MVG 13 Acacia Open Woodlands
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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 xx. Gidgee woodlands NSW, QLD, SA, NT (number of NVIS descriptions) 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