Australian forest profiles: Acacia
The genus Acacia occurs naturally on all continents except Europe and Antarctica and contains more than 1,500 species. Australia has around 955 species, making it the nation’s largest genus of flowering plants. The genus is remarkably varied in appearance, habit and location, encompassing species with small creeping forms and trees that can grow as tall as 30 metres. Australia has more than 50 species of forest-forming acacias.1
Acacias became known as wattles in Australia because their branches were used to build huts with what the early British settlers called the ‘wattle and daub’ method. This involved weaving acacia branches among stakes to create a frame that could be plastered with mud.
Many acacias produce spectacular golden flowers and are a conspicuous feature of the Australian landscape. An acacia was one of the first Australian plants collected by Europeans.
Covering 10.4 million hectares and accounting for 7% of the total native forest area, acacia forests are Australia’s second most common forest type. They predominantly form woodlands (Table 1) in regions where average annual rainfall is less than 750 millimetres. In wetter areas, they can form open forests, usually dominated by a single species: in Tasmania, for example, blackwood (A. melanoxylon) dominates extensive stands of swamp forest on poorly drained sites.
Where are Australia’s acacia forests?
Acacia forests occur in all Australian states and the Northern Territory (Figure 1), with the largest areas in Queensland and Western Australia. They are especially common and conspicuous in arid, semi-arid and dry sub-tropical areas, where they usually occur as low woodlands. Mulga (Acacia aneura) is the dominant species in many parts of the arid and semi-arid zone, and also occurs as an understorey species in some eucalypt forests in eastern Australia. Brigalow (A. harpophylla) is widespread in Queensland and northern New South Wales, forming dense forests on flat or undulating country with clay soils.
Patches of lancewood (Acacia shirleyi) grow as forest on rocky outcrops and steep slopes in the tropics and sub-tropics, whereas myall (A. pendula) dominates forests in semi-arid country adjacent to streams or saltbush shrublands. Gidgee (A. cambagei) is widespread in areas where average annual rainfall is less than 500 millimetres and is even found in the Simpson Desert in central Australia.
Figure 1: Acacia forest distribution, 2008
Source: National Forest Inventory, 2008. Note: The resolution of forests shown on this map has been enhanced to ensure their distribution is visible at the national scale.
Trees without leaves
Acacias produce leaves of great variety. Often, however, what look like leaves are not really leaves at all but phyllodes — flattened leaf stalks — that look and function like leaves. Some phyllodes are large — up to 30 centimetres long; others are so small they are hard to see. Some stems, such as those of A. glaucoptera, have distinctive wide wings with tiny phyllodes. Several acacias have bipinnate (fern-like) leaves, made up of a large number of small leaflets (pinnules) along a central stalk.
Australia’s floral emblem
Golden wattle (Acacia pycnantha) is Australia’s national floral emblem and is part of Australia's coat of arms. National Wattle Day is celebrated on 1 September each year.
| Woodland | Open | Closed | Total | |
|---|---|---|---|---|
| Acacia | 7,059,000 | 3,306,000 | 0 | 10,365,000 |
| Total native forest | 99,007,000 | 44,120,000 | 4,270,000 | 147,397,000 |
| Note: Crown cover is the area of ground covered by tree canopies, ignoring overlaps and gaps within individual canopies. It is usually measured from above using aerial photographs or other remote sensing imagery. In Australia, woodland has a crown cover of 20–50%, open forest a crown cover of 50–80%, and closed forest a crown cover of greater than 80%. | ||||
Ownership and management
Less than 5% of acacia forests are in nature conservation reserves. Most (86%) occur on leasehold or private land (Table 2).
| ACT | NSW | NT | Qld | SA | Tas | Vic | WA | Australia | |
|---|---|---|---|---|---|---|---|---|---|
| Leasehold land | 0 | 1 143 | 1 255 | 4 206 | 159 | 0 | 0 | 570 | 7 334 |
| Multiple-use public forests | 0 | 6 | 0 | 44 | 0 | 36 | 13 | 6 | 105 |
| Nature conservation reserves | 0 | 54 | 1 | 271 | 73 | 14 | 25 | 55 | 492 |
| Other Crown land | 0 | 29 | 13 | 109 | 3 | 1 | 0 | 478 | 634 |
| Private land | 0 | 99 | 227 | 1,219 | 3 | 21 | 3 | 15 | 1 586 |
| Unresolved tenure | 0 | 1 | 0 | 212 | 1 | 0 | 0 | 0 | 214 |
| Total | 0 | 1 333 | 1 496 | 6 060 | 239 | 72 | 41 | 1 123 | 10 365 |
| Note: Totals might not tally due to rounding. The six forest tenure categories above are defined in MIG (2008,pp xvii-xviii) Source: MIG (2008) |
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Values and uses
Wood
The arts and craft industry makes substantial use of the attractive timbers of some Acacia species, particularly blackwood. In the past, black wattle (Acacia mearnsii), silver wattle (A. dealbata) and brown salwood (A. celsa) have been harvested for their timber or to produce paper pulp.
Acacias have also been harvested and used for poles, posts and rails and for small-scale wood-turning. Some, such as lancewood, burn slowly and evenly and are valued as firewood.
