Botanical name

Acacia subcontorta Maslin, Nuytsia 18: 169, fig. 8 (2008)

Description

Multi-stemmed shrubs or trees (1-) 2-6 (-8) m tall, crowns ąrounded, sub-dense (sparser on oldest plants) and spreading (1-) 3-5 m across, trunks somewhat contorted and reaching about 10-15 cm diameter at breast height, main branches somewhat contorted and ąhorizontally spreading, young plants obconic with a dense sub-rounded crown. Bark grey, thin, fibrous and finely longitudinally fissured on trunks and main branches. Branchlets terete, obscurely ribbed, light brown at extremities, glabrous (except new shoots and sometimes in axil of mature phyllodes). New shoots green, resinous (but not sticky), pale yellow sericeous (often intermixed with some microscopic, red-brown resin hairs) when young but the hairs soon becoming silvery white, the indumentum sometimes obscured by resin when shoot is first initiated. Stipules caducous. Phyllodes linear to very narrowly elliptic, 6-11 cm long, (2-) 3-5 (-8) mm wide, moderately coriaceous to sub-rigid, rather wide-spreading to ascending or ąerect, straight to very shallowly recurved, commonly with a thin layer of non-sticky resin, adolescent phyllodes grey-green to bluish with a distinct silvery sheen due to an indumentum of appressed silvery white hairs (indumentum most dense on youngest phyllodes), oldest phyllodes green and glabrous or sub-glabrous; parallel longitudinal nerves fine, very close together and of uniform prominence or the central one slightly more evident than the rest, anastomosing minor nerves absent; marginal nerve discrete, resinous, yellowish or red-brown; apices acute to acuminate, curved to uncinate, innocuous; pulvinus 2-3 mm long, yellow to yellow-brown and transversely wrinkled when dry. Gland not prominent but sometimes slightly raised above margin, situated on upper margin of phyllode 0.5-1 (-3) mm above the pulvinus, circular, 0.3-0.6 mm in diameter, often brown (when dry). Inflorescences simple or sometimes rudimentary racemes (with 2 spikes) to 2 mm long; peduncles 2-6 (-8) mm long, sparsely to moderately densely hairy when in flower (hairs mostly appressed to sub-appressed but sometimes patent), glabrous or sub-glabrous when in fruit; spikes numerous and showy, 20-35 mm long, golden to light golden, buds slightly resinous and densely flowered (i.e. flowers close together) but becoming sub-densely flowered at anthesis (more space between the individual flowers so that the receptacle is often seen); receptacle puberulous with sparse to dense, spreading, white or pale yellow (rarely light golden) hairs but often glabrous when in fruit. Bracteoles linear-spathulate, c. 1 mm long, equal in length to calyx, the claws linear, the laminae c. 0.5 mm wide, inflexed and slightly thickened. Flowers 5-merous, rather small (petals c. 1.5 mm long), slightly resinous (but not sticky); calyx 1/3 to ˝ length of corolla, dissected for c. ˝ its length into oblong, sparsely hairy lobes which are inflexed and slightly thickened abaxially at apex, as flowers mature the sepals may sometimes irregularly split (sometimes to the base); petals glabrous, nerveless or very obscurely 1-nerved; ovary densely white tomentulose. Pods narrowly oblong to linear, flat but turgid to ąsub-terete, straight-edged or very shallowly constricted between seeds (occasional deep constrictions may occur on a few pods), 1-7 cm long, 6-9 mm wide, coriaceous-crustaceous to woody, glabrous or appressed-hairy (hairs white, moderately dense to sub-sparse and often embedded in a resinous, non-sticky, matrix when young), straight to shallowly or strongly curved, occasionally irregularly sigmoid or openly coiled, dark brown; marginal nerve not or scarcely thickened, yellow to light brown. Seeds longitudinal in pods, obloid to ellipsoid, large (5-7 mm long and 4-5 mm wide), compressed (3 mm thick), very dark brown to almost black, dull except slightly shiny at the centre (associated with areole); areole narrowly obloid to narrowly ellipsoid, 1.5-2 mm long and c. 0.7 mm wide, slightly shiny; pleurogram continuous, bordered by a ąobscure, narrow, very slightly raised, sometimes slightly shiny band of tissue; funicle thread-like and folded beneath a thickened, waxy terminal aril which is white except light green (when fresh) at attachment to seed.

