The Tree
Page 41
But all is not so simple. Key players in photosynthesis are the stomata, the holes in the leaf surface that allow the carbon dioxide to enter. Rising levels of carbon dioxide, and increasing photosynthesis, stimulate the stomata to close. This, of course, will slow the increase in photosynthesis. So whatever effect the increase in photosynthesis may have in reducing atmospheric carbon dioxide will be limited. At some point, metaphorically speaking, the trees will cry, “Enough!” They could simply be overwhelmed.
And as carbon dioxide increases so, too, will temperature—and this complicates the picture again, in many ways. Heat in general increases the rate of all chemical reactions, and since metabolism is the sum total of the body’s chemistry, rising temperature means faster metabolism. Rising temperature should stimulate photosynthesis along with everything else, which is fine: the warmer it gets, the faster the trees should absorb carbon dioxide, and so prevent further warming.
But—there is always a but!—rising temperature will also stimulate respiration, the burning of sugars; and respiration may speed up more quickly than photosynthesis does, and if it gets too warm there could be net loss of carbon from any one plant. Worse: the many creatures that live in the soil—bacteria, fungi, invertebrates—will also respire faster as temperature rises. Since they feed primarily on leaf litter, they will break it down faster—and so release the carbon it contains more rapidly. Thus rising temperature could easily cause a net loss of carbon from the forest as a whole—partly from the trees, partly from the ground. Finally, if temperatures rise too high, essential enzymes within the plant will be damaged, and the plant will start to die. As this happens, photosynthesis will stop and the living parts decay. Then, the net release of carbon into the atmosphere will become massive—and the temperature will rise even more. Again, there is some direct experimental evidence to support this scenario.
Of course, tropical trees are adapted to heat and should not be killed by the kind of temperatures that are envisaged; and all trees can adapt to change to some extent, given time. But what’s truly serious, not to say terrifying, is the rate of climate change. We may well see significant temperature rises in the next half century, or even sooner if the global-dimming hypothesis turns out to be correct. Few trees indeed could adapt in such a time, and none could make the necessary, generation-by-generation genetic changes. Only the creatures with short lifetimes—bacteria, flies, perhaps mice—can effect significant change in such a period. So we could see a massive die-off, pushing even more carbon dioxide into the atmosphere, resulting in higher and higher temperatures, leading to more deaths, and so on.
Then there’s the matter of water. If plants have a design fault, it lies with their stomata. The same neatly guarded apertures that allow the essential carbon dioxide to enter also, inevitably, allow evaporating water to leave. This, of course, is exacerbated by rising temperatures. So plants that are warmed more than they are used to, in regions where water is the limiting factor (which is often the case even in rain forests—for many rain forests have dry seasons), will wilt and eventually die: the precise opposite of what is desired. Again, there is direct evidence from the field. Scientists in Brazil, including Dr. Yadvinder Malhi of Oxford University, have deprived whole acres of trees of water by covering the ground in polyethylene panels, so that most of the rain runs off into drains around the edges. As might be predicted (but these things always need to be demonstrated), the trees quickly show signs of suffering. Growth stops—meaning that respiration starts to exceed photosynthesis, meaning that carbon dioxide, in net, is being lost.
As a final botanical complication, it seems likely that when conditions favor more photosynthesis—more carbon dioxide, more warmth (within limits), and more water—the epiphytes, the mass of ferns, bromeliads, orchids, and others that grow on trees, will grow more quickly than the trees themselves. This may not matter for the well-being of the planet as a whole—so long as some plants absorb more carbon, it does not matter which they are. But overgrowth of epiphytes would be bad for trees, and would profoundly affect overall forest ecology (it could in theory increase biodiversity, though all change is more likely to be destructive in the short term). This last complication proves another general point: that prediction is immensely difficult because we just don’t know enough about the physiology of wild creatures in general, let alone the result of all their interactions; and if we did know enough, the computers that help to make sense of masses of data would be very hard pressed indeed to simulate reality reliably, on the local as well as on the grandest scale.
Then there is fire. More and more fires these days are started by human beings, deliberately or inadvertently, although even when deliberate and for legitimate agricultural or conservational purposes, they sometimes get out of hand. But fires, commonly though not exclusively triggered by lightning, are part of nature. In the places where they naturally occur—virtually everywhere that has anything to burn and is not permanently wet—the local plants (and animals, to a greater or lesser extent) tend to be adapted to them. Grasses need to have their tops burned off if grazing animals do not do the job for them, or the tops become senescent and stifle the fresh growth beneath. As we have already seen, many trees are highly fireproof, like redwoods and eucalypts, and the seeds of many pines and other species will not germinate unless first effectively cooked, whereupon they “know” they can sprout in the nutrient-rich ash provided by their immediate predecessors.
But as with every input—including water, general warmth, light, carbon dioxide, and many minerals that are in small doses essential—there can be too much of a good thing. Fire is lethal to the trees that are not adapted to it, of course—and even for those that need it, timing and intensity are all. If, for whatever reason, the fires come too frequently, or too rarely, or burn too intensely, then the best-adapted trees are overwhelmed. Human beings are altering the world in ways that are very much against the interests of wild trees, upsetting even those that seem well adapted.
Human beings have been using fire to influence vegetation probably for about 500,000 years; that, at least, is the age of the oldest fires that are thought to have been started by humans. Human beings in those earliest days were not modern, even anatomically. They had smaller brains than we have. But they knew about fire. Sometimes our ancestors set fire to the local plants, as Australian Aborigines may still do, to drive wild animals into traps or simply to freshen the vegetation (grasses) to attract more grazers. Sometimes they set fire to trees just to get rid of them, so they could then sow crops or plant grass for cattle in the ashes. Sometimes people just seem to want a better view. As the world becomes more and more crowded and complicated, there are many conflicts of interest, each requiring a different attitude to fire. Conservationists in general want to keep trees but recognize the need for occasional fires—for example, to stimulate germination. Tour operators and people who live in the country commonly want to keep trees but hate the idea of fires, which reduce some of the forest to ashes and threaten their houses. Farmers commonly want to get rid of trees—but prefer it if the ones that do remain do not catch fire and burn their farms. People in general tend to feel that fires are a bad thing, and politicians have an eye to their votes.
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 hec
tares, 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.