The Secret Life of Trees

by Colin Tudge

  There are many other fine genera of trees in the Sterculiaceae, too, as well as some shrubs and climbers. Pterygota has wooden fruits almost as big as a croquet ball, packed with winged seeds. It includes some fine, tall avenue trees and others that yield valuable dark-flecked yellowy timber. Heritiera from South-East Asia yields the dark, much-valued hardwood mengkulang. Theobroma cacao is cocoa: Theobroma means ‘food of the gods’. Cola nidia and C. acuminata are the trees that provide cola nuts, the original ingredient of the soft drinks, the political and economic influence of which this past hundred years can hardly be overestimated.

  Perhaps most remarkable of all the extraordinary trees in the Bombacaceae family is the baobab tree, Adansonia digitata, also known as the monkey-bread tree. Of the eight remaining species, seven are indigenous to Madagascar; one also grows naturally in Africa; and one more is indigenous to northern Australia. It used to be thought that the baobab was another ancient Gondwanan genus, with just a few species surviving on various Gondwanan landmasses. But DNA studies suggest the genus arose in Madagascar, long after the break-up of Gondwana – and that a few seeds managed to float across the Indian and Pacific oceans and took hold in Africa and Australia. These things happen. Several species have also been distributed by human beings and, for example, there are now plenty of baobabs in India.

  Their appearance is extraordinary. They are not on the whole outstandingly tall (up to about 20 metres or so) but the trunk is swollen with water, rilled to bursting like an over-stuffed sausage, and can be huge: up to 10 metres in diameter, or 33 metres in girth. The mop of twisted (but often vast) branches at the top look more like roots. Thus have arisen various myths. One has it that all the animals were given trees of their own. The baobab went to the hyena – who was so disgusted by it that he turned it upside down. Another version has it that the first ever baobab was extremely beautiful – and far too proud of its beauty. So the gods, to punish its conceit, stuck it back in the ground the wrong way up. The story echoes that of Arachne in Ancient Greece – far too proud of her sewing for the gods’ liking, and turned into the world’s first spider.

  In truth, the trunks serve as giant water-butts, so the baobab is marvellously resistant to drought. The spongy timber is of little use, but the rest of the tree is very valuable indeed. The enormous flower buds – ‘like balls of pale-green suede’, says Mrs Cowen – open to form big, creamy-white flowers which appear at midnight one day in July and are wilted by morning. (Many plants flower remarkably briefly: clearly they have enormous confidence in the animals that pollinate them.) After the flowers come the fruits, white and gourd-like. The woody shell is rich in protein and in Africa is used to feed livestock, while in India, Gujurati fishermen tie them to their nets as floats, and monks employ them as water pots. The seeds provide valuable oil. Each seed is surrounded by pulp (which goes powdery), and is extremely rich in vitamin C; in both Africa and India it makes a cooling drink which protects against scurvy, and is otherwise medicinal. The pulp is held in position around the seeds by small fibres that are used for stuffing cushions. The leaves are eaten too, and strong rope is made from the bark. In Gujurati the baobab is known as gorak chinch after a monk, Gorak, who taught his disciples under the shade of one. In Zimbabwe Phytotrade Africa is marketing baobab products – part of its broad initiative to gain economic value from native plants for the benefit of local people.

  Nobody knows how long a baobab may live. They are so vast, so monumental, the biggest seem to be thousands of years old. But they grow very quickly, and perhaps 500 or so is the limit. Age is particularly hard to judge because, with age, they grow hollow – and then, perhaps with further hollowing, they are put to all kinds of uses. Often they are filled with water, as village reservoirs. The hollow spaces may be as big as a fairly sizeable suburban dining room – and some in their time have served as pubs and post offices. Spookily, Africans sometimes inter their dead in hollow baobabs whereupon, in the dry heat, the bodies mummify without further treatment. It is wise to take care when entering a hollow baobab.

