The Secret Life of Trees

by Colin Tudge

  There can be enormous variation between 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. But 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 broadleaf trees, such as oak, 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. Conifers adopt the same idea – but use totally opposite physics. In conifers the extra reinforcement is on top of the big horizontal branch. The timber added above is ‘tension wood’: it is acting 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 upwards to 3 metres 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 heart-wood is very good at resisting pressure – it has high ‘crushing strength’ – while the 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 colour 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. 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 learnt, 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 we were (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 burrs. Timber grows around the burrs and then 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, and turners, interested in decoration, love burr wood, and will pay hugely for it.

  Trees grown in forests 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 woods tend to be straight and tall as towers while the pampered specimens in Kew Gardens, with no deer and horses to browse their lower branches and armies of gardeners over 200 years to keep competitors out of their light, are spherical as golf balls – albeit 20 metres or so in height. The oaks of ancient windswept Scottish hillsides commonly had bent branches – of particular use to ship-builders, who could fashion the keel and the prow around the natural shape.

  Finally, the timber may vary in colour and figure depending on soil and even on infection. Wood may be coloured 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 colour 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–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 wine-makers speak of yeast as ‘the noble rot’. The fungus that decorates it from within may rot the wood, to be sure – but when the fungus itself is killed off, it remains in colourfully suspended animation effectively for ever, 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 build enormous numbers of Mosquito light bombers, which incorporated ash, spruce, birch and balsa, each minutely specified. Timber still plays an enormous part in modern aircraft – and of course in ships and boats, even those with fibreglass 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, then there are endless capricious twists of pattern and colour 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 marvellous ways, we could not have designed such a material with such micro-architecture in a thousand years, or ten thousand, and indeed 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.

  So these are the generalizations. In the next four 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.

  II

  All the Trees in the World

  5

  Trees Without Flowers: The Conifers

  Some living bristlecone pines are as old as all written history

  Among 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’.* Conifers first appeared on earth nearly 300 million years ago, in the Permian, 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 diplodocus and iguanadons, 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 (and some are down to one). In all, only about 630 species of conifer are known. Doubtless there are many more still to be found, not least in the uplands of South-East 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, beginning in the 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 – climber, 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 rivalled (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 on 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 rainforest where conditions are somewhat less easy – clambering for example up the hillsides of South-East 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 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 sub-boreal 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 3,000 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 understorey trees in the shade, including that sombre denizen of English graveyards, the yew, and the cypress-like Thujopsis of Japan, which grows slowly at first as an understorey tree until it overtops its neighbours.

  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 centres 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 websites 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 – in plantations. But apart from a few Eurasian types on the Himalayas, the only living native is Nageia wallichiana of the southern hemisphere podocarp family in the Western Ghats in the south-west of the country. The reason might be historical. Ancient India was wiped clean about 60 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 in to 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 furthest 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 flavour 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: never hermaphrodite, 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 affa
ir. 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 favoured by turners). Firs, pines, 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 coloured, 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 pre-dinosaur; 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 even more difficult).