by Colin Tudge
A young yew—with perhaps another two thousand years to live.
The terms “wood” and “timber,” like the word “tree,” tend to be used in different ways. Some people define wood loosely, and some more narrowly. Loosely, the word refers to the hard skeletons of conifers and flowering (angiosperm) trees. But some botanists and foresters don’t like to think of monocot wood—the kind that comes from palms and bamboo and so on—as “true” wood because it has a quite different structure. For them, true wood comes only from conifers and broad-leaved angiosperms—the broadleaves being all the dicots, from magnolia to oak and teak.
This description focuses on “true” wood, as in conifers and broadleaves. In both cases, the basic component of the wood, which makes it both functional and strong, is the conducting tissue, the basic plumbing. This tissue is of two main kinds. On the inside is the xylem, a mass of tubes that carry water with dissolved minerals up from the roots to the leaves. In broadleaves, most of the xylem tubes are open all the way along, but in conifers they are interrupted by perforated plates (and this is the chief difference between the two types). The second group of conducting tissues form the phloem, strings of cells that carry the products of photosynthesis (organic materials of various kinds, which basically are variations on a theme of sugar) out from the leaves, downward and outward to the rest of the plant. The tissues of the phloem are on the outside. Collectively the phloem forms a cylinder, enclosing the solid column of xylem within.
So all in all, you can imagine wood as a close-packed bundle of straws, bound tightly together into a solid whole. But now add one more element to the image. Imagine that swords were thrust into the bundle from the outside, slicing between the bundles—running from the outside toward the middle. The “medullary rays” run in just this fashion from the center to the outside. These blades of tissue provide some linkage between the different elements of the xylem and phloem and also act as a food store for the whole trunk. By carrying material inward and outward, the rays enable the trunk to increase in diameter as the tree grows. More generally, they help to ensure that the trunk is itself a larder, to be drawn on as required.
But where does the growth come from? How can the trunk increase in thickness and yet be continuously functional? Here is where the subtlety really begins. Between the xylem and the phloem is a thin layer of tissue known as cambium, which forms a sheath, running from roots to leaves. The cambium is stem-cell tissue: the kind whose job it is to generate more tissue. It generates more xylem vessels on the inside, and more phloem vessels on the outside. So the tree grows thicker year by year—and yet the trunk is always functional. Always, fresh xylem and phloem are coming on line. Herbaceous plants and young trees, of course, have some thickness to them from the outset. A tomato stalk grows thicker as the season wears on—more and more cells are produced, all puffed up by water pressure within the cells. But only conifers and broadleaves have the complete sheath of cambium, not far from the surface, that allows the tree to go on getting thicker year by year, perhaps for centuries. This is the phenomenon of “secondary thickening.” Other trees that are not conifers or broadleaves may practice secondary thickening up to a point. Cycads do. The lycophyte tree, Lepidodendron, apparently did. (Palms don’t. In general, they begin life short and fat and stay at the same thickness until they are twenty or so meters tall.) But no trees apart from conifers and broadleaves have taken secondary thickening to such a peak. It is the final requirement and accomplishment of true treedom (at least up to now).
The cells that form the tubes of the xylem soon die. In fact, in order to become fully functional, they need to die. They lose their living cytoplasm; all that is left is the cell wall, cellulose stiffened with lignin. However, as time passes, the cells both of the xylem and phloem not only die but lose their function as conducting tissue. Clearly, in any one tree trunk, the xylem closest to the center is the oldest; it may have been laid down ten, a hundred, even a thousand years earlier. But xylem that is more than a decade or so old tends to be increasingly blocked, not least with tannins. So the center of a tree becomes increasingly solid. Not only are the individual cells dead, the whole structure loses its ability to transport water. Phloem is the mirror image of xylem: its oldest vessels are on the outside, and they become crushed as new phloem tissue is laid down inside them.
But although their days as conducting tissues are over, the very dead xylem in the core of the tree and the crushed phloem on the outside do not cease to be functional. The very dead, commonly tannin-soaked xylem within becomes the “heartwood”; and the newer xylem outside it, still serving as plumbing, forms the “sapwood” (because indeed it is full of sap). The heartwood truly provides the skeleton of the tree; it is what enables it to become big. The crushed phloem, outside, becomes incorporated into the bark, providing essential protection. “Bark” in general means everything that lies outside the cambium; the inside layers consist of living phloem, but the layers beyond that are dead. We get a hint of the life that lies just beneath the surface of the tree through the phenomenon of “cauliflory”: the way in which many tropical trees in particular, including the cacao tree, produce flowers and then fruit straight from the trunk or biggest branches.
