Botanical name

Acacia bromilowiana Maslin, Nuytsia 18: 141, fig. 1 (2008)

Common name

Bromilow's Wattle

Description

Gnarled and normally erect shrubs or trees commonly 2-5 (-6.5) m tall with diameter at breast height to about 12 cm, occasionally (in very favourable sites) 10-12 m tall with diameter at breast height to about 30 cm, with 1 or 2 sub-straight to somewhat crooked main stems that branch at c. 1 m or more above the ground, the crown dense, rarely (on Balfour Downs Station) domed sprawling shrubs 1-2 x 3-4 m with crown extending to the ground, sometimes clonal (see Variation below). Bark grey to black, longitudinally fissured and fibrous (peels off in long strips) on main stems and upper branches. Branchlets terete, very obscurely ribbed, glabrous, reddish, sometimes pruinose. New shoots invested with a dense layer of ferruginous microscopic hairs when first initiated but soon becoming glabrous and green or lightly pruinose. Phyllodes asymmetrically lanceolate to narrowly elliptic, broadest near or below the middle, lower margin ąstraight to shallowly concave, upper margin convex, (8-) 10-14 (-18) cm long, (13-) 20-40 (-45) mm wide, coriaceous, straight to shallowly falcately recurved, slightly undulate when very broad, glabrous, grey-green to sub-glaucous or glaucous (except young shoots); parallel longitudinal nerves numerous, very fine, close together, 3-5 slightly more pronounced than the rest and none confluent with the margin at the base, a few anastomosing (observe at x10 magnification or higher); apices sub-acute to obtuse, not pungent; pulvinus (2-) 3-6 (-8) mm long, smooth or shallowly transversely to longitudinally wrinkled, normally with a few low-profile longitudinal ridges when dry, dull reddish or reddish brown (? sometimes yellow). Gland inconspicuous, situated on upper margin of phyllode 0-2 mm above pulvinus. Inflorescence normally a short raceme 5-15 mm long bearing two spikes, sometimes growing out at apex with subsequent inflorescences single and simple (i.e. not racemose) within axil of phyllodes, raceme axes compressed and glabrous; peduncles 5-15 (-20) mm long, glabrous; receptacles glabrous, sometimes longitudinally wrinkled when dry, invested with short emergent processes near flower scars when in fruit (but only one fruiting sample seen); spikes 25-45 mm long and 9-10 mm wide, bright light golden, flowers close together to form a dense spike. Bracteoles 1-1.5 mm long, claws linear, ciliolate and pale coloured, laminae ovate, inflexed, small (c. 1 mm long), light brown and acute to apiculate. Flowers 5-merous, 2 mm long; calyx 2/5-ž length of corolla, shortly dissected into oblong, inflexed lobes which are thickened abaxially, the tube sparsely to moderately hairy, truncate at base; petals glabrous, nerveless. Pods (few seen) narrowly oblong, flat, scarcely raised over seeds, straight-edged or very slightly constricted between seeds, 2-7 cm long, 7-10 mm wide, papery, glabrous, yellowish, nerveless or very obscurely nerved. Seeds (few seen) transverse or longitudinal in the pods, ovoid, 3.5-4 (-4.2) mm long, 2.5-3 mm wide, slightly shiny, brown; pleurogram very indistinct, widely "u"-shaped; funicle flattened, creamy white, expanded into a small sub- terminal aril.

Characteristic features

Gnarled, normally erect shrubs or trees with 1 or 2 sub-straight to somewhat crooked main stems, rarely (on Balfour Downs Station) domed and sprawling shrubs. Bark longitudinally fissured and fibrous (peels off in long strips). Branchlets glabrous, reddish, sometimes pruinose. New shoots with a dense layer of ferruginous microscopic hairs when first initiated. Phyllodes asymmetrically lanceolate to narrowly elliptic, broad (normally 2 cm or more wide), coriaceous, grey-green to glaucous (green on new shoots), parallel longitudinal nerves numerous, very fine and close together, a few anastomosing; pulvinus mostly 3-6 mm long, often dull reddish or reddish brown, smooth or finely wrinkled. Inflorescence normally a short raceme (5-15 mm) bearing two spikes; peduncles 5-15 (-20) mm long glabrous; spikes long (25-45 mm) and densely flowered. Pods (few seen) narrowly oblong, flat, broad (7-10 mm), papery, glabrous, yellowish. Seeds (few seen) transverse or longitudinal in the pods, brown; funicle flattened, creamy white, expanded into a small sub-terminal aril.

