The Secret Life of Trees

by Colin Tudge

  The Avicenniaceae family includes Avicennia, which is described in the discussion of mangrove swamps in Chapter n. Until recently, Avicennia was sometimes placed within the Rhizophoraceae, alongside what is perhaps the best-known of the mangrove genera, Rbizo-phora. Now it is clear that its resemblance to Rhizophora is an example of convergent evolution – two plants with a similar way of life, adopting similar solutions to it.

  The Myoporaceae family – about 150 species in four genera from Australia and the south Pacific, with a few in South Africa, south Asia and Mauritius – are mainly trees and shrubs. Best known are the highly drought-resistant genus of Eremophila, which includes the colourful emu bushes and some useful timber trees; and Myoporum, also with some pleasant, aromatic shrubs and some timber trees.

  Ecologically and economically, though, the Avicenniaceae and Myoporaceae families are minor. Of supreme importance in all ways is the Oleaceae family. It includes 600 or so species in twenty-nine genera, grows in tropical and temperate climates, and includes climbers, shrubs and trees. The shrubs include privet, lilac, forsythia and jasmine. The trees include the sixty or so species of ash (Fraxinus); and the twenty or so species of olive (Olea).

  The European ash, Fraxinus excelsior, is one of the great trees. If it weren’t for human beings changing the landscape so radically, present-day, post-Ice Age Britain would be covered virtually from end to end with forests of Scots pine, oak, or mixtures of oak and ash. The timber is creamy white to light brown, sometimes with dark heartwood that is marketed as ‘olive ash’. F. americana is the American or white ash. The timber of both species is wonderfully springy –excellent for oars, tool handles and baseball bats – and also lends itself to steaming and bending: and so it turns up too in bentwood furniture, umbrella handles, and so on. F. ornus is cultivated in Sicily for the sweet gum that it exudes, known as ‘manna’.

  The olive of the Mediterranean, Olea europea, is the species grown for its delectable, oily fruits – not just a treat in those parts but traditionally a staple. The trees are generally small and misshapen but other olive species from Kenya, Tanzania and Uganda such as O. hochstetteri and O. welwitschii grow to 25 metres and provide valuable ‘olivewood’: pale brown with attractive dark curly streaks, highly resistant to abrasion, and valued for floors, furniture, sculpture, turning and veneers.

  The family Bignoniaceae includes some very important trees – although most of the family are climbers and many more are mere shrubs. The 800 species (in 113 genera) are mostly native to South America but others come from throughout the tropics and subtropics, and a few are temperate. The forty-odd species of Jacaranda from South American include J. mimosaefolia, a native of Brazil but grown throughout the tropics and subtropics for its beautiful blue flowers. Jacaranda has fine timber too: pleasantly scented, often patterned with dull purple streaks, and favoured for pianos in Egypt for some reason. Catalpa is one of the Bignoniaceae that grows in temperate climes and in Britain is sometimes called the ‘Indian bean’ because its fruits are long pods, although it is neither a bean nor (I believe) Indian. The extraordinary sausage tree, Kigelia, has fruits that are indeed like big fat salamis hanging from the rafters of some Italian kitchen, though biscuit-coloured. I have seen it in India and my wish list includes a possible meeting in its native Africa. The tulip tree, Spatho-dea campanulata (not to be confused with Liriodendron from the Magnoliaceae), comes from tropical Africa but was introduced to India in the late nineteenth century both for its shade and for its marvellous masses of scarlet flowers, which earned it the soubriquet of ‘flame-of-the-forest’. Some Bignoniaceae are useful timber trees. Paracetoma peroba from the coastal forest of Brazil grows to 40 metres and its pale, golden-olive heartwood is known as white peroba and is highly favoured for furniture, boats, floors and veneers. Even catalpa provides fence posts.

  Now comes scope for possible confusion, which I will do my best to ameliorate. In traditional texts you will find two families that have long been thought to be closely related: the Verbeneacae and the Lamiaceae. (Confusion is compounded because the Lamiaceae was formerly known as the Labiatae, and members of the Lamiaceae are still commonly called ‘labiates’, at least by people like me, in whom old habits die hard.) The Verbenaceae family was named after the perennial Verbena and also included Lippia, the lemon verbena, and Lantana, the South American ornamental that is now the mother of all pests, taking over tropical waysides and forests just about everywhere it has been introduced, and spurned by elephants, which gives it even more scope to grow. The traditional Verbenaceae was best known, however, for Tectona grandis, alias teak: which, when both quantity and quality are taken into account, is by far the most valuable of tropical hardwoods. The Lamiaceae (formerly Labiatae) included a few big woody plants including the fifty-seven known species of the genus Gmelina from India, Malaysia and Australasia, such as the all-purpose timber tree Gmelina arborea. But the Lamiaceae family was best known for its range of culinary herbs: mint, oregano, basil, rosemary, sage, lavender, thyme.

