The Tree (11 page)

By the same token, different species have different patterns of grain and “figure.” Grain refers to the narrow stripes that run along the length of the wood and appear when the growth rings are cut across, and figure involves the general appearance, whether the growth rings are cut across or not. These variations represent differences in microarchitecture. Clearly, it is vital to a tree that its wood should be functional. Equally clearly, the very fine details of structure do not matter too much—especially in the heartwood, whose only tasks are to provide strength and bulk. We can imagine, then, that trees contain a variety of genes that in some way or other influence grain and figure—to the extent that experts can (usually, and at least in theory) identify the species of any kind of timber from these patterns. The small genetic variations that cause these differences do not matter to the tree.

There can be enormous variation among the different individuals of any one species, too, which again is partly genetic. Grain and figure may vary, just like human fingerprints. There may be no specific benefit from such variation. But if there is no great natural selective pressure not to vary, then variations will creep in. Genomes are not commandments, which say exactly what to do come what may. Genes present options. They operate in dialogue with the environment. So the same tree, grown under different circumstances, could grow in very different ways; and the effects of the different circumstances are reflected in the timber.

Thus growing timber responds to stresses and strains and pressures just as the bone of mammals may do. A big horizontal branch puts enormous strain on the point of contact with the trunk. In coniferous trees, such as pine, you will often see that the base of the branch, where it meets the trunk, is not round. It will be oval: the branch beneath is bolstered by “compression wood,” like a corbel in a cathedral holding up a beam. Broad-leaved trees adopt the same idea—but use totally opposite physics. In broadleaves the extra reinforcement is
on top
of the big horizontal branch. The timber added above is “tension wood”: it acts as a guy rope.

Around the base of tropical trees from many different families you commonly see buttress roots, which take many forms; commonly and bizarrely they are like the fins of a rocket ship, thin vertical triangles of timber protruding from the sides and sometimes extending upward to three meters or more. They can be impressive structures. Yet they are not truly buttresses, for buttresses are under compression; the buttresses of cathedrals support the walls by pushing against them. The buttresses of tall tropical forest trees are again like guy ropes, under tension. More generally, a tree exposed to the wind somehow “knows” that it is being shaken and grows thicker.

Heartwood is usually very different from sapwood. Often heartwood is very good at resisting pressure—it has high “crushing strength”—while sapwood has high tensile strength. The archers of medieval England made their longbows from the timber of yew, from the particular part of the trunk where heartwood meets sapwood. The former is dark in color and has great crushing strength; the latter is lighter and very flexible. With the dark heartwood on the inside and the light sapwood on the outside, the yew bow gave tremendous spring. Result: a very powerful bow. Indeed, the English archers made short work of the French knights at Crécy in 1346, and again at Agincourt in 1415. You would think the French knights would have learned, but apparently not. Unfortunately, the best yews for the purpose came from Spain, with whom the English, at least later, were also intermittently at war. However, “total war” is a twentieth-century concept, and as late as the eighteenth century the great English navigator James Cook was able to replenish his ships at French-owned ports in the Pacific, even though England was (again) at war with France. So perhaps the English had less trouble buying Spanish yews than might be imagined. Business is business.

Sometimes there is internal tension in a tree that contributes to its strength, in the same way that steel under tension is sometimes used to reinforce concrete. Eucalyptus is often like this. When eucalypts are cut the tensed tissues within them are free to uncoil, and the timber may split even as the tree is falling; and when a eucalyptus burns (which it will do when the flames are hot enough, even though eucalypts as a whole are fire-adapted) it may explode, both with the tension and because of the oils trapped in its timber.

The grain makes its way around the bases of branches growing from within; and the cut branch bases form the knots in wood. Some trees, including oaks and redwoods, produce anomalous masses of buds that come to nothing but persist to form burls. Timber grows around the burls, and its grain may be all over the place. The grain may go this way and that, too, around the bases of trees and in parts of the buttress roots. Builders want straight-grained wood, for maximal strength and predictability. But makers of veneers, as well as turners, interested in decoration, love burl wood, and will pay hugely for it.

