Botanical name

Acacia robeorum Maslin, Nuytsia 18: 161, fig. 6 (2008)

Common name

Robe's Wattle

Description

Diffuse, spreading, openly branched, multi-stemmed, glabrous shrubs 2-3 m tall, main stems slightly crooked, crowns not dense. Bark grey on main stems, light bronze on upper branches, smooth. Branchlets terete, very obscurely ribbed, yellowish to pale orange-brown or bronze, not pruinose. Stipules spiny, present on young plants but normally absent or very few on mature plants, 1.5-3 (-4) mm long. Phyllodes linear to narrowly oblong or linear-oblanceolate, 15-25 (-35) mm long, (1-) 2-3 (-4) mm wide, l:w = (5-) 6-10 (-14), rather thick, fleshy and breaking with a clean snap upon bending (when fresh), longitudinally wrinkled when dry, straight to shallowly incurved , characteristically bright green; midrib obscure or more commonly not evident, lateral veins not evident; apices rounded and excentrically mucronulate. Gland not overly prominent, situated on upper margin of phyllode 0-0.5 mm above the pulvinus, circular to elliptic, 0.4-0.5 mm long, 0.4-0.5 mm wide. Inflorescences simple, initiated on new shoots with a single peduncle within axil of developing phyllodes, subtending phyllodes fully developed before heads reach anthesis; peduncles (8-) 10-17 mm long; heads light golden, 40-60-flowered. Bracteoles similar to sepals. Flowers 5-merous; sepals free, 2/3 length of petals, linear-spathulate; petals nerveless. Pods oblong to narrowly oblong, flat but rounded over the seeds along the midline, mostly straight-edged, (1.5-) 2.5-4 (-6) cm long, 6-8 (-9) mm wide, papery, straight or slightly curved. Seeds transverse in the pods, obloid to ellipsoid, ovoid or globose, 3-4 mm long, 2.2-3.2 mm wide, somewhat compressed, dull or slightly shiny, uniformly black or sometimes very obscurely mottled dark brown; areole 'u'-shaped and very small at centre of seed; funicle short, thick, cream coloured, very slightly expanded into a small terminal aril.

Characteristic features

Openly branched, glabrous shrubs 2-3 m tall, upper branches light bronze coloured. Branchlets yellowish to pale orange-brown or bronze, not pruinose. Stipules spiny, present on young plants but usually absent from mature plants. Phyllodes short and narrow (mostly 15-25 x 2-3 mm with l:w = 6-10), linear to linear-oblanceolate or narrowly oblong, bright green, not pruinose, rather fleshy (fresh phyllodes breaking with a clean snap when bent), longitudinally wrinkled and midrib normally not evident when dry. Gland not overly prominent, 0.4-0.5 mm long. Inflorescences simple, initiated on new shoots with a single peduncle within axil of developing phyllodes, subtending phyllodes fully developed before heads reach anthesis; heads golden, 40-60 flowered. Pods papery, oblong to narrowly oblong, flat but rounded over seeds along midline, not or scarcely constricted between seeds. Seeds transverse in the pods, black, sometimes very obscurely mottled dark brown; aril small.

Distribution and ecology

Occurs in northwest Western Australia where it ranges from the Pilbara eastwards to the Rudall River National Park in the Little Sandy Desert. In the Pilbara it extends from the vicinity of Marble Bar and Pardoo Station (at south western extremity of the Great Sandy Desert), south to Ethel Creek Station (northeast of Newman). A specimen in inflorescence bud collected on Barrow Island from a localized population of about 10 plants appears to be referable to A. robeorum; however, future research is required to determine the provenance of these Barrow Island plants thereby confirming if they are recent naturalized introductions or indigenous to the island. Grows on skeletal sand or sandy loam (pH 6.5 - 7) over granite, laterite or quartz, in spinifex hummock grasslands, sometimes along drainage lines.

Flowering and fruiting period

It is possible that flower initiation is dependant upon the timing and/or intensity of rainfall. Extent collections show plants in flower from August to September. Pods with mature seeds have been collected from late October to late November.

Taxonomy

The species was treated as A. aff. victoriae by Maslin (1981) and as a narrow phyllode variant of A. synchronicia by Maslin (1992 and 2001); until recently it was known under its phrase name Acacia sp. Rudall River (B.R. Maslin 2046A).

Affinities

Acacia robeorum is a member of the 'A. victoriae group' of species (see Maslin 1992 for discussion); other members this group that occur in the Pilbara include A. aphanoclada , A. cuspidifolia, A. glaucocaesia, A. synchronicia and A. victoriae . Acacia robeorum is most closely related to the more widespread A. synchronicia with which it sometimes grows. In the field A. robeorum is easily distinguished from A. synchronicia by its bright green, non-pruinose phyllodes which are thicker and more fleshy (they break with a clean snap when bent), and its yellowish to pale orange-brown or bronze, non-pruinose branchlets; in A. synchronicia the phyllodes are grey-green to glaucous and sometimes pruinose, more thinly textured (they do not break with a clean snap when bent) and the branchlets are darker coloured and commonly lightly pruinose at their extremities. Furthermore, A. robeorum commences flowering a month or more before A. synchronicia . The phyllodes of A. robeorum are normally narrower than those of A. synchronicia , however, there is overlap for this character and plants with phyllodes 3-4 mm wide need to be carefully examined in order to apply the names accurately. In A. robeorum the phyllodes are always clearly wrinkled when dry, their midrib and lateral nerves are either very obscure or superficially absent and the basal gland is not overly prominent. In narrow phyllode forms of A. synchronicia the dry phyllodes may also sometimes be wrinkled (but finely so) with the nerves obscure or superficially absent, however, the basal gland is larger and more prominent. Furthermore, A. synchronicia often has some inflorescences which are paired within the phyllode axils (always single in A. robeorum), the seeds are mottled (not often mottled in A. robeorum), the pods are often slightly broader, the spiny stipules are generally longer (although on mature plants of both species it is not uncommon to find specimens without any stipules at all) and the phyllodes of the two species are differently shaped.

Notes

Acacia robeorum, unlike A. synchronicia , is not a troublesome (increaser) plant for pastoralists in the Pilbara.

Conservation status

Not considered rare or endangered.

Origin of name

The species name acknowledges Robe River Iron Associates who provided generous financial support to the study of Pilbara Wattles through the West Angelas Coondewanna West Environmental Offsets Agreement. This support enabled the authors to undertake a five year study of the Pilbara Acacia flora which has resulted a number of publications (including the present one) to be produced. More recently Robe River Iron Associates through its assets manager Pilbara Iron, in collaboration with two other mining companies, has provided considerable financial support to a three-year study of the complex Acacia aneura (Mulga) group.

References

Maslin, B.R. (1981). Acacia. pp. 115-142. In: J. Jessop (ed.) Flora of Central Australia. pp. 537. (A.H. and A.W. Reed: Sydney.)

Maslin, B.R. (1992). Acacia Miscellany 6. A review of Acacia victoriae and related species (Leguminosae: Mimosoideae: Section Phyllodineae). Nuytsia 8: 285-309.

Maslin, B.R. (2001). Acacia. In: A.E. Orchard and A.J.G. Wilson (eds) Flora of Australia. Volume 11A. pp. 536. (ABRS/CSIRO Publishing: Australia.)