In trees that grow seasonally, the addition of xylem and phloem is intermittent. In a typical temperate tree, the new xylem laid down in spring is wide but thin-walled, while the summer xylem is narrower but thicker-walled. The differences can be seen clearly and result in a series of concentric “growth rings.” Typically there is one growth ring per season, and so the age of the tree can be gauged. In good growing years the growth rings are broad. In bad growing years, they are close together. Thus, knowing the age of the trees, it is possible to work out the climate of past years. If we cut a mature tree in, say, 2006, we can see what the weather must have been like in, say, the 1850s. Some growth rings might be particularly far apart, and some particularly close together. If we have a piece of timber we know was cut sometime in the late nineteenth century but we don’t quite know when, we can see which of its growth rings correspond in width to the ones of the tree felled in 2004—and which, therefore, correspond to the 1850s. We can then count back and work out when the tree was planted. Then we can overlap that older tree with one that is even older, and so on back. This is the principle of dendrochronology—judging past climates, and the general ages of things, by examining the growth rings of successively older trees. Dendrochronology has provided some remarkable insights in archaeology. (Tropical trees in places where there are distinct wet and dry seasons also show growth rings. Trees where the climate is constant do not.)
Many trees have a layer of secondary cambium, outside the principal cambium layer, with the specific job of producing cork. Cork cells (like xylem cells) are born to die; they finish up small, with thick, impermeable cell walls. Cork is wonderful material: it is light; it is waterproof (hence preventing excessive water loss); it helps repel pests; and it is relatively fireproof. All trees have some corky cells in their bark, and some have a great deal of it. Trees that are most likely to be exposed to fire tend to have the thickest cork—like, of course, the beautiful cork oaks (Quercus suber) of the Mediterranean and the baobabs of Madagascar, Africa, and Australia (which are also used for cork). The one snag from the tree’s point of view is that cork is also airproof, and thus prevents exchange of gases. But it tends to be interrupted by passages of only loosely bundled cells, known as lenticels, which let air through.
Bark too, of course, compounded from formerly functional phloem and custom-built cork, is highly evolved and adapted. Much of the variation is not explicable in terms of function; it just is the way things have turned out. It can be used (by experts) to identify species, just as the pattern of the timber itself can. But bark does have many adaptive features. Some bark, for example, is highly impregnated with tannins to repel pests. The bark of redwood trees is not corky like cork oak but is fireproof nonetheless; it is fibrous and up to nearly a foot thick. Other
s, like Enterolobium ellipticum (it has no common English name), which must endure periodic fires in the dry forest of the Cerrado in Brazil, have huge ridges of corky bark. I suspect the ridges help to create an updraft, which carries the heat up and away from the trunk.
Many trees shed their bark, sometimes in great swaths, which can be helpful in various ways. Some (especially tropical forest trees) seem to shed it in an attempt to get rid of epiphytes, which can grow in great abundance on their trunks and branches and so weigh the tree down and block its light. The bark of eucalyptus is rich in oils and resins and burns quickly and fiercely. Oddly, this is an antifire device. The bark is shed, commonly in shreds, and builds up around the tree as litter. Other plants find it difficult to grow through the chemically rich, dark brew, and so there may be little or no undergrowth. When the bushfires rage they race quickly through the oily, resiny tinder on the ground—and a quick, hot flame is far less damaging than a cooler but slower one. The bark beneath the wisps that are shed is smooth and iron hard, difficult for fire to take hold in. By shedding their bark, London plane trees shrug off the polluting city soot, so they do well as urban trees. This cannot have been an adaptation—the parent species of this hybrid evolved long before cities did—but it is a good example of “preadaptation”: a feature that evolved earlier in some other circumstance, coming by chance into its own.
Clearly, different species produce different timbers. Some are very light and fast-growing. Some are very dense and on the whole tend to grow more slowly. Quite a few are heavier than water, such as lignum vitae and various species of Olea, the genus of the olive. Some timbers are black, some creamy white, some yellow, and some distinctly red.
These are the broad differences. To some extent they seem easily explicable. For instance, pioneer trees—those that invade newly available space quickly—need to grow fast. But since they are soon likely to be overtaken by other trees and will then be overwhelmed, they do not need to be strong enough to endure for a long time. So their timber, typically, is strong and light. A classic pioneer of this type are various species of Cecropia, whose big, silvery, horse chestnut–like leaves are such a feature of tropical forest that has been opened up by storms or logging. But nature cannot be second-guessed—we cannot assume that it will always follow our logic. So it is that some pioneer trees endure the later invasions of other species and are extremely long-lived—like redwoods; and some are not only long-lived but also have very hard timber—like mahogany. The baobab tree of Madagascar (and Africa and Australia), on the other hand, has extremely soft wood, like a classic pioneer, but commonly lives for five hundred years or more. Many other trees begin life in the shade as part of the understory, grow slowly up to the canopy (or wait for a gap to open), and then endure perhaps for centuries. Their wood is likely to be dense and strong, to enable them to live a long time. Some long-lived trees bend with the wind; others outface it. In Britain, the flexible ash and the resilient oak have become symbols of different life strategies. But other differences—including, perhaps, color—seem mostly down to chance. The prime requirement is to produce an organism that works. Many of the genes will have odd effects in addition to the ones that seriously contribute to survival. It is hard to see how it matters to a tree whether its timber is black or white or red or a pleasing buff, but the genes that influence color may be doing something truly useful as well—for example, repelling pests. Or they may not. Provided their side effects do no serious harm, these genes will be passed on through the generations, with whatever eccentricities they bring with them.
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 ma
sses 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.
