| Mangium and Other Fast-Growing Acacias for the Humid Tropics |
Mangium is not the only Acacia species that comes from the humid tropical region of northern Australia, Papua New Guinea, and eastern Indonesia. In or near its native habitat, notably in northern Australia, are also found Acacia aulacocarpa, A. auriculiformis, A. baker), A. cincinnata, A. crassicarpa, A. hylonoma, A. polystachya, A. solandri, and A. spirorbis.
These trees may have a much wider promise in tropical forestry than has been previously recognized. They have wood properties similar to those of mangium and may also prove to be fast growing. They appear to be adaptable, colonizing species that may grow with little help in some problem soils that are poor in nutrients. They produce root nodules profusely and often survive on land low in nitrogen and organic matter where most other tree species fail.
Few of these species have been tried in plantations, but they seem able to withstand root competition from nearby trees and may adapt well to cultivation. They produce seeds profusely and almost continually from an early age. All produce excellent fuelwood and other useful wood products. Their potential qualities-especially the probable capacity to enrich poor soils-make them worthy of pilot-scale testing. They may prove particularly valuable in slowing erosion and in reclaiming deforested sites in a wide array of climates from warm temperate to tropical. A more detailed discussion of each of them follows.
Although never utilized extensively outside Australia and Papua New Guinea, Acacia aulacocarpa+ seems to have promise for use in other tropical areas. It has been successfully introduced into lowland Malaysia and is being tried in plantations there. Moreover, it is reportedly showing remarkable promise in trial plots on the infertile white sands (spodosols) of Guyana, reaching an average height of around 12 m in 3 years.
Acacia aulacocarpa is related to Acacia crassicarpa (see below) and more distantly to Acacia auriculiformis (also below). It has a wide geographic range, from coastal districts of central New South Wales through eastern Queensland to the northern part of the Northern Territory and southern New Guinea. In the more temperate part of its range it is a small tree, growing best on rainforest margins and along streams, where it forms dense stands. It also commonly occurs on poor soils in eucalypt woodlands. In New Guinea it attains a height of 40 m and occurs mainly in wooded savannas with other Acacia and Melaleuca species.
Its leaves (phyllodes) are much shorter than those of mangium. Its pale-yellow flowers occur in spikes; its oblong pods are transversely veined, about 2 cm wide, and have undulate margins. Often the trees have poor form, but some specimens are straight bunked. Strict seedsource selection as well as extensive provenance trials are suggested before plantations are established.
Native stands of this species are found from near sea level to 900 m in climatic zones that are tropical or subtropical and humid or subhumid. The trees can withstand high temperatures-exceeding 32°C for an average of 280 days a year and 38°C for 70 days in some areas. Its native habitat gets few or no frosts. A few stands occur in wetter areas of the semiarid climatic zone in northern Australia where in some years the annual rainfall may be as low as 500 mm. However, more commonly, the average annual rainfall measures between 900 and 4,000 mm, an extremely wide range.
The trees grow in widely varying soils, including deep sands (stabilized sand dunes, for instance), ultisols, spodosols, and hard and gravelly clays, as well as in humus loams of good quality.
In Australia Acacia aulococarpa wood is marketed with that of mangium and three other acacias under the trade names black wattle or brown salwood. The narrow sapwood is pale yellow. The heartwood is dark brown, hard, strong, moderately durable to durable, and moderately heavy (specific gravity about 0.6). It is used for framing, weatherboards, and joinery, and is an attractive timber for furniture and cabinetmaking. It is also a good fuel.
Acacia auriculiformis ranges from Queensland (north of about 15° S), to the northern parts of the Northern Territory, across southern New Guinea to the Kei Islands of Indonesia. It occurs in closed forests along streams, adjacent to mangroves, behind beaches, and in more open forests, often on sand. It is a resilient, vigorously growing small tree, reaching 30 m tall under favorable conditions in the northern part of its range.
The tree will grow, with practically no maintenance, in a wide range of deep or shallow soils. It produces profuse bundles of root nodules and often survives on land so low in nitrogen and organic matter that most eucalypts and other tree species fail.
Acacia auriculiformis looks much like Acacia aulacocarpa and Acacia crassicarpa (see below), but the fine veins of the phyllodes are anastomose (interconnected), the pods are narrower and more undulate than those of Acacia aulacocarpa, and the funicle encircles each seed.
Acacia auriculiformis thrives in hot climates with mean annual temperatures of 26°C and above. Although it survives in areas with less than 1,500 mm annual rainfall, it is better suited to climates with annual rainfall from 2,000 mm to more than 2,500 mm and with several dry months. Normally it is cultivated only at low elevations.
In trials in Papua New Guinea, trees have reached 6 m in 2 years (diameter 5 cm) and up to 17 m in 8 years. In Malaysia, transplanted seedlings on clay soils have reached a height of 9-12 m after 3 years; on nutrient-poor, sandy soils they reached 6 m in 3 years. In comparison trials in Sabah, Acacia auriculiformis grew at a similar rate to mangium.
The trunks of Acacia auriculiformis are generally crooked. However, the native plants show considerable variation in form. In Papua New Guinea, for instance, straight-stemmed specimens have been discovered (see illustration) and marked for seed collection. And in Sabah some straight-stemmed individuals have also been observed. Thus, through selection and breeding there seem to be good prospects for improving this species for plantation production.
As a source of wood pulp Acacia auriculiformis has definite potential. Tests concluded in Australia have shown that the wood of 10-year-old trees (grown in Papua New Guinea) yields a high amount of pulp with excellent papermaking properties. The wood is well suited for fuel, and the tree is already established in fuelwood plantations in Indonesia.
