Acacia Woodlands and Shrublands


Photo 1 Acacia mulga herbfield

Acacia woodlands and shrublands occupy large areas of the arid zone with Mulga A. aneura having by far the largest representation. Only spinifex communities occupy a larger area. Mulga generally occupies soils of intermediate fertility and, though found on a variety of soils, it is most commonly found on red earths where large stands occur.

Mulga communities and forms are quite variable, shrublands are often described and separated according to their understorey. "Perennial Mulga" has perennial grasses as the major understorey component and "annual Mulga" has an understorey composed of annual herbs and grasses.


Photo 2 Acacia Grove Mulga


 
  Mulga growing on gentle slopes forms groves that are aligned with the contours with bare areas between groves. Water run-off flows across the bare intergrove areas and is trapped by the litter and grasses beneath the Mulga groves. Small stands of Mulga are also found between sand dunes, in Triodia pungens communities and on lower non-spinifex hill slopes.

Ranges as tall open-shrublands with a Senna and Eremophila understorey . Witchetty bush also occurs on foothills and plains south, east and west of Alice Springs as sparse-shrubland with grass understorey.


Photo 3 Acacia Witchetty bush shrubland

  


Photo 4 Acacia Gidgee

Gidgee A. georginae shrublands cover large areas of central Australia east and north-east of Alice Springs and into Queensland on calcareous soils. The understorey usually consists of open tussock grassland, including Mitchell Grass (Astrebla spp.) and occasionally herbland. Gidgee patches also occur in inter-dune flats in the southern Simpson Desert, increasing towards Queensland. A small area of scattered Gidgee occurs near the SA border on New Crown Station. Although Gidgee may make good "top feed" for cattle, over much of its range it contains the poison 1080, especially in the seeds. Gidgee may exude an unpleasant smell, especially during wet weather.


Photo 5 Acacia Ironwood woodland

Ironwood A. estrophiolata woodlands are found on plains at the base of hills and adjacent to watercourses where higher nutrient alluvial loamy sands occur. The understorey consists of annual and perennial grasses, if present the middle stratum contains open Acacia and Senna shrubs. Whitewood Atalaya hemiglauca and Hakea spp. along with the dominant Ironwood form the canopy.

Dead Finish A. tetragonophylla / Witchetty Bush sparse-shrubland is a minor community which extends from the Petermann Ranges to Kulgera along the NT/SA border and south of Alice Springs in valleys between ranges. A.amobia and Bastard Mulga A. stowardii are also minor shrubland communities with very restricted distributions. Also in this category is Waddy Wood A.peuce) , this large tree is restricted in the NT to a small reserve on the north-western edge of the Simpson Desert.

Total Area

145 422 km2

Presence in Conservation Reserves

Alice Springs Protection Area, Alice Springs Telegraph Station, Arltunga, Davenport Murchison, Dulcie Ranges National Park, Emily and Jessie Gap Nature Park, Ewaninga Rock Carvings, Finke Gorge National Park, Illamurta Springs, Kuyunba, Mac Clark, Native Gap, Ruby Gap, Trephina Gorge, Uluru-Kata Tjuta National Park, Watarrka National Park, West MacDonnell National Park.


Map 1 Acacia woodland and shrubland distribution (Wilson et. al. 1990)
 

Threats

Acacias are fire sensitive. Spinifex is highly flammable and fire tolerant, the most susceptible Acacia communities are those which have a spinifex understorey or abut a spinifex community. Mulga is particularly at risk from fire as it is often closely associated with spinifex. Frequent fires kill mature trees and seedlings along the interface of the two communities allowing spinifex to intrude into the mulga stand eventually replacing it.

Introduced herbivores impact on seedlings survival through trampling and browsing though anti-herbivory mechanisms are present as poison in Gidgee and very prickly juveniles in waddy wood.

Further reading