  There are still more outstanding Bombacaceae. Bombax is the genus of the various kapok trees: their seeds are hairy (like the cotton plant, of which they are of course distant relatives) and the hairs are used for stuffing mattresses, sleeping bags, Mao Tse Tung-style quilted jackets, and what you will. Kapok trees are widely grown. I have enjoyed the shade of some truly magnificent Bombax ceiba growing as specimen trees in Belém in Brazil, defining the boundaries of the cathedral garden there. India’s red silk cotton, Bombax malabarica, is found throughout India and into Malaysia and Myanmar, and is now cultivated widely in the tropics, including Africa. Several other trees are also called ‘silk cotton’, with various coloured flowers (white, yellow), some of which belong to the Bombacaceae and some of which do not. The form of the cottony seed has been widely adopted, and clearly has evolved more than once.

  Like Sterculia, Bombax produce their flowers when the tree is bare of leaves. Those of the red silk cotton are bright red through pink to orange, and fall to the ground to be eaten by deer – or by the villagers, who put them in curries. The finger-like fruits that follow the flowers harden and split to release the cottony seeds which, before the wind does its work, seem to smother the tree in cotton wool. Crows, bulbuls, mynahs, rosy pastors, sunbirds and flower-peckers flock to feast on the oily seeds, though monkeys seem to be deterred by the vicious spikes on the smooth grey trunk and branches. The wood, whitish, soft and light, is known as ‘simul and is used for dugout canoes, floats, matches and coffins.

  The tree with one of the lightest timbers of all (though not quite the lightest) is also from the Bombacaceae family: balsa, Ochroma pyramidale, native to tropical America from Mexico through to Brazil (and also Cuba), and now planted in India and Indonesia. Every child knows balsa as the stuff of model-making. Grown-ups use it for rafts, aircraft, insulation, equipment for water sports, and of course for theatre and film props, to reduce damage to actors. The main commercial source nowadays is Ecuador.

  Finally we might mention the durian, Durio zibethinus, whose fruits are huge and spiky. Some find them delectable: another food for the gods. But they stink (shades of Sterculia again) and are expressly forbidden on aeroplanes.

  In its new form, then – subsuming the Tiliaceae, Sterculiaceae and Bombacaceae – Malvaceae emerges as a truly remarkable family of truly remarkable trees. But the Malvales order also includes another arboreal family that is clearly discrete from the Malvaceae: the Dipterocarpaceae.

  The dipterocarps are hugely various: 680 or so species in sixteen or seventeen genera (different taxonomists split them slightly differently). The principal genera are Shorea (by far the most important), Dipterocarpus and Dryobalanops. Mostly the dipterocarps come from south or South-East Asia – Malaysia is the focus, with 465 species in fourteen genera – but there are also forty-nine species in three genera in Africa, and one each in South America and the Seychelles. Between them the different types grow from coast to uplands in many (tropical) climates and in soil both fertile or – often – extremely infertile. Some grow in dry land but on the whole they prefer the wet: there are many kinds in swamps and the biggest grow where there is year-round moisture. I am told there is an old plantation of dipterocarps outside Kuala Lumpur where many individuals exceed 60 metres.

  Dipterocarp means ‘two-winged fruit’ – and indeed they have fruits roughly like sycamore keys, though often much larger, and sometimes brightly coloured. People make wide use of dipterocarps. The fruits of many Shorea and some Dryobalanops are boiled as vegetables. The seeds of both Shorea and Pachycarpae are extremely rich in fat – up to 70 per cent – which is similar to cocoa butter though harder, and is much favoured for chocolate and cosmetics. (And note again the loose phylogenetic relationship between the dipterocarps and the cocoa tree, in the Sterculiaceae. Biochemistry runs in dynasties.) Most dipterocarps too produce useful resins; and a form of camphor comes from Dryobalanops, used as incense.

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sp; But above all, the dipterocarps dominate the international market in tropical timber. Shorea species from South-East Asia are marketed as ‘meranti’. Various kinds are sold as light-red or dark-red meranti – red indeed and finely figured, and used for all purposes. Another group of Shorea feature as white or yellow meranti, also with many uses, from floors to ships, plywood and veneers. Mersawa and krabak are two more, similar, South-East Asian timbers from the genus Anisoptera. Kapur from Malaysia and Indonesia is the genus Dryobalanops. Altogether, the Dipterocarpaceae are a formidable family – comparable in their part of the world with the oaks and beeches from further north.