Plantations of Australian acacias have been established in more than 70 countries. Mangium (A. mangium) occurs naturally in Queensland, Papua New Guinea and eastern Indonesia and is a major plantation species in tropical countries as well as in the Northern Territory. Queensland’s natural populations of this species therefore constitute a valuable genetic resource for the global plantation industry.
Mangium comprises 97% of Australia’s acacia plantations. The remaining plantings in Australia comprise a few hundred hectares of A. decurrens and A. melanoxylon and even smaller areas of 15 other species.
Mangium plantations Melville Island
With a land area of 5 788 km2, Melville Island is Australia’s second largest island after Tasmania and the larger of the two Tiwi Islands north of Darwin. The entire island is Aboriginal freehold land. Until recently, it remained mostly untouched by large-scale modern development, the exception being a small area of trial plantations and a somewhat larger area of exotic Pinus caribaea (about 2 200 hectares), which was planted in the period 1975–1997. Relatively high rainfall (ranging from 1 400 mm/yr in the east to 2 000 mm/yr in the northwest), warm to hot temperatures (16–29˚ in July to 25–35˚C in November), gentle terrain, low elevation (<102 m above sea level) and deep red and yellow sandy soils have attracted the establishment of short-rotation plantation forestry using Acacia mangium. First established in the late 1990s, these plantations have now expanded to more than 20 000 hectares.
Source: Davidson et al. (2007).
Environmental
Because of their wide distribution, acacia forests are important for biodiversity conservation and the maintenance of ecosystem processes. The Brigalow Belt in northern New South Wales and Queensland, for example, supports at least 148 species of reptiles, 13 of which are rare or endangered, and 328 species of birds, 24 of which are threatened. Mature brigalow constitutes the sole food source of the vulnerable northern imperial hairstreak butterfly (Jalmenus evagoras subspecies eubulus), which is restricted to about 30 locations in the Brigalow.
Indigenous uses
Indigenous people have long used acacias for fuel, medicine, musical instruments (such as clap-sticks), tools and weapons (notably boomerangs and spearheads), and in ritual practices. The seeds of about 50 Australian dry-zone Acacia species are a traditional food. Acacia seeds were ground into flour to make cakes. Witchetty grubs, an important source of food, can be found under the bark and in the roots of some species. Indigenous knowledge of acacias is an invaluable guide to their potential wider use.
Other uses
Acacia forests supply industries based on cut flowers and oils and are also commonly used in land rehabilitation. Mulga is an important fodder tree across semi-arid northern Australia, sustaining stock in times of drought. Species such as black wattle were once used in Australia as a source of tannin for treating leather and are still used for this purpose overseas. Acacia seeds have been developed as a bush food used to flavour cakes, biscuits and ice cream.
Nitrogen Fixation
Acacias fix atmospheric nitrogen with the aid of symbiotic bacteria (rhizobia) that live in nodules on the roots. While this process occurs in nature, a wattle seed inoculant, Wattle GrowTM, has been developed to improve survival of planted acacias and reduce the amount of seed required to vegetate a given area. Containing four elite strains of Bradyrhizobium, it is effective on a range of wattle species used for revegetation in southeastern Australia.
References and further reading
Boland, D.J., Brooker, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A., McDonald, M.W. and Turner, J.D. 2006, Forest Trees of Australia. Fifth Edition. CSIRO Publishing, Melbourne.
Chapman, A. R., Conn, B. J., Court, A. B., Cowan, R. S., George, A.S., Keith, D. A., Kodela, P. G., Leach, G. J., Lewington, M. A., McDonald, M. W., Maslin, B. R., Pedley, L., Ross, J. H., Tame, T. M. and Tindale, M.D. 2001, Flora of Australia Volume 11A, Mimosaceae, Acacia part 1. Melbourne: ABRS/CSIRO Publishing, Melbourne.
Chapman, A. R., Conn, B. J., Court, A. B., Cowan, R. S., George, A.S., Keith, D. A., Kodela, P. G., Leach, G. J., Lewington, M. A., McDonald, M. W., Maslin, B. R., Pedley, L., Ross, J. H., Tame, T. M. and Tindale, M.D. 2001, Flora of Australia Volume 11B, Mimosaceae, Acacia part 2. ABRS/CSIRO Publishing, Melbourne.
Davidson, J., Parsons, M. and Thackway, R. 2007, Trends in the establishment of Australia’s plantations assessed from the perspective of vegetation condition states. Proceedings of the International Symposium on Forest Soils and Ecosystem health, Noosa, Australia, 19 – 23 August.
Harwood, C. E. 1994, Human food potential of the seeds of some Australian dry-zone Acacia species. Journal of Arid Environments 27:27-35.
Maslin, B. R. and McDonald, M. W. 1996, A key to useful Australian Acacias for the seasonally dry tropics. CSIRO Publishing, Melbourne.
MIG (Montreal Process Implementation Group for Australia (2008). Australia’s State of the Forests Report, Bureau of RuralSciences, Canberra.
Websites
www.flora.sa.gov.au/id_tool.html
www.daff.gov.au/forestsaustralia
Footnotes
1 Australia's definition of forest is 'an area dominated by trees having usually a single stem and a mature or potentially mature height exceeding 2 m and with existing or potential crown cover of overstorey strata about equal to or great than 20%.'