Botanical name

Acacia arida Benth., London J. Bot. 1: 370 (1842)

Common name

False Melaleuca and Arid Wattle

Aboriginal name

Gunhawii (Yindjibarndi), Gunabree or Barlbirriny (Ngarluma), Pirnkalya (Nyangumarta), Kunalya (Kurrama) and Kunaka (Banyjima)

Description

Obconic, glabrous shrubs normally 0.5-2 (-3) m tall, with few to many slender stems arising from the base (reaching 5 cm in diameter at maturity), spreading by subterranean runners and resprouting from base following fire; crowns dense with soft and delicate foliage, rarely a few plants with an open, wispy habit like A. orthocarpa. Bark light-grey to mid-grey except towards the ends of the branches which are light brown, smooth. Branchlets light brown, reddish brown or yellowish at extremities. New shoots light green, resinous, not or only slightly sticky. Phyllodes linear, usually (2-) 2.5-6 (-8) cm long, 1-4 (-6) mm wide, flat, not rigid, erect, straight to shallowly incurved, minutely punctulate (i.e. covered with microscopic circular white dots - observe at x10 magnification or higher), dull, light green (at least when young) to distinctly sub-glaucous or grey-green, with a sweet fragrant smell when young; superficially nerveless or with an obscure, central midrib, occasionally 1 or 2 less prominent longitudinal nerves parallel to midrib; apex acute to obtuse, terminated by a small, blunt brown hard point. Gland obscure, situated on upper margin of phyllode at distal end of pulvinus. Inflorescences simple; peduncles (3-) 4-10 (-15) mm long; spikes not showy (scattered over plant), (10-) 20-30 mm long and 3-5 mm wide when dry, golden, flowers densely arranged. Flowers 5-merous; calyx dissected for 1/3-½ its length; buds resinous. Pods terete or flat or (at least when slightly immature) sub-quadrangular, neither raised over nor constricted between seeds, (3-) 4-6.5 (-8) cm long, (3-) 4-6 mm wide, erect, woody, opening elastically from the apex (dehisced valves shallowly to strongly recurved), straight or slightly curved, resinous (but not sticky, even when young), obscurely longitudinally nerved, yellow-brown to brown (but with a conspicuous blue tinge of resin when young), narrowed at base, apices obtuse to acute and often slightly hooked. Seeds retained in pods following dehiscence, obliquely placed within distinct chambers separated by narrow partitions, narrowly obloid, 5-5.5 mm long, 2 mm wide, laterally compressed (3 mm thick), glossy, dark brown to blackish except pleurogram bordered by a narrow band of dull yellow tissue; areole 3 x 1.5 mm; funicle-aril narrowly turbinate, straight, dull cream to pale yellow but tinged greenish brown at attachment to seed.

Characteristic features

Obconic, multi-stemmed, glabrous shrubs with soft, delicate crown foliage, spreading by subterranean runners and resprouting from base following fire; crowns dense. Phyllodes linear, relatively short (mostly 2.5-6 cm long), flat, erect, not rigid, puncticulate with microscopic circular white dots; nerveless or with 1 or few very obscure parallel longitudinal nerves; apices not spiny; with a sweet fragrant smell from young phyllodes. Spikes not showy, golden. Pods erect, terete to flat or sub-quadrangular, woody, obscurely longitudinally nerved, opening elastically from the apex with the dehisced valves shallowly to strongly recurved. Seeds retained in pods following dehiscence, obliquely placed within distinct chambers separated by narrow partitions, laterally compressed; funicle-aril narrowly turbinate.

Distribution and ecology

Occurs in northwest Western Australia where it ranges from North West Cape to the Pilbara region; it is also found in the more northerly Kimberley region. Acacia arida has a widespread, scattered distribution within the Pilbara but is not an especially common or conspicuous element of the vegetation of the region, except on many hilltops ('sky islands') in the Hamersley Range where it is often the dominant shrub. Grows in spinifex country in often skeletal sand along shallow, seasonal watercourses or on tops and slopes of hills and at the base of breakaways.

Flowering and fruiting period

It is likely that the flowering and fruiting in this species is influenced by the timing and intensity of rainfall events. Flowers (often sporadically) between February and November (but not all plants will be in flower during this period) with the main flush occurring between May and August. Pods with mature seeds have been collected between July and November with most likely to be present in October. Tyler (1988) reports that pods can develop many months after flowering (up to one year).

