Botanical name

Acacia steedmanii subsp. borealis Maslin, Nuytsia 18: 164, 166, fig. 7 (2008)

Common name

Northern Steedman's Wattle (preferred common name) and Roadside Wattle (in the Pilbara)

Description

Obconic shrubs or small trees 1.5-4 (-5) m tall, crown rounded and open or dense, with numerous, slender, erect, main stems arising from ground level. Bark smooth and conspicuously pruinose on young plants, with age becoming grey and fibrous towards base of stems (where it more or less forms a stocking) with the upper branches red or yellow and frequently pruinose. Branchlets glabrous, slightly to moderately pruinose. Phyllodes narrowly elliptic to oblanceolate, uniformly and gradually narrowed towards the base, (4-) 5-8 (-10) cm long, (6-) 8-15 (-20) mm wide, rarely to 28 mm wide at the Rudall River outside the Pilbara, thinly coriaceous, mostly wide-spreading, normally straight but a few shallowly recurved, glabrous, green or sub-glaucous; midrib and marginal nerves fairly prominent, lateral veins scarcely visible when fresh (except with transmitted light) but becoming evident upon drying, openly anastomosing (deflexed at their ends to intersect the marginal nerves); apices somewhat abruptly narrowed and obtuse, very rarely acute; pulvinus distinct, 4-7mm long. Glands 2 or 3 on upper margin of phyllodes, the lowermost 0-3 mm above the pulvinus and normally 1 or 2 additional, often slightly raised glands near or above the centre of the phyllode. Inflorescences racemose or sometimes paniculate (especially at ends of branchlets); racemes 10-40 mm long, the axes glabrous; peduncles 2-4 mm long, glabrous, rather stout, finely to rather coarsely longitudinally wrinkled when dry; heads showy and fragrant, globular, 5-6.5 mm in diameter, bright golden, containing 40-50 densely arranged flowers. Bracteoles light to mid-brown. Flowers 5 merous; calyx � length of corolla, very shortly dissected into inflexed, minutely ciliolate lobes; calyx tube glabrous, dark red-brown when dry; petals c. 1.6 mm long, glabrous, 1-nerved. Pods linear, gently raised over seeds and normally slightly constricted between them (occasional deep constrictions occur on some pods), 4-12 cm long, usually 5-6 mm wide (see note under Variation below), with up to 12 seeds per pod, firmly chartaceous, straight to slightly curved, glabrous, dark brown. Seeds longitudinal in the pods, obloid-ellipsoid, 4.5-6 mm long, 2.5-3.5 mm wide, slightly shiny, black; funicle thread-like, cream-coloured, completely encircling the seed in a single or double fold (expanded length 30-40 mm long); aril cream-coloured.

Characteristic features

Multi-stemmed, glabrous shrubs or small trees, the stems and branches conspicuously pruinose (at least on young plants). Phyllodes narrowly elliptic to oblanceolate, gradually narrowed towards the base, mostly 5-8 cm long and 8-15 mm wide, thin-textured, midrib and marginal nerves fairly prominent, lateral veins somewhat evident upon drying and openly anastomosing, apices normally obtuse. Glands 2 or 3 on upper margin of phyllodes. Racemes 10-40 mm long; heads showy and fragrant, 40-50-flowered; peduncles short (2-4 mm), glabrous, rather stout, finely to rather coarsely longitudinally wrinkled when dry. Pods linear, gently raised over seeds and normally slightly constricted between them, mostly 5-6 mm wide, thin-textured, dark brown. Seeds 4.5-6 mm long, completely encircled by a cream-coloured funicle 30-40 mm long (expanded length).

Distribution and ecology

Occurs in northwest and north-central Western Australia from the Pilbara region (restricted to the central Hamersley Range area) eastwards though the Little Sandy Desert to the Rudall River National Park and near the Carnarvon Range. In the Pilbara it is found in skeletal, red, stony clay-loam over massive banded ironstone. It shows its best development (sometimes forming thickets) along floodplains, drainage lines and in water gaining sites. It is common along road verges and similar water gaining sites in many of the places where it occurs.

Flowering and fruiting period

Flowering extends from late June to early September with the main flush in July and August. Pods with mature seeds have been collected in mid-October and it would be expected that mature fruits would be found on the plants until November.

Variation

Specimens of this subspecies from the Rudall River were described as A. steedmanii in Maslin (1981) and noted as a variant under A. validinervia in Maslin (2001).

Taxonomy

Acacia steedmanii comprises two subspecies but only subsp. borealis occurs in the Pilbara; the typical subspecies (subsp. steedmanii) occurs in southwest Western Australian.

Affinities

Subspecies borealis is related to A. validinervia which occurs well to the east of the Pilbara (in the ranges of central Australia) and which is distinguished by its thicker, generally wider phyllodes (mostly 15-40 mm) with a more pronounced reticulum, heads with more numerous flowers (50-80) and darker-coloured bracteoles, usually wider pods (7-8 mm), and wider seeds (3-5 mm) which are encircled by a generally shorter funicle (6-30 mm long, expanded length). Subspecies borealis is also related to an undescribed entity from the Little Sandy Desert that was called A. aff. validinervia in Maslin (1981) and noted as a variant under A. validinervia in Maslin (2001). This entity is currently ascribed the phrase name Acacia sp. Lake Disappointment (S. van Leeuwen 2865) and is characterized by its long phyllodes, large flower heads and spindly, whipstick growth form.

Notes

Subspecies borealis can resprout from a basal woody rootstock following fire. It probably has a relatively fast growth rate and regenerates from seed.

This is an attractive species with its white stems (at least when plants are young) and showy, fragrant, golden heads, and as such would be well-suited for amenity plantings and in home gardens in arid areas.

Conservation status

Not considered rare or endangered.

Origin of name

The species is named for Harry Steedman (Hall 1978). The subspecies name is derived from the Latin borealis (northern) and alludes to the distribution of this taxon relative to the typical subspecies.

References

Hall, N. (1978). Botanists of the Eucalypts. pp. 180. (CSIRO: Melbourne.)

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