Botanical name

Acacia sibirica S. Moore, J. Linn. Soc., Bot. 34: 189 (1899)

Common name

False Witchetty Bush (preferred common name) and Bastard Mulga

Description

Obconic or rounded shrubs or small trees 1.5-5 (-6), with few to many ąstraight or slightly crooked, spreading-erect main stems from the base, crown dense and spreading. Bark grey or sometimes blackish, longitudinally fissured and fibrous at base of oldest stems otherwise smooth (fissured bark rarely extending to upper branches). Branchlets glabrous. New shoots light green, resinous but not sticky. Phyllodes variable in shape and size, linear to narrowly oblong or narrowly elliptic, (2-) 3.5-10 (-13) cm long, usually 1-5 mm wide, rarely to 7 mm (see discussion below), l:w = 8-60 (-100), ąrigid, somewhat coriaceous, erect or spreading, straight to shallowly or moderately incurved, very slightly shiny, glabrous, green to grey-green or sub-glaucous ; parallel longitudinal nerves numerous, fine, close together and of uniform prominence; apex mostly acute to sub-acute with a hard, brown, non-spiny mucro. Inflorescences simple or rudimentary racemes (with axes to about 1 mm long), scattered over the plants and not especially showy; peduncles (3-) 5-12 (-18) mm long, glabrous; spikes (8-) 10-25 (-30) mm long, longer than the peduncles, bright golden, the flowers small and not especially densely arranged. Flowers 5-merous; calyx shortly dissected, ź-1/3 length of corolla. Pods linear to narrowly oblong, (3-) 4-9 (-10) cm long, 3-5 (-7) mm wide, flat but slightly raised over seeds, straight-edged to shallowly constricted between seeds (occasional deep constrictions sometimes occur), straight to shallowly curved, papery to thinly coriaceous, glabrous, brown, very obscurely nerved, marginal nerve not thickened. Seeds longitudinal to longitudinally oblique in the pods, ellipsoid to obloid-ellipsoid, about 4-5 mm long, 2.5-3 mm wide, dark brown to black; aril folded and creamy white.

Characteristic features

Multi-stemmed, glabrous shrubs or small trees. Bark grey or sometimes blackish, longitudinally fissured and fibrous at base of oldest stems otherwise smooth. Phyllodes variable in shape and size, mostly 3.5-10 cm long and 1-5 mm wide with l:w = 8-60, ąrigid, parallel longitudinal nerves numerous, fine, close together and of uniform prominence, apex acute to sub-acute and non-spiny. Spikes not especially showy, mostly 10-25 mm long (longer than the peduncles), the flowers small and not especially densely arranged in spikes. Calyx less than ˝ length of petals. Pods linear to narrowly oblong, not exceeding 7 mm wide, papery to thinly coriaceous. Seeds longitudinal to longitudinally oblique.

Distribution and ecology

Widespread in arid Australia where it occurs in all mainland States except Victoria. In the Pilbara A. sibirica is common in the Hamersley Range between Newman and Quarry Hill (about 100 km west of Tom Price), but there are also scattered occurrences in the east of the region and north to near Munjina, Nullagine and east of Marble Bar. Commonly grows in skeletal soils of stony or rocky ridges and breakaways.

Flowering and fruiting period

Seemingly flowers and fruits in response to the timing and intensity of rainfall. Flowers mostly from May to June or sometimes July, although plants with few flowers have also been collected in August and September. Pods with mature seeds have been collected between late September and mid-October.

