MVG 13 - ACACIA OPEN WOODLANDS

Gidgee woodlands (Acacia cambagei) near Coopers Creek, Innamincka, SA (Photo: M. Fagg)

Overview

Many Acacia woodlands are dominated by a single Acacia species, which may co-occur with

other members of the genus, either in the understorey or in transitional segments of

environmental gradients.

Species composition and the identity of the dominant species vary along rainfall gradients, with

substrate and to a lesser extent rainfall seasonality (Beadle 1981; Johnson and Burrows 1994).

MVG 13 comprises several subgroups including: “gidgee” and “myall” woodlands that occur as

mean annual rainfall declines below 550 mm and “mulga” which dominate woodlands on stony

or sandy landscapes across the arid zone and across the northeast semi-arid landscapes adjoining

savanna.

Mulga vegetation takes on a variety of structural expressions and is consequently classified

partly within MVG 6, in accordance with the Kyoto Protocol definition of forest cover in

Australia (trees > 2 m tall and crown cover > 20%, foliage projective cover > 10%); and partly

within MVG 13 where the tree canopy layer is sparser or MVG 16 where the woody dominants

are not predominantly single-stemmed

Facts and figures

Major Vegetation Group MVG 13 - Acacia Open Woodlands

Major Vegetation Subgroups (number of NVIS descriptions)

xx. Gidgee woodlands NSW, QLD, SA, NT 22. Semi-arid myall woodlands NSW, QLD 24. Arid myall woodlands NSW, SA, WA 20. Stony mulga woodlands and shrublands NSW, QLD, NT, SA, WA 23. Sandplain Acacia woodlands and shrublands NSW, QLD, NT, SA, WA

Typical NVIS structural formations Woodland (low) Open woodland (mid, low)

Number of IBRA regions 36

Most extensive in IBRA region Est. pre-1750 and present: Mulga Lands (Qld and NSW)

Estimated pre-1750 extent (km2) 320 981

Present extent (km2) 314 040

Area protected (km2) 23 815

Acacia aneura (Mulga) woodland western NSW (Photo: B. Pellow)

Structure and physiognomy

Woodland structure varies from 5 – 20 m tall with canopy cover 10 -30% along the regional

rainfall gradient and with local soil moisture gradients between rises and depressions (Johnson

and Burrows 1994; Neldner et al. 2013).

Open woodlands are distinguished from shrublands (MVG 16) by the predominantly single-

stemmed growth form of the woody dominants.

Typically the structure of this MVG comprises a canopy dominated by Acacia species, a smaller

shrub layer and a ground layer of perennial grasses and forbs and ephemeral species.

Canopy emergents, where present, include scattered eucalypts and casuarinas.

Leaves of the dominate layer vary from microphyll phyllodes or leaves (2.5 – 20 cm2) to

nanophyll phyllodinous needles (0.25 – 2.5 cm2), (Johnson & Burrows 1994).

Understories may include open layers of shrubs with typically sparse ground layers of tussock

grasses and forbs that vary depending on canopy cover, rainfall and substrate.

In myall woodlands an open layer of chenopod shrubs and other woody plant species and an

open to continuous groundcover of grasses and herbs may occur (Keith 2004; NSW Scientific

Committee 2005).

Indicative flora

A single species of Acacia typically dominates, but may co-occur with other members of the

genus as sub-dominants or occasionally co-dominants.

Five Major Vegetation Subgroups are recognised, of which two also occur in MVG 6 and in

MVG 16. Three of the subgroups are dominated by Acacia in the ‘Microneurae’ group within

section Plurinerves of the genus, with chenopods and/or tussock grasses primarily of tribes

Chlorideae and Sporoboleae on fine-textured alkaline soils. Two other subgroups are dominated

by Acacia species from section Juliflorae of the genus, with tussock grasses primarily of tribes

Aristideae and Andropogoneae on coarse-textured acid soils (Johnson & Burrows 1994).

o Gidgee woodlands (also represented in MVG 6) - Acacia cambagei is the primary

dominant, and there may be scattered emergent eucalypts (section Adnataria). Co-

occurring shrubs are sparse but may include species of Alectryon, Atriplex, Carissa,

