Botanical name

Acacia sp. Mulga short phyllodes (B.R. Maslin et al. BRM 9276)

Common name

Hilltop Mulga

Description

Shrubs or small obconic trees 3-5 m tall, with 1-3 main stems which divide into several ascending stems at or just above ground level, stems to 10 cm diameter at breast height, crowns somewhat rounded and sub-dense. Bark grey, thin, longitudinally fissured on main stems of mature plants otherwise smooth. Branchlets sericeous between ribs which are coated with a thick, crenulated layer of opaque resin (which is milky blue-grey or sometimes yellowish in colour) that often obscures the indumentum which is lost with age. New shoots enveloped by resin when first initiated, as shoot expands they are light khaki brown and shiny and the resin is most pronounced on phyllode margins and on the nerves of the lamina. Phyllodes narrowly elliptic to obovate or oblanceolate, 20-40 (-45) mm long, (6-) 7-12 mm wide, ascending to erect, straight, light grey tinged blue-green, sometimes glaucous, minutely sericeous between nerves, parallel longitudinal nerves numerous, the nerves ąwidely spaced, resinous and of uniform prominence, anastomoses absent or few, surface sometimes completely covered by opaque resin which obscures the nerves and indumentum; marginal nerve not prominent but discrete, narrow, resinous, yellowish (best seen by observing phyllodes edge-on); apex obtuse (sometimes sub-acute when phyllodes narrow). Gland not conspicuous, situated on upper margin of phyllode at distal end of pulvinus. Inflorescences (few seen) simple; peduncles 6-10 mm long, sparsely appressed white-hairy intermixed with red-brown resin microscopic hairlets; spikes 10-20 mm long. Flowers 5-merous; calyx very reduced (about 1/6 length of corolla), sepals free, oblong and about 0.3 mm long, buds resinous. Pods 1.5-3.5 cm long, 7-9 (-10) mm wide, flat, glabrous, obscurely longitudinally reticulate, with a narrow but distinct marginal wing 1-1.5 mm wide. Seeds oblique in pods.

Characteristic features

Shrubs or small trees which occur high in the landscape. Branchlets with a thick, crenulated layer of resin over the ribs. Phyllodes relatively short and broad (mostly 20-40 x 7-12 mm), obtuse or almost so, light grey tinged blue-green, sometimes glaucous, sometimes completely covered by opaque resin, multistriate with longitudinal nerves of uniform prominence; marginal nerve not prominent but discrete, resinous, yellowish. Spikes 10-20 mm long. Calyx very reduced (about 1/6 length of corolla), sepals free, oblong and about 0.3 mm long. Pods (immature) narrowly winged.

Distribution and ecology

Widespread in Western Australia where it extends from the Pilbara region south through the Ashburton district to the goldfields; it also extends eastwards to the Little Sandy Desert and far western Northern Territory. It is not overly common in the Pilbara where it has a scattered distribution that extends from the central Hamersley range around West Angelas to east of Newman. In the Pilbara this species is known only from the upper slopes and tops of rocky hills such as Mt Windell, Mount Robinson and Shovelanna Hill, where it grows in skeletal soil. It forms localized populations and is not uncommon in the places where it grows.

Flowering and fruiting period

The paucity of gathering makes it difficult to determine the phenology of this entity. As with other taxa of Mulga this one presumably flowers and fruits in response to the timing and intensity of rainfall. One plant with flowers at anthesis has been collected in early July, at which time some very immature pods were also present.

Taxonomy

Hilltop Mulga is a member of the taxonomically complex Mulga (A. aneura) group of species. The taxonomic status of this entity is currently being assessed but indications are that it most probably represents a new species. The present description of this widespread species is based on just the Pilbara occurrences. Previously this species was known as the Hilltop variant or A. minyura and was given the formal phrase name 'Acacia sp. Hamersley Range hilltops (S. van Leeuwen 3552)' at the Western Australian Herbarium.

Affinities

Acacia sp. Mulga short phyllodes (B.R. Maslin et al. BRM 9276) is related to A. minyura (which also occurs in the Pilbara) which is most readily distinguished by its smaller phyllodes. It may superficially resemble short phyllode forms of A. ayersiana which are distinguished by having phyllodes that tend to be more acute and with nerves of ąunequal prominence (phyllodes generally obtuse and nerves of uniform prominence in Hilltop Mulga). Although the branchlet ribs and phyllode nerves of A. ayersiana are resinous, the resin is not as thick or crenulated as in Hilltop Mulga.

Conservation status

Not rare or endangered.

Origin of name

The phrase name Acacia sp. Mulga short phyllodes (B.R. Maslin et al. BRM 9276) identifies this taxon at the W.A. Herbarium.