Thickpod Salwood Northern Wattle

Acacia crassicarpa A. Cunn. ex Benth.

Thick-pod salwood is a tree that often reaches 20 m in height but may attain heights of 30 m on favourable sites or it may occur as a shrub less than 1 m tall on adverse sites. The bole may be up to half the tree height in length, with maximum dbh development in the range of 50-60 cm. The crown is usually dense, comprised of large, dull, broadly falcate foliage.

In Australia this species is endemic to Queensland and extends from a number of islands in the Torres Strait south to the Burdekin region. There are southern outliers near Mackay and on Whitsunday Island. Populations become less frequent with increasing distance from the coast, but populations extend to the Atherton Tableland. This species also occurs in the southern lowlands of New Guinea.

In Australia, thick-pod salwood is most common on lowland, coastal or near-coastal sites, including sandy levees near seasonally dry creeks and on coastal foredune systems. The main soil types are acidic, alluvial or collu-vial sands or sandy loams. On the Atherton Tableland soil types are clay or clay loams. Rock substrates include granite, schists and volcanic types.

Thick-pod salwood occurs in open forests or woodlands or sometimes on the margins of closed rainforests. Associated species include ear-pod wattle (A. auriculi-formis), other salwoods (A. celsa, A. mangium), red wattle (A. flavescens), A. leptocarpa, A. polystachya, carbeen (E. tessellaris), white gum (E. platyphylla) and swamp box (Lophostemon suaveolens).

Related species: Thick-pod salwood (sect. Juliflorae) is a member of the 'A. aulacocarpa group'. Within this group it belongs in a subgroup with A. lamprocarpa, A. midgleyi, A. peregrinalis and A. wetarensis (McDonald and Maslin 2000). These species are distinguished by their mode of pod dehiscence in which seed is shed via the ventral suture; other members of the A. aulacocarpa group shed seed via the dorsal suture. Thick-pod salwood differs from its close relatives in having larger broadly falcate phyllodes, up to 27 X 4.5 cm, which narrowly tapered and elongate at the base, golden-yellow flower spikes in clusters of 2-6 per axil, broader woody pods and terete seeds that have a long, many-folded aril.

Publication: Hooker's London J. Bot. 1, 379 (1842). Type: Lizard Island, Queensland, Aug. 1820, collected by the botanist Alan Cunningham during Phillip Parker King's voyage of the HMS Mermaid.

Names: Botanical—from the Latin crassus (thick) and the Greek carpos (a fruit), in reference to its woody pods. Common—salwood probably alludes to the colour of the heartwood and its similarity to Sal (Shorea robusta).

Bark: Rough, moderately to deeply longitudinally furrowed, brown weathering to grey.

Foliage: Cotyledons—oblong, 6-8 mm long. Seedling—first leaf pinnate, second leaf bipinnate, at the third leaf stage the petiole elongates and flattens and a bipinnate leaf persists at the apex, phyllodinous by about the fourth leaf stage. Adult—

phyllodes, broadly falcate (broadest below the middle and curved along both margins) with a long narrow taper to the base, 8-27 X 1-4.5 cm, glabrous, leathery, longitudinal nerves numerous and parallel, with 3-7 nerves more prominent than the rest, the minor nerves close together and non-anastomosing; gland small, basal.

Inflorescences: Simple, axillary spikes, 2-6 per axil, up to 12 cm long, flowers subdensely arranged along rachis, light golden to pale yellow; flowers 5-merous, sepals fused. Flowers Apr.-Jun.

Fruits: Pods, oblong to narrowly oblong, flat, woody, with transverse to transversely oblique nerves, 4-12 X 2-4.5 cm. Seeds transversely aligned in pod, obloid to ovoid, more or less terete, glossy, black, aril cream to white, many-folded and up to 2 cm long when fully expanded. Mature Sept.-Nov.

Wood: Sapwood pale yellow-brown, heartwood reddish brown, density 500-805 kg m-3; uses include heavy construction, furniture, flooring and veneer; plantations for pulp wood production have been established in South-East Asia by the paper pulp industry.

Climate: Altitudinal range: mainly near sea level to 150 m but up to 700 m on the Atherton Tableland; Hottest/coldest months: 32-34°C/17-21°C; Frost incidence: low (except sites on the Atherton Tableland); Rainfall: 800-3600 mm per year, summer max.

Distinctive features: Tree with a dense crown of large, dull, grey-green, broadly falcate foliage; large phyllodes which taper narrowly at the base, with 3-7 prominent longitudinal nerves and numerous, parallel, closely spaced, non-anastomosing minor nerves; 2-6 spikes per axil, up to 12 cm long; thick and woody pods which dehisce seed from the ventral suture; seeds terete with a long, many-folded aril.

Acacia crassicarpa 1. Bark 2. Seeds 3, 4. Pods 5. Tree, Lizard Island, Qld 6. Seedling 7. Intermediate phyllodes 8. Flowering sprig 9. Tree, Merluna Station, Qld 10. Adult phyllodes 11. Phyllode nervation

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