Acacia aulacocarpa (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Acacia aulacocarpa A. Cunn. ex Benth.

Protologue: Lond. Journ. Bot. 1: 378 (1842).

Family: Leguminosae - Mimosoideae

Chromosome number: 2n= 26

Synonyms

• Acacia aulacocarpa A. Cunn. ex Benth. var. macrocarpa Benth. (1864),
• A. lamprocarpa O. Schwarz (1927),
• Racosperma aulacocarpum (A. Cunn. ex Benth.) Pedley (1987).

Vernacular names

• Brown salwood, hickory wattle, New Guinea brown wattle, New Guinea wattle (En).

Origin and geographic distribution

A. aulacocarpa occurs naturally in Australia, Papua New Guinea and Indonesia. It extends from northern New South Wales, eastern Queensland and the northern parts of the Northern Territory in Australia to southern Papua New Guinea and adjoining areas of south-eastern Irian Jaya. It has been tested in most countries of South and South-East Asia.

Uses

The wood is used as wattle, e.g. for heavy construction, furniture, flooring, cabinet-making, boat-building and panelling. It has long been used for native building posts and has a reputation of being durable. It is also used for fuel. It has excellent potential as a source of fibre for pulping and paper-making industries, producing one of the strongest bleached kraft pulps among acacias. A. aulacocarpa is used in reforestation of poor soils.

Properties

Nutrient content of the foliage per 100 g dry matter is: N 2.2 g, P 0.09 g, K 0.74 g, Ca 0.43 g, S 0.31 g, Mg 0.26 g, Cu 0.9 mg, Zn 4.5 mg, Mn 28.1 mg, Al 8.1 mg, B 3.5 mg. Dry matter digestibility and protein content of the foliage are low, making it unsuitable as a fodder. The weight of 1000 seeds is 12.5-25 g.

The sapwood of A. aulacocarpa is narrow, pale yellow to straw-coloured, distinct; heartwood pale olive-brown to grey brown, often attractively streaked with grey bands. The wood is hard, strong, and moderately heavy with a basic density of 600 kg/m³, an air-dry density of 645-720 kg/m³ at 12% moisture content, and an energy value of 21 600 kJ/kg. In a test, wood of a 12-year-old tree gave a screened pulp yield of 55.4% with Kappa number 19.3. The sapwood is susceptible to attack by Lyctus borer and the heartwood has low to moderate durability in contact with the ground. Charcoal made from A. aulacocarpa wood has a density of 500 kg/m³ at 1.25% moisture and an energy value of 37 100 kJ/kg.

See also the table on wood properties.

Description

• Shrub to slender, large tree, 3-40 m tall, bole branchless for up to 21 m, up to 1 m in diameter, sometimes fluted.
• Bark hard, brownish, about 1 cm thick, longitudinally fissured, peeling in long strips, inner bark red to pink, branchlets angular.
• Phyllodes straight or falcate, acute or subacute, 5-15 cm × 0.6-3.5 cm, 3-12 times as long as wide, glabrous, greyish-green or dull grey, with 3 prominent longitudinal veins somewhat crowded towards the lower margin at base, usually not yellowish, and with many parallel, not anastomosing, secondary veins; pulvinus 4-6 mm long with an ellipsoid basal gland.
• Inflorescence a spike, 2-6 cm long, yellow, 1-3 together; peduncle 2-8 mm long, scurfy.
• Flowers 5-merous, bisexual; calyx broadly cupular, 0.5-1 mm long, membranous, with broad, obtuse, scurfy lobes 0.2-0.3 mm long; corolla 1.2-1.9 mm long, lobed to the middle, glabrous, 2-3 times as long as the calyx; stamens many, 2.5-3 mm long; ovary 0.5 mm long, shortly pubescent or scurfy.
• Pod oblong, up to 10 cm × 2 cm, light brown, coriaceous to subwoody, with prominent obliquely transverse, dark brown veins, glabrous, often twisted when old.
• Seed elliptical-oblong, 5-8 mm × 2.5-3.5 mm, shiny black, transverse or oblique in pod, with pale terminal aril.

Growth and development

Mature seeds germinate readily. After the cotyledons have fully unfolded, a pinnate leaf with 8-10 leaflets emerges. This is followed by a bipinnate leaf. A second bipinnate leaf follows, and usually it is from this leaf position onwards that the flattened petiole expands to form a phyllode, but with a bipinnate leaf remaining intact at the tip. Following this stage, seedlings develop to full phyllode stage, producing phyllodes without intact bipinnate leaves. The adult foliage form is reached about 6 weeks after germination.

