by Colin Tudge
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, chick peas, groundnuts (alias 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 ploughed 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, chick peas) 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; eighteen 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 metres 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 3). 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 black-wood, 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 tips. 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 behaviour 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 harbour 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 coloured 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 each other, 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? Here 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).
Many acacias are cultivated for many purposes. A. auriculiformis, A. mangium and the Australian blackwood, A. melanoxylon, are the most favoured timber trees. The Australian blackwood grows wild in Queensland and New South Wales to a height of up to 30 metres; nonetheless it grows as an understorey tree, beneath the giant eucalypts known as mountain ash, which may grow to nearly 100 metres. 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 favour 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 favour 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
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Then there is the genus Leucaena. The twenty-two known species grow wild in the Americas from Peru to Texas, from sea level to 3,000 metres. Native people eat the edible pods for their garlic flavour. 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. leucocepbala, 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-flavoured coffee is ‘shade’ 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 shortlist 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 centimetres in diameter: massive. Such trees, grown in appropriate plantations, help to take the burden of human needs and ambitions from the wild forests, and if burnt 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 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, Liriodendrori) 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 favoured for turnery. The boldly striped zebrano from West Africa comes from various species of Microberlinia: again favoured for carving. Purpleheart from South America, figured like fine tweed, is a species of Peltogyne: used to make apparatus for the gymnasium, 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. lnstia 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. pseudacacia, otherwise known as the false acacia or black locust, which was introduced to Europe in the 1700s and selected for the navy – as ‘shipmast locusts’. These have been cultivated intensely (and apparently 250,000 hectares of them are planted in Hungary). Tipuana tipu, sole member of its genus, is the pride of Bolivia; also known as a rosewood, it grows up to 20 metres as a street tree, as a windbreak and for fodder, both in Bolivia and Argentina.
Many legumes are grown as ornamentals. The suburban favourite is Laburnum. The tropical American Enterolobium, of which the huge-leaved E. cyclocarpum is 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 which 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 flavour 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 harbours 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 forty-four species of Prosopis are mostly generally resistant to drought and salt – which makes them promising candidates for the many million hectares worldwide now spoiled by salinity, brought about by over-zealous irrigation. Prosopis includes the mesquites: P. glandulosa is the North American kind, favoured for its aromatic charcoal that adds flavours to the barbecue. P. cineraria of tropical India also provides charcoal, plus firewood, fodder, green manure and goat-proof thorny fences. The extraordinary P. tamarugo of Chile is slow-growing but widely planted for its resistance both to salt and 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 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 3,000 years for its beauty, and for dye and medicines.
Many leguminous trees provide food. Several species from the huge tropical genus lnga have edible fruits and seeds, including the ice-cream bean, I. laurina.(The genus lnga 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. Thus: 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 vapours (or so it is claimed) and make you feel ill; while neem trees are restorative. So the further the man travelled, 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 have 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 are 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 eighty-five 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 westwards so 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 on to 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 seventy-six species of pears (Pyrus), are close relatives of apples: some species 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 metres tall, 5 metres in girth (about 1.5 metres in diameter). Cydonia is the quince and Eriobotrya is the loquat.
