General Plant Info
Acacia aulacocarpa, also known as "Brown Wattle", "Hickory Wattle", "Bush Iron Bark Wattle" and "Brown Salwood", is a shrub to a slender, large tree, 3-40 m tall, occurring naturally East of the Great Dividing Range from North Queensland to Northern New South Wales. Despite its extensive distribution it is a relatively uncommon species as populations tend to be locally confined to creek banks or run-on sites near rock outcrops.
The species name ‘aulacocarpa’ is derived from the Greek aulakos (a furrow) and carpos (fruit), referring to the prominent furrowing and thickened transverse bands on the pods.
Two varieties are recognised. Acacia aulacocarpa var. fruticosa, a bushy shrub to 3 m tall
Acacia aulacocarpa var. aulacocarpa, usually a tree 10-20 m tall but ranges from a shrub of 4 m on xeric sites to a large tree to 40 m tall in tropical rainforest.
Acacia aulacocarpa has a discontinuous distribution along coastal areas and adjoining tablelands of the Great Dividing Range in eastern Qld from the Mt. Windsor Tableland ( c. 25 km east of Daintree), to just south of Brisbane and into northern N.S.W. near Grafton. Recorded also from Qld continental islands of Hinchinbrook, North Keppel and Deloraine (Whitsunday Group) Islands. Despite its extensive distribution it is a relatively uncommon species as populations tend to be locally confined to creek banks or run-on sites near rock outcrops There is considerable variation in habit between some populations, e.g. plants ocurring in coastal areas of North Keppel Island have a compact, domed habit only 0.5 m tall whereas those from along rivers in forests at Finch Hatton Gorge are medium-sized trees to 12 m tall with trunks to 40 cm diameter at breast height. Spindly, sparsely foliaged shrubs 3 m tall with a basal stem diameter of only 1–2.5 cm occur on residual trachyte plugs such as Ngun Ngun (these plants were described by White in 1946 as A. aulacocarpa var. fruticosa ). Plants of a similar habit occur on skeletal soils at Port Clinton, the type locality of A. aulacocarpa
Shrubs 0.5–2 m tall or small trees 2–8 (–15) m tall, single-stemmed or sparingly branched near base, crowns spreading and blue-green. Bark smooth except very shallowly rimose on trunks of largest trees. Branchlets acutely angled for 10–25 cm (occasionally more) below the apex, slender to sub-stout, glabrous.
Phyllodes mostly dimidiate to subfalcate, (3–) 5–12.5 cm long, 0.7–3.5 cm wide, glabrous, subglaucous to glaucous with a slight sheen; longitudinal nerves numerous, close (3–4 per mm) and parallel, not anastomosing; pulvinus 4–8 (10) mm long.
Inflorescences simple, 1–2 (–4) per axil; peduncles mostly 4–8 mm long, glabrous; spikes 2–5 (–6) cm long, bright golden. Flowers 5-merous; calyx shallowly dissected; ovary densely hairy.
Pods 1.5–8 cm long, usually 0.8–1.5 cm wide, straight to shallowly curved, dehiscing along dorsal suture, woody, conspicuously resinous, reddish-brown, prominently nerved, the nerves broad, sharply defined, prominently raised, oblique to longitudinally oblique and sparingly anastomosing. Seeds oblique, ovoid to obloid or ellipsoid, 3.5–5 mm long, 2–3 mm wide; funicle/aril 2–6 mm long (unextended).
The low toxicity of these Acacia extracts and their inhibitory bioactivity against bacteria validate Australian Aboriginal usage of A. aulacocarpa as antiseptic agents and confirms their medicinal potential.
Fodder: The potential for fodder production is limited; the phyllodes are rather unpalatable to stock, being eaten only during very dry periods, and the predicted in vivo digestibility is low, 33%
Fuel: The wood has an energy value of 21600 kJ/kg and is suitable for firewood. Charcoal made from A. aulacocarpa wood has a density of 500 kg/cubic metre at 12.5% moisture and an energy value of 37 100 kJ/kg
Fibre: Excellent potential as a source of fibre for pulping and paper-making industries, producing one of the strongest bleached kraft pulps among acacias
Timber: The sapwood of A. aulacocarpa is narrow, creamy yellow to straw-coloured, distinct; heartwood pale olive-brown to grey-brown, often attractively streaked with grey bands. The heartwood is reddish-brown, hard, heavy (600-800 kg/cubic metre basic density), moderately durable and tough. Used as a construction timber, for furniture and cabinetwork, flooring, boat building, tool handles, boxes and crates, joinery and turnery.
Tannin or dyestuff: The inner bark may be used for tannin production
Shade or shelter: The light to moderate crown makes A. aulacocarpa useful for shade planting.
Ornamental: The tree is occasionally planted as an ornamental.
Trees generally start to flower after 3 years. Insects, especially bees, are believed to be the main pollinating agent. Seeds mature 4-5 months after flowering; it is not unusual for A. aulacocarpa to produce 2 seed crops a year. 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 6-31 deg. C. It can tolerate only very mild frost. Found in a diverse range of vegetation associations, mainly open forest, and is one of the few acacia species extending into rainforest. The species has been described as an early secondary species, ‘proclimax’ species or secondary species dominating disturbed or successional forests and ‘permanent gap’ conditions such as steep, unstable slopes and watercourse margins. It 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. The soils are freely to imperfectly drained, usually acid or very acid and of low fertility
Soil type: Found frequently on yellow soils, red or yellow podzolics that are usually acidic or very acid (pH 3.5-6) and of low fertility, and on sandy clay soils; tolerates a wide pH range
In general, the seedlings are ready for transplanting 3-4 months after sowing. A spacing of 3-4 x 3-4 m is considered suitable for firewood and pulpwood plantations. A. aulacocarpa competes well with weeds, including Imperata cylindrica. In plantations with 2-3 x 2-3 m spacing, it will totally suppress the weed 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. Trees attain 12-16 m in height and 11-14 cm in diameter in 4 years. An evergreen species, the main and lateral shoots grow almost throughout the year, but growth may stagnate during the very hot and dry season. The tendency of A. aulacocarpa to have a fluted stem may reduce its value for purposes such as veneer. Selection of a suitable provenance will be necessary to obtain optimum results Seed storage behaviour is orthodox. There are 40 000-80 000 viable seeds/kg. Apart from infestation by powdery mildew in the nursery, trees are sometimes attacked by Sinoxylon spp., which girdles small stems and branches less than 2 cm in diameter, causing them to break at the point of attack. Attack by a stem pinhole borer (Lyctus spp.) has been reported in Sabah, Malaysia.