Variation

The Pilbara plants of this widespread species are extremely variable, especially with respect to phyllode dimensions which are often longer and/or broader than on plants elsewhere. In some Pilbara populations the phyllodes are of a constant size but in others considerable variation occurs. The patterns of variation are both extreme and confusing and further study is needed to determine if formal taxa can be recognized. The phyllodes vary from long, narrow and linear (generally 5-10 cm x 1-2 mm with l:w = 25-60) to short, broad (generally 3.5-7 cm x 3-5 mm with l: w =8-15) and narrowly oblong to narrowly elliptic . There are a few specimens (mostly from the central Hamersley Range area in the vicinity of West Angelas) where the phyllodes are 7 mm wide. In some cases these broader than normal phyllodes are likely to represent juvenile foliage but this probably does not explain all of them and it is not known if these represent part of the normal range of variation within A. sibirica or whether they are some sort of intergrade with A. kempeana. Similar broad phyllodes occur also on a few plants outside the Pilbara. Plants with long, linear phyllodes were referred to A. stowardii variant 1 by Maslin (1982) and predominate in the area from Tom Price and Paraburdoo east to Rhodes Ridge. However, the more widespread short, broad phyllode forms (referred to by Maslin 1982 as A. stowardii variant 2) are also found in this same region. Because there is no apparent sharp distinction between these two phyllode forms (i.e. there appears be continuous variation from one form to the other, and neither form appears to be confined to particular habitats) formal taxa have not been described to accommodate the observed variation. This is regrettable because plants at each end of the variational range look very different on account of differences in phyllode length and width.

Taxonomy

Acacia sibirica belongs to a taxonomically complex small group of species of which five are found in the Pilbara region, namely, A. adsurgens, A. atkinsiana, A. kempeana, A. rhodophloia and A. sibirica. In the Pilbara the situation is further complicated because several of these species seemingly hybridize. These species are characterized by their flat, finely multi-striate phyllodes with the nerves very close together, flowers arranged in obloid or cylindrical spikes, calyx shortly dissected and pods flat and thin-textured.

The above description is based entirely on Pilbara occurrences of this widespread species.

It is unfortunate that the well-known name A. stowardii must now be replaced by an earlier one, A. sibirica.

Affinities

Acacia sibirica is most closely related to A. kempeana and these two very variable species are perhaps only arbitrarily distinguished, based on phyllode and pod width, A. kempeana having generally wider pods and wider phyllodes (see A. kempeana for discussion). Plants of A. sibirica with long, narrow phyllodes superficially resemble A. adsurgens which is distinguished by having generally longer phyllodes with a slightly pronounced central nerve (nerves of uniform prominence in A. sibirica), generally paler yellow spikes with flowers more densely arranged, a longer calyx relative to the corolla and peduncles which are about as long as the spikes (peduncles shorter than spikes in A. sibirica). The broader phyllode forms of A. sibirica are often very difficult to distinguish from A. rhodophloia in the absence of knowing bark characters (bark red Minni Ritchi in A. rhodophloia). A possible hybrid between A. rhodophloia and A. sibirica is described separately as A.? rhodophloia x sibirica; this entity grows with A. rhodophloia and only rarely with A. sibirica (see A.? rhodophloia x sibirica for discussion).

Sterile specimens of A. sibirica which have very narrow phyllodes may possibly be confused with Mulga (A. aneura var. tenuis) but are distinguished most easily by their glabrous branchlets (branchlet extremities minutely appressed -hairy in var. tenuis, carefully observe at x10 magnification or higher), free sepals and generally narrower pods. Acacia sibirica sometimes co-occurs with A. thoma and care needs to be taken not to confuse the two species (see A. thoma for discussion).

Notes

Acacia sibirica resprouts from the base following fire or mechanical damage cause by clearing. Plants from Balfour Downs Station have also been observed to regenerate from seed following fire.

Although the phyllodes may contain very low levels of cyanogenic glycoside the species is unlikely to be dangerous to stock (Maslin et al. 1987).

Conservation status

Not considered rare or endangered.

Origin of name

The botanical name is from the Latin sibirica (of Siberia) referring to the type locality, Siberia Soak, an abandoned Goldfields townsite north of Kalgoorlie in Western Australia.

References

Maslin, B.R. (1982). Studies in the genus Acacia (Leguminosae: Mimosoideae) - 11. Acacia species of the Hamersley Range area, Western Australia. Nuytsia 4(1): 61-103.

Maslin, B.R., Conn, E.E. and Dunn, J.E. (1987). Cyanogenic Australian species of Acacia: a preliminary account of their toxicity potential. pp. 107-111. In: J.W. Turnbull (ed.) Australian Acacias in developing countries. Proceedings of an international workshop held at the Forestry Training Centre, Gympie, Australia, 4-7 August 1986. ACIAR Proceedings No. 16. pp. 196. (Australian Centre for International Agricultural Research: Canberra.)