Variation

Phyllode size is somewhat variable. For example, within a single population (near Rippon Hills) they ranged from 1-3 mm wide and 3.5-7.5 cm long. See also discussion below under Affinities.

Affinities

Acacia arida is very closely related to A. orthocarpa and the relationship between the two species requires further study. These two species are immediately distinguished from all other Pilbara wattles by their phyllodes that are superficially ±nerveless and more particularly are covered with numerous, circular, often whitish stomata, which are clearly seen at x10 magnification. Other distinctive (but not necessarily diagnostic) features include their pedunculate golden spikes, woody pods which are normally terete and narrowed at both ends, laterally compressed seeds that are seated in distinct, oblique chambers within the pods, and a straight (non-convoluted), turbinate funicle/aril which retains the seeds attached to the pods following dehiscence (pods dehisce by splitting elastically from the apex with the dehisced valves recurved). Most, but not all, Pilbara plants can be attributed to one species or the other by the transverse sectional shape and width of their phyllodes, flat and normally 1-4 mm wide in A. arida, terete to sub-terete and 0.5-1 mm wide in A. orthocarpa. Defined in this way A. arida is generally a low, multi-stemmed shrub with a dense crown and, based on somewhat limited field observations, appears to spread vegetatively by subterranean runners and regenerates from a basal root stock following fire. Acacia orthocarpa on the other hand is commonly a taller plant with an open wispy habit and is single-stemmed or sparingly branched at the base; however, it sometimes has a multi-stemmed, bushy growth form similar to that of A. arida. Limited field observations suggest that A. orthocarpa is normally killed by fire, regenerating from seed and not spreading by runners (although it does occasionally resprout from the base as in A. arida). There are, however, a few Pilbara (and Kimberley) plants that cannot be confidently assigned to either species using on the above combinations of characters; these plants are characterized by having long (9-13 cm), flat (but very narrow) phyllodes and sometimes at least, have a wispy open growth form. Acacia arida probably also has some affinities to A. arrecta.

Rare putative hybrids between A. arida and both A. ancistrocarpa and A. stellaticeps have been reported from near Roebourne (but these have not been recollected in recent years).

Notes

Acacia arida is very attractive shrub that has considerable horticultural potential. It is currently grown as an ornamental at Karratha annex of the West Pilbara College of TAFE and in several Shire of Roebourne gardens. This species preference for rocky habitats suggests that it would be a suitable for use in waste dump revegetation programs.

As noted by Ken Walker (in Fox and Dunlop 1983) pre-treating seed with boiling water can substantially increase the rate of germination and final germination percentages to almost 90% after six days.

As reported by Juluwarlu Aboriginal Corporation (2003) Yindjibarndi peoples used A. arida in various ways. The phyllodes were burnt and the smoke used to deter insects (and to punish disrespectful children). Phyllodes were also soaked or boiled in water and the liquid used as a jami (medicine), especially for the eye irritations (Young 2007). Smoke produced by burning phyllodes was also important in purification ceremonies for young males amongst Ngarluma people.

Conservation status

Not considered rare or endangered.

Origin of name

The botanical name is derived from the Latin aridus (dry) presumably in reference to the annotation on the type specimen, collected by Allan Cunningham in September 1819 - "parched desert shores of Cambridge Gulf, NW Coast" [of Western Australia]. The common name False Melaleuca refers to superficial resemblance of this shrub to a Melaleuca plant, particularly in respect to the smoky grey-green appearance of the phyllodes and the multi-stemmed habit.

References

Fox, J.E.D. and Dunlop, J.N. (1983). Acacia species of the Hamersley Ranges, Pilbara Region of Western Australia. Mulga Research Centre. Occasional Report No. 3. pp. 94. (Western Australian Institute of Technology: Bentley.)

Juluwarlu Aboriginal Corporation (2003). Wanggalili: Yindjibarndi and Ngarluma Plants. pp. 128. (Juluwarlu Aboriginal Corporation: Roebourne, Western Australia.)

Tyler, J.P. (1998). The Dampier Salt Guide to Pilbara Plants for the Garden. pp. 148. (Dampier Salt (Operations): Karratha.)

Young, L. (2007). Lola Young: Medicine Woman and Teacher. Complied by Anna Vitenbergs. pp. 160. (Fremantle Arts Centre Press: Fremantle.)