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Tropical eucalypt woodlands/grasslands (Eucalyptus orgadophylla), north of Hughenden, Qld


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MVG 12 - Tropical Eucalypt Woodlands/ Grasslands



Tropical eucalypt woodlands/grasslands (Eucalyptus orgadophylla), north of Hughenden, Qld (Photo: M. Fagg)

Overview

  • Also known as tropical savannas.

  • Occupy up to a quarter of the Australian landmass (Russell-Smith et al. 2014).

  • Contains the so-called tall bunch-grass (Beard et al. 2013) savanna woodlands and tropical woodland of far north Western Australia, and related eucalypt woodland and open woodland communities in the Northern Territory and in far north Queensland, including Cape York Peninsula.

  • Dominant eucalypts include species of Corymbia and Eucalyptus, notably the subgenera Fibridia and Leprolaena, but the tree canopy also includes a range of tropical non-eucalypt genera.

  • Many of the trees and shrubs are dry-season deciduous, with even some eucalypts exhibiting semi-deciduous leaf phenology.

  • Typified by a suite of annual C4 grasses (notably Sorghum species.) but is replaced by woodlands with hummock grass understories as mean annual rainfall declines or on more stony sites.

  • Seasonally fire-prone due to curing of the annual grasses during the dry monsoonal winters.

  • An evolutionarily recent biome thought to have assembled within the last 5 to 10 million years.

  • Used extensively for rangeland beef production (Russell-Smith et al. 2014).

Facts and figures

Major Vegetation Group

MVG 12 - Tropical Eucalypt Woodlands/Grasslands

Major Vegetation Subgroups

(number of NVIS descriptions)

7. Tropical Eucalypt forests and woodlands with an annual grassy understorey NT, QLD, WA. (19)

Typical NVIS structural formations

Woodland (mid)

Tussock grassland (mid).



Number of IBRA regions

14

Most extensive in IBRA region

Est. pre-1750 and present: Victoria Bonaparte (NT, WA)

Estimated pre-1750 extent (km2)

115 503

Present extent (km2)

112 481

Area protected (km2)

14 436



Tropical eucalypt woodlands/grasslands (Erythrophleum chlorostachys; Eucalyptus tectifica; Sorghum intrans) NT (Photo: D. Napier)

Structure and physiognomy

  • Consist of an open tree layer with a continuous grassy ground layer (Lehmann et al. 2011; Murphy and Bowman 2012) and are characterised by:

    • eucalypt-dominated woodland and open woodland, transitioning to savannah open forest (MVG 4) as rainfall increases;

    • various non-eucalypt genera in the tree canopy stratum or subcanopy stratum; and

    • understorey composed of a largely continuous cover of tussock grasses exhibiting the C4 photosynthetic pathway, a diversity of other herbaceous species and a variable shrub layer (Dyer et al. 2001; Williams et al. 2002; Lehmann et al. et al. 2011).

  • Most types of this MVG are dominated by Eucalyptus species. Broad leaved, pan tropic trees and shrubs may occur in the upper-mid storeys and may form the tree stratum of some savannas (Wilson et al. 1990: Williams et al. 1996).

  • Some broad-leaved taxa and some eucalypts may be semi-deciduous or wholly deciduous in the monsoonal dry season.

  • Many of the grasses have annual life cycles, although some are perennial (Beard et al. 2013).

  • In drier areas there may be mosaics of wooded areas with open patches of tall savanna grassland.

  • Treeless grasslands occurring on extensive areas of heavy clay soils, or in sites of impeded drainage (Wilson et al. 1990: Williams et al. 1996) are assigned to MVG 19.

Indicative flora

  • Tree layer is dominated by eucalypt genera Eucalyptus (subgenera Fibridia and Leprolaena, with Symphyomyrtus section Adnataria becoming important in northern Queensland) and Corymbia (section Rufaria) usually with two or three species occurring in various combinations. Widespread species include Eucalyptus tectifica (Darwin box), Eucalyptus tetrodonta (Darwin stringybark), Eucalyptus megasepala, Eucalyptus miniata, Eucalyptus phoenicea, Corymbia dichromophloia, Corymbia foelscheana, Corymbia latifolia, Corymbia flavescens, Corymbia polycarpa, Corymbia nesophila, Corymbia clarksoniana, Corymbia grandifolia, Corymbia bleeseri and Corymbia ferruginea (Brock 2001; Fox et al. 2001; Beard et al. 2013; Neldner et al. 2014).

  • Non eucalypt genera may be prominent in the tree canopy or subcanopy. Widespread species include Brachychiton diversifolius, Erythrophleum chlorostachys, Lysiphyllum cunninghamia, Callitris intratropica, Brachychiton diversifolia and various species of Acacia, Alphitonia, Livistona, Melaleuca, Petalostigma and Terminalia (Brock 2001; Beard et al. 2013; Neldner et al. 2014). Adansonia gregorii (boab) may be prominent in parts of the southern Kimberley and Victoria River region (Beard et al. 2013).

  • The understorey understorey includes scattered shrubs from any of the above genera, as well as species of Cochlospermum, Grevillea, Persoonia, Calytrix, Planchonia, Jacksonia and Buchanania obovata.(Brock 2001; Beard et al. 2013; Neldner et al. 2014).

  • Grass species dominate the ground layer and include species of Sorghum, Schizachyrium, Chrysopogon, Themeda, Heteropogon, Sehima, Astrebla, Dichanthium, Iseilema Aristida, Panicum, Scleria, Digitaria and Thaumastochloa. (Williams et al. 2002; Beard et al. 2013; Neldner et al. 2014). Many of these species have annual life cycles and all have C4 photosynthetic pathways