Botanical name

Acacia trudgeniana Maslin, Nuytsia 18: 180, fig. 11 (2008)

Common name

Trudgen's Wattle

Description

Erect shrubs (0.5-) 2-5 m tall, with 1 or 2, straight or crooked main stems, lateral branches short and wide-spreading. Bark corky, thick, longitudinally furrowed, light grey (but surface often blacked by fire) except dull pale yellow in furrows. Branchlets terete, finely ribbed, glabrous, light brown to dull yellow tinged orange (often dull pale green at extremities), not pruinose. New shoots light green. Stipules spiny but often only the bases persisting (as blunt knob-like protuberances) with age, 4-8 mm long, patent, straight, brown, rigid. Phyllodes inequilaterally ovate to elliptic or rarely obovate, upper margin prominently rounded and much longer than the shallowly convex to shallowly sigmoid lower margin, (1.5-) 3-5 cm long, (8-) 15-30 (-37) mm wide, coriaceous, undulate, distinctly sub-glaucous (but not pruinose), aging dull green; midrib evident (but not raised), excentric (near lower margin), yellow (when dry); the minor nerves forming a close reticulum; apices short-acuminate and ending in a short, straight, rigid, brown, spiny, excentric cusp 1-3 mm long. Gland situated on upper margin of phyllode at distal end of pulvinus, 1-2 mm long, not raised. Inflorescences (few seen) elongated racemes that extend beyond the phyllodes, 1 or sometimes 2 per axil; raceme axes 3-15 cm long, dull purplish red, glabrous; peduncles 7-20 (-25) mm long, twinned (rarely 3) and often with a phyllode within their angle (these phyllodes are initiated as small, bract-like structures), glabrous, dull purplish red; heads globular, light golden, 20-25 flowered, (the purplish red petals may be seen between the exerted anthers). Bracteoles absent. Flowers 5-merous, glabrous, dull purplish red when in bud; sepals slightly less than � length of petals, free or almost so, linear to narrowly oblong; petals 2 mm long, nerveless, glabrous. Pods (few seen) sub-straight to curved into an open � circle, strongly raised over seeds and slightly to moderately constricted between them, occasional deep constrictions sometimes occur, 2-5 cm long, 8-10 mm wide, thinly crustaceous to slightly coriaceous, light brown, not pruinose, glabrous. Seeds (few seen) longitudinally oblique in the pods, obloid-ellipsoid to almost globose, c. 5 mm long, c. 4 mm wide, dull (not shiny), dark brown, except sometimes very dull pale yellow around the tiny pleurogram near base of seed; funicle light brownish, folded on top of seed but not encircling them, not expanded into an aril.

Characteristic features

Erect shrubs (0.5-) 2-5 m tall, with 1 or 2, straight or crooked main stems and short, wide-spreading lateral branches, plants appearing �gnarled when main stems crooked. Bark thick and corky. Branchlets not pruinose. Stipules spiny but often only the bases persisting with age. Phyllodes mostly 3-5 cm long and 15-30 mm wide, very asymmetric with midrib situated near lower margin, minor veins forming a net-like reticulum, coriaceous, undulate, sub-glaucous (but not pruinose) aging dull green, terminated by a needle-sharp tip. Inflorescences racemose (3-15 cm long); raceme axes , peduncles and flower buds dull purplish red; heads 20-25-flowered. Sepals free. Pods sub-straight to moderately curved. Seeds not encircled by aril. Flowering commencing in September/October.

Distribution and ecology

Of scattered occurrence in northwest Western Australia where it extends from Winning and Yanrey stations located inland from Exmouth Gulf, northeast through the Hamersley Range to Balfour Downs Station in the eastern Pilbara region; it also occurs north of the Hamersley Range near Mt Montagu in the Millstream-Chichester National Park, from the Yule River crossing on North West Coastal Highway south of Port Hedland, and from Barrow Island off the Pilbara coast. It commonly forms small, localized populations in the places where it occurs. Grows on red sand or brown loam with a mantle of ironstone pebbles over spinifex ground cover, on flat or low-undulating plains.

Flowering and fruiting period

Flowering commences between early September and early October but because of the paucity of specimens it is not known when it ceases (perhaps around December). Pods with mature seeds have been collected in November, but only a single mature-fruiting specimen has been seen.

Variation

A dwarf form of this species, 0.5 m tall, occurs at one locality on Barrow Island; the phyllodes on this entity are smaller than normal (15-25 mm long, 8-10 mm wide). On the mainland, plants with smallest phyllodes are found on a few specimens from West Angelas (25-35 mm long and 10-20 mm wide).

Taxonomy

Until recently this species was known under its phrase name, Acacia sp. Mount Hilditch (M.E. Trudgen 19134).

Affinities

Acacia trudgeniana is most closely related to A. inaequilatera and can be easily confused with this species on account of its thick corky bark, spiny stipules, coriaceous, reticulately-veined, spine-tipped phyllodes which are markedly asymmetric (midrib near lower margin), brightly coloured, long racemes, peduncles and petals, and its free sepals. However, A. inaequilatera is most readily distinguished by its pruinose branchlets, pruinose, often more blue-grey phyllodes, slightly more flowers per head (30-40), generally more strongly curved pods with seeds that are partially or wholly encircled by the funicle, earlier flowering period (May-August) and its generally more gnarled, diffuse growth form. The two species grow in close proximity in some places but they are not known to co-occur. Acacia inaequilatera is far more common in the Pilbara than is A. trudgeniana. Acacia marramamba is also related to A. trudgeniana but it is distinguished by having a more straggly growth form, non-corky bark, sparingly and openly anastomosing minor nerves on the phyllodes and brownish red raceme axes, peduncles and flower buds. These three species along with A. pyrifolia and A. strongylophylla (which does not occur in the Pilbara) comprise the informal 'Acacia pyrifolia group'.

Notes

A fire-tolerant species with its thick protective bark and its ability to resprout by epicormic growth following burns. Also has the capacity to resprout after the above ground biomass has been removed through bulldozing and other clearing operations.

One plant of A. trudgeniana has been observed to emit a delicate, faint honey odour (?from flowers) in the heat of the afternoon.

This species provides good wildlife protection and bird nests have been observed in a number of plants.

Conservation status

Not considered rare or endangered.

Origin of name

This species is named for Malcolm Trudgen, botanist, who has a particular interest in the Pilbara and the family Myrtaceae. Over the past 30 years Malcolm has collected extensively in the Pilbara during the course of his many vegetation and flora surveys of that area (e.g. Trudgen and Casson 1998, Trudgen 2002). It was Malcolm who first drew our attention to the existence of this new species, although he was not the first to collect it.