Botanical name

Acacia ayersiana Maconochie, J. Adelaide Bot. Gard. 1: 182, fig. 2 (1978)

Common name

Broad leaf Mulga

Aboriginal name

Marndarru (Banyjima)

Description

Rounded large shrubs maturing to obconic trees, (2-) 3-8 (-10) m tall and with a dense, often rounded crowns 3-6 m across, much-branched at or near base, main stems rather straight and to about 14 cm diameter at breast height. Bark grey to black, longitudinally fissured on mature main stems. Branchlets with dense silvery appressed hairs between the red-brown resinous ribs (resin not thick and prominent as in A. minyura) at extremities, glabrous with age. New shoots pale yellowish green with a silvery sheen, at initiation the very young phyllodes are densely appressed-hairy (the hairs are very pale yellow but soon age silvery, also minute, scattered, reddish resin hairs present) and have prominent resinous marginal nerves and normally some reddish resinous longitudinal nerves clearly evident. Phyllodes normally narrowly elliptic, sometimes lanceolate or (in very short phyllode forms) oblanceolate, normally 5-10 cm long and 7-13 (-18) mm wide, 30-40 mm long and 5-9 mm wide on short phyllode variant, coriaceous, commonly erect, straight or shallowly recurved, minutely sericeous (hairs often difficult to see without magnification), silvery grey-green to sub-glaucous; parallel longitudinal nerves numerous, 3-6 nerves slightly more pronounced than the rest (best observed on young phyllodes where the nerves dry brownish), the nerves sometimes resinous (resin most evident on young phyllodes but it is not thick as in A. minyura), anastomoses absent or few; marginal nerve discrete, narrow, often resinous, yellow to light brown, this nerve can be seen when the phyllode is observed in plane view and is most obvious on young and adolescent phyllodes towards the ends of the branchlets; apex acute to acuminate, straight to sub-uncinate, innocuous. Gland inconspicuous, situated on upper margin of phyllode 0-2 mm above the pulvinus. Inflorescences simple; peduncles 2-5 mm long, minutely silvery appressed -hairy (hairs often difficult to see without magnification); spikes 10-30 mm long, bright light golden; receptacle glabrous. Flowers 5-merous; sepals free or shortly united. Pods �oblong, flat, 15-40 mm long, 10-15 mm wide (including wings), chartaceous, straight, reticulately nerved, minutely appressed -hairy (hairs difficult to see without magnification); marginal wing distinct (i.e. intra-marginal nerve 1-2 mm from edge of phyllode), with a short basal stalk. Seeds transverse in the pods, obloid, 5-6 mm long, 3 mm wide, glossy, dark brown; aril small and white.

Characteristic features

Large round shrubs maturing to obconic trees. Branchlet apices with dense silvery appressed-hairy between the red-brown resinous ribs (resin not thick and prominent as in A. minyura). Phyllodes broadest at or just below middle (normally 7-13 mm wide) and narrowed at both end, rather broad, straight or shallowly recurved, silvery grey-green to sub-glaucous, with numerous, parallel longitudinal nerves of which 3-6 are more pronounced than the rest (best observed on young phyllodes where the nerves dry brownish), marginal nerve narrow but discrete and normally yellow or brown when dry (best observed by viewing phyllode side-on), apices acute to acuminate. Sepals �free. Pods �oblong, short and broad (15-40 x 10-15 mm), marginal wing distinct, flat, papery, reticulately nerved, minutely appressed -hairy (hairs difficult to see without magnification), with a short basal stalk. Seeds transverse in the pods.

Distribution and ecology

Occurs in the arid zone where it extends from Western Australia to South Australia and southern parts of Northern Territory. In the Pilbara A. ayersiana is most common in the Hamersley Range between Tom Price and Newman, but there are scattered occurrences to the east of Newman. It grows in often stony red-brown loam or loamy clay on colluvial flats, gentle slope or low rocky hills, and is commonly found in mixed Mulga communities dominated by A. aneura.

Flowering and fruiting period

The paucity of Pilbara gathering makes it difficult to accurately determine the phenology of this species. It apparently flowers and fruits in response to the timing and intensity of rainfall. A single collection with flowers at anthesis has been collected in February (presumably in response to summer rains) and another with mature flower buds in May. Plants with mature seed occur from about mid-September to October.

Variation

Specimens with atypically short phyllodes (30-40 mm long, 5-9 mm wide and narrowly elliptic to narrowly oblanceolate) occur at West Angelas and near Mt Bruce (the typical form of the species also occurs in this area). The taxonomic status of these individuals needs to be determined but it is possible that these narrow phyllode individuals are part of an intergrade between A. ayersiana and A. aneura var. intermedia which also occurs in the West Angelas area. These plants with atypically narrow phyllodes appear to fall into two classes defined by phyllode width, namely, 4-6 mm wide and (2-) 3-4 mm wide, and they vary from linear (on narrowest phyllodes) to linear-elliptic. Plants with the broader phyllodes look like a narrow phyllode form of A. ayersiana, whereas plants with the narrowest phyllodes may some look remarkably similar to var. intermedia (see that variety for discussion). Short phyllode forms of A. ayersiana superficially resemble Acacia sp. Mulga short phyllodes (B.R. Maslin et al. BRM 9276).

Taxonomy

This species is a member of the Mulga group. The above description is based on Pilbara specimens only.

Affinities

Acacia ayersiana is generally distinguished from all other Pilbara Mulga taxa by its broad phyllodes, which are obviously narrowed at each end. Other characters useful in identifying this species are found its phyllode nerves. Unlike most other Mulgas where the phyllode nerves are of uniform prominence those of A. ayersiana are often of unequal prominence with some slightly more pronounced than the rest (this character is best observed on young or adolescent phyllodes where the nerves generally dry a brownish colour due to the presence of microscopic resin hairs). The winged pods of A. ayersiana indicate that it has affinities with A. aneura var. microcarpa which has much narrower phyllodes.

Notes

Acacia ayersiana is generally killed by fire but can regenerate from the base of the trunk.

Indigenous peoples used the timber from Broad-leaf Mulga to make digging stick, clubs and other tools.

Conservation status

Not considered rare or endangered.

Origin of name

The botanical name is derived from the location of the type specimen which was collected in 1973 at Ayers Rock, Central Australia (now Uluru).