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Tropical Deforestation
In many parts of the world the loss of tropical forests has reached severe proportions. Africa, Asia, and Latin America have vast areas of once-forested regions that are now denuded. Wholesale removal of tree cover threatens the already fragile soils. Without trees and their root systems to retain moisture, heavy rains on hillslopes become torrents that erode the land and produce devastating floods that despoil highways, dams, bridges, towns, villages, and farmlands.
There is no end in sight to the destruction of forests: local farmers cut and burn trees to gain land for growing crops; villagers and entrepreneurs harvest wood for fuel as escalating prices preclude the use of kerosene, electricity, and gas for cooking and heating; herdsmen clear more forest and shrub land for grazing; and timber is now the Third World's fourth- or fifth- largest export.
Some countries-Haiti, for instance-have essentially eliminated their forests, while deforestation proceeds rapidly in many others. The denuded land is often lost to coarse, almost useless grasses (such as Imperata cylindrica) or to a wilderness of shrubs and scrubby trees of little value to commerce or villagers. These enormous, virtually unproductive areas are increasing. Some 25 million hectares of Imperata wastelands already cover Thailand, the Philippines, and Indonesia. Traditional reforestation with pines, eucalypts, and other well-known species should be applied more extensively, but the magnitude of the problem demands examination of less well-known species as well.
Tree Legumes
To the general public, legumes usually connote vegetables for the dining table. But to plant scientists, they include not only vegetables but also thousands of shrubs, vines, and tree species. Indeed, the family Leguminosae is the third largest in the plant kingdom. Many of its species are tropical trees whose potential in plantations remains largely untested.
Yet leguminous trees appear to have special attributes that make them particularly promising for combatting devastating tropical deforestation. For example, many are among nature's pioneering plants and colonize newly cleared land. It seems ecologically wise to exploit their innate abilities and use them for revegetating eroding or weed-smothered terrain, halting erosion, and providing a protective ground cover for the regeneration of slower-growing forest species. Furthermore, many everyday wood requirements can be met by small, fast-growing tree legumes. Cultivating them might thus reduce logging elsewhere and thereby help spare the last remnants of the natural forests.
An important characteristic of most legume species is the nodulation on their roots. Within these peppercorn-sized swellings bacteria convert nitrogen gas from the air into soluble compounds that the plant absorbs and utilizes. Thus, for average growth, leguminous plants usually require no nitrogenous fertilizer. Some produce such amounts of nitrogenous compounds-usually in the form of protein in their foliage-that they make excellent forage crops, and the decay of their fallen leaves makes the soil around them nitrogen rich.
Acacias
There are about 1,100 species in the genus Acacia. Most are shrubs or small trees of the dry savannas and arid regions of Australia, Africa, India, and the Americas. A few-such as Acacia mearnsii, a source of tannin for the world's leather industry-are suited to cool, moist areas, both in temperate regions and in tropical highlands. There is, however, a small third group of acacias, which is native to the lowland wet tropics. These species are adapted to hot, humid conditions, and, although comprehensive studies have not been made, they appear to have the characteristic acacia robustness and adaptability. Furthermore, they tend to grow quickly, their wood is hard, dense, and useful for many purposes; and the species examined so far all fix nitrogen.
Because of their innate competitiveness, tropical acacias are probably easy to establish and cultivate. On their natural range many occur in dense, pure stands, suggesting that they might be grown in monoculture without serious pest problems. Some can possibly be direct seeded, (avoiding the expense of nurseries and of transplanting fragile seedlings), and with some, even aerial seeding may prove feasible.
Mangium is native to three small
areas of Queensland, Australia, the southwestern portion of New Guinea, and the
Molucca islands of eastern Indonesia. In the last decade it has been planted
widely in Sabah, Malaysia (where it is, for instance, the principal species in a
200,000-hectare reforestation scheme), but so far it is little known elsewhere.
Mangium
Trees often perform in unexpected ways when introduced to new locations. Growth rates, health, and usefulness in their natural environments are poor guides for predicting performance when they are grown as exotics.
