The Tree

by Colin Tudge

  On more practical matters: plantation rubber trees generally start producing useful amounts of latex at around seven years; are producing maximally by age fifteen; and are generally chopped down and replanted after thirty years (when they are about 20 meters tall). So the grower has about twenty-three productive years out of thirty. In the past the timber was simply burned, but it is a pleasing reddish brown and strong, and in recent decades has become a major crop. Malaysia and Thailand now export nearly $1.5 billion worth of rubberwood furniture. Southeast Asia could harvest more than 6.5 million cubic meters of rubberwood per year—almost as much as the entire timber harvest of Central America.

  In Malaysia I found the rubber plantations in many ways attractive: green and aromatic shade in a colonnade of trunks, and all as cool as Sussex. But the work—slashing the bark every day and replacing the receptive cups—is desperately tedious, and recruitment of labor is difficult. My solution would be to make the work more interesting—by integrating smaller plantations with mixed farming, as I have seen in China. Indeed, rubber trees do lend themselves to agroforestry. Other crops (including valuable herbs) may be grown among them, and their shade is good for livestock. As a bonus, they produce big, oily seeds—which are thrown spectacularly for several meters as the fruit dries and splits. In the wild, in Amazonia, these seeds are dispersed by large river fish. In plantations, I would have thought they would be ideal for turkeys. I would hate to be a traditional rubber tapper, and I feel sorry for those who are (in Malaysia, their daughters take off for the electronics factories). But a mixed exercise in agroforestry, with other crops and livestock, is a different proposition all together. A grand challenge: most interesting.

  Then there are the Rhizophoraceae—the family of Rhizophora, the red mangrove trees. Mangrove forests grow at the edge of the sea throughout the tropics: in northern Australia, Southeast Asia, western Africa, around the Red Sea, along the north coast of South America, on both coasts of Central America, and in the Caribbean. Although mangroves occupy only seventy thousand or so square miles of the earth’s surface, they are hugely important ecologically and economically. Around their roots breed a host of sea creatures, including many ocean fish. Local people take as much from the mangrove forest as forest people take anywhere—fuel, timber, fruits—and fish, too. Offshore from the mangroves, typically, are the sea grasses—food for fish, mollusks, manatees, and marine iguanas—and beyond them lie coral reefs, which in diversity of wildlife are second only to tropical forests. The mangroves, if left intact, protect the sea grasses and the reefs. Since the mangroves, sea grasses, and corals are all nurseries for marine creatures, the consequences of mangrove destruction extend through all the oceans. Yet they are being destroyed—to make way for marinas and promenades, for lagoons to raise tropical shrimp for Western supermarkets, and even, it sometimes seems, just for the sake of it. In Panama in 2003 I was shown a mangrove that had been filled in with rubble to provide a park for containers, of the kind used for oceangoing cargo ships. The local government bought the idea from some entrepreneur on the grounds that it would provide employment. The only employee when I was there was a man with a gun, to keep people off. The entrepreneur, having pocketed the taxpayers’ money and destroyed the mangrove forest (and the wild creatures, and the livelihoods of the people who were living there), was about to sell his barren dump to the Chinese. That’s business, apparently.

  To return to the natural and saner world: about eighty species of trees have mastered the adaptations needed to live in the intertidal zone. Of these, thirty or forty are core species that turn up in most mangrove forests; and of these the most important overall are seven or eight species of Rhizophora.

  The third great arborescent family of Malpighiales is the Salicaceae. It is named for the genus Salix, the willows. There are about 400 species—although as outlined in Chapter 1, they are hard to identify and very prone to hybridizing, so it will always be effectively impossible to say exactly how many there really are. They range from small shrubs to big trees, and from the tropics to the extreme north. Up cold and windy mountains, and on the edge of glaciers up toward the Arctic, they are often the chief woody species. In such territory, they send out underground stems to form vast clones: a wood that in effect is a single plant. Thus the creeping willow, S. repens, colonizes marshland and begins its transition into forest. Most willows like the edges of rivers, where they are commonly planted to stabilize the banks. They may serve, as reed beds do, to purify the water. Some are ornamental: the original weeping willow in particular, S. babylonica, originated in China, drooping languidly over lakes and lazy rivers as if specifically intended for patterned tea sets.

