2981

11: Auxiliary plants

Acacia aulacocarpa A. Cunn. ex Benth.

London Journ. Bot. 1: 378 (1842).

LEGUMINOSAE - MIMOSOIDEAE

2n = 26

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

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

Acacia 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.

The wood of Acacia aulacocarpa is suitable as firewood, for construction and flooring, boat building, furniture and cabinet work, tool handles, boxes and crates, joinery and turnery. 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. Acacia aulacocarpa is used in reforestation of poor soils.

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 Acacia 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 Acacia aulacocarpa wood has a density of 500 kg/m³ at 1.25% moisture and an energy value of 37 100 kJ/kg.

Shrub to slender, large tree, 3—40 m tall; trunk up to 1 m in diameter, sometimes fluted. Bark hard, brownish, about 1 cm thick, longitudinally fissured, peeling in long strips. Phyllodes straight or falcate, acute or subacute, 5—15 cm x 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 x 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 x 2.5—3.5 mm, shiny black, transverse or oblique in pod, with pale terminal aril.

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. Acacia 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 Acacia auriculiformis A. Cunn. ex Benth. attained only 7.8 m, and Pinus caribaea Morelet and Paraserianthes falcataria (L.) Nielsen failed completely.
Acacia 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 Acacia aulacocarpa to produce two seed crops per year.

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

The main occurrence of Acacia 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.
Acacia 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 Eucalyptus tetrodonta F. v. Mueller, on the edges of semi-arid woodland with Eucalyptus polycarpa F. v. Mueller and Eucalyptus papuana F. v. Mueller. On rain forest fringes it is often associated with Eucalyptus pellita F. v. Mueller, Eucalyptus intermedia R. Baker, Acacia cincinnata F. v. Mueller, Acacia mangium Willd. and Acacia polystachya A. Cunn. ex Benth. On the swampy coastal plains of north-eastern Australia and south-western Papua New Guinea, it occurs with Acacia mangium and Acacia crassicarpa between wet depressions dominated by Melaleuca spp.
Acacia 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 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 x 3—4 m is considered suitable for firewood and pulpwood plantations.

Acacia aulacocarpa competes very well with weeds including Imperata cylindrica (L.) Raeuschel. In plantations with 2—3 m x 2—3 m spacing, it will totally suppress Imperata 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.
Acacia 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.

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.

Acacia 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.

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.

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.

Because of its good fuel, timber and pulping characteristics, Acacia 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.

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.

K. Pinyopusarerk

Pinyopusarerk, K., 1997. Acacia aulacocarpa A. Cunn. ex Benth.. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea