The Tree

by Colin Tudge

  Many acacias are cultivated for many purposes. A. auriculiformis, A. mangium, and the Australian blackwood, A. melanoxylon, are the most favored timber trees. The Australian blackwood grows wild in Queensland and New South Wales to a height of up to 30 meters; nonetheless it grows as an understory tree, beneath the giant eucalypts known as mountain ash, which may grow to nearly 100 meters. The dark, black-flecked timber of Australian blackwood is highly valued for everything from boats to billiard tables. Other acacias are grown for chipping and for pulp. As noted above, too, various members of the parasitic families of Loranthaceae and Santalaceae favor acacias of Africa and Australia as hosts; and these acacias, accordingly, are grown as host trees in sandalwood plantations, proving that foresters can be opportunist too. Some acacia seeds are highly nutritious: A. colei and A. tumida were introduced to the Sahel for firewood and shade but are now showing promise for human food. Various acacia seeds in Australia are finding favor as fashionable “bush food.” Acacias throughout their range provide hugely important browse for wild mammals from antelope to elephants—and fodder for domestic livestock. Some provide valuable gums and medicines. Some are used in perfumery. Some, however, are highly toxic—both seeds and leaves. Several are valued ornamentals.

  Then there is the genus Leucaena. The 22 known species grow wild in the Americas from Peru to Texas, from sea level to 3,000 meters. Native people eat the edible pods for their garlic flavor. Many can be grown as shrubs and are valued for fodder—but they do raise a problem since they contain exotic amino acids that do not normally form part of animal proteins, and which when eaten may lead to the loss of hair and hooves. Leucaena also provides some of the world’s fastest-growing trees. Best known is L. leucocephala, which was first brought out of the Americas four centuries ago and is now grown worldwide for fodder but also for timber. L. diversifolia, originally from highland Mexico, is now used widely to provide shade in coffee plantations (the best-flavored coffee is shade-grown coffee; it grows more slowly than in open sunlight, and slow is good) and for firewood and green manure. L. esculenta also comes from highland Mexico and has edible pods.

  But foresters are not content with the wild species, and Leucaena is one of a fairly long short list of trees (it’s a long list, but short relative to the total number of species) that have been intensively cultivated and bred. “Breeding” means selecting the best, and crossing different species to produce hybrids that (with luck) combine the best qualities of both. Many of the hybrids between different species of Leucaena are fertile, and a few can be reproduced by cuttings (so it does not matter if they are fertile or not). Thus hundreds of crosses made between sixteen different species of Leucaena at the University of Hawaii have produced some highly desirable hybrids. One, between L. leucocephala and L. diversifolia, known as L. x spontanea, takes just twelve years to grow into a tree with a trunk 40 centimeters in diameter: massive. Such trees, grown in appropriate plantations, help take the burden of human needs and ambitions from the wild forests, and if burned for fuel they are “carbon neutral,” and so do not contribute (in net) to global warming. On the other hand, Leucaena can be very nasty weeds. L. leucephala in Hawaii is one such. This particular species may well be a hybrid that arose in Mexico—a hybridization mediated by human hand.

  Then there are the 200 or so known species of Dalbergia. They include shrubs and climbers but also provide some of the world’s most prestigious timbers, valued for xylophones, piano keys, and billiard tables. Sadly, many Dalbergia species are endangered in the wild through deforestation; but some are widely cultivated. Indian rosewood, source of fabulous veneers, is D. latifolia (although Burmese rosewood, equally fabulous and sometimes known as narra, is a different legume, Pterocarpus indicus). D. sissoo, known as the sissoo or shisham, is native to the gravelly foothills of the Indian Himalayas. It grows slowly and crooked but is amazingly resistant to searing temperatures, drought, and frost, and it is hugely valued locally for fodder, fuel, charcoal, and medicines, while its flowers provide bees with nectar for honey. Sissoo also provides beautiful, dark brown timber. The Forestry Research Institute at Dehra Dun has a gun carriage of sissoo: I can see it in my mind’s eye thundering across the maidan, pulled by frantic horses, urged through the dust by equally frantic soldiers in scarlet and brass, some bursting with glory and others cursing their luck. The African blackwood is D. melanoxylon. The Brazilian tulipwood (not to be confused with the tulip tree, Liriodendron) is D. decipularis. In France it was known as bois de rose and made some of the finest furniture for Louis XV and Louis XVI.