  Willows belong in the Northern Hemisphere, although one (S. mucronata) crosses the equator in Kenya. Many kinds live in western China, which is truly one of the world’s great centers of diversity; but, like so many trees, these Chinese willows have not yet been properly studied. Willows are nearly all dioecious (males and females on separate plants) and produce catkins that are usually pollinated by insects, although wind probably plays a part. Their seeds are tufted and float on the wind. The different forms of willow find many traditional uses: the thinner twigs (especially of osiers, S. viminalis) for baskets, coracles, and hurdles; the bigger timbers for construction. Female clones of S. alba var. caerulea are the sole source of wood for cricket bats (though sadly threatened from time to time by the bacterium Erwinia salicis). The finished cricket bat is a botanical extravaganza: white willow for the blade, bamboo and rubber for the handle, plus twine to bind the handle and glue to hold it in the blade, both of which may come from plants, and linseed to keep the bat supple. Willow is also an important fuel wood, now much vaunted as a source of biomass to supply energy without exacerbating global warming. Finally, the bark of willow is particularly rich in salicin, the root molecule of salicylic acid, the stuff of aspirin—of proven use as an analgesic and anti-inflammatory, and now favored to reduce blood clotting and guard against thrombosis.

  Also in the Salicaceae and closely related to willow is poplar (Populus), the twenty-nine or so species of which include the aspens, like North America’s quaking aspen, P. tremuloides. Poplars are hugely favored for plantations worldwide, for matchsticks and paper pulp. They also serve as windbreaks and (like eucalypts) help to dry out wetlands, acting like wicks. A major task now is to conserve their genetic diversity, as the wet river banks that they favor are drained and contained. The levees of the Mississippi have greatly reduced the natural regeneration of the native P. deltoides. Attempts are afoot worldwide to conserve the diversity in arboretums. In Europe, EUGORGEN, the European Forest Genetic Resources Programme, is intended to conserve the natural diversity of the black poplar, P. nigra, and holds nearly 2,800 clones from nineteen countries. In the Pacific Northwest, GreenWood Resources holds one hundred stands of P. trichocarpa, to counteract the losses along the Columbia and Willamette rivers and their tributaries. Efforts to conserve poplars in situ include the Tarim River nature reserve in the Xinziang autonomous region of China, largely intended to conserve the remaining third or so of the original P. euphratica, and a plan supported by the International Poplar Commission and the United Nations to conserve the native variety of P. ciliata in the Himalayan foothills of India. Such efforts are heartening. Populus, however, is only one among many thousands of genera of trees, and the vast majority are receiving no help at all. Even if they escape extinction, few will escape severe genetic diminishment.

  There are some other notable Salicaceae too—including some that are traditionally placed in the Flacourtiaceae, which Judd enfolds within the Salicaceae. Among them are the Maracaibo boxwood, Gossypiospermum praecox, from Cuba, the Dominican Republic, Colombia, and Venezuela—highly favored for specialist tasks, not least for the working parts of pianos, and odoko from West Africa, species of Scotellia, hard and tough and excellent for floors.

  Several other families in the Malpighiales are worth a passing mention. The Violaceae in temperat
e regions manifests as herbs, including violets; but in the tropics it produces some fairly mighty trees. The Malpighiaceae, for whom the order is named, include the Barbados cherry. The Clusiaceae are best known as the family of Saint-John’s-wort, which finds favor as an antidepressant. But it also includes some handy trees—not the least being the 200 or so species of Garcinia, which include the mangosteen, G. mangostana, a native of Malaysia with fruits the size of a Ping-Pong ball, whose brownish-purple leathery skins enclose treacly and truly delicious (I can vouch) creamy-white segments. An excellent fruit, though the trees are slow-growing and not easy to cultivate.

  The Ochnaceae family includes the ekki tree from West Africa, Lophira alata, from whose timber (for some reason) the tracks of the Paris Metro are made. The Metro trains have rubber tires. I venture that few who ride the Paris Metro realize their debt to the trees of the Malpighiales.