Distribution and ecology

Restricted to the Pilbara region of northwest Western Australia where it is known only from a few disjunct hilltop populations in the Hamersley Range (from Tom Price through Ophthalmia and Hancock Ranges to Newman) and also from Balfour Downs Station, about 150 km northeast of Newman. A similar disjunction between the Hamersley - Ophthalmia Ranges and the Balfour Downs area occurs also in A. subcontorta and A. catenulata subsp. occidentalis. In the Hamersley Range A. bromilowiana typically grows in skeletal stony soils over massive banded ironstone pavements high in the landscape, on Balfour Downs Station it also grows high in the landscape but usually below the ironstone rim of breakaways in skeletal pallid zone (highly weather) soils and gravels. The Hamersley Range populations occur on steep slopes, ridge tops and breakaways (often in gullies and sheltered places) that comprise a substrate of banded ironstone (Hamersley Group - Brockman Iron Ore Formation) or massive basalts (Jerrinah Formation - Fortescue Group). These localities are dominated by very open low eucalypt woodlands (Eucalyptus leucophloia, Corymbia hamersleyana) over spinifex (e.g. Triodia pungens, T. wiseana). On Balfour Downs Station the species occurs as a series of disjunct populations over a ten kilometres range where it forms pure stands in skeletal loamy soil, on the dissected slopes of low ferruginous breakaways of the Billygoat Land System (van Vreeswyk et al. 2004). In these Balfour Downs populations there is limited litter or spinifex between plants and thus they are afforded protection from fire which probably accounts for their presumed old age.

Flowering and fruiting period

Flowering commences in early July and continues until about mid-August. A number of collectors have commented on the large numbers of inflorescence spikes found on the ground under the plants during the flowering period (the peduncles having abscised from the racemes soon after the flowers have opened) in contrast to the negligible number of flowering inflorescence spikes actually on the plants. Mature pods occur from about September to October (few pods have been seen which makes it difficult to accurately determine the fruiting phenology).

Variation

Acacia bromilowiana shows considerable variation in its growth form. In the Hamersley Range it is normally an erect shrub or small tree 2-5 (-6.5) m tall with one, or sometimes two, rather crooked main stems and a dense crown that occupies the upper half of the plant; in the Hancock Range, however, plants from a small, sheltered gully attained a height of 12 m. It is assumed that this tall stature has resulted from the population escaping the ravages of fire for a considerable length of time. On Balfour Downs Station the plants are similarly protected from fire but in this case they grow as low-domed shrubs 1-2 m tall, with thick, gnarled and twisted trunks and spreading crowns that extend to the ground.

On most plants the phyllodes vary from 20-40 mm wide, however, on one gathering from the Ophthalmia Range they were atypically narrow, ranging down to about 13 mm wide. Some collectors have noted that on living plants the pulvinus is reddish in colour (but normally drying brownish), however, on some herbarium material the pulvinus is yellowish and it is not known if this colour results from drying.

Judging from field observations and genetic testing (Margaret Byrne, pers. comm.) it appears that most (perhaps all) occurrences of A. bromilowiana in the Hamersley Range represent a series of clonal populations. However, plants on Balfour Downs Station appear not to be clonal, although this proposition has not been tested.

Affinities

Acacia bromilowiana appears to be most closely related to A. hamersleyensis with which it often grows in the Hamersley Range. Acacia hamersleyensis , however, is often a multi-stemmed shrub (oldest plants rarely assuming a gnarled spreading habit somewhat similar to some forms of A. bromilowiana) which is readily recognized by its simple (not racemose) inflorescences, hairy peduncles and pods (glabrous in A. bromilowiana), sericeous new shoots with pale yellow or silvery appressed hairs (the distinctive layer of dense, red-brown minute hairs that occur on young new shoots of A. bromilowiana is absent) and its seeds which are oblique (not transverse or longitudinal) in the pods and which have a band of dull yellow tissue surrounding the small central areole (this band of tissue is absent in A. bromilowiana). Other characters useful in recognizing A. hamersleyensis include its yellowish to light brown or orange branchlets (seemingly always reddish in A. bromilowiana), normally narrower phyllodes which only rarely exceed 20 mm in width and which never have anastomosing nerves (careful examination of phyllodes at x10 magnification or higher is needed to observe the relatively few anastomoses that occur on the phyllodes of A. bromilowiana), its commonly shorter peduncles (3-7 mm long) and its often longer (mostly 5-10 mm), more slender and more coarsely wrinkled, orange or yellow pulvinus.

Notes

This species was only recently discovered (from Weeli Wolli Creek in 1984) and described.

Conservation status

Acacia bromilowiana is listed as a Priority 4 taxon on the Department of Environment and Conservation's Declared Rare and Priority Flora List.

Origin of name

This species is named for Robert (Bob) Neil Bromilow, who, as Technical Officer to Stephen van Leeuwen (see A. leeuweniana) provided excellent field and laboratory assistance over 17 years in the Pilbara. The support provided by Bob was pivotal in the completion of several large scale botanical surveys the most noteworthy being the 'Botanical Survey of Hamersley Range Uplands', 'Biological Survey of the South-western Little Sandy Desert' and the current 'Pilbara Biological Survey'. Bob was also instrumental in maintaining and curating the collection at the now disbanded Pilbara Regional Herbarium in Karratha.

References

van Vreeswyk, A.M.E., Payne, A.L., Leighton, K.A. and Hennig, P. (2004). An Inventory and Condition Survey of the Pilbara Region, Western Australia. pp. 423. Technical Bulletin No. 92. (Department of Agriculture: Perth, Western Australia.)