  The reddish-brown, dark-etched Rhodesian “teak” is Baikiaea plurijuga, greatly favored for turnery. The boldly striped zebrano from Western Africa comes from various species of Microberlinia: again favored for carving. Purpleheart from South America, figured like fine tweed, is a species of Peltogyne: used to make apparatus for gymnasiums, skis, and billiard-cue butts. Among the thirty or so species of Albizia from Africa are some that produce timber for big ships and jetties, floors, and veneers. Various species of Pterocarpus provide hard, dark timbers, including P. indicus, which we have already met. Instia palembanica is cultivated as “Borneo teak.” In Brazil, various leguminous genera (and a few that are not legumes) provide the valued timber collectively known as angelim (as outlined in Chapter 2). The entire vast country of Brazil is named after one of its own leguminous trees: brazilwood, Caesalpinia echinata.

  Robinia was apparently tropical in origin, but just four closely related species survive—not in the tropics but in North America. R. hispida is the ornamental “rose acacia.” Best known is R. pseudoacacia, otherwise known as the false acacia or black locust, which was introduced to Europe in the 1700s and selected for the navy—as “shipmast locust.” These have been cultivated intensely (and apparently 618,000 acres of them are planted in Hungary). Tipuana tipu, the sole member of its genus, is the pride of Bolivia; also known as a rosewood, it grows up to 20 meters as a street tree, as a windbreak, and for fodder, both in Bolivia and Argentina.

  Many legumes are grown as ornamentals. The suburban favorite is Laburnum. The tropical American Enterolobium genus includes the huge-leaved E. cyclocarpum, known as “monkey ears” or sometimes as “elephant’s ear.” The round-leaved Judas tree is Cercis siliquastrum. The forty or so species in the genus Parkia that grow widely in the tropics are glorious umbrella-shaped forest trees that hold out their flowers to be pollinated by bats and dangle their bright fruits to be dispersed by birds. The Malaysians eat the seeds of Parkia as petai, which has a strong flavor of garlic (which persists in the urine). The 400 or so species of Mimosa provide great benison, from thorn fences to their much-admired pom-pom flowers. The rain tree of India is Samanea saman. For reasons best known to itself, the rain tree seems to encourage epiphytes—which most trees seem to go to some lengths to get rid of. Indeed, it is often grown for its epiphytes. It also harbors the lac insect, which periodically sprays the ground with water (or so it seems); and this, presumably, gives rise to the tree’s common name. The asoka, Sarraca indica, is planted around Buddhist and Hindu temples, where its yellowy-red blossoms are religious offerings. Asokas are said to blossom more vigorously when given a good kicking by young women. Don’t we all.

  Many other leguminous trees serve many more workaday functions. The 44 species of Prosopis are generally resistant to drought and salt—which makes them promising candidates for the many million acres worldwide now spoiled by salinity, brought about by overzealous irrigation. Prosopis includes the mesquites: P. glandulosa is the North American kind, favored for its aromatic charcoal that adds flavor to the barbecue. P. cineraria of tropical India also provides charcoal, plus firewood, fodder, green manure, and goatproof thorny fences. The extraordinary P. tamarugo of Chile is slow-growing but widely planted for its resistance both to salt and to very low rainfall.

  Many legumes are toxic and many are medicinal (these are two sides of the same coin). One of several “ordeal” toxins used in Africa in various kinds
of initiation rites is the famous red bark from Erythrophleum. The fifty-two species in the genus Sophora provide hardwoods but also toxins and medicines: the Japanese pagoda tree, S. japonica, has been cultivated in China for more than three thousand years for its beauty, and for dye and medicines.