In forests, trees grow straight and tall, anxious for the light—which on the whole is how builders like it. Trees grown in open spaces may spread themselves like a Persian cat on a feather bed and take all manner of wondrous forms. Thus the beeches of England’s many fine forests tend to be straight and tall as towers, while the pampered specimens in Kew Gardens, with no deer or horses to browse their lower branches and armies of gardeners laboring for over two hundred years to keep competitors out of their light, are spherical as golf balls—albeit twenty meters or so in height. The oaks of ancient windswept Scottish hillsides commonly had bent branches—of particular use to shipbuilders, who could fashion the keel and the prow around the natural shape.

Finally, the timber may vary in color and figure depending on soil and even on infection. Wood may be colored by minerals—blue or green by nickel, red or black by iron. Some trees, like the zebrawood
Microberlinia
(another of the family Fabaceae), are beautifully striped naturally. Others are striped with color by fungal infections—red, black, whatever. Infections are not all bad. In the tulip frenzy of seventeenth-century Holland, striped blooms were the most highly prized—and the stripes were caused by a virus. (Viruses were not identified as discrete organisms until the twentieth century. But the craftsmen and breeders of earlier centuries had a good working knowledge of disease and knew how to produce striped flowers to order.) Cheeses are beautifully veined by
Penicillium
and other fungi; and winemakers speak of a botrytis fungus as “the noble rot.” The fungus that decorates a tree from within may rot the wood, to be sure—but when the fungus itself is killed off, it remains in colorfully suspended animation effectively forever, and again, the results are highly prized by turners.

Thus wood is not only wonderful, it is also endlessly various. If humanity had only one kind of timber to draw upon, it could think itself blessed (although we are an ungrateful lot). But in practice we have many thousands—a tree for every job, and for every decorative caprice. If timber is appropriately grown and selected, its qualities can be as tightly specified as steel, and it can be used for the most exacting tasks. Thus in the Second World War the British de Havilland company built enormous numbers of Mosquito light bombers, which incorporated ash, spruce, birch, and balsa, each minutely specified. Timber still plays some part in modern aircraft—and of course in ships and boats, even those with fiberglass hulls. Timber, too, could undoubtedly replace much of the steel now used in the biggest buildings—which surely would be friendlier to the planet, since it takes far less energy to prepare a beam of teak, say, than to make a girder. Furthermore, timber is composed primarily of the element carbon, which is derived from the atmosphere in the form of carbon dioxide. It therefore serves as a carbon “sink”: a wooden beam will lock up the carbon of which it is composed for as long as the building that it helps to form continues to stand. After the building has run its course, the timber can be recycled. The prestige buildings of the future, like those of the distant past, could with great advantage be constructed of timber.

On the other hand, if you want a fruit bowl or a bureau that is both unique and beautiful, and does not have to endure the strains of a plane or a boat or a tower, there are endless capricious twists of pattern and color to draw upon.

Yet all this benison is merely a bonus; for the real point of wood is to enable plants to grow big and lift their photosynthesizing leaves high into the air and sunshine, yet keep them bathed in water drawn from the earth. Although we can grow trees, and some people can fashion them in many marvelous ways, we could not have designed such a material with such microarchitecture in a thousand years, or ten thousand, and indeed we have not done so; for even our most remarkable modern synthetics do not begin to compete, in versatility and functionality and beauty, with what nature has provided. Such is the power of evolution.

These are the generalizations. In the next six chapters, I want simply to wallow in the glories: an overview of all the trees (at least, the conifers and angiosperms) that nature has left us with.

Some living bristlecone pines are as old as all written history.