  Frankincense and Myrrh, Oranges and Lemons, Maples, Mahogany and Neem: ORDER SAPINDALES

  The Sapindales are closely related to the Malvales. As in the Malvales, reclassification is in train, so the traditional list of eleven Sapindales families is now reduced to eight. Six of those families — one of them now much expanded – contain trees that are at least intriguing, if minor (ecologically and economically), and others that are of supreme importance both in the wild and to humanity.

  Minor but intriguing is the Simaroubaceae family, a hundred species of trees and shrubs in twenty-one genera from throughout the tropics and subtropics. They are biochemically potent and widely deployed in medicine, especially Quassia from Africa; while Picramnia from the Americas was once exported to Europe to treat erysipelas and venereal disease. The white syringa of Africa, Kirkia acuminata, grows to 18 metres, provides useful timber, and also has swollen roots that store water which knowledgeable locals tap in times of drought. Best known in the gardens and streets of the West is the tree of heaven, Ailanthus altissima. It was first imported to England from its native China in the mid eighteenth century and thence to the rest of the West. It resists pollution, can grow to 30 metres sometimes in less than twenty years, and has compound feathery leaves like an ash, although each may be nearly a metre long.

  Minor too by world standards is the Burseraceae family – but extremely intriguing, as the family of both frankincense and myrrh. In all there are about 500 species, in seventeen genera, throughout the tropics but mainly in Malaysia, tropical America and Africa, and many provide resins and aromatic oils for perfumes, soaps, paints, varnish and incense. Frankincense comes from Boswellia carteri, of Somaliland; myrrh is from various Commiphora species, notably C. abyssynica, which grow and are now cultivated in Arabia and Ethiopia. The gifts of the Magi to the infant Christ in Bethlehem were exotic indeed. The first known government-sponsored plant-collecting expedition was in search of myrrh: reliefs on the Temple of Deir el Bahair at Karnak show myrrh trees being transported from the Land of Punt, around 1495 BC. The Burseraceae family also provides some useful timbers, widely used in Malaysia and Africa. Outstanding is gaboon of Africa, Aucoumea klaineana, used for everything from cigar boxes to sports gear and high-class furniture. Sometimes gaboon is mottled and striped, and then it is highly valued for veneers.

  The Rutaceae family is named after the rue, Ruta graveolens, a small aromatic shrub, toxic but also medicinal, and grown in herbal gardens for centuries. It pops us here and there in the plays of Shakespeare, who surely was a competent naturalist, while his son-in-law was an outstanding apothecary. But the Rutaceae overall include about 900 species in 150 genera that grow throughout all the warmer reaches of all continents, including Australasia. By far the best known and important is the genus Citrus: C. limon, the lemon; C. medica, the citron; C. aurantium, the sour Seville orange – a variety of which is also the source of bergamot, the stuff of Earl Grey tea; C. sinensis, the ordinary, sweet orange; C. reticulata, different varieties of which are mandarins, satsumas and tangerines; C. aurantifolia, the lime (not of course to be confused with Tilia); and C. paradisa, the grapefruit. Closely related to Citrus is Fortunella, the genus of the kumquat, which turns up sometimes pickled on smart dinner tables, though not in my opinion to very obvious advantage.

  The Rutaceae also provide some valuable timbers. Various species of Flindersia feature as the cinnamon-coloured ‘Queensland maple’ – again for prestige furniture, gunstocks, oars, and what you will. Southern silver ash from Eastern Australia, F. schottiana, is just as versatile, but pale yellow. From South America comes pau marfin, Balfourodendron riedelianum: tough, pale, flexible; wonderful for oars, tool handles, shoe lasts, furniture, marquetry. Ceylon satinwood, Chloroxylon swietenia, from the southern Indian subcontinent is called satinwood because it seems to shimmer like folded silk, and is highly prized for panels and veneers. Pale-golden West Indian satinwood, Fagara flava, was used widely in the eighteenth century by the great English makers of fireplaces and furniture: Adam, Sheraton and Hepplewhite.

  But satinwoods have largely been ousted from favour by members of the Meliaceae family. The fifty-one genera (with 550 species) include several outstanding timber trees – indeed their present and historical importance can hardly be over-emphasized. World star of the families for almost four centuries is the genus Swietenia: the American or ‘true’ mahoganies. There are three species – closely related, and often hybridizing in the wild where their ranges overlap. S. humilis, with small leaves, prefers drier country, and spreads north into Mexico. S. mahogoni, the first of the trio to be named (by the Austrian botanist Gerard von Swieten) is from the Caribbean and Southern Florida. S. macrophylla, the big-leaf mahogany from Brazil and Honduras is the giant: an emergent tree, towering above the seasonally dry rainforest canopy at about 70 metres, with huge buttress roots to reinforce a trunk that can be 3.5 metres in diameter.

