Botanical name

Acacia subtiliformis Maslin, Nuytsia 18: 173, fig. 9 (2008)

Common name

Witarra

Description

Erect, spindly (wispy), single-stemmed shrubs 2.5-3.5 m tall, crown openly branched, stems about 1.5 cm diameter at breast height, the upper branches scarred where phyllodes have fallen. Branchlets terete, glabrous, light brown or red-brown but soon aging grey, finely ribbed (rib most evident immediately below insertion of phyllodes), lenticellate (lenticels scattered, circular, dull yellow), resinous (but not sticky). Stipules persistent, narrowly triangular, 0.7-1.5 mm long, 0.2-0.3 mm wide at base, erect and appressed to branchlets, often enveloped and obscured by resin, dark brown. Phyllodes asymmetrically oblong to oblong-elliptic, lower margin ąstraight, upper margin often shallowly convex, very small, 2-3.5 (-5) mm long (include mucro), 1.2-1.8 mm wide, l:w = 1-2.3, sub-crowded, slightly thickened, wide-spreading, straight, glabrous , green; midrib not prominent, slightly raised (when dry) and extending from pulvinus to mucro, often with a less pronounced nerve parallel to midrib on its upper side (rarely also on lower side); apices rounded and abruptly terminated in a short (0.3-0.5 mm long) but distinct, excentric, slender, rigid, subulate, pungent, straight or shallowly recurved, brown point; pulvinus very indistinct (rudimentary or seemingly absent), to 0.2 mm long, yellowish. Gland not prominent, situated on upper margin of phyllode 1-1.5 mm above the base (near to, or below, centre of phyllode). Inflorescences simple, 1 per axil; peduncles glabrous, 6-8 mm long, dark red (at least when fresh), base ebracteate; heads globular, c. 5 mm in diameter (when dry), densely 40-50 flowered, golden. Bracteoles sub-peltate, 1-1.2 mm long, the claws linear, hairy on margins and adaxially, the laminae sub-circular, inflexed at right angles to claw, slightly thickened and fimbriolate (hairs straight and silvery white). Flowers 5-merous; sepals 2/3-ž length of petals, free to base, shape, size and indumentum same as bracteoles; petals 1.5 mm long, glabrous, free, very obscurely nerved; ovary glabrous. Pods narrowly oblong, flattened but rounded over seeds, ąstraight-edged or slightly constricted between seeds, 20-40 mm long, 3-4 mm wide, firmly chartaceous, straight, glabrous, slightly shiny, brown to greyish brown, sometimes slightly pruinose, the marginal nerve narrow, with a distinct basal stalk about 3 mm long, apex acute. Seeds longitudinal in pods, ellipsoid, small (about 3 mm long and 2 mm wide), turgid (2 mm thick), slightly shiny, black but mottled dull pale yellow, with a narrow band of dull pale yellow tissue bordering the pleurogram and around the periphery of the seed; areole very small (c. 0.3 mm long and the same across), located near centre of seed, dull yellow; pleurogram very obscure, 'u'-shaped; funicle thread-like, not expanded into an aril, c. 2-2.5 mm long, reddish brown.

Characteristic features

Erect, spindly, wispy, single-stemmed, glabrous shrubs, the upper branches scarred where phyllodes have fallen. Branchlets lenticellate, resinous (but not sticky). Stipules persistent, very small (0.7-1.5 mm long), erect, appressed to branchlet and often obscured by resin. Phyllodes asymmetrically oblong to oblong-elliptic, lower margin ąstraight, upper margin often shallowly convex, very small (mostly 2-3.5 x 1.2-1.8), sub-crowded, green; midrib not prominent; apices terminated in a short but distinct, excentric, slender pungent point; pulvinus very indistinct. Inflorescences simple, 1 per axil; peduncles dark red (when fresh), base ebracteate; heads globular, 40-50 flowered. Sepals free. Pods narrowly oblong, flattened but rounded over seeds and ąstraight-edged or slightly constricted between seed, rather small (20-40 x 3-4 mm), firmly chartaceous, with a distinct basal stalk about 3 mm long. Seeds black but mottled pale yellow, funicle not expanded into an aril.

Distribution and ecology

Confined to the Pilbara region in northwest Western Australia where it is known from only six populations within the Hamersley Ranges, namely, four from the Hancock Range (where plant numbers exceed 10,000 in one of the populations) and two in the Ophthalmia Range (where plant numbers exceed 20,000 in the two populations). Acacia subtiliformis occurs in low undulating country on calcareous rises adjacent to drainage lines. Soil analysed from one of the Ophthalmia Range populations was shown to be very low in total phosphorus, relatively high in exchangeable calcium and measured pH 8.9 (S. van Leeuwen, pers. comm.). The dominant vegetation at most sites is characterised by Eucalyptus mallees over scattered Melaleuca and Petalostylis shrubs over scattered hard spinifex.

Flowering and fruiting period

Flowers in July and August. Pods with mature seeds have been collected in mid October.

Taxonomy

This species was discovered very recently, in 1995 and until recently was known under its phrase name, Acacia sp. Calcrete Hamersley Range (B.R. Maslin 8564).

Affinities

Closely related to A. minutissima (see that species for distinguishing features). These two species are related to A. maitlandii which is readily recognized by its distinguished from both by its much longer phyllodes (i.e. 7-25 mm). Acacia subtiliformis has a very distinctive wispy growth form and there are a number of taxa in the Pilbara which are similar in this regard. Some, like A. aphanoclada, A. orthocarpa and A. subtiliformis, warrant recognition as distinct species because they possess characters other than growth form which set them apart from their closest relatives. However, wispy variants are also known to occur in A. bivenosa and A. hamersleyensis but based on current knowledge these entities appear not to warrant recognition as separate taxa.

Conservation status

Acacia subtiliformis is listed as a Priority 3 taxon on the Department of Environment and Conservation's Declared Rare and Priority Flora List.

Origin of name

The species name is derived from the Latin subtilis (thin, slender, fine) and forma (shape, figure) in reference to the characteristic wispy, spindly growth form of the species. The common name is derived from the Nyiyaparli Native Title Claimant Group whose land on which all populations of Acacia subtiliformis occur. In Nyiyaparli Witarra translates to skinny which is an apt description for the stems of this species.