  STAR FRUIT AND COACHWOOD: ORDER OXALIDALES

  The Oxalidales is yet another order that has been subject to serious reclassification. The family for which the order is named, the Oxalidaceae, is primarily tropical and subtropical and is known to northerners primarily through wood sorrel (Oxalis acetosella). It does include a few small trees, however, one of which is the star fruit or carambola, Averrhoa carambola. The fruit is juicy, sometimes sweet and sometimes acid; is deeply ribbed and thus star-shaped in cross section; and has lately become fashionable outside its native Indonesia. Modern DNA studies also place the Cunoniaceae within the Oxalidales; these include Ceratopetalum apetalem, a tall (18 to 24 meters) and valuable timber tree from New South Wales known as coachwood (or lightwood or scented satinwood). Its browny-pink, fragrant timber is used for lots of things, especially joinery and moldings, but also for gunstocks, shoe heels and musical instruments. Thus the wood sorrel emerges as yet another homely herb with wildly exotic relatives.

  TREES FOR FODDER, FUEL, FLOWERS, AND BEAUTIFUL TIMBERS: ORDER FABALES

  Within the Fabales order are four families, but the only one that need delay us is the Fabaceae—and they should delay us plenty because, together with the grass family, Poaceae, the Fabaceae is the most important plant family of all, both ecologically and economically. It is also the third largest, with 18,860 or so known species in 630 genera, but the inventory is still rising fast. Only the orchids and daisies have more species. Fabaceae used to be called Leguminosae—the pods are called “legumes.” The old family name is now officially defunct but the adjective “leguminous” and the informal noun “legume” (applied to the whole plant as well as to the pods) live on.

  Fabaceae take every form: herbs such as vetches, clovers, and alfalfa; vines like peas and runner beans, which climb by twining and/or with tendrils; woody climbers like Wisteria; and shrubs like gorse and broom. In addition, many of the tropical genera in particular make fine trees that are of huge significance worldwide in forests and savannahs, while providing every kind of service for humanity and our domestic animals.

  The special trick of the Fabaceae—although not all do it—is to retain colonies of nitrogen-fixing bacteria of the genus Rhizobium in special nodules within their roots. The nitrogen fixers are to a large extent self-nourishing and provide nitrogen-rich leaves and seeds even in poor soils—and “nitrogen-rich” in practice generally mean “protein-rich.” Thus many legumes are especially valued for food, including all the pulse crops (peas, beans, lentils, chickpeas, peanuts, and many a tree); and even more provide outstanding fodder, whether growing wild or cultivated (when the trees also provide shade for the grazing animals). Even when not eaten, the nitrogen-rich leaves are often dug or plowed into the soil to make green manure. Equally to the point, nitrogen-fixing plants are generous with their nitrogen: they enrich the soil around them. So clover and alfalfa and vetches have long been used to enrich pastures, nourishing the grass that grows alongside; and pulse crops (beans, peas, lentils, chickpeas) complement cereals (which, of course, are also grasses); and leguminous trees of many kinds are the greatest of all the candidates for agroforestry, which offers one of the principal hopes for a sustainable world.

  Many genera are outstanding, but perhaps the most important, ecologically and economically, are the acacias. The 1,300 or so species grow almost throughout the tropics and subtropics: more than 950 in Australia, where they are known as wattles; another 230 or so in the New World; 135 in Africa—mainly out on the savannah, where, flat-topped, they are often the only source of vital shade; 18 more in India; and a few others dotted around Asia and endemic to odd islands. Not all in the genus Acacia are trees (some are shrubs or woody climbers), but a great many are.

  Some acacias thrive in the wet—some in the American tropics live in rainforest; and some, like A. xanthophloea, survive periodic flooding. But most thrive in harsh, dry environments and have many adaptations to extreme aridity. Some, like A. eriloba of Africa, have extremely long taproots, stretching down to aquifers as much as 12 meters below the surface. Some have very small leaves or have replaced their leaves with flattened leaf-stalks (petioles) known as “phyllodes” (as in the celery pines, described in Chapter 5). Generally acacias shed their leaves when it’s very dry, sometimes all at once, sometimes progressively as the aridity increases—never having more than the conditions will support; but some desert kinds produce fresh leaves before the rains return, to the delight and benefit of camels, antelopes, giraffes, and the nomadic tribes of Africa who need fodder for their cattle, sheep, and goats.

