MVG 6 - ACACIA FORESTS AND WOODLANDS Acacia aneura woodland near Yenloora, Qld (Photo: M. Fagg) Overview Almost all Acacia forests and woodlands are dominated by a single Acacia species, which co- occurs with other members of the genus only in transitional segments of environmental gradients. Other genera may occur as subdominants. 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). Where the Acacia stratum is dense, the understorey and ground layer are typically sparse, scattered shrubs and tussock grasses are present in more open stands. MVG 6 comprises several subgroups including: “brigalow” and “gidgee”, which dominate forests or woodlands that are restricted to subtropical and nearby tropical eastern Australia on productive clay soils in sub-humid climates; and “mulga” and “lancewood”, which dominate woodlands on stony landscapes around the eastern fringe of 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. Temperate humid Acacia forests tend to be part of dynamic mosaics within eucalypt forest landscapes, often have emergent eucalypts within them and share virtually all of their species with surrounding eucalypt forests. In this national scale classification, they are grouped with MVGs 2 and 3. Acacia-dominated vegetation in southern, central and Western Australia takes on structural expressions of open woodland and shrubland, and is therefore assigned to MVGs 13 and 16. Extends from central-northern New South Wales in an arc around the northeast of Australia’s semi-arid zone through central Queensland and into the Northern Territory. Facts and figures Major Vegetation Group MVG 6 - Brigalow Forests and Woodlands Major Vegetation Subgroups 13. Brigalow forests and woodlands NSW, QLD, (number of NVIS descriptions) xx. Gidgee woodlands NSW, QLD, SA, NT 14. Upland tropical Acacia Woodlands QLD, NT 20. Stony mulga woodlands and shrublands NSW, QLD, NT, SA, WA Typical NVIS structural formations Open forest (low) Woodland (mid and low) Number of IBRA regions 71 Most extensive in IBRA region Est. pre-1750 and present: Murchison (WA) Estimated pre-1750 extent (km2) 495 059 Present extent (km2) 408 632 Area protected (km2) 36 064 Acacia harpophylla (brigalow) forest, 60km east of Tambo, Qld (Photo: M. Fagg) Structure and physiognomy Structure varies from open forest usually 10 – 25 m tall with canopy cover exceeding 30% to woodland 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. 2014). Canopy emergents, where present, are primarily members of the box group of eucalypts (Eucalyptus section Adnataria), with bloodwood eucalypts (genus Corymbia) more common in the tropics (Johnson and Burrows 1994). In areas where the vegetation is regrowing from recent clearing or fire, Acacias may form dense thickets, which thin gradually over time. This is especially the case for Brigalow forests and woodlands, which regenerate from sucker stems that emerge from underground rhizomes (Beadle 1981). Leaves of the dominate layer vary from notophyll phyllodes or leaves (20 – 45 cm2) to nanophyll phyllodinous or cladodinous needles (0.25 – 2.5 cm2), with the latter predominating in drier areas (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. Indicative flora A single species of Acacia typically dominates, which co-occurs with other members of the genus only in transitional segments of environmental gradients. Co-dominants or emergents include Eucalyptus, Casuarina and various ‘vine scrub’ genera. The ground layers are generally sparse and comprised of tussock grasses and forbs. Associated species include shrub species such as Eremophila, Dodonaea, chenopods such as Atriplex, Maireana, Sclerolaena and grasses such as, Aristida, Austrostipa and Triodia. Four Major Vegetation Subgroups are recognised, of which two also occur in MVG 13 and one in MVG 16. Two of the subgroups are dominated by Acacia in the ‘Microneurae’ group within section Plurinerves of the genus, with 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 Brigalow forests and woodlands - are dominated by Acacia harpophylla and Casuarina cristata, while Acacia cambagei, Acacia argyrodendron, and species of Geijera and Eremophila may co-occur at the drier end of the rainfall gradient (Johnson and Burrows 1994; Keith 2004). In higher rainfall areas, vine scrub genera such as Alectryon, Brachychiton, Cadellia, Carissa, Lysiphyllum, Terminalia and Ventilago occur and the vines Capparis, Cissus and Jasminum, may be scattered within the canopy or midstorey. Various eucalypts (section Adnataria) may emerge above the canopy. The groundcover is typically sparse with specie sfrom the genera Abutilon, Brunoniella, Einadia, Enchylaena, Sida and grasses primarily of the Chlorideae and Sporoboleae tribes (Johnson and Burrows 1994). o Gidgee woodlands (also represented in MVG 13) - include Acacia cambagei as the primary dominant, and there may be scattered emergent eucalypts (Eucalyptus 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. Acacia tephrina also occurs with Acacia cambagei in the wetter northern parts of its range. 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. 2014). o Upland tropical Acacia woodlands - are most commonly dominated by Acacia shirleyi, which is replaced by Acacia catenulata on slightly deeper soils and Acacia petraea in drier areas (Johnson and Burrows 1994). The Acacia canopy may include emergents of Corymbia, with Eucalyptus of section Adnataria becoming more prominent in the subtropics. Small trees and shrubs below the Acacia canopy include species of Petalostigma, Erythropleum and Macropteranthes, while the groundcover is grassy and open with species primarily of the Aristideae and Andropogoneae tribes (Beadle 1981; Johnson and Burrows 1994). o Stony mulga woodlands and shrublands (also represented in MVG 13 and 16) -Acacia aneura (mulga) is the primary dominant, typically present in monospecific stands, or with scattered eucalypts (section Adnataria) emergent above the canopy. 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 (Johnson and Burrows 1994; Neldner et al. 2014) Environment Occurs largely within the 500-900 mm annual rainfall belt with a predominance of summer rainfall, although winter rainfall peaks occur in the south of its distribution where the climate in western areas is more arid (Pulsford, 1984; Johnson, 1997), and annual rainfall may exceed 1000 mm in the north where there is a strong winter drought (Johnson and Burrows 1994). Brigalow and Gidgee assemblages occur on fine textured alkaline clays and clay loams. The clay soils often have a micro relief pattern, referred to as gilgai or melon holes, resulting from lateral mass movement associated with wetting and drying, with the depressions sometimes intermittently filling with water (Keith 2004) Upland tropical Acacia and mulga assemblages are associated with coarsely textured acid loams and sandy loams (Isbell, 1962; Bui and Henderson, 2003) Different subgroups occur on contrasting substrates: o Brigalow forests and woodlands occur on lowland plains and depressions with deep fine textured alkaline soils from central Queensland to central northern New South Wales where mean annual rainfall is between 500 and 750 mm. 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 New South Wales. o Upland tropical Acacia woodlands occur on shallow coarse-textured acid soils typically on dissected uplands of central and northern Queensland and west to the northern Territory where mean annual rainfall is between 500 and 750 mm. o Stony mulga woodlands and shrublands occur on open plains with light-textured loams and hard subsoils, high levels of iron and low phosphorus