeable the huge drops penetrate and eventually carve deep gulleys.

Then again, people seeking to establish new forests too quickly have sometimes approached the task too eagerly. Thus in China, zealous peasants in recent years have sometimes begun by plowing the land they wish to plant—and if this is on a hillside, which it often is, then by the time the trees have established themselves the newly bared soil has already washed away. Forests in general work best when they are diverse and when they are given time to do their own thing. Attempts to push ahead too quickly, or to oversimplify the planting, can cause the kinds of disasters that give proactive forestry a bad name. A surprising variety of vested interests are ranged against trees and forests. It is as well not to give them further ammunition.