Enchylaena, Eremophila, Senna and Terminalia. In the swales of red desert dunefields,

Acacia georginae becomes the sole dominant tree, while in the higher rainfall areas

Acacia argyrodendron is the sole dominant. The groundlayer comprises a sparse cover of

tussock grasses including species of Astrebla, Enteropogon, Paspalidium and Sporobolus

(Johnson and Burrows 1994; Keith 2004; Neldner et al. 2013).

o Semi-arid myall woodlands - Acacia pendula is the sole dominant species, often

populated with the mistletoe Amyema quandang, although eucalypts (section Adnatria),

Alectryon, Casuarina and other species of Acacia may be present as subordinates or

scattered emergents, especially in the north. The more extensive southern occurrences

have understories dominated by chenopod genera including Atriplex, Chenopodium,

Enchylaena, Maireana, Rhagodia and Sclerolaena, although many of these may be

eliminated by heavy grazing leaving a grassy groundlayer assemblage of Austrostipa,

Chloris, Eragrostis and Sporobolus with forbs of the Asteraceae and Brassicaceae (Keith

2004). Chenopods are less common in the northern part of the distribution, where

Eremophila maculata becomes a more conspicuous shrub and Astrebla and Dichanthium

become more prominent grass genera (Johnson and Burrows 1994).

o Arid myall woodlands - Acacia papyrocarpa dominates to the west of the Flinders

Ranges, while the related Acacia loderi dominates to the east. These may co-occur with

Casuarina pauper, Myoporum platycarpum, Pittosporum angustifolium and Acacia

aneura. Shrubs include species of Senna, Eremophila and Grevillea on deeper soils or,

where calcrete is close to the surface, Maireana sedifolia, other Maireana species,

Atriplex vesicaria and numerous Sclerolaena species. Widespread grasses include species

of Austrostipa, Austrodanthonia and Enneapogon (Beadle 1981; Johnson and Burrows

1994; Beard et al. 2013).

o Stony mulga woodlands and shrublands (also represented in MVGs 6 and 16) - Acacia

aneura (mulga) is the primary dominant, often present in monospecific stands, but unlike

most other Acacia assemblages, commonly co-occurs with other Acacia species or with

scattered Eucalyptus or Casuarina. Widespread congeners include Acacia victoriae and

Acacia tetragonophylla, while Acacia estrophiolata Acacia grasbyi, Acacia

quadrimarginea, and Acacia xiphophylla may be regionally abundant in parts of central

and western Australia The sparse shrub layer includes species of Atalaya, Eremophila,

Senna, Dodonaea and Maireana. The ground cover varies inversely with overstorey cover

and responds to rain. It is dominated by tussock grasses including species of Aristida,

Enneapogon and Eragrostis, with Austrostipa in the south (Johnson and Burrows 1994;

Neldner et al. 2013).

o Sandplain Acacia woodlands and shrublands (also represented in MVG 16) - include

Acacia aneura with Acacia calcicola, Acacia ligulata, Acacia murrayana, Acacia

ramulosa and Acacia tetragonophylla in various combinations across the distribution of

this subgroup, along with species of Dodonaea, Eremophila, Grevillea, Hakea and Senna,

with Acacia ancistrocarpa in the north. The ground layer includes a diversity of

ephemeral Asteraceae, hummock grasses (Triodia species), and/or tussock grasses such as

Aristida, Enneapogon, Eragrostis and Austrostipa (Johnson and Burrows 1994; Keith

2004).

Environment

Occurs largely within the 150 mm to 550 mm annual rainfall range across arid and semi-arid

areas of Australia.

The climatic conditions are generally dry, hot summers, with cool to warm winters. Rainfall is

variable although maximum falls are either in summer (northern) or in winter (southern).

In southern Australia rainfall occurs in winter while in northern Australia rainfall is linked to the

monsoons of summer ((Hodgkinson 2002).