5

Trees Without Flowers: The Conifers

A
MONG THE CONIFERS
are the world’s tallest trees (California’s coastal redwoods), the oldest (California’s bristlecone pines), and some of the most drought resistant (a cypress in the midst of the Sahara), while various species fill some of the vastest forests in the world’s most extreme and dramatic landscapes. Yet the conifers that are left to us are, as botanists are wont to say, “relicts.”
^*1
Conifers first appeared on earth nearly 300 million years ago, in the Permian period, long before the dinosaurs, and their heyday lasted until at least 50 million years ago, well into the Tertiary, which zoologists chauvinistically call “the age of mammals”; and so the earlier (but already ancient) types were browsed by diplodocuses and iguanodons, while their descendants saw the world’s first elephants and horses and cats, and the world’s first squirrels and primates cavorted in their branches. Over all that time the conifers have given rise to scores of families, containing goodness knows how many genera and species. Now only eight families are left to us, with seventy genera—three-quarters of which have only five species apiece, or even fewer (some are down to one). In all, only about 630 species of conifers are known. Doubtless there are many more still to be found, not least in the uplands of Southeast Asia and Venezuela, but they are still vastly outnumbered by the 300,000 or so species of flowering plants. What’s left, therefore, is but a shadow of what there has been: “relicts” is the word. But what’s left, nonetheless, is magnificent and endlessly intriguing.

In truth, since the early Cretaceous, about 100 million years ago, the conifers have been steadily upstaged by the angiosperms. All conifers are woody. Most are trees, although some are ground huggers. None live as epiphytes; and just one makes a living as a parasite. There are about fifty times more species of flowering trees than of coniferous trees; yet most flowering plants are herbs, which between them have adopted every known form and way of life—vine, liana, annual, perennial, epiphyte, aquatic plant, and several thousand species that live as parasites.

Broadly speaking, conifers now flourish in conditions that flowering plants find especially difficult. Between them they can hack any kind of climate, from tropical to almost arctic. Of course, they will not grow in extreme desert or at the poles—no tree can—but in Siberia the spruces are matched only by birches (sometimes) in the extreme north, and in Canada the pines are rivaled (sometimes) only by aspens. On the whole conifers are excellent pioneers, invading soil that has been variously devastated and has not yet built up fertility. But in good or adequate soils and in reliable climates, where growing should be easy, conifers tend to be ousted by angiosperms. So there are no native conifers at all in the vast tropical forests of central Africa and Amazonia. Yet conifers do thrive in highland tropical rain forests, where conditions are somewhat less easy—clambering, for example, up the hillsides of Southeast Asia.

In practice, in the wild, conifers come into their own where the soils are poor or badly drained, or where conditions are in other ways uncertain. Often they succeed by forming mutually helpful relationships with toadstool-like fungi (basidiomycetes); these invade the roots of the trees, but in a benign fashion, and hugely extend their absorptive powers. Such symbiotic associations are called “mycorrhizae.” Broadleaves too form many such symbioses, but many conifers seem particularly adept at them. Conifers also often thrive in places that are particularly beset by fire. The cones of coastal redwoods and of many pines will not release their seeds unless the cones are first cooked in a forest fire (though if it’s too hot it burns them up completely).

In general, conifers are light lovers—an odd thought, as you wander through the green shade of the redwood forest, or peer through the close-set boles of some spruce plantation, or contemplate the long, dark winter months of spruce and pine in the subboreal forests of the Baltic or the truly boreal forests of Alaska and Canada, Scandinavia and Russia. So in general they can do well in the company of angiosperms when they are tall enough to overshadow them, but not when they are overshadowed themselves. To this end they have a trick. Many can grow very tall very quickly. Thus one of the biggest of the living giant sequoias is called the Boole Tree (another great tree with a personal name) and is thought to be around three thousand years old. But when space was cleared around it, other sequoias leapt in—and within a hundred years were just as tall as the mighty Boole. If these newcomers are spared, they will spend the next few thousand years growing thicker.

This, too, is why the conifers of the far north tend to be tall and thin: the sun is always low in the sky, so they get most of their light from the side. Their steeple shape (says Dr. Farjon) is not primarily a way of keeping off the snow, as is often suggested. Conifers of lower latitudes, where the light comes from overhead, tend to be shorter and flat on top—like the lovely and characteristic stone or umbrella pines of southern Europe (
Pinus pinea
), which turn up in the background of Mediterranean paintings. Perhaps, too, conifers feature on tropical hillsides because the slope gives them a grandstand view of the sun (though as Dr. Farjon points out, this would be the case only in the hours after dawn and before dusk). All generalizations are dangerous in biology, however. Some conifers do grow happily as understory trees in the shade, including that somber denizen of English graveyards, the yew, and the cypresslike
Thujopsis
of Japan, which grows slowly at first as an understory tree, until it overtops its neighbors.