  Many leguminous trees provide food. Several species from the huge tropical genus Inga have edible fruits and seeds, including the ice-cream bean, I. laurina. (The genus Inga is huge. Exactly how many kinds there really are, at least in tropical America, is the subject of research discussed in Chapter 12.) Tamarind is the fleshy fruit of Tamarindus indica, which adds astringency to curries and pickles, of which the Indians tell a charming story: A man set out on a long journey, but his wife didn’t want him to go. So she asked the local guru how she might hasten his return. “Make him promise,” said the guru, “to sleep every night under a tamarind tree on the outward journey, and to lodge beneath a neem tree every night on the way home.” The man kept his promise. But tamarind trees exude toxic vapors (or so it is claimed) and make you feel ill; while neem trees are restorative. So the farther the man traveled, the worse he felt; and as he got nearer to home again, he felt better and better. There are many morals in this tale, beyond doubt. (The neem or nim is from the mahogany family, Meliaceae, of which more later.)

  It would be easy to fill several volumes with leguminous trees. But we should move on.

  APPLES, PLUMS, ELMS, FIGS, AND CECROPIAS: ORDER ROSALES

  The Rosales has been radically reorganized these past few years, thanks mainly to molecular studies. Judd recognizes seven families—all of which have interesting trees and deserve discussion. But the relationships between those families, and the plants that they include, are often surprising—and the modern taxonomy differs enormously from most traditional treatments of only a decade or so ago. The position is still fluid because in many ways the Rosales is especially tricky, with enormous morphological variety on the one hand and a strong tendency to hybridize on the other, which sometimes makes it hard to tell where one group ends and the next begins. But the following reflects the state of play.

  Fittingly, the most primitive of all the Rosales families, apparently the sister to all the rest, is the Rosaceae, with its simple round flowers evolved to attract generalist insect pollinators—flies for the smaller kinds, bees for the bigger ones, and long-tongued moths for the biggest. Among the 3,000 species of the Rosaceae are many lovely and useful herbs, like cinquefoil and strawberry. But most (three-quarters) of the 85 genera include woody plants. Some are mainly shrubby, as in the roses (Rosa), or the blackberries, raspberries, and loganberries (Rubus). But many are very significant trees.3

  All of the most important temperate tree fruits are from the Rosaceae family. The apples are the genus Malus. All the hundreds (actually, thousands) of varieties that are generally eaten are variations on a theme of M. domestica, which, so recent studies in Oxford confirm, has been selected over many a century from the wild Asian M. sieversii. The various wild ancestors had small fruits, like modern crab apples, but as cultivation spread westward they became bigger, until by the Middle Ages we had recognizably modern fruit. Domestic varieties of apples stay constant from generation to generation (or century to century) because they are reproduced as cuttings and grown by grafting desirable varieties onto robust rootstock. Thus all the Cox’s Orange Pippins in the world, for instance, are a clone of the first ever Cox that was bred in the nineteenth century. Several other Malus species are kept as ornamentals. The 76 species of pears (Pyrus), are close relatives of apples; some species are grown for fruit, some for ornament (some have pleasantly silvery foliage, for instance, sometimes weeping), and valuable smooth pale-golden timber for much-prized kitchen furniture and for woodcuts. Pear trees can grow big: 18 meters tall, 5 meters in girth (about 1.5 meters in diameter). Cydonia is the quince and Eriobotrya is the loquat.

  The genus Prunus abounds with good things. P. dulcis is the almond. P. armeniaca is apricot. P. avium is the sweet cherry (known as the bird cherry when wild) and P. cerasus the sour cherry. P. persica is the peach and P. domestica the plum. Many Prunus species are grown as ornamentals—notably the Japanese flowering cherries—and several are noted for their fine reddish timber. In the wild, Prunus and Crataegus, the hawthorn, are early to put in an appearance as new forests establish themselves, although P. serotina, the black cherry, grows on in mature deciduous forest. P. avium, the bird cherry, grows widely in Europe, including Britain, where it is apparently native.

  Other ornamental trees (or near trees) include Amelanchier, which the Americans call the serviceberry or shadbush; the flowering quince (Chaenomeles); Cotoneaster, always known as cotoneaster; the hawthorn, Crataegus, otherwise known as the quickthorn or may tree; the firethorn (Pyracantha); the roses, of course, in the genus Rosa—many thousands of cultivars hybridized from about nine wild ancestors; Sorbus, which includes the mountain ash, or rowan, and also the whitebeam; and the florists’ favorite, Spiraea. Some have other uses, too. Hawthorn in particular is Britain’s favorite hedging plant, layered—the branches cut halfway across, then laid sideways—to form an impenetrable thorny barricade—or sometimes left to grow into a big mature tree in the hedge, as elms often were.