  In general acacias do well in soils that are poor and disturbed—and so they are excellent colonizers: for example, the Australian blackwood, A. melanoxylon. Some, like Australia’s A. auriculiformis, tolerate toxic or highly acid soils. Many acacias are adapted to fire, including most of those in Australia. In some, fire stimulates germination; in others (including some from Africa), it promotes coppicing (regeneration of shoots). On the other hand, some dryland kinds withstand freezing. In some the seeds are known to remain viable in the ground for up to sixty years. Some reproduce by apomixis (a form of parthenogenesis: the new tree grows from an unfertilized ovule). Some spread themselves by suckers, as many trees do (for example, willows, poplars, elms, and redwoods).

  Their pioneering hardiness is both an asset and a menace. It is good for land reclamation—and so in Australia A. auriculiformis is used to colonize acid mine dumps. But it also means acacias make excellent weeds. We see the worst and the best of them when foresters or gardeners take them from one continent to another—for all nature is unpredictable and nothing more so than the behavior of “exotics.” Most plants or animals die when taken to new places, which they are not adapted to. Some settle in and become naturalized, and whatever the native wild species may think of the invaders, they can be economically valuable—so it is that Australia’s A. mangium, for example, has become a valued timber tree in India. But some become rampant and are hugely destructive—and so Australia has its rabbits, cats, and foxes and also A. nilotica, from Africa, plus others from America. Australia has got its own back, however, with exports of immensely destructive acacias to Africa, Portugal, and Chile (and, of course, of eucalypts to absolutely everywhere, and possums to New Zealand).

  Like various other members of the Fabaceae, acacias have formed some close symbiotic (“mutualistic”) relationships with ants. In fact, different acacias have clearly formed such relationships independently, more than once. Thus many acacias have thorns, typically at the bases of their leaf stalks; and some species in Central America have “swollen” thorns that are hollow and accommodate colonies of ants. A. melanoceros houses ants of the genus Pseudomyrmex. In Africa, whistle-thorn acacias such as A. seyal have resident colonies of Crematogaster. Ant acacias often provide board to go with the lodging, in the form of protein-rich food stores. The ants, in turn, rid their hosts of pests—not only insects but also, presumably, browsers: for few would risk ants up their snouts. (I have had ants up my arm in India, picked up from an epiphyte, where they create airy chambers by sewing the edges of the leaves
together. I wouldn’t fancy them up my snout either.)

  Taken all in all, acacias are wonderfully integrated socially. Below ground many (though not all) house nitrogen-fixing bacteria to aid with nourishment. Typically, too, they also form mycorrhizae in association with fungi, which further increases their nutritional efficiency. Many harbor ants for housekeeping. They employ a variety of insects—flies and beetles but mostly bees—and sometimes birds to pollinate their flowers; and Africa’s A. nigrescens may be pollinated at least in part by giraffes. In some species a variety of animals help to spread their seeds: some have brightly colored arils (fleshy exteriors) around their seeds to attract birds; others increase the attraction by suspending their seeds beneath the pods—in some the seeds are dispersed by antelopes and elephants, passing through their guts. Thus an acacia tree is a veritable hotel; or perhaps it should be seen as the ultimate networker, with a host of mutually beneficial associations with representatives from just about every other class of organism.

  As we will see, too, in Chapter 13, acacias also team up with one another, issuing chemical warnings to their fellows that giraffes are on the prowl. Clearly this is necessary. In recent years giraffes have been introduced to places in South Africa where giraffes do not naturally live; and they have all but wiped out the native A. davyi, at least where the trees are accessible, because, apparently, these acacias are not well adapted to giraffes. Here again we see the menace of introduced species, and also a clash of conservation aims: do we prefer big mammals or native trees? This is one more reason why so many trees of all kinds are endangered—including thirty-five of the acacias (which is almost certainly an underestimate).