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

Seed harvesting and collection

In India and Indonesia seeds are collected from plantations or orchards by locally hired climbers who cut off branches and strip the fruits into bags. After air drying, the pods are hand threshed and seeds are cleaned. Seeds can be produced from the 18 to 20-month-old trees, but the flowering and fruiting seasons are quite different based on the locations. In Indonesia fruits ripen in July, while in Papua New Guinea they ripen in September (Krisnawati et al. 2011).

Seed extraction and cleaning

Seeds can be extracted manually after sun-drying for several days (24-48 hours) until the pods turn brown/black and split. The drying temperature should remain below 43°C to avoid loss of seed viability (Krisnawati et al. 2011).

The pods should be processed as soon as possible after harvest. If pods and seeds are not thoroughly dried, the best storage during transport is cloth bags. Otherwise, the heat and humidity encourage the development of fungi. In India, seeds are extracted manually after sun drying for several days until the pods turning brown and/or black and split. Pods and seeds should not be left long to dry in the sun, as temperatures over 43°C can reduce viability. In Malaysia, pods are dried in a simple drying chamber equipped with an electric heater and a domestic fan (Adjers and Srivastava 1993). Seed moisture content should be reduced below 13% to prevent fungus development. Extraction with flailing thresher followed by winnowing as described in Doran et al. (1983) is suitable for this species. Because threshing of pods and seeds produces a highly irritating dust, workers need protection (Adjers and Srivastava 1993). In Fig. 6, fruits are shown.

^{Fig. 6. Fruits of A. mangium.}

Seed storage

The hard impermeable seed coat confers A. mangium seed long viability under almost any conditions if seeds are kept dry and free from insect pests. FAO (1987) recommended storing A. mangium seeds in sealed, air-tight containers in a refrigerator between 0-5°C temperatures. Supriadi and Valli (1988) recommended using clean jerry cans or small jars that could be closed tightly for storing seed. These jars can be stored in a dry, cold storage especially designed for forest tree seeds. This technique has been used to store seeds of A. mangium for several years without serious problems (Adjers and Srivastava 1993).

Dormancy and pretreatment

Germination in A. mangium is inhibited by hard impermeable seed coat. To obtain even and quick germination in nursery, it is necessary to use scarification or some other pretreatment to make the permeable testa being moistened. The most common and practical pretreatment method is the hot water treatment (Adjers and Srivastava 1993). Seeds are pretreated by immersion in boiling water for 30 seconds followed by soaking in cold water for 24 hours alternatively, they can be manually scarified. Germination rate is high (75-90%) after this treatment.

Seed germination and nursery practices

Seeds can be sown in seedbeds, germination trays (wet towel method) or directly in containers. Adjers and Srivastava (1993) gave a detailed description of nursery techniques. The optimum seedling container size for best results is 300 cc. For substratum in container use either top soil mixed with compost or a mixture of tropical peat or rice husk (between 70:30 and 90:10, depending on the characteristic of peat). The optimum height of seedlings for out-planting is 25-40 cm which can be achieved in 12 weeks with proper fertilizer applications. After 3-4 weeks for proper hardening, the seedlings will be ready for planting in 15-16 weeks after sowing in nursery.

Vegetative propagation

A. mangium stem cuttings can be easily rooted if cutting materials from 6 to 12 month-old seedlings are used. Rooting percentage drastically reduced with older planting stocks.

At six-month age of stock plant, rooting percentage was 71% and at 24 months it reduced to 15% (Darus 1993). High air humidity (70-90%) and fairly constant temperature (28°C) required in the rooting chamber. Use of cuttings with one half or one phyllode and applications of auxins such as 500-1,000 ppm IBA or a hormone rooting powder improved rooting. Rooting medium with high pH (5.8-neutral) and high water holding capacity increased rooting (Darus 1993). Micropropagation technique for A. mangium has also been successfully developed and reported. For optimum induction of multiple shoots, Murashige and Skoog basal medium supplemented with 0.5 mg/l of BAP was found to be most suitable (Dams 1993).