Despite their limitations, conifers are a huge presence through most of the world—in the Americas, Eurasia, Australasia, and, in the past, so the fossils show, in Antarctica. In the north, the greatest centers of conifer diversity are California, Mexico, a slice of eastern China that embraces Sichuan and Yunnan and extends up to the eastern Himalayas, Japan, and Taiwan. Taiwan even has a genus named after it, the cypress relative
Taiwania.
In the Southern Hemisphere, the greatest diversity of conifers is not on the great southern continents but in New Caledonia, an island about the size of Wales or Massachusetts, slap in the midst of the South Pacific, halfway between Australia and Fiji. The Web sites for New Caledonia dwell primarily on its beaches and nightclubs. Some mention its unique wildlife, but none refers specifically to its fabulous trees (particularly its araucarias). Ah, well.

India, however, is strangely deprived of wild, native conifers. Conifers grow very well in India—on plantations. But apart from a few Eurasian types in the Himalayas, the only living native is
Nageia wallichiana
of the Southern Hemisphere podocarp family in the Western Ghats in the southwest of the country. The reason might be historical. Ancient India was wiped clean about sixty million years ago by the huge Deccan volcanoes, which buried a great part of the subcontinent in lava. The angiosperms, by then well established, seem to have been the first to get back into the devastated land (although this idea clearly does not chime well with the conifers’ reputation as outstanding pioneers).

Conifers are also largely absent from oceanic islands—the kind that arise as volcanoes (such as Hawaii) as opposed to those that are fragments of continents (such as New Caledonia). At least, conifers may be found on volcanic islands that are close to continents, but the farthest from any continental shore is the juniper
Juniperus brevifolia,
on the Azores. The seeds of pines and most other living conifers are winged and wind-blown, and do not generally travel far over oceans. But juniper seed cones are fleshy and tasty (they are the “berries” that flavor gin) and are eaten and dispersed by birds—which not only travel vast distances but can also control their landing, as wind-blown seeds cannot.

Conifer means “cone-bearing.” All cones are either male or female; they are never hermaphroditic, as many flowers are. Many conifers bear both male and female cones on the same individual (“monoecious”), while others (like yews and most podocarps) have only one sex per tree (“dioecious”). For conifers, reproduction is often a leisurely affair. Many weeks may pass between the transfer of pollen from the male cone to the female and actual fertilization, when the pollen tube grows into the ovule. The female cone, when fertilized, may take several years to mature. Some conifers shed their mature cones, and some retain them on the tree. In some, like the knobcone pine (
Pinus attenuata
) and the Monterey pine (
Pinus radiata
), the growing branches may envelop and eventually encase the old cones (which makes for some interesting patterns when the timber is sawn across, again much favored by turners). Firs, pines, true cedars, and so on have the classical cones we all admire and many like to collect; but in others, like the yews, the lower part of the seed coat grows up around the developing seed to form a fleshy “aril,” superficially like a fruit. In junipers, the cone scales fuse and become succulent or pulpy, imitating a berry. In
Podocarpus,
the basal parts of the cone below the developing seed swell up to form a brightly colored, fleshy receptacle. The fleshy “fruits” of yews, junipers, and podocarps could well have evolved early in conifer history, to be dispersed by birds and sometimes by mammals. Mammals are ancient, after all—dating from the Triassic, which again is predinosaur; and birds date from the Jurassic.

In the timber trade, conifers are lumped together as “softwoods,” while the broadleaves are “hardwoods.” This is rough and ready, for some conifer timbers (like yew) are far harder than some angiosperm timbers (like balsa)—but no conifer timber is as hard as the hardest broadleaves, some of which can be worked sharp as steel, like oak and mahogany, and some of which are too hard to be worked at all except with tungsten and diamond tools that are too expensive to be worth the trouble (some hardwoods are even spiked with silica, which makes them that much more difficult).

