Botanical name

Acacia orthocarpa F. Muell., J. Proc. Linn. Soc., Bot. 3: 136 (1859)

Common name

Pilbara Weeping Wattle (preferred common name), Needle-leaf Wattle and Straight-podded Wattle

Aboriginal name

Yartupu (Nyangumarta)

Description

Commonly wispy, graceful, ąumbrella-shaped, glabrous shrubs or small trees 2-6 m tall with an open crown (the ultimate branchlets sometimes ąpendulous) and single-stemmed or sparingly branched at ground level (stems often weak and bending easily in breeze), sometimes multi-stemmed ąumbrella-shaped shrubs 1-3 m tall with a bushy crown (habit not unlike that of A. arida), normally regenerating from seed following fire (bushy form rarely resprouting from base as in A. arida). Bark grey and thin, smooth except fibrous and longitudinally fissured at base of oldest stems. Branchlets slender. New shoots green, resinous, not or only slightly sticky. Phyllodes terete to sub-terete (but sometimes drying irregularly quadrangular or rarely flat), (5-) 6-15 cm long (wispy variant) or (1.5-) 2-6 (-7) cm long (bushy variant), 0.5-0.9 mm in diameter, slender, delicate and soft, shallowly to moderately curved, puncticulate with minute white dots (visible at x10 magnification), sometimes longitudinally furrowed when dry, dull green to grey-green or sub-glaucous; nerveless or very obscurely longitudinally multi-striate; apex not spiny. Gland inconspicuous, situated on upper surface of phyllode at distal end of pulvinus. Inflorescences simple; peduncles (2-) 4-10 (-18) mm long; spikes not showy, 10-30 mm long, deep yellow, densely flowered. Flowers 5-merous; calyx membranous, shallowly to deeply dissected; buds resinous. Pods terete to compressed (often sub-quadrangular or flattish prior to maturity), narrowed at both ends, 3-9 cm long (to 13 cm in Kimberley region), 3.5-5 mm wide, often erect, woody, opening elastically from the acute apex with the dehisced valves shallowly recurved, straight to very shallowly curved, resinous (but not sticky), nerveless or sparingly and obscurely longitudinally nerved (nerves obscured by resin), light brown to yellow-brown (yellow-green just prior to maturity); margins not thickened. Seeds longitudinally oblique in the pods, seated in distinct chambers separated by narrow partitions, obloid, laterally compressed, 4.5-5 mm long, often retained in pods following dehiscence, shiny, dark brown to blackish (tinged greenish just prior to maturity); pleurogram bordered by a narrow band of dull yellow tissue; funicle/aril turbinate, straight, creamy white (but tinged green-brown at attachment to seed).

Characteristic features

Wispy, graceful shrubs or small trees with open crowns (ultimate branchlets sometimes ąpendulous) and one or few main stems, sometimes multi-stemmed shrubs with a bushy crown (this habit not unlike that of A. arida), normally regenerating from seed following fire. Phyllodes mostly terete to sub-terete, slender, mostly 6-15 cm long (2-6 cm in bushy variant), delicate and soft, dull green to grey-green or sub-glaucous, sometimes lightly resinous, ąnerveless, not spiny. Spikes 10-30 mm long. Pods terete to compressed, narrowed at both ends, often erect, ąstraight, light brown to yellow-brown, resinous, nerveless or sparingly and obscurely longitudinally-nerved (nerves obscured by resin), opening elastically from apex with dehisced valves shallowly recurved. Seeds retained in pods following dehiscence, longitudinally oblique to oblique in distinct chambers separated by narrow partitions, laterally compressed; funicle/aril turbinate, straight and creamy white.

Distribution and ecology

Occurs in the Pilbara and Kimberley regions of Western Australia extending eastwards through Northern Territory to Queensland. The species has a scattered but widespread distribution in the northern and central Pilbara and is recorded from a number of the islands in the Dampier Archipelago and on the Burrup Peninsula. Grows on skeletal sand or sandy loam (pH 7.5-8.5) associated with granite or Brockman Iron Formation pediments in spinifex country or in siliceous coastal sand and granophyritic soils in the Dampier Archipelago and on the Burrup Peninsula. It is often found high in the landscape on hills and ridges, sometimes along drainage lines although in the west Pilbara it may grow on the plain immediately behind coastal dunes. It may form dense populations on the slopes, and at the base, of hills.

Flowering and fruiting period

It is likely that flowering and fruiting in this species is influenced by the timing and intensity of rainfall events. It flowers (often sporadically) between April and early November, but not all plants will be in flower during this period. Pods with mature seeds have been collected between September and October (few flowers are often present on plants during this time), but it is probable that they would be present also in November and December on some plants. Young plants can flower and fruit with one year (Tyler 1988).

Variation

As noted in the above description the Pilbara plants of A. orthocarpa are commonly tall, single-stemmed or few-stemmed and wispy with an open crown. However, it can also be multi-stemmed with a low stature and a reasonably dense, bushy crown; this growth form is not dissimilar to that of A. arida and like that species it may resprout from the base following fire. This shrubby form is found on islands of the Dampier Archipelago, the Burrup Peninsula and in a few other places around the Pilbara. Limited field observations suggest that the two forms of A. orthocarpa do not co-occur and each can form localized, discontinuous stands. The shrubby form of A. orthocarpa may superficially resemble young plants of A. exilis (see that species for discussion).

Affinities

Acacia orthocarpa is very closely related to A. arida and the relationship between the two species requires further investigation (see A. arida for discussion). The traditional (perhaps artificial) separation of these two species is maintained here, namely, phyllodes terete to sub-terete and 0.5-0.9 mm wide in A. orthocarpa, flat and normally 1-4 mm wide in A. arida. These characters enable most, but not all, Pilbara specimens to be confidently ascribed to one or other of these species: see A. arida for further discussion. Acacia orthocarpa probably also has some affinities to A. arrecta. A rare putative hybrid between A. orthocarpa and A. ancistrocarpa occurs on Woodstock station. Around Nullagine care needs to be taken not to confuse A. aphanoclada with A. orthocarpa (the typical spindly form) because both species have a similar growth form and long, slender phyllodes, and form small populations on the rocky hills in that region. These two species are not at all closely related and upon close inspection A. aphanoclada is readily recognized by its globular heads which are arranged in racemes, pruinose branchlets, flat, non-puncticulate phyllodes with a single midrib and papery pods. The bushy form of A. orthocarpa is superficially similar to A. tenuissima and although the two species can easily be confused they are not particularly closely related; see A. tenuissima for the differences.

Notes

This species has potential in cultivation, not only the common wispy form but also the low shrubby form.

The ash (junpa) from burnt phyllodes was used with tobacco to make a chewing quid by aboriginal people of the Nyangumarta language group.

Conservation status

Not considered rare or endangered.

Origin of name

The botanical name is derived from the Greek ortho (straight) and carpos (fruit) referring to the straight pod.

References

Tyler, J.P. (1998). The Dampier Salt Guide to Pilbara Plants for the Garden. pp. 148. (Dampier Salt (Operations): Karratha.)