Different subgroups occur on contrasting substrates:

o Gidgee woodlands occur on deep fine texted soils in landscape depressions, replacing

Brigalow woodlands as mean annual rainfall declines below 550 mm in central and

western Queensland, and adjoining areas of the Northern Territory, South Australia and

northern NSW (Johnson and Burrows 1994; Keith 2004)

o Semi-arid myall woodlands are distributed on mostly grey clay soil plains, often of

ancient alluvial origin, within a belt defined by the 375 mm and 550 mm isohyets to the

east of Australia’s arid zone stretching from the Riverina district in the south to Clermont

in central Queensland (Johnson and Burrows 1994; Keith 2004).

o Arid myall woodlands occur within the 150 to 280 mm annual rainfall belt across the

southern arid zone from the north-western fringe of the Nullarbor Plain to far western

New South Wales on calcareous loams and clay loams, usually with a shallow calcrete

subsoil (Beadle 1981; Johnson and Burrows 1994).

o Stony mulga woodlands and shrublands occur on open peneplains with light-textured

loams and hard subsoils, high levels of iron and low phosphorus where mean annual

rainfall is between 200 and 500 mm from central Queensland to the west Australian coast

(Johnson and Burrows 1994; Keith 2004).

o Sandplain Acacia woodlands and shrublands occur on siliceous red sandplains, mainly

on the crests and slopes of dunes, across the central and western deserts (Johnson and

Burrows 1994; Keith 2004).

Geography

Dominates large areas of semi-arid and arid Australia particularly Western Australia, the

Northern Territory, South Australia, Queensland and New South Wales.

Largely occur on extensive undulating plains and downs, low hills and valleys of the rangelands.

Largest areas occur in the Queensland (91 014 km2), South Australia (73 939 km

2) and Western

Australia (61 309 km2).

Change

Approximately 2% of the estimated pre-1750 extent cleared accounting for 0.7% of total

clearing in Australia mainly as a result of pastoral activities in Queensland and New South

Wales.

Approximately 7 000 km2 cleared since European settlement.

Modified by clearing for pastoral activities, change in local drainage systems, increased local

grazing pressure by providing stock watering, an overall increase in grazing through the

introduction and spread of feral animals.

High levels of grazing in Myall woodland has led to the replacement of chenopod shrubs and tall

perennial tussock grasses with short perennial grasses and unpalatable annual herbs (Beadle

1948), as well as introduction of exotic herbs and grasses such as Bromus spp., Erodium spp.,

Lepidium africanum, Medicago spp., Sisymbrium spp. and Trifolium spp. Introduced shrubs,

notably Lycium ferocissimum (African boxthorn) have also invaded some areas occupied by this

community (NSW Scientific Committee 2005).

Extensive invasion of Buffel grass (Cenchrus ciliatus) across arid and semi-arid parts of

Queensland and Northern Territory is making Acacia woodlands more regularly prone to fires,

threatening the persistence of trees and shrubs.

Less than 30% of Myall woodland remains in New South Wales (NSW Scientific Committee

2005) with much of the clearing taking place during droughts to provide emergency stock food

(Keith 2004)

Overgrazing by cattle and feral vertebrates has degraded extensive areas of Gidgee and Mulga

woodlands, resulting in a lack of regeneration, soil compaction and erosion, reduction of woody

debris, as well as disruption of grove-intergrove patterns critical to water movement, resource

capture and landscape function (Tongway and Ludwig 1990).

Foremost threats are over-grazing (including cattle, sheep and feral animals) and changes to fire

regimes (NLWRA 2001b).

Tenure

Acacia Open Woodlands occur largely on leasehold land.

New South Wales: leasehold, protected areas and very small areas on freehold land

Northern Territory: leasehold land,protected areas, some freehold land, little other crown land

Queensland: leasehold land, protected areas, isolated areas on freehold land

South Australia: leasehold land, protected areas, little on freehold land

Western Australia: leasehold land, protected areas, some other crown land

Key values

Biodiversity, several endangered ecological communities and species are known from temperate

regions of this MVG.

Maintenance of landscape function and soil productivity.

Remnant populations of a wide range of vertebrate and invertebrate species.