Acacia mangium exemplifies this. In 1966 this tree was introduced to Sabah, Malaysia, from its natural range along the margins of tropical moist forests in Queensland, Australia. It grew so well that it was tested in plantations. There, mangium grew as fast, or faster than, Gmelina arborea and Eucalyptus deglupta, both among the most rapidly growing useful trees in the humid tropics. In 14 years mangium grew up to 30 m tall, with a trunk 40 cm in diameter.
The trees grow so quickly that on good, logged-over sites in Sabah the canopy can close within one year of planting at a spacing of 3 m x 3 m.
On poor sites mangium notably outperformed the other species tested. On disturbed or burned sites, on degraded oxisols (laterite) underlain with volcanic rock, on soils so worn out that shifting cultivation had been abandoned, and on hillslopes infested with weeds such as Imperata cylindrica and Eupatorium species, mangium has grown vigorously. Sabah foresters have now converted more than 15,000 hectares of degraded lands into productive mangium plantations.
Although Sabah is so far the only region with a major mangium planting program, trial plots have been established in other parts of Malaysia and in other countries: Papua New Guinea in 1969, Nepal in 1976, the Philippines in 1977, Bangladesh in 1978, Hawaii in 1979, Cameroon and Costa Rica in 1980, and Indonrsia in 1980 and 1981.
From the successes generally reported in these various areas it seems likely that mangium has the potential to become a widespread and important tropical resource.
Mangium grows with relatively good form in plantations. The main bole is usually straight and clear. The tree nodulates profusely, coppices, and responds to silvicultural manipulation.
Plantations are easy to establish. Natural seed production is prolific. To extract the seeds from the pods requires some expertise, but dry seeds store well and germinate readily after simple, but essential, pretreatment. Nursery procedures are uncomplicated, and, like other acacias, the tree can probably be direct sown on many sites.
A noteworthy feature is mangium's ability to grow on soils with pH as low as 4.2. This is important because such acid soils are widespread in the tropics and is an attribute that distinguishes mangium from some other leguminous trees such as leucaena, which require a pH above 5.5.
Mangium's wood is a dense, all-purpose hardwood with an attractive, medium-brown colon It is of high quality and has been likened to black walnut in properties (see chapter 4). It has potential for sawn timber, molding, furniture, veneer, firewood, and charcoal. The wood is also promising for pulp, paper, and particle board.
Although mangium's wood is
considerably denser than most other fast-growing species, its volume growth in
Sabah ranks with that of the most productive trees known. (This chart, courtesy
of J. Davidson, is based on volume-yield data reported in the literature.)
The trees are useful for shade, ornamental purposes, screening, boundaries, and windbreaks, as well as for use in agroforestry and erosion control. The leaves can serve as forage for livestock. The tree generates considerable amounts of fallen branches and dead leaves that can be gathered for fuel-an important benefit in view of the current firewood shortages in many tropical countries.
In sum, mangium seems to be a species worth testing widely in the tropics, especially on problem lands. Chapters 2, 3, and 4 will discuss the plant's characteristics, cultivation, and utilization.
Mangium's Limitations
The extent to which mangium will adapt to climatic conditions differing from those in its natural habitat has yet to be determined. Nevertheless, successful growth will probably be limited to low-altitude, humid, tropical regions where rainfall is high and temperatures are equable year-round.
Soil moisture is an important consideration in choosing a site to cultivate mangium. Rainfall in its native habitat and in Sabah is reliable and is normally more than 2,000 mm annually. Soil moisture usually remains high throughout the year. Prolonged dry periods slow up or halt the tree's growth.
Mangium may possibly produce allergens or irritants. The dust from the pods that are pounded during seed extraction causes a respiratory reaction in some people, and, while it is possible that the pollen may be allergenic to those individuals, no hint of this has been reported.
It seems unlikely that such a useful tree could become a weed, but this is a possibility, especially if poor provenances are introduced.
Other Acacias of the Australasian Tropics
Like mangium, some other acacias from the rainforests of northern Australia, Papua New Guinea, and eastern Indonesia also seem to be hardy, irrepressible, and suited to a wide range of soils, climates, and environments. In chapter 5 of this report, Acacia aulacocarpa, Acacia auriculiformis, Acacia cincinnata, Acacia crassicarpa, and others are singled out.
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