The Tree (22 page)

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).

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.