Pastoral production.

List of key management issues

Grazing pressure from both domestic stock and feral animals. Trampling and grazing by large

herbivores has a number of impacts.

Interactions between stocking rates and drought cycles, implementing pastoral management

systems that avoid cumulative legacies of soil degradation caused by sustained overgrazing into

successive drought episodes.

Location and access to artificial watering points, which are associated with intensification of

herbivore activity and associated impacts.

Control of introduced grasses on pastoral lands to limit their influence on fire regimes and native

groundcover.

Fire regimes, recognising that some introduced grasses such as Cenchrus ciliaris (buffel grass)

burn with greater intensity than native pasture grasses, thereby altering regeneration responses of

trees and shrubs.

Indirect effects of climate change.

Feral animals including pigs, goats, cats and foxes.

Long term monitoring to inform future management strategies.

References

Australian Surveying and Land Information Group (1990) Atlas of Australian Resources. Volume 6

Vegetation. AUSMAP, Department of Administrative Services, Canberra, 64pp. & 2 maps.

Beadle NCW (1948) Vegetation and pastures of western New South Wales. Soil Conservation Service,

Sydney.

Beadle N.C.W. (1981) The Vegetation of Australia. Cambridge Univ. Press, Cambridge, 690pp.

Beard J.S., Beetson, G.R, Harvey J.M. Hopkins A.J.M and Shepherd D.P. (2013) The Vegetation of

Western Australia at 1:3,000,000 Scale. Explanatory Memoir. Second Edition. Science Division,

Department of Parks and Wildlife, Western Australia.

Hodgkinson K.C. (2002). Fire regimes in Acacia woodland landscapes: effects on functional processes

and biological diversity . In Flammable Australia. The fire regimes and biodiversity of a continent.

(ed Bradstock R.A., Williams J.E and Gill A.M.) pp. 259 - 280. Cambridge University Pres,

Cambridge

Johnson R.W. and Burrows W.H. (1994) Acacia open forests, woodlands and shrublands. In:

Australian Vegetation (ed. R.H. Groves) pp. 257-290. Cambridge Univ. Press, Cambridge.

Keith D. (2004) Ocean Shores to Desert Dunes. The native vegetation of New South Wales and the

ACT. Department of Environment and Conservation (NSW), Hurstville

National Land and Water Resources Audit (2001) Australian Native Vegetation Assessment 2001.

National Land and Water Resources Audit, Canberra, 332pp.

Neldner, V.J., Niehus, R.E., Wilson, B.A., McDonald, W.J.F. and Ford, A.J. (2014). The Vegetation of

Queensland. Descriptions of Broad Vegetation Groups. Version 1.1. Queensland Herbarium,

Department of Science, Information Technology, Innovation and the Arts.

NSW Scientific Committee (2005). Final Determination Myall Woodland in the Darling Riverine

Plains, Brigalow Belt South, Cobar Peneplain, Murray-Darling Depression, Riverina and NSW

South Western Slopes bioregions - endangered ecological community listing. New South Wales

Scientific Committee, Hurstville.

National Land and Water Resources Audit (2001b) Rangelands—Tracking Changes, National Land

and Water Resources Audit, Canberra.

Tongway D. J. and Ludwig, J. A. (1990) Vegetation and soil patterning in semi-arid mulga lands of

Eastern Australia. Australian Journal of Ecology 15, 23–34.

Data sources

Interim Biogeographic Regionalisation for Australia (IBRA), Version 6.1.

Land Tenure in Australia's Rangelands (1955 to 2000), National Land and Water Resources Audit.

National Vegetation Information System, Version 4.1.

1996/97 Land Use of Australia, Version 2.

Collaborative Australian Protected Areas Database – CAPAD 2004 – Terrestrial.

Notes

Additional large areas identified in NVIS 3 from improved data for Western Australia, South

Australia, Queensland and the Northern Territory; also gap-filling (non-NVIS) data in South

Australia.

See the Introduction to the MVG fact sheets for further background on this series.

Semi-arid myall woodlands (Acacia pendula), western NSW (Photo: B. Pellow)