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Electronic Flora of South Australia species Fact Sheet

Family: Leguminosae
Acacia suaveolens

Citation: C. L. Willdenow, Sp. Pl. 4:1050 (1806). Mimosa suaveolens Sm., Trans. Linn. Soc. Lond. 1:253 (1791)

Derivation: suaveolens (Latin)--sweet smelling.

Synonymy: Racosperma suaveolens (Smith)Martius, Hort. Reg. Monac. Semin. (1835).

, Mimosa suaveolens

Common name: Sweet wattle

Description:
Erect, dull, bluish-green, stiff, glabrous, sometimes spindly shrubs, 1-3 m high; branchlets acutely angled, triangular appearing flattened; bark smooth greenish.

Phyllodes narrow-oblong to linear, oblanceolate, 7-12 cm long, 4-8 mm broad, narrow-thick, straight or curved, 1-veined more or less central, apex usually ending in a small straight or curved mucro; glands basal.

Inflorescences axillary, glabrous racemes much shorter than phyllodes, enclosed in large scaly imbricate bracts when young; flower-heads scented, pale yellow, 6-1O-flowered; flowers 5-merous.

Legumes oblong, 2-4 cm long, 12-20 mm broad, flat, obtuse, coriaceous, purplish-brown, pruinose; margins thickened. Seeds transverse in legume; funicle short, with a few short folds thickened into a small fleshy aril.

Distribution:  A restricted occurrence in the lower South-Eastern region in the hundred of Comaum just north-east of Penola. In low open forest, associated with Eucalyptus baxteri, Acacia mearnsii, A. melanoxylon, and a low shrub understorey. Soils: sandy neutral yellow duplex. Rainfall approximately 700 mm. Also Qld, N.S.W., Vic. and Tas.

S.Aust.: SL, SE.

Conservation status: Lang & Kraehenbuehl (1987) consider this species to be Vulnerable.

Flowering time: May — September.


SA Distribution Map based
on current data relating to
specimens held in the
State Herbarium of South Australia

Biology: No text

Related taxa: Allied to Acacia iteaphylla which has similar inflorescences and phyllodes but differs in habit and legume features. Also allied to A. subcaerulea Lindl., a W.Aust. endemic species.

Taxonomic notes: A brief eulogy of A. suaveolens in Victoria is given by Galbraith (1960).

The early development of the floral apex of A. suaveolens was discussed by Newman (1936). The author interprets the legume as a single laminar structure which by incurving and adpression of its margins encloses a single cavity, the ovules arising on the incurved margins of this laminar structure.

For the results of seed chipping and hot water treatments on the germination of seeds see Clemens et al. (1977).

The effect of scarification treatment on the germination of seeds of A. suaveolens was studied by Auld (1986a). This paper also gives references to earlier germination tests on 51 other species of Acacia. The paper, which covers tests of germination from plants from different sites over a 3 year period, showed that there were differences between sites and years. In most cases the total viability was greater than 90% (lowest 73%) but the innate dormancy due to the impermeable seed coat was also very high, more than half the samples being greater than 90% impermeable. Scarification of the seed was the most effective way of breaking dormancy and was more effective than boiling water or the use of microwave treatment. The generally small percentage of seeds that did not germinate after treatments were frequently found to be defective -- the damage possibly caused by insects which had fed on the developing seed.

The distribution, ecology and conservation status of A. suaveolens may be found in Morrison et al. (1983). This species only just reaches the far south-east of South Australia which provides the western-most point of its range. It has an almost continuous sub-coastal distribution from western Victoria to Marlborough in Queensland.

A prostrate broad-phyllode form was recognised around Sydney and a slender narrow-phyllode form at Myall Lakes, north of Sydney. Flowering is initiated in a geographical sequence beginning from Feb.-March in Queensland to late May in western Victoria and South Australia. Although not rare, conservation status varies with State and locality as many habitats are now over run by agriculture and suburbia. The very small occurrence in South Australia is not conserved.

Not surprisingly the seed crop varies between years and sites, Auld (1986a) and Auld & Myerscough (1986). It reached its peak 1-4 years after establishment and then declined. Pod production fell from a mean of about 16 per plant for the first couple of years to less than 5 between 14-16 years old.

Seed predation by insect damage was consistently high ranging from 20 to 60% and frequently 40% or more, Auld (1983).

Soil heating through fire caused flushes of seed germination, Auld (1986b). Temperatures less than 60(C left the seed dormant and optimum temperatures for breaking dormancy were between 60 and 80(C. The temperature also affected the depth in the soil to which the seed was heated. Seedlings could emerge from depths up to 8 cm but beyond that success was greatly reduced and was nil at 14 cm. In the field, observations of seedlings confirmed that post-fire emergence is concentrated over a small range of soil depths directly related to the duration and intensity of heating.

A synthesis of the life cycle of A. suaveolens was presented by Auld (1987). Survival of seedlings and adults was studied for several years at seven sites near Sydney. As the adults are killed by fire and do not regenerate vegetatively all individuals represent plants established from seed in, often, even-aged stands after fire.

The fate of seedlings germinating after fire was followed as well as the fate of adults in stands of known age. Moisture stress was the largest single cause of seedling death, about 90%, seedlings eaten 9% and falling litter 1%. There was a more gradual decline of plants that did survive seedling stage but again drought stress seemed to be important, perhaps aided by fungal or nematode infection.

The plants can flower in their second year and need 2-5 years to reach maturity and a further 6 years to maximise seed input to the soil seed bank.

Before seed dispersal up to 35% can be destroyed by insect predation and up to 36% by external grazers. Of mature seed about 83% was dispersed by ants, 13% was incorporated in the soil by other means and 4% was eaten. Of the seed taken by ants 62% was in the top 0-5 cm of soil. Auld estimates that adult plants would disappear from the flora 20-25 years after a fire, though seeds persist well beyond this point, and that a long no-fire period of up to 50 years would not eliminate it. However very long fire free periods of 70-100 years could lead to the loss of A. suaveolens completely. The species would be favoured by medium to high intensity burns every 10-30 years.

Morrison & Myerscough (1989) studied the seed producing effort of A. suaveolens over a 3 year period. It was found that only 44% of flowers produced ovaries, only 25% of these ovaries produced fruits and 60% of these fruits aborted at early stages of formation. The number of inflorescences per plant was related to the rainfall during bud initiation. The number of inflorescences produced was inversely related to plant age.

Cultivation: A hardy shrub which does well in poor sandy soils near the coast. An attractive ornamental with pale yellow, sweet-scented flowers which burst out from the coveting bracts; conspicuous bluish pods are produced after flowering.

Author: Not yet available

References:

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