Trees attain 12-16 m in height and 11-14 cm in diameter in 4 years. A. aulacocarpa was tested in Guyana on strongly leached, white quartz sandy soil with pH 4.7 and an annual rainfall of 2400 mm. It grew to 12.5 m tall in 3 years, while A. auriculiformis A. Cunn. ex Benth. attained only 7.8 m, and Pinus caribaea Morelet and Paraserianthes falcataria (L.) Nielsen failed completely.

A. aulacocarpa is an evergreen atmospheric nitrogen fixing species. The main and lateral shoots grow almost throughout the year, but growth may stagnate during the very hot and dry season. Trees generally start to flower after 3 years. The main period of flowering is from February to April in subtropical Australia and from April to June in tropical parts of its natural range. Insects, especially bees, are believed to be the main pollinating agent. Seeds mature 4-5 months after flowering. It is not uncommon for A. aulacocarpa to produce two seed crops per year.

Other botanical information

Two varieties have been distinguished: var. aulacocarpa and var. fruticosa C.T. White. The former is a tree, phyllodes with crowded nerves, 7-15 cm long, 4-12 times as long as wide; calyx 0.7-1 mm long; pod usually 1.5-2 cm wide; the latter is a bushy shrub up to 3 m tall, phyllodes with less crowded nerves, 5-10 cm long, 3-5 times as long as wide; calyx 0.5-0.6 mm long; pod 1-1.2 cm wide. Var. fruticosa is restricted to southern Queensland. These and other differences between populations in humid and dry areas are the focus of current taxonomic attention.

The most closely related species is A. crassicarpa A. Cunn. ex Benth., whose distribution overlaps that of A. aulacocarpa in northern Queensland and Papua New Guinea. A. crassicarpa has very narrow (2-4 mm), long fruits, whereas A. aulacocarpa has contorted, wider (1-2 cm) ones. A. aulacocarpa is also closely related to A. auriculiformis which has contorted fruits with undulate margins. A. aulacocarpa has phyllodes without anastomosing secondary veins, in A. auriculiformis these veins are somewhat anastomosed. A. aulacocarpa may infrequently hybridize with A. crassicarpa.

Ecology

The main occurrence of A. aulacocarpa is in warm to hot humid and sub-humid zones of the tropics and subtropics, at the latitudinal range 6-30 °S, and it extends from near sea level in New Guinea up to about 1000 m altitude in Australia. Mean annual rainfall ranges from 500-3000 mm with a monsoonal distribution. The mean minimum temperature of the coolest month is 10-21 °C and the mean maximum temperature of the hottest month is 29-38 °C.

A. aulacocarpa is mainly a species of open forest and woodland, but with limited extension into rain forest. In open forest it grows in association with Eucalyptus miniata Cunn. ex Schauer and E. tetrodonta F. v. Mueller, on the edges of semi-arid woodland with E. polycarpa F. v. Mueller and E. papuana F. v. Mueller. On rain forest fringes it is often associated with E. pellita F. v. Mueller, E. intermedia R. Baker, Acacia cincinnata F. v. Mueller, A. mangium Willd. and A. polystachya A. Cunn. ex Benth. On the swampy coastal plains of north-eastern Australia and south-western Papua New Guinea, it occurs with A. mangium and A. crassicarpa between wet depressions dominated by Melaleuca spp.

A. aulacocarpa grows in a wide topographical range including undulating highlands, ridges, and steep rocky slopes, as well as on the flat and gently undulating terrain of coastal plains and foothills. It is found frequently on yellow earths, red or yellow podzolics that are usually acid or very acid and of low fertility, and on sandy clay soils. It tolerates a wide pH range.

Propagation and planting

Propagation is generally by seed, although cuttings and air layering can also be used. Seeds have a hard seedcoat which requires heat treatment or nicking to break dormancy. Immersion in boiling water for 1 minute is a suitable treatment. Treated seeds are sown in germination beds and seedlings are transplanted into polythene bags when they reach the 2 leaf-pair stage. Seeds can also be sown directly into polythene bags. Young seedlings should initially be kept at 50% sunlight, but this can be increased to 70% once the seedlings are established. Excess shading often results in attack by mildew and other fungi and damping off. In general, the seedlings are 25-30 cm tall and ready for transplanting 3-4 months after sowing. A spacing of 3-4 m × 3-4 m is considered suitable for firewood and pulpwood plantations.

Husbandry

A. aulacocarpa competes very well with weeds including Imperata cylindrica (L.) Raeuschel. In plantations with 2-3 m × 2-3 m spacing, it will totally suppress I. cylindrica grass within 2-3 years. However, weed control is necessary in the first 2 years to help establishment.

