The Tree

Description

A leguminous tree species of the subfamily Mimosoideae, Acacia mangium grows as tall as 30 m, with a straight bole that may be unbranched for more than half its total height. Mature trees are usually more than 15 m high, but on adverse sites they may not reach 10 m. Stem diameters up to 90 cm have been measured in the natural forests of Queensland and Papua New Guinea. Slight fluting is generally present in the lower bole, reducing the tree's value for veneer. Trees grown in the open have globular crowns, but in plantations crowns are columnar. A type of self-pruning in which the lower branches die begins at an early age. Older trees have a thick, rough, hard bark that is furrowed longitudinally and varies from dark brown to fawn.

Newly germinated mangium seedlings have compound leaves made up of many leaflets. This juvenile foliage is like that of Leucaena, Albizia, and other species of the subfamily Mimosoideae. However, after a few weeks mangium no longer produces these true leaves. Instead, the leaf stalk and main axis of each compound leaf flattens and is transformed into a "phyllode." Phyllodes look like the leaves of common plants. They are simple, parallel veined, and exceptionally large-up to about 25 cm x 10 cm. These give mangium trees an entirely different appearance from most species of the subfamily Mimosoideae, including many other Acacia species.

Mangium's inflorescence is a loose spike, up to 10 cm long, made up of small white or cream flowers. It seems probable that neighboring trees normally cross-pollinate the flowers, but self-pollination is also possible.

After fertilization the flowers develop into green pods that darken to become blackish-brown at maturity. Initially straight, the pods twist and intertwine into irregularly spiralled clusters. In Queensland the time between flowering and seed maturity is about 7 months, though there is some evidence for a shorter maturation time in its northern range; in Sabah 6 months is considered average.

Acacia mangium. (Drawing by Gillian Rankin, courtesy of Queensland Herbarium, Indooroopilly, Queensland, Australia) ]

Seeds are arranged longitudinally within the pod. A bright-orange ribbon, known as a funicle, attaches each seed to the pod. Ripe pods dehisce along a single margin, and the mature, black, hard-coated seeds (3-5 mm long) hang out on their orange funicles. Within a few days, especially in strong winds, the funicles separate from the pod, and the seeds, with their orange ribbons still attached, fall to the ground. The colorful funicle seems to attract ants and birds, and this may help to disperse the seed.

Mangium flowers precociously in Sabah, and viable seed can be harvested 24 months after planting.

Natural Distribution and Habitat

Mangium is indigenous to northeastern Australia, Papua New Guinea, and eastern Indonesia (the Moluccas and Irian Jaya). Populations extend from a northern limit in Irian Jaya of 0°.50'S to the most southern occurrence in Queensland, Australia, at about 19°S.

The best-known stands are in Australia, where mangium occurs discontinuously along the eastern coast of Queensland between Ingham and the Jardine River. In this region the trees are typically found at elevations below 100 m, but two stands occur at elevations of 450 m and 720 m. While the Indonesian and Papua New Guinean occurrences are not well documented, they also appear limited to low elevations. They stretch from Taliabu in the Moluccas of Indonesia to Wuroi and the Oriomo River in the Western Province of Papua New Guinea.

In its native habitat mangium is found on the fringes of mangrove and melaleuca stands as well as in riverine forests grading into grassland. It does not occur in mature rainforest but is most often found on the forest margins. Normally, the trees occur in small groups; only occasionally do they dominate large areas.

Mangium is a pioneer species. It establishes itself after sites have been disturbed. For instance, it commonly grows beside tracks and roadsides and along the edges of sugarcane plantations in Queensland. Ground fires, a characteristic of its native habitat, often scar its bark, and fire appears to play a vital ecological role by periodically disturbing sites for its natural establishment.

Temperature

In its native habitat in Queensland, the hottest month (December or January) has a mean maximum temperature of 31°-34°C, and the coolest month has a mean minimum temperature of 12°-16°C. Sabah, too, has high temperatures, with similar maximums, but minimums there range from 22° to 25°C.

Mangium does not normally occur in areas that receive frost. However, it will survive light frost.

Rainfall

Mangium is a tree for wet sites. Dry spells slow its growth drastically, and currently available provenances (geographic seed sources) are probably unsuitable for areas with long dry seasons.

In mangium's native habitat, the annual rainfall varies from 1,000 mm to more than 4,500 mm. A notable concentration of the trees occurs in the Mission Beach-Tully area of Queensland that receives about 4,400 mm per year, with a relatively dry period (about 700 mm) for 4 months. A pattern of a relatively dry winter and spring and a wet summer and autumn is typical of all Australian areas to which mangium is native.

In Sabah the mean annual rainfall varies from 1,500 to 3,100 mm. Successful plantations have been established in localities receiving more than 2,000 mm of rain annually. Soil moisture usually remains high throughout the year.

Shade Tolerance

Like most pioneer species, mangium grows best in full sunlight. In shade, it remains stunted and spindly.

Soils

Mangium grows satisfactorily on eroded, rocky, thin mineral soils and also on deeply weathered or alluvial soils. In Queensland the trees are generally found on acidic ultisols and only rarely on soils derived from basic rocks. On Seram (Indonesia) the species is reported to occur on ultisols (red-yellow podsols) with a calcareous substrate. In Sabah the species has been planted both on entisols and ultisols, many of them acidic, with a pH as low as 4.5.

Phosphate levels are as low as 0.2 ppm on some soils where mangium grows, but rapid growth rates cannot be expected on such sites.

Symbioses

Mangium, like most legumes, forms a mutually beneficial symbiosis with soil bacteria of the genus Rhizobium. The bacteria penetrate young rootless in the aerated surface soil layers and multiply to form nodular swellings on the root surface. In the nodules the bacteria absorb nitrogen gas from air in the soil, transforming it into nitrogen-containing organic and inorganic compounds. Mangium normally has large and prolific nodules and usually requires no nitrogen fertilizer because the Rhizobium bacteria alone provide enough nitrogenous compounds to sustain the tree's growth.

A symbiotic relationship with the mycorrhizal fungus Thelephora ramariodes has been identified in Sabah. Fungi of this type benefit plants by helping them absorb micro- and macronutrients, especially phosphorus. This allows the trees to grow better in soils deficient in readily available minerals.

The plant's recent success in widely differing parts of the tropics suggests that suitable bacteria and fungi for these symbioses are widespread.