Acacia mangium Willd. - A Fast Growing Tree for Tropical Plantation

Tree morphology

A. mangium is a kind of evergreen tree, up to 30 m tall. The general feature is presented in Fig. 2. The bole can be unbranched for more than half of the total tree height. It is sometimes fluted at the base and the tree diameter rarely exceeds 50 cm. Bark is rough and furrowed, either grey or brown in color. Small branches are winged. It may be reduced to a small tree or large shrub of 7-10 m on unfavorable sites. The bark surface is rough, furrowed longitudinally, and varies in colour from pale grey-brown to brown. The lower bole is sometimes fluted. Detailed botanical description refers to Pedley (1975).

^{Fig. 2. Feature of Acacia mangium. 1-Habit of young tree; 2-flowering twig; and 3-pods (Source: DFSC seed leaflet).}

Leaves (phyllodes)

Borne on very acutely angled, glabrous and stout branchlets, the mature phyllodes of A. mangium are very large, normally 11-27 cm long and 3-10 cm broad. They are dark green, glabrous on a glabrous pulvinus 0.6-1 cm long. The phyllodes are characterized by four (rarely three or five) main longitudinal nerves, basally confluent but distinct from lower margin, minor nerves strongly anatomizing to form a prominent reticulum (Maslin and McDonald 1996). A gland (extra floral nectary) is conspicuous at the base of the phyllode (Fig. 3).

^{Fig. 3. Phyllode of A. mangium with four longitudinal veins.}

Growth phenology

A. mangium is able to grow throughout the year if conditions are suitable. In Thailand, it has been observed that growth appears to slow down or cease in response to the combination of low rainfall and cool temperatures in January-February. Trees start to grow actively again in April before the start of the wet season (Atipanumpai, 1989).

Flowering phenology

Flowering in Acacias is precocious. A. mangium starts to flower and produce seeds 18-20 months after planting (Mergen et al. 1983). Mature fruits occur 3-4 months after flowering period. The time from the onset of flower buds to pod maturity is about 199 days (Zakaria 1993). Flowering phenology differs throughout its natural and planted range. In natural habitat, its flowers are present during February to May in Australia and the seed matures in October-December (Sedgley et al. 1992). Farther north the fruits mature earlier with seed available from July in Indonesia, and late September in Papua New Guinea (Skelton 1987; Turnbull et al. 1983).

As an exotic, the normal flowering cycle may be disrupted and flowering can occur throughout the year. However, a distinct peak is usually discernible (Awang and Taylor 1993). The peak is reported to be June-July in Peninsular Malaysia (Zakaria and Kamis 1991), January in Sabah (Sedgley et al. 1992), October-November in Taiwan (Kiang et al. 1989) and September in Thailand (Kijkar 1992).

Inflorescences, flowers and fruits

The inflorescence consisting of many tiny flowers, occur as rather loose spikes up to 10 cm long, singly or in pairs in the upper axils (Fig. 4). The whitish (or cream) flowers are in rather loose spikes 5-12 cm long on peduncles 0.6-1 cm long, singly or in pairs in the upper axils. The seed pods are linear, tightly coiled when ripe, sometimes tightly spirally coiled, slightly woody, 7-8 cm long and 0.3-0.5 cm wide.

The seeds are black and shiny, longitudinal, ovate to oblong 3-5×2-3 mm with a yellow or bright orange (rarely red) funicle folded to form an oily, fleshy aril beneath the seed (Fig. 4).

^{Fig. 4. A. mangium inflorescence in blooming stage.}