Acacia aneura - a desert fodder tree
Acacia aneura is known as mulga in its native Australia where it
is one of the best known species in the genus. Mulga is the Aboriginal word for
a long narrow shield made of acacia wood. It is probably the most important
woody forage plant in Australia because it is palatable, abundant and widespread
in regions of low rainfall. Its use as an exotic. however, has been restricted
by its relatively slow growth rate and its limited capacity to regenerate after
fire or severe branch lopping.
Acacia aneura repented with
permission, M. Simmons, 1981. Inset map shows natural distribution of mulga in
Australia (Turnbull et al. 1986).
BOTANY:
Acacia aneura F. Muell. ex Benth. is one of many thornless
acacias endemic to Australia. It occurs as a 10-15 m tall, often single stemmed
tree in higher rainfall areas but is a 2-3 m high shrub in dry situations or on
very shallow soils. Its form and phyllode morphology are exceptionally variable
(Midgley and Gunn 1985). The phyllodes range from short and needle-like to long
(20 cm), broad (1 cm) and net. Very fine hairs give the foliage an attractive
silverygrey appearance.
Small yellow flowers form spikes 1.5-2.0 cm long Thin. flat
membranous pods. 2-5 cm long, usually with an obvious narrow wing along their
edge, contain dark brown seeds. each with a small pale aril at the base.
Flowering depends on favorable weather conditions and only late
summer flowering followed by winter rain leads to seed set (Davies 1976).
ECOLOGY:
Mulga is the one of the dominant species in Australian shrub
woodlands. Natural populations extend over an area of 1.5 million km²
chiefly in the arid climates where the annual rainfall is 200-250 mm. Mulga
ranges in elevation from sea level to 300 m elevation. In many of the drier
parts of its distribution mulga occurs as the only species in groves up to 50 m
wide and 400 m long with intergrove areas acting as water catchments to provide
substantial run-on water.
In the eastern part of its range in northern New South Wales and
Queensland mulga is found in semiarid conditions with a mean annual rainfall of
300-500 mm. It experiences hot summers and cool winters with light frosts. Soils
supporting mulga are usually acidic sands or sandy loams, which permit easy
filtration of water into the upper horizons, but are usually very low in
nitrogen and available phosphorus (Turnbull 1986). Acacia aneura can live for
more than 50 years, it is drought-tolerant, but very fire sensitive (Kube 1987).
PROVENANCE TRIALS:
The wide variability in soils and climate together with a high
degree of polymorphism suggests that major provenance differences will occur in
growth rates and drought and frost tolerance. International provenance trials
were initiated in 1984 by FAO and CSIRO Division of Forestry and Forest
Products. Canberra (Midgley and Gunn 1985) and trials were established in South
Asia the Middle East, Africa and South America.
WOOD USE:
The heartwood of mulga is dark brown with contrasting markings
of golden yellow; the sapwood is white. The wood is very hard, heavy (850-1100
kg/m³) and durable in the ground: it turns well and takes a high polish
(Boland et al. 1984). Mulga also makes an excellent firewood and charcoal. In
Australia the wood has been used extensively for fence posts but a log size
rarely exceeding 2 m x 25 cm usually restricts the use of the wood to small
turnery items.
FODDER:
In many parts of Australia mulga forms a significant part of a
sheep's diet at all times of the year but without supplementary high quality
feed it supplies protein and energy barely sufficient for maintenance of
dry-range sheep (Goodchild and McMeniman 1987). Phyllodes have a high crude
protein level (11-16%), low phosphorus content (0.05-0.12%) and good
palatability (Turnbull et al. 1986, Vercoe in Boland, 1987). Excessive grazing
may result in the death of mulga.
OTHER USES:
Mulga can be used in arid areas to provide shelter and shade,
its attractive silvery grey foliage makes it a popular choice for amenity
plantings. The Australian Aborigines ground the mulga seed for flour. The seeds
have a protein content comparable to dried split peas or peanuts (Caffin et al.
1980). Aborigines also used the resinous phyllodes of desert mulga form as an
adhesive resin (Turnbull et al. 1986).