  Or this, at least, was the pristine state. There is little or no forest left where S. mahogoni and S. humilis once lived. The big-leaf mahogany is still to be found in the forests of southern Amazonia – anywhere between one big tree in every 10 hectares to three trees per hectare – and might still, some say, be harvested sustainably and profitably from the wild. It is difficult to increase the proportion in the wild because mahoganies are light-lovers, and need open ground or clearings to get going. If they are simply planted among other trees they fail. But big-leaf mahogany and S. mahogoni are now widely grown in plantations, particularly in tropical Asia and Oceania, and increasingly in their homelands in the American tropics. Yet they have suffered enormously in plantations from an insect pest, the mahogany shoot-borer, a species of Hypsipyla, which bores into the shoots and turns what should be a straight proud tree into a mean shrub. The pest can be controlled; but largely because it is perceived as a problem, the world’s plantations of mahogany are only one twentieth those of teak. Wild mahogany is now listed by CITES, and trade in general is more and more regulated. But it will be interesting to see what happens to mahogany, wild and cultivated, over the next few centuries.

  Several close relatives of true mahogany are also fine timber trees: pride of India, Melia azedarach; African walnut, Lovoa trichilioides; South American cedar, species of Cedrela; Asian-Pacific red cedars, species of Toona; the sapele, Endophragma cylindricum; and Endophragma and Khaya, which are sometimes called ‘African mahogany’. Several trees that have nothing to do with Meliaceae at all are also sold as ‘mahogany’, including several dipterocarps (of the genus Shored) and the occasional eucalypt.

  Less closely related to Swietenia, yet still within the Meliaceae, is the wondrous neem tree, Azadirachta indica, which featured earlier as the antidote to noxious tamarind. The neem grows to 20 metres or more, with evergreen, roughly ash-like leaves, much valued for their year-round shade. It is native to south Asia, but its deep roots enable it to thrive on dry, poor soil and it has been planted and become naturalized throughout all tropical Asia, while the British took it to Africa in the early twentieth century to slow the spread of the Sahara. It has also been taken to Fiji, Mauritius, Saudi Arabia, and all tropical and subtropical America, including Florida, Arizona and California. In the USA there are neem plantations.

  There are many outstanding chemists among plants, but the neem is among the greatest of all. For centuries, indeed for thousands of y
ears, the Indians have treasured the neem for medicines: it features in some of the most ancient Hindu texts. Many Hindus begin the new year by chewing neem leaves. Many clean their teeth with neem twigs. They treat skin disorders with its juice, and drink infusions as a tonic. Gum from the bark is used for dye. Neem has also proved active against more than 200 species of insects, preventing them from feeding, inhibiting their reproduction – discouraging egg-laying and disrupting the development of any eggs that are laid. As if to make the point a plague of locusts in India in 1959 destroyed just about everything that grew, except the neems. The timber is termite-proof, and perfect therefore for hot climates, for everything from furniture to tool handles. Indians put the leaves in cupboards, to safeguard the contents. The leaves also make good fodder, while the seeds are 45 per cent oil and provide excellent seed cake for livestock, or oil for lamps. Very properly, the neem is venerated. Many Indian place names incorporate its name: Neemuch, Neemrana, Nemawar, and hundreds more. The neem is said to have been blessed with nectar, sent from heaven.

  Science has reinforced the folk law. Different parts of the neem, but particularly the seeds, contain a host of potent organic compounds shown to be active against just about everything pestilential: bacteria, fungi, viruses, nematodes and mites, as well as insects. A powerful spermicide is in there too, raising hopes in some circles of an effective male contraceptive. Yet the extracts do not seem to harm mammals (including people) or birds. Outstanding among the compounds studied so far is azadirachtin. It is now being incorporated into commercial pesticides not only because it seems innately effective but also as part of a general swing away from industrial chemistry to biotech, based on natural processes and materials.