WHO’S WHO AMONG THE CONIFERS

The classification of conifers has wavered a little these past few decades. A common traditional taxonomy recognized eight families: Araucariaceae, Cephalotaxaceae, Cupressaceae, Pinaceae, Podocarpaceae, Sciadopityaceae, Taxaceae, and Taxodiaceae. Most modern taxonomists, however (including Aljos Farjon), merge the Taxodiaceae with the Cupressaceae, reducing the list to seven. But some (including Dr. Farjon) divide the Podocarpaceae into two—splitting off the “celery pines” into the Phyllocladaceae. So now we are back to eight.

Note, in the following account, that three of the eight families contain only one genus, and only three families contain more than ten genera: the Cupressaceae with thirty, the Pinaceae with eleven, and the Podocarpaceae with eighteen. In all, Dr. Farjon now recognizes seventy genera—many with only one or a very few species. This is typical of ancient groups that have found just a few niches in the modern world—“few” being a relative term that in practice means extensive and various.

Of the eight families, the Podocarpaceae live mainly in the Southern Hemisphere, and the Araucariaceae is exclusively southern. The rest are primarily or exclusively northern (although, of course, human beings have introduced trees from all families to just about everywhere). In the deep past all the world’s landmasses were grouped in two vast supercontinents, Gondwana to the south and Laurasia to the north. So it is tempting to speculate that the Podocarpaceae and the Araucariaceae originated in Gondwana, and the other families in Laurasia. But their places of origin are far from certain. In the past, as we will see, there were araucarians in the north as well as the south—the south is where they just happen to have survived. The modern cypress family, Cupressaceae, occurs all over the world. I
like
the idea that the Araucariaceae and Podocarpaceae are Gondwanan, and perhaps the Cupressaceae, and that the rest are Laurasian. But I have sometimes been forced to acknowledge that some of the things I would like to be the case aren’t.

The relationships among living conifers, and between the living and the extinct, are not easy to pin down. Morphology (structure) is the main guide to relationships, but the various conifers seem to have too few distinctive features, and sometimes the most important structures—like the cones of podocarps—are too reduced to make much sense of. Often it is hard to decide whether any one particular feature is “shared, derived” and signifies close relationship or is merely “primitive” and common to everybody. Molecular studies should help to clarify things, but as yet there seem to be too few. So it is not yet entirely clear whether the various conifer families, as now recognized, all form truly coherent groups (true clades); and it is far from clear how and to what extent the various recognized living families relate to one another. Thus some taxonomists show the Pinaceae as outliers—the sister group of all the rest. But others group the Pinaceae with the Podocarpaceae and Taxaceae. I have described the families in no special order.

K
AURIS
,
THE
M
ONKEY
P
UZZLE
,
AND THE
L
ONG-LOST
W
OLLEMIA
:
F
AMILY
A
RAUCARIACEAE

The Araucariaceae were very various in dinosaur times (245 to 65 million years ago), and they lived all over the world. Now there are just forty-one species left, in three genera
—Agathis, Araucaria,
and
Wollemia—
all in the Southern Hemisphere. The greatest variety of Araucariaceae is on the magical Pacific island of New Caledonia, perhaps the most pristine remaining fragment of ancient Gondwana.

Among the twenty-one species in the genus
Agathis
is one of the mightiest trees of all:
Agathis australis,
the kauri of New Zealand’s North Island. The grandest of the grand is Tane Mahuta: it is 51.5 meters tall, its lowest branches are nearly 18 meters above the ground, and its trunk is 13.77 meters in girth—nearly 4.5 meters in diameter—which means it would touch all four walls if planted in an average suburban living room. I have stood at the huge buttressed feet of Tane Mahuta. It is surrounded by other enormous trees, but it makes them seem ordinary. Its trunk rises out of the gloom like an iceberg in the Southern Ocean. The mass of epiphytes it holds aloft in its great spreading boughs is a fantastical, floating garden. It must have supported entire dynasties of lizards and invertebrates who never went anywhere else and might have thought, if they could think at all, that Tane Mahuta was the whole world.