Botanical name

Acacia aneura var. pilbarana Pedley, Fl. Australia 11B: 489 (2001)

Common name

Mulga

Note

Acacia aneura and related species form a very complex and polymorphic assemblage of taxa with A. aneura itself the most variable (see Miller et al. 2002 for discussion). These species are currently under review therefore the classification here of Pilbara entities referable to A. aneura is provisional. Currently 10 varieties are recognized within A. aneura (Pedley 2001) of which six appear to occur in the Pilbara. These varieties encompass most of the variation in Mulga in the Pilbara and are the entities upon which this Wattles of the Pilbara treatment of Mulga is based. It is often difficult to distinguish between the varieties, and between A. aneura and related species (especially A. ayersiana and A. paraneura) on account of seemingly intermediate forms. In past literature it is often not possible to know to which of the varieties the information applies. Therefore, we have assembled these data under the heading of A. aneura. The data presented below for A. aneura var. pilbarana is derived from our field and herbarium studies of only the Pilbara plants of this entity.

Description

Rounded shrubs maturing to obconic trees 4-10 m tall, often single-stemmed and normally dividing at 0.3-3 m or more above ground level into 2 or few main stems, the bole straight or almost so, the main stems spreading-erect and dividing into numerous ąstraight branches; young plants often with a pseudo-conifer growth form. Branchlets ribless or very obscurely ribbed at extremities (ribs occasionally slightly resinous with translucent resin). New shoots light brown to dark red-brown at initiation. Phyllodes terete to compressed or slightly flattened (but thicker than those of var. tenuis), 40-100 mm long, 1-1.2 mm wide, dull green but aging grey-green to pale sub-glaucous or sometimes ąglaucous, sub-straight to incurved, sometimes shallowly sinuous or sigmoid; parallel longitudinal nerves numerous, fine, of uniform prominence, close together, resinous (resin evident on young phyllodes but absent from mature phyllodes). Gland 0-3 mm above the pulvinus. Inflorescences simple; peduncles (2-) 3-13 mm long; spikes 10-30 mm long. Sepals free, linear-spathulate. Pods (15-) 20-60 mm long, 8-12 mm wide, nerveless to very obscurely nerved, sub-glabrous with minute appressed hairs, light brown; marginal wing absent.

Distribution and ecology

Variety pilbarana is most common in the Hamersley Range (especially on the Coondewanna Flats near West Angelas) and the valley of the Fortescue River. Grows in red-brown clay-loam or sandy loam, mainly on alluvial flats, creeks and other water-gaining sites. Sometimes extending from the flats to adjacent gentle slopes or low rises. The overall range of var. pilbarana is given by Pedley (2001) as occurring in the Pilbara and Ashburton regions of Western Australia.

Flowering and fruiting period

Flowers mainly from March to May, following summer rains, but also in June and July (winter rain). Pods with mature seeds have been collected in late September and October.

Taxonomy

The relationship between A. aneura var. pilbarana and its very close relative, A. aneura var. tenuis, is under review in connection with the Understanding Mulga project. The wingless pods and the translucent resin on the branchlets are important characters uniting the two entities. For practical reasons in Wattles of the Pilbara we are following Pedley (2001) in regarding the Pilbara plants of var. pilbarana as having ąterete phyllodes and those of var. tenuis as having flat phyllodes. It is noted, however, that outside the Pilbara var. tenuis has both flat and terete phyllodes. Although our on-going studies do suggests that two different taxa exist within the Pilbara we are unsure of their rank, the differences between them are subtle, and phyllode transverse sectional may ultimately prove not a reliable character for distinguishing them. There are suggestions that plants of var. pilbarana will tolerate water-logging and resprout from the base following fire whereas var. tenuis does not tolerate water-logging and is killed by fire. However, until the taxonomy is resolved it is not possible to say with whether or not these biological and ecological differences are real.

Conservation status

Not considered rare or endangered.

Origin of name

The varietal name is taken from the area where this variety predominates.

References

Miller, J.T., Andrew, R.A. and Maslin, B.R. (2002). Towards an understanding of variation in the Mulga complex (Acacia aneura and relatives). Conservation Science Western Australia 4(3): 19-35.

Pedley, L. (2001). Acacia aneura. pp. 314-322. In: A.E. Orchard and A.J.G. Wilson (eds) Flora of Australia. Volume 11B. pp. 536. (ABRS/CSIRO Publishing: Australia.)