  Of late, however, taxonomists have found that the old-style Verbenaceae was not a clade; more of a mixed bag. Most of its members were more closely related to other families, or needed to be placed in their own families. Much more to the point, about two-thirds of the traditional Verbenaceae now seem to belong in the Lamiaceae. So now Verbenaceae is a much less interesting family. It still includes lemon verbena, verbena and lantana. But the jewel of the family, Tectona, now finds itself grouped with mint, thyme, oregano and the rest in the Lamiaceae. Modern taxonomy makes some strange bedfellows. The Lamiaceae now emerge as a huge and cosmopolitan family with nearly 7,000 species in more than 250 genera.

  Teak grows naturally in mixed tropical forests in a range of conditions, although it is generally supposed to prefer a climate with a dry season, as opposed to uninterrupted rain. It grows naturally alongside many other species in forests in India, Myanmar, Thailand and Laos – although it may not be native to India: Hindu monks may have brought it in from Indonesia in the fourteenth century. There are huge plantations too throughout the tropics, not least in Brazil. Some Indian foresters feel that teak, for all its magnificence, is somewhat overplayed: at the start of 2004, to help restore the balance, the director of the Forestry Research Institute in Dehra Dun, Dr Padam Bhojvaid, organized a conference to re-focus attention on some of the other 400 or so Indian species (out of more than 4,000 Indian natives) that have been put to use this past few thousand years. But the attractions of teak are clear. Its timber is strong and durable, enduring the great outdoors without treatment, and weathering to a characteristic, sober grey. As a bonus, it is available in long lengths. The Mesopotamians recognized its worth in the third millennium BC. The Europeans came to it somewhat later, in the sixteenth century, and from then on used it to build much of their navies. It is now grown in plantations throughout the tropics, not least in Brazil, and is the subject of huge research endeavours in genetics, tissue culture and the control of pests and diseases. A particular target is a moth that regularly defoliates the trees – which is disfiguring and also reduces growth rate. By 1998 the total area of teak worldwide was estimated at 28 million hectares – although the lion’s share of this was still in natural forest. Tectona grandis is the valuable teak. Four other known species (T. australis, T. hamiltoniana, T. philippinensis, T. ternifolia) can be ecologically but not economically important locally.

  Teak grows slowly – traditionally harvested in India at eighty-year intervals

  Holly: ORDER AQUIFOLIALES

  The only family in the Aquifoliales order is the Aquifoliaceae. It contains about 400 species in three genera – and about 97 per cent 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 broadleaved 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 ri
ch 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 south–eastern United States make ‘black drink’ from the leaves of I, vomitoria. Holly timber is hard, white, smooth and much prized.

  Daisies and Just a Few Trees: ORDER ASTERALES

  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 metres, and provides short lengths of very heavy timber with grey-white sapwood and orange-brown heartwood, which looks good, wears well and is favoured for everything from floors to animal carvings. The muhuhu also smells pleasantly and its oil is distilled as a substitute for sandalwood – and it is even exported to India, as an aromatic fuel for cremations.

  Handsome in winter and with fine white timber: the holly

  III

  The Life of Trees

  11

  HOW Trees Live

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

  A century or so before Aristotle the Presocratic 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 (H2O) is compounded from two atoms of the element hydrogen and one of oxygen. Carbon dioxide (CO2) is one carbon with two oxygen. Ammonia (NH3) 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 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 sulphur. 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 catalogue of additional minerals to fill out the details, including metals such as calcium, sodium, potassium, magnesium, iron, zinc, and manganese; and non-metals 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 sulphur. 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 non-scientists 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 Presocratic 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 by either 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 harbour 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 H2O, 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 manoeuvring, 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 centre.

  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 labour, 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) come mainly from the ground – sometimes from deep below the ground – and so must be carried upwards 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 metres tall. The roots may add a great deal more to their length: those of trees such as eucalypts that may live in semi-desert, and the native trees of Brazil�
�s dry Cerrado, may reach down for tens of metres. The longest known roots of all belong to a South African fig: 120 metres. The whole vast and intricate structure is evolved to bring air and water together in the presence of sunlight; and the water and attendant minerals come mainly from the earth.

  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 myth-makers 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 (1608–47) 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.