  Five closely related genera within the Rosaceae (though none of the important ones mentioned above) have developed symbiotic relationships with nitrogen-fixing bacteria that live in nodules in their roots. The bacteria are not Rhizobium, as in the Fabaceae, but Frankia. In all, plants from about ten families harbor nitrogen-fixing Frankia in nodules in their roots (one of the chief of which is the alder, Alnus). It would be tempting to suggest that the Fabaceae also started out with Frankia in their roots but that these were later displaced by Rhizobium. Only if this were so, we would expect the most primitive Fabaceae to harbor one or other of the two bacteria. In fact, the most primitive Fabaceae do not have nitrogen-fixing bacteria at all. Thus it seems that nodules to harbor Frankia and nodules to harbor Rhizobia are independent, parallel inventions—yet another, stunning case of convergent evolution. We also see, yet again, the propensity of organisms—one might almost say their eagerness—to cooperate.

  Then there are the Rhamnaceae, the family of the buckthorns: 850 species in 45 genera: often thorny; some trees, some shrubs, some climbers; and, again, sometimes with nitrogen-fixing bacteria in their roots. We are familiar with a few Rhamnaceae in the temperate north: buckthorn is Rhamnus; Ceanothus is a highly fashionable ornamental. But the Rhamnaceae come mainly from the tropics, where many are useful. Hovenia dulcis is the raisin tree. Ziziphus jujuba is the Indian jujube. The jujube grows fast on dry, poor land to form red-flowered trees that are often scrubby but can reach 24 meters. Its timber is good, and it burns well; its prickly branches make serviceable fences; its leaves and twigs are fodder for camels and goats; its wild green fruits make sherbet, sold in the markets and (it’s said) much loved by students; and it is cultivated for its fruit, used for seasoning, cooked with sugar, or stored in oil or sugar syrup. Perhaps most of all, though, the jujube is a fine host for the lac insect, which sucks its sap and exudes a reddish resin over the whole surface of the twigs—which yields a dye and also becomes shellac, once used for gramophone records and still favored for polishes and for lacquer. The jujube illustrates a general principle: how much use is made of plants that outsiders would scarcely notice, by people who know about them; how entire economies and cultures can flourish under our noses without us noticing; and how easily and often those ways of life are swept aside—for what developer would care about the wild jujube trees? The lac insect also feeds on the peepul, the rain tree, and the mango.

  Both the Rosaceae and the Rhamnaceae families are at the edge of the Rosales order, however. The remaining families all seem to group roughly together in one great clade. And what families they are.

  First come the Ulmaceae, the family of the elms (Ulmus) and the favored park tree Zelkova. There are 6 genera, and about 40 species, all trees, mostly in the temp
erate north. Elms until recent times were so common in England they largely defined the lowland landscape: they dominate John Constable’s Suffolk landscapes in the east, and in the west were known as the “Wiltshire weed.” They commonly grew in hedges and formed fine trees whose timber was often used in great slabs—for example, to make the buttock-molded seats of rural wooden armchairs and the sides of wheelbarrows. In the 1970s, Britain’s elms were struck down by Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi and carried and introduced beneath the bark by bark beetles of the genus Scolytus. Within a few years, despite the best efforts of foresters and biologists, mature elms had all but disappeared. The original types hang on as hedgerow bushes—but as soon as they reach a critical height, within the beetle’s flying zone, they are attacked again and die off. New resistant strains are being developed, but England’s lowlands will never be the same (although, of course, the transformations wrought by urbanization and agribusiness are far more dramatic). Dutch elm disease occurs on mainland Europe and in America as well, where the fungal pathogen is carried both by Scolytus and Hylurgopinus.

  There are also the Celtidaceae, which include the hackberry or sugarberry (Celtis), whose colorful fruits are for the birds that disperse them, and are rarely eaten by humans. But hackberry is used for timber and grown as an ornamental.