Agroforestree Database

Local names:
 English (thick-podded salwood,red wattle,Papua New Guinea red wattle,northern wattle,brown salwood)

Acacia crassicarpa is a small- to medium-sized tree 25 m (max. 30) tall; bole often straight and branchless for about 13-18 m; up to 50-60 cm in diameter; crown heavily branched and spreading. Bark dark or grey brown, hard with deep vertical furrows; inner bark is red and fibrous.

Phyllodes falcate, 8-27 x 1-4.5 cm, greyish-green, glabrous; primary veins 3-5, prominent, longitudinal, tending to run into the lower margin at the base; secondary veins parallel, not anastomosing, crowded; pulvinus, 4-20 mm long with a circular gland at the top.

Inflorescence a bright yellow spike, 4-7 cm long, clustered in groups of about 2-6 in the upper axils; peduncle 5-10 mm long, rachis thick; flowers pentamerous, bisexual; calyx broadly cupular, 0.5-0.7 mm long, lobes concave, lobed to about halfway down; corolla widely spreading, glabrous, 1.3-1.6 mm long, 2-3 times as long as the calyx; stamens 2-3 mm long; ovary shortly pubescent, more densely hairy at the top.

Pod woody, ovoid-oblong, flat, 5-8 x 2-4 cm, glabrous, dull brown, transversely veined but hardly reticulate. Seed oblongoid, 5-6 x 2-3 mm, black, arranged separately in separate compartments; areole large and almost closed; funicle folded and thickened, forming a long, pale creamy-yellow aril below the seed.

The generic name acacia comes from the Greek word ‘akis’ meaning a point or a barb, 
and the specific epithet is from Latin, 'crassus' (thick) and  'carpus'  (fruit), in reference to its thick pod.

Ecology

The species is found in warm to hot humid and sub-humid zones in the lowland tropics. In Australia, it is commonly found immediately behind beaches, on coastal plains and foothills. It appears to be tolerant of salt spray and soil salinity. In Papua New Guinea and Irian Jaya, Indonesia, A. crassicarpa is found on the gently undulating terrains on well-drained, strongly acid soils, and on imperfectly drained soils that flood in the wet season. On former rainforest wetlands with a sandy loam soil it is superior in growth to Acacia mangium. In the southern coastal lowlands of Queensland the species occurs in the understorey of open forest and in open woodland dominated by Eucalyptus pellita, E. tereticornis or E. tessellaris. On frontal sand dunes it is found as a wind-sheared shrub or small tree, behind Casuarina equisetifolia and associated with Alphitonia exelsa. On Cape York Peninsula, it is associated with Eucalyptus tetrodonta, Allocasuarina littoralis and Melaleuca spp. In Papua New Guinea, northern wattle occurs frequently with Acacia aulacocarpa, A. auriculiformis and A. mangium.

Native range
Australia, Indonesia, Papua New Guinea

Tree management

Under favourable conditions seedlings grow rapidly reaching 25-30 cm in 3-4 months. A. crassicarpa is one of the fastest growing tropical Acacia species. It appears to maintain active shoot growth almost the year round, although a few months of stagnation may occur in the dry season. On sites dominated by Imperata cylindrica or other weedy plants, weed control is necessary in the first 1-2 years to ensure establishment. Coppicing ability varies with cutting height and provenance, but is not a suitable method of regeneration, since trees do not coppice well. In open situations, the crown is strongly branched and casts a moderate shade. Preliminary observations indicate that A. crassicarpa is resistant to low intensity fires. An aboveground dry biomass of 207 t/ha can be obtained in 3 years with mean annual rainfall of 1 500 mm, and 40 t/ha in 3 years on poorer sites.

Spacing of 3 x 3 m (1 100 trees/ha) to 4 x 4 m (625 trees/ha) is suitable for land reclamation, fuelwood and pulpwood plantations.

The seeds of A. crassicarpa have a hard seed coat, low moisture content and are well-suited to storage in an air-tight container at room temperature (24°C). The seeds weigh about  36 400 seeds/kg.

The species is found in warm to hot humid and sub-humid zones in the lowland tropics. In Australia, it is commonly found immediately behind beaches, on coastal plains and foothills. It appears to be tolerant of salt spray and soil salinity. In Papua New Guinea and Irian Jaya, Indonesia, A. crassicarpa is found on the gently undulating terrains on well-drained, strongly acid soils, and on imperfectly drained soils that flood in the wet season. On former rainforest wetlands with a sandy loam soil it is superior in growth to Acacia mangium. In the southern coastal lowlands of Queensland the species occurs in the understorey of open forest and in open woodland dominated by Eucalyptus pellita, E. tereticornis or E. tessellaris. On frontal sand dunes it is found as a wind-sheared shrub or small tree, behind Casuarina equisetifolia and associated with Alphitonia exelsa. On Cape York Peninsula, it is associated with Eucalyptus tetrodonta, Allocasuarina littoralis and Melaleuca spp. In Papua New Guinea, northern wattle occurs frequently with Acacia aulacocarpa, A. auriculiformis and A. mangium.

Seeds remain viable for many years and heat treatment or nicking of the seed coat is required to break dormancy. Immersion in boiling water for 1 minute is also a suitable treatment. Treated seeds are sown in germination beds, and seedlings with 2 pairs of leaves can be transplanted into polythene bags containing a mixture of soil and river sand. They are raised under partial shade, then in the open, and planted out when stem height reaches 25-30 cm. Inoculation of nursery seedlings with a selected Rhizobium strain prior to planting out is recommended for maximum nodule development. Vegetative propagation through air-layering has given promising results in Thailand.

Cuttings may be successful for trees up to 3 years old but older trees are difficult to root. Marcotting is effective for older specimens.

The tree can be planted for weed control and is often cited as an effective species for the rehabilitation of land infested with Imperata cylindrica. In Papua New Guinea, it is reported to be a very vigorous colonizer of degraded soils following shifting cultivation. Although suitable for planting for land reclamation, it is too competitive to grow in combination with annual crops.

The wood dries out moderately rapidly, burns well and is useful for firewood and charcoal, although it sparks at ignition and produces some fly ash and smoke during flaming. Its energy value is 22 600 kJ/kg.

Fibre:  A. crassicarpa is suitable for kraft pulping, with a moderate screened pulp yield of 47%, and excellent pulpwood productivity (300 kg per cubic metre). Fast-grown plantation wood of lower density may prove more suitable for pulp production than wood from native trees.

Timber:  The sapwood is pale yellowish-brown and heartwood golden-brown. The wood is strong and durable with a density of 670-710 kg/cubic metre. It is suitable for a wide range of sawn timber end-uses including light structural and decorative purposes. Examples include construction, furniture, flooring, board and boat building and tobacco curing firewood as is the case in  Tanzania.

Shade or shelter:  A. crassicarpa provides suitable shade.

Nitrogen fixing:  A. crassicarpa is a vigorous atmospheric nitrogen fixer and nodulates well with a group of related Rhizobium strains.

Soil improver:  Leaves decompose slowly and are useful as mulch.