With its dense foliage-which remains through the hot season- Acacia auriculiformis makes a useful shade tree and soil cover crop. It withstands city heat better than most broad-leaved trees and requires little attention. It is, however, a messy species and is being removed from Singapore's streets because of its excessive litter.
All leaves are shed annually, and in China the abundant litter of branches and dried leaves, both of which are useful as household fuel, annually amounts to 4.5-6 tons per hectare. On Java as many as 3 tons of leaves and 2 tons of twigs and branches per hectare have been measured beneath the trees.
In the 1960s China obtained seeds of Acacia auriculiformis from Australia. The tree has since become an important afforestation species in southern China. According to a recent report plantations on Hainan Island are showing annual growth up to 3-4 m in height and 2-3 cm in diameter. The wood is used primarily for fuel but is employed for making farm tools and furniture as well.
In Australia Acacia bakeri has been utilized in a limited way as cabinet timber. It is native to a small subtropical area (25°-29°S) at low elevations in or near remnants of rainforests, usually on loamy soils. A century ago specimens up to 45 m tall and 1 m in diameter were recorded, but at present they rarely exceed 8 m in height. This is because the lowland rainforests of southern Queensland and northern New South Wales where Acacia baker) once occurred extensively have been almost completely destroyed.
Little is known of the biology of Acacia baker). Its almost elliptical phyllodes are up to 13 cm long and 6 cm wide with 3 prominent longitudinal veins. Its flowering is irregular in late spring, and its pods mature at the wettest time of the year (February), the seeds sometimes germinating in the pod before they fall.
The wood is porous, open grained, not very durable, and tends to discolor and warp on seasoning. It produces considerable dust on turning.
Because of the destruction of most natural stands, seed is likely to be difficult to obtain.
This fast-growing tree reaches about 25 m in height and 30 cm in diameter in favorable situations, but it is commonly a small tree of only 8-10 m. It colonizes disturbed areas, and in dense stands its form is good. Probably it is short-lived. No production figures are available, but it seems to have potential as a short-rotation industrial wood.
Acacia cincinnata has a discontinuous distribution in Queensland, between 16° and 28°S. In the northern part of its range the climate is hot, humid, and mainly frost free; in southern localities it is warm, humid to subhumid, and with occasional frosts. Throughout much of its area this species grows on the rainforest fringes, but in the south it is mainly associated with open eucalypt forests.
Because of its wide range of native habitats, it is important to select seed from areas with climate similar to that of the planting site.
Commonly occurring close to the beach, Acacia crassicarpa seems to tolerate salt spray and high salinity in the soil. It tends to occupy drier and less-favorable habitats than either mangium or Acacia aulacocarpa.
A small tree or large shrub, Acacia crassicarpa usually grows to heights of 6-15 m, although trees taller than 30 m have been reported in Papua New Guinea. It is found mainly in the coastal lowlands of northern Queensland and on the Oriomo Plateau of Papua New Guinea at elevations ranging from near sea level to about 700 m.
Acacia crassicarpa has close affinities with Acacia aulacocarpa but a more restricted geographic range, occurring from about 20°S latitude in coastal Queensland to about 8°S in southern New Guinea. In Queensland it is often found in open eucalypt woodlands on slightly drier and less-fertile situations than those occupied by Acacia aulacocarpa. In New Guinea the two species occur in similar ecological ranges.
Acacia crassicarpa differs from Acacia aulacocarpa in that it has oblong pods, more than 2.5 cm wide, and with margins that are not undulate. Its phyllodes tend to be larger, with long pulvinuses that have a yellowish tinge, particularly when dry.
Acacia hylonoma occurs in an extremely limited area in lowland rainforest southeast of Cairns, Queensland (about 17° S), where annual average rainfall is about 2,000 mm. It grows to about 18 m tall with a diameter of 20 cm. It is reported to flower profusely and sets abundant seed. It has not been exploited in any way.
Although not now used commercially, Acacia polystachya may be of value as a tree crop for infertile sandy soils. It occurs naturally in northeastern Australia, where its geographic range is similar to that of mangium. It tends to grow in drier situations than mangium, however, and is most common in open eucalypt forest and in stunted forests near the sea. It also occurs in riverine rainforests and on granitic soils, where it reaches 24 m in height with a diameter of 50 cm.
Natural hybrids of Acacia polystachya and Acacia auriculiformis have been found in Australia, and hybrids with mangium are also likely.
Usually a small tree, Acacia solandri is found in light rainforest or more open communities close to the sea. It also occurs on islands off the coast of Queensland as well as in Melanesia. Specimens with a clear bole of 20 m have been recorded from a Vanuatu (New Hebrides) rainforest. A specimen with a diameter of 80 cm has been reported from a Queensland rainforest. Acacia solandri is closely related to Acacia spirorbis (see below), and the two are expected to behave similarly. Indeed, it seems likely that the two are identical and that one of them is misnamed.
A small tree, Acacia spirorbis may reach 15-20 m in height and often 40-60 cm in diameter. It is native to New Caledonia and Vanuatu and grows on the drier sides of the islands (1,500-2,000 mm annual rainfall) on neutral (pH 6-7), coral-derived soils. It also colonizes wetter sites (2,000-3,000 mm annual rainfall) having acid (pH 5-6), basalt-derived soils. On Erromanga (19°S) it has colonized large areas, forming almost pure stands on better coral- and basalt-derived soils, on the drier side of the island (1,600-2,000 mm, temperature 16°-30°C, altitude 0-400 m).
Its timber is hard, dense, and durable in contact with the ground. The government of Vanuatu plans to use it for energy production, either as charcoal or firewood. It probably occupies the same niche as mangium, but it has not yet been tested in plantations. The trees are believed to fix nitrogen; they set seeds prolifically and should be easy to raise.