ESTABLISHMENT:
For good germination. seed (50,000110,000/kg) should be
scarified by mechanical abrasion or immersed in undiluted sulfuric acid (95%
36N) for 30 minutes and then thoroughly washed in water. Alternatively,
immersion in hot water (90°C) for 1 minute will usually break dormancy
(Doran and Gunn 1987). Seeds sown in a germination tray are ready for separating
into containers within 10 days. The potting mix needs to drain freely but have
good moisture holding capacity (Kube 1987).
Nursery growth is slow with seedlings often taking 6-8 months to
reach 20 cm tall. When transplanted to the field the seedlings usually require
several months without severe moisture stress to survive and in arid areas may
need supplementary irrigation. Established seedlings have the ability to survive
severe drought. They develop a long tap root and an extensive lateral root
system in the top 30 cm of the soil. Acacia aneura needs to be protected from
browsing animals while young.
GROWTH:
Growth rate is generally slow but is related to moisture
conditions. In central Australia planted specimens receiving an average of 370
mm of rainfall a year grew in ten years into multi-stemmed shrubs 3 m tall and
2-4 cm dbh with a crown diameter of 2 m (Kube 1987). Cultivated specimens
receiving regular irrigation have reached 10 m tall and 10 cm dbh in 10 years.
In trials where rainfall is relatively high, the Charleville, Queensland
provenance a broad phyllode form, has grown more rapidly than provenances from
central Australia (Ryan and Bell 1989). Trees with different phyllode forms have
been observed to have different growth rates (Fox 1980).
SYMBIOSIS:
A. aneura forms nodules with Rhizobium with which it exhibits a
degree of specificity (Roughley 1987). Ectomycorrhizal associations have been
observed and there is almost certainly VA mycorrhizal symbiosis (Reddell and
Warren 1987).
PESTS AND DISEASES:
In its natural habitat A. aneura is subject to partial
defoliation by a range of insects and root damage by termites. Termite damage
was light (4% mortality) to moderate (30% mortality) to two provenances aged 18
months in a trial in Zimbabwe (Mitchell 1989).
WEEDINESS:
With its relatively slow growth rate and irregular seeding
habits A. aneura is unlikely to become a serious weed.
PRINCIPAL REFERENCES:
Boland, D.J., M.H. Brooker, G.M. Chippendale, N. Hall, B.P.M.
Hyland, R.D. Johnston, D A. Kleinig and J.D. Turner. 1984. Forest trees of
Australia. 4th Ed. Nelson-CSIRO, Melbourne.
Boland, D.J. (ed). 1987. Trees for the tropics: Growing
Australian multipurpose trees and shrubs in
developing countries. ACIAR
Monograph No. 10. ACIAR. Canberra, Australia.
Caffin. N.,R. Bell. G. Nitchie, S. Weston and N. Ho. 1980.
Protein and mineral content of several species of Acacia seeds. Mulga Research
Centre Annual Rep. No. 3, 1979. W. Australian Inst. of Tech., Bentley,
Australia. p. 43-44.
Davies, S.J.J.F. 1976. Studies of the flowering season and fruit
production of some arid zone shrubs and l trees in western Australia. J. of
Ecology 64:665-687.
Fox, J.E.D. 1980. Stability in mulga stands in times of drought.
in Mulga Research Centre Annual Report No. 3, 1979. W. Australian Inst. of
Tech., Bentley, Australia.
Midgley, S.J. and B.V. Gunn. 1985. Acacia uneura seed
collections for international provenance trials. Forest Genetic Resources
Information 13:21-29.
Simmons. M. 1981. Acacias of Australia. Thomas Nelson.
Melbourne.
Turnbull. J.W. (ed). 1986. Multipurpose Australian trees and
shrubs. ACIAR Monograph No. 1. ACIAR. Canberra, Australia.
Turnbull. J.W. (ed). 1987. Australian acacias in developing,
countries. ACIAR Proceedings No. 16.
ACIAR. Canberra
Australia.
