The Tree (32 page)

H
OLLY
: O
RDER
A
QUIFOLIALES

The only family in the Aquifoliales order is the Aquifoliaceae. It contains about 400 species in three genera—and about 97 percent of them are in the genus
Ilex:
the trees and shrubs known as hollies. Most of them live in tropical mountains, but they are very widely spread and include only one of two (at most) species of evergreen broad-leaved trees in Britain. Many are cultivated for their glossy and often prickly foliage and for their bright red or yellow fruits (distributed by birds), but they are rich in caffeine and some are cultivated for medicine. The leaves of
I. paraguariensis
are brewed to make maté, high in caffeine, while the native people of the southeastern United States make “black drink” from the leaves of
I. vomitoria.
Holly timber is hard, white, smooth, and much prized.

D
AISIES AND
J
UST A
F
EW
T
REES
: O
RDER
A
STERALES

Twelve families and nearly 30,000 species make up the Asterales. The Asteraceae family—formerly known as the Compositae—contains most of them, and may be the biggest plant family of all (although some say the orchids have more species). Among the Asteraceae are many ornamentals, such as daisies, chrysanthemums, and marigolds, and edible and medicinal herbs, including endive, artichoke, sunflower, Jerusalem artichoke, dandelions, and lettuce. There are few convincing trees, although a few (including some from the Brazilian Cerrado) do provide timber that is locally useful. The muhuhu,
Brachylaena hutchinsii,
is one of the few bona fide trees, and an impressive one. It comes from the coastal belt of East Africa and the highlands of Tanzania and Kenya, grows to 25 meters, and provides short lengths of very heavy timber with gray-white sapwood and orange-brown heartwood, which looks good, wears well, and is favored for everything from floors to animal carvings. The muhuhu also smells pleasant, and its oil is distilled as a substitute for sandalwood—it is even exported to India, as an aromatic fuel for cremations.

Handsome in winter and with fine white timber: the holly.

Mangroves: how can trees grow with their feet in the sea?

11

How Trees Live

A
CENTURY OR SO
before Aristotle, the pre-Socratic philosophers of Greece proposed that all material things, including those that are alive, are composed of just four elements: earth, air, fire, and water. This sounds quaint to modern ears, and is sometimes taken as evidence that the Greeks, for all their sophistication, were in truth rather primitive. Yet in this, as in so many things, they were spot on.

Nowadays, to be sure, the word “element” does not mean what the old Greeks meant by it. Chemists now recognize about a hundred basic elements, of which all the tangible components of the universe are constructed. Thus water (H
₂
O) is compounded from two atoms of the element hydrogen and one of oxygen. Carbon dioxide (CO
₂
) is one carbon with two oxygen. Ammonia (NH
₃
) is one nitrogen with three hydrogen. And so on.

Early in the nineteenth century came the revelation that flesh, too, is chemistry: that it is not extra-special “vital” stuff but is made from the ordinary elements of the universe. Thus the science of biochemistry was born. Carbohydrates such as sugars, starch, and cellulose, and fats from sources including fish, vegetable oils, and waxes are all compounded exclusively from carbon, hydrogen, and oxygen. Proteins are made from carbon, hydrogen, oxygen, and nitrogen, with a touch of sulfur. The stuff of which genes are made, DNA, and its companion nucleic acid, RNA, is compounded of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

In practice life is not so simple, and virtually all organisms also need a fairly long catalog of additional minerals to fill out the details, including metals such as calcium, sodium, potassium, magnesium, iron, zinc, and manganese, as well as nonmetals or quasi-metals such as molybdenum, boron, chlorine, and (in animals) iodine. But the bulk of all flesh is compounded from the big six: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Carbon is the key player, forming the core structure of all life’s most characteristic molecules. The grand word “organic,” at its most basic, simply means “containing carbon.” I have known nonscientists to object to such discussion: to “reduce” life to mere chemistry, they argue, is to demean it. But this is to misconstrue the nature of chemistry. That the simple elements, suitably arranged, can give rise to living things shows how wonderful they really are. Life, in all its extraordinariness, is implicit in the fabric of the universe. We can only guess what else the universe might be capable of.

All this seems to make the pre-Socratic Greeks look a little silly. The tangible universe and all living things within it are constructed from a hundred elements, in a billion billion combinations, each interacting with all the rest. What sense can it make to suggest that everything is made from air, fire, earth, and water?

All the sense in the world, is the answer—at least when we are talking of trees.

EARTH, WATER, AIR, AND FIRE

Living tissue is complicated: it is built from many different components. More to the point, it is “alive.” It is constantly replacing itself, even when it seems to stay the same. It is not a thing but a performance. The physical components from which living tissue is built are acquired in the form of nutrients; and the incessant self-renewal—metabolism—requires a constant input of energy.