An 8-10-year rotation is recommended for pulpwood plantations, and a 15-20-year rotation for saw logs.

A. aulacocarpa does not coppice well, but there is evidence that trees from Queensland respond to coppicing better than those from Papua New Guinea. The coppicing mechanism is not well understood.

Diseases and pests

Apart from infestation by powdery mildew in the nursery, trees are sometimes attacked by a Sinoxylon sp. that girdles small stems and branches of less than 2 cm in diameter only, causing them to break at the point of attack. Attack by a stem pinhole borer (Lyctus sp.) has been reported in Sabah, Malaysia.

Yield

A. aulacocarpa has shown considerable variation in growth and yield. In general, provenances from Papua New Guinea grow much faster than those from Australia. At a planting site in southern Thailand, a seedlot from Oriomo, Papua New Guinea, produced an above-ground dry biomass of 103 t/ha in 3 years, twice as much as that produced by material from Queensland. Papua New Guinea provenances have also shown satisfactory growth in Sabah.

Genetic resources

The Australian Tree Seed Centre, Commonwealth Scientific and Industrial Research Organization (CSIRO, Canberra), has a good coverage of genetic material from the natural range in Australia and Papua New Guinea. Seed from Papua New Guinea provenances is also available from the Forest Research Institute in Lae. Seed from seed orchards established in Thailand is now available.

Breeding

Current breeding programmes are limited to a small number of progeny trials, which are being converted into seed orchards in Australia, Indonesia, Malaysia and Thailand.

Prospects

Because of its good fuel, timber and pulping characteristics, A. aulacocarpa has great potential as a plantation species in the humid and sub-humid tropics. Its tendency to have a fluted stem may reduce its value for some purposes, like veneer. Its light to moderate shade makes it also useful for shade and ornamental planting. Further study of the coppicing mechanism is warranted.

Literature

• Awang, K. & Taylor, D.A., 1993. Acacias for rural, industrial and environmental development. Proceedings of the 2nd meeting of the Consultative Group for Research and Development of Acacias (COGREDA) held at Udorn Thani, Thailand, February 15-18, 1993. Winrock International and Food and Agriculture Organization, Bangkok, Thailand. 258 pp.
• Clark, N.B., Balodis, V., Fang Guigan & Wang Jinxia, 1991. Pulping properties of tropical acacias. In: Turnbull, J.W. (Editor): Advances in tropical acacia research. Proceedings of an international workshop held in Bangkok, Thailand, 11-15 February, 1991. ACIAR Proceedings No 35. Australian Centre for International Agricultural Research (ACIAR), Canberra, Australia. pp. 138-144.
• Nielsen, I.C., 1992. Mimosaceae (Leguminosae-Mimosoideae). Acacia. In: de Wilde, W.J.J.O., Nooteboom, H.P. & Kalkman, C. (Editors): Flora Malesiana. Series 1, Vol. 11. Foundation Flora Malesiana, Leiden, the Netherlands. pp. 34-64.
• Pedley, L., 1978. A revision of Acacia Mill. in Queensland. Austrobaileya 1(2): 148-149.
• Sim Boon Liang & Gan, E., 1991. Performance of Acacia species on four sites of Sabah Forest Industries. In: Turnbull, J.W. (Editor): Advances in tropical acacia research. Proceedings of an international workshop held in Bangkok, Thailand, 11-15 February 1991. ACIAR Proceedings No 35. Australian Centre for International Agricultural Research (ACIAR), Canberra, Australia. pp. 159-165.
• Thomson, L.A.J., 1994. Acacia aulacocarpa, A. cincinnata, A. crassicarpa and A. wetarensis: an annotated bibliography. CSIRO, Division of Forestry, Canberra, Australia. 131 pp.
• Vercoe, T.K., 1987. Fodder potential of selected Australian tree species. In: Turnbull, J.W. (Editor): Australian acacias for developing countries. Proceedings of an international workshop held at the Forestry Training Centre, Gympie, Queensland, Australia, 4-7 August 1986. ACIAR Proceedings No 16. Australian Centre for International Agricultural Research (ACIAR), Canberra, Australia. pp. 95-100.
• Visaratana, T., 1991. Coppicing ability of some Australian tree species in Thailand. In: Turnbull, J.W. (Editor): Advances in tropical acacia research. Proceedings of an international workshop held in Bangkok, Thailand, 11-15 February 1991. ACIAR Proceedings No 35. Australian Centre for International Agricultural Research (ACIAR), Canberra, Australia. pp. 43-47.

145, 162, 289, 297, 509, 649, 650, 672. timbers

Authors

• K. Pinyopusarerk