Characteristic features

Multi-stemmed, spreading shrubs or trees with rather crooked stems, main branches somewhat contorted and ąhorizontally spreading. New shoots green, resinous (but not sticky), pale yellow sericeous when first initiated with the hairs soon becoming silvery white, indumentum sometimes obscured by resin when shoot is first initiated. Phyllodes linear to very narrowly elliptic, mostly 3-5 mm wide, moderately coriaceous to sub-rigid, straight to very shallowly recurved, adolescent phyllodes grey-green to bluish with a distinct silvery sheen due to an indumentum of appressed silvery white hairs (densest on youngest phyllodes), aging green and glabrous or sub-glabrous; parallel longitudinal nerves fine, and numerous, marginal nerve resinous and yellowish or red-brown, apices acute to acuminate and curved to uncinate. Peduncles relatively short (mostly 2-6 mm); spikes showy, golden, buds slightly resinous; receptacle with sparse to moderately dense spreading white or pale yellow (rarely light golden) hairs but often glabrous when in fruit. Pods 6-9 mm wide, turgid to ąsub-terete, coriaceous-crustaceous to woody, glabrous or appressed white-hairy (hairs often embedded in a resin when pods young), dark brown. Seeds large (5-7 x 4-5 mm), very dark brown to almost black.

Distribution and ecology

Occurs in Western Australia where it has a scattered distribution from the Pilbara region south to Lorna Glen Station (about 150 km northeast of Wiluna) then east to Thryptomene Hill in the Gibson Desert (about 600 km east-northeast of Wiluna). Acacia thoma has a somewhat similar distribution pattern. In the Pilbara A. subcontorta occurs on Balfour Downs and Ethel Creek stations (northeast of Newman) and also in the Hamersley Range near Giles Point, about 150 km to the west (a similar disjunction between the Hamersley Ranges and the Balfour area also occurs in A. bromilowiana and A. catenulata subsp. occidentalis). Acacia subcontorta is often common in the places where it occurs. In the Pilbara this species grows on stony hardpan plains that are typically characterised by very shallow red-brown loamy soils with a surface mantle of ironstone pebbles and cobbles over a consolidated ferruginous hardpan. These hardpans are typically found on gently undulating plains or between extensive Mulga groves. The hardpans tend to be characterized by overland sheet flows of water and are thus run-off sites; these sites occur in the Zebra, Spearhole, Washplain and Wanna Munna Land Systems (Van Vreeswyk et al. 2004). Further south in the eastern Gascoyne district and adjacent deserts, A. subcontorta is also associated with hardpan habitats although the soils appear to be somewhat sandy, especially in the Lorna Glen and Carnegie area. These hardpans may contain calcareous elements and where drainage is more defined, or where there exists a drainage foci such as a clay pan, A. subcontorta may occur on the fringing sandy levee banks. Open Mulga woodlands, characteristically grove-intergrove Mulga, with limited spinifex understorey forms the typical vegetation association in which this species occurs.

Flowering and fruiting period

Flowers from May to July and pods with mature seeds have been collected from late August to mid October.

Variation

There appears to be considerable variation in the growth form of this species. In better sites (i.e. along drainage lines) it may grow to a small tree reaching 6 (-8) m in height but on drier sites such as hardpan plains (e.g. in places on Balfour Downs Station) it appears to be more shrubby, ranging in height from 1-3 m tall. These differences in stature are most likely related to the depth of the hardpan.

Taxonomy

Until recently this species was known under its phrase name Acacia sp. Balfour Downs (B.R. Maslin 8526).

Affinities

Acacia subcontorta has affinities with A. citrinoviridis which is common in parts of the Pilbara and which is most readily distinguished by its broader pods (10-15 mm) that are densely citron sericeous (this colour is most evident on young pods, the hairs may turn silvery white as the pod matures); also, it has more obviously falcate phyllodes that are normally broader (mostly 7-13 mm) with yellow (never red) marginal nerves. The habit and habitat of A. citrinoviridis also differ from that of A. subcontorta. Acacia citrinoviridis normally has a single, straight main trunk, attains a height of 3.5-12 (-18) m tall and is normally found on the banks and adjacent floodplains of major river systems (infrequently along minor water courses). Acacia subcontorta is multi-stemmed and commonly of lower stature, has somewhat contorted main branches and favours drier hardpan sites. Narrow phyllode forms of A. catenulata subsp. occidentalis may superficially resemble A. subcontorta; these two taxa grow together near Giles Point on the Wanna Munna Flats in the Hamersley Range, and in the absence of pods care needs to be taken not to confuse the two taxa (see A. catenulata subsp. occidentalis for differences).

Conservation status

Not considered rare or endangered.

Origin of name

The species name is derived from the Latin sub (somewhat) and contortus (twisted) referring to the form of the stems and main branches.

References

van Vreeswyk, A.M.E., Payne, A.L., Leighton, K.A. and Hennig, P. (2004). An Inventory and Condition Survey of the Pilbara Region, Western Australia. pp. 423. Technical Bulletin No. 92. (Department of Agriculture: Perth, Western Australia.)