For animals, nutrients and energy seem to amount to the same thing. Both must be supplied in their food. Animals get most of their energy by breaking down carbohydrates and fat. They acquire this energy-rich provender either by eating other animals or by eating plants—or both, as human beings do. Organisms like us, which need their food ready-made, are called “heterotrophs.” But the buck has to stop somewhere—and in most earthly ecosystems, it stops with plants. Plants make their own carbohydrates, fats, proteins, and everything else they need from raw materials—simple chemical elements, and the simplest possible chemical compounds. They obtain the energy to do this from the sun. They are “autotrophs”: self-feeders.

The key to autotrophy is photosynthesis. Within their leaves plants harbor the wondrous green pigment known as chlorophyll. Chlorophyll traps units of energy—photons—from sunlight. Then, acting as a catalyst, it uses the photon energy to split molecules of water. Where there was H
₂
O, now there is H plus O. The O—oxygen—floats away into the atmosphere as oxygen gas. If it weren’t for photosynthesis, there would be no oxygen gas at all in the atmosphere, and creatures like us could never have evolved at all.
¹
The interesting bit in this context is the hydrogen, which is then combined, within the leaf, with carbon dioxide gas from the atmosphere. Thus simple organic acids are created, compounded from carbon, hydrogen, and oxygen. These simple compounds, with a little more maneuvering, are transformed into sugars (the simplest carbohydrates). When the sugars are modified a little more, they become fats. Add nitrogen, and they can be made into proteins. Incorporate a few other chemical elements, and all the components of living tissue can be made. Chlorophyll itself is basically a protein, with some magnesium at its center.

Green plants are engines of photosynthesis. It is what they do, their raison d’être, and we should be properly grateful that it is, for without their ingenuity and labor, insouciant heterotrophs like us could not exist. Trees are the greatest of nature’s engines of photosynthesis. Their need to photosynthesize explains the whole, vast, elaborate architecture of the tree. Leaves are the meeting place of carbon dioxide (wafting in from the air), water (drawn up from the ground), and sunlight. All are brought together in the presence of chlorophyll, which acts as host and mediator. Leaves archetypically are flat and thin, to expose the chlorophyll within them to as much sunlight as possible. The chlorophyll is held in loosely bound cells in the middle layers of the leaf—a spongy arrangement, so air can circulate freely. The air enters through perforations underneath the leaf, known as “stomata,” which open and close according to conditions (generally closing when it is too dry and the leaf is in danger of wilting, and also, typically, when it is dark). All green plants do all this—but trees, the greatest of plants, hold their leaves as high in the sky as possible, for maximum exposure to air and sun. The water (and minerals) comes mainly from the ground—sometimes from deep below the ground—and so must be carried upward through all the length of the roots and trunk and branches to the leaves aloft.

Yet the trunk of the tallest trees—redwoods and Douglas firs and some eucalypts—may be 100 meters tall. The roots may add a great deal more to their length: those of trees such as eucalypts that may live in semidesert, as well as the native trees of Brazil’s dry Cerrado, may reach down for tens of meters. The longest known roots of all belong to a South African fig: 120 meters. The whole vast and intricate structure is evolved to bring air and water together in the presence of sunlight.

So the old Greeks were absolutely right. Trees, at least, are compounded from earth, water, and air, and the sun that powers the whole enterprise is the greatest fire of all, at least in our corner of the universe. Other ancient mythmakers conceived of trees as the link between earth and sky, and they were right too. That is exactly what they are.

But how can a tree take water from such depths to such heights?

THE PROBLEMS OF WATER

Some plants, especially epiphytes, which often grow high above the forest floor, derive some or most of their water from the air. Some trees do this too: extraordinarily, the mighty redwoods of California get about a third of their water from the morning fogs that sweep in from the Pacific. Mostly, however, trees draw water up from the ground, through the conducting vessels of the xylem, coursing through their trunks and branches. It would be wonderful with X-ray eyes to see a forest without the timber. It would be a colony of ghosts, each tree a spectral sheath of rising water.

But how does the water rise up to the leaves? After the Italian physicist Evangelista Torricelli (1608–1647) showed that air has weight, some suggested that water is driven up through plants by the pressure of the atmosphere bearing down upon their roots. Yet it was obvious from the start that this could not work. Many atmospheres of pressure would be needed to drive water from the depths of the earth to the top of a big tree. Now it seems that the water is not pushed up to the leaves but is sucked up from above by a combination of osmosis and evaporation. The sap in the cells of the leaf interior is a concentrated solution of minerals and organic materials, and water from the conducting vessels flows into them by osmosis. Because the cells of the leaf interior are open to the air (via the stomata), the water within them evaporates and exits via the stomata (and to some extent through the leaf surface in general). As water is lost, so the sap that remains in the leaf cells becomes more concentrated—and so more water is drawn from below.

So water is not pumped from below but dragged from above by the leaves, up through the vessels of the xylem, not in a crude and turbulent gush but in millions upon millions of orderly threads. Each liquid thread is only as thick as the bore of the conducting vessels: the biggest are 400 microns across (0.4 of a millimeter) and most are far smaller than this. The tension within them is enormous: the threads are taut as piano wires. Yet, except under conditions of severest stress, they do not break. Water molecules cling tightly together. Their cohesive strength is prodigious. Were it not so, trees could not pull water from below, and could not grow so tall; but in practice the forces are such that a tree could grow to a height of nearly two miles if the tensile strength of water was the only constraint on its growth. Even as things are, the threads of water may sometimes break—an accident known as “cavitation”—leaving a space in the vessel that a plumber would call an air lock and a surgeon would call an embolus. Given time and favorable conditions, plants can eventually fill this space again, and normal service is resumed. Otherwise, if cavitation is too great, the tissues that depend on the vessels may die. Parasites such as the mistletoes increase the tendency to cavitation because they take in the conducting vessels of their hosts and extract water from them by transpiring more quickly than the host, thus creating even greater osmotic tension than the host itself. Sometimes in these conditions the water supply gives out. Mistletoes are wonderful and have launched a thousand myths, but they may kill their hosts, not least by desiccation.

The final evaporation of water from the leaves, out through the stomata, should perhaps be seen as a side effect of the whole mechanism. The evaporation may bring benefit because it cools, as sweating does, and it’s hot out in the sun. Some drought-tolerant plants practice a refined form of photosynthesis known as crassulacean (because it was first discovered in succulent plants of the genus
Crassula,
much beloved of gardeners) that is designed (or evolved) to reduce the loss of water. In such plants, the stomata open only at night. The carbon dioxide that is taken on board during the night is then put into temporary chemical storage and is released again the next day, when the sun comes out and photosynthesis can resume. No tree that I know practices crassulacean photosynthesis, so it is not directly relevant here—except to say that these plants at least demonstrate that it is possible to live out in the sun without overheating, even when water is not lost by evaporation. So it seems that most plants (including all trees) lose water through the stomata simply because this is very difficult to avoid—or, at least, the loss is a price worth paying to maximize the efficiency of photosynthesis. The
point
of the plant’s architecture—all those conducting vessels, all those perforated leaves—is to bring the Greek elements together: to present water to the sun, in the presence of air. But it is hard to bring them together without losing water, and sometimes losing more than the plant would like.

The overall effect is a flow of water from the roots through the vessels, to the leaves, and out to the atmosphere: trees act like giant wicks. The final loss of water by evaporation is called “transpiration”; and the total flow of water from soil to atmosphere is the “transpiration stream.” The overall magnitude of this stream, especially when several trees are gathered together, can be prodigious; and its effect on soil and climate, and thus on surrounding vegetation and landscape, is critical to all life on earth, including ours. (I discuss this further in Chapter 14.)

So to the earth, the fourth of the four Greek elements. In this context, earth means soil.

THE SOIL

Air and water provide the carbon, oxygen, and hydrogen that are the most basic materials of plants. The soil provides all the rest: nitrogen, phosphorus, sulfur—which (apart from carbon, hydrogen, and oxygen) are the materials required in the greatest amounts—and a host of metals. All these extra elements from the soil are collectively known as “minerals.” If any of these essential minerals is lacking (or, indeed, if carbon or water is lacking) then growth is restrained, if not impossible: whichever ingredient it is that is deficient, and holding up the rest, is called the “limiting factor.” In most soils, the most likely limiting factors are nitrogen, phosphorus, and potassium: and these are the three standard components of the artificial fertilizers used in agriculture, which accordingly are packed in bags marked “NPK” (K being the chemical symbol of potassium).

In truth, such fertilizers should probably contain sulfur as well; but the fields and forests, at least of industrialized countries, have been well if dubiously served these past two hundred years by the sulfur-rich smoke from coal-burning factories, which has kept crops and trees alike well supplied. Now that factories are burning cleaner fuels, crops could well become short of sulfur, and we are likely to see fertilizer bags marked “NPKS.” Nitrogen, too, has rained on plants from on high, mainly in the form of the ammonia and nitrate from car exhaust; in fact, there has been enough nitrogen from such pollution to keep the forests of Europe growing steadily, even when the timber and fruits are harvested. It is indeed an ill wind that blows no good. On the other hand, sulfur and nitrogen in the form of sulfuric and nitric acids fall as acid rain, and have often proved extremely harmful. To enrich the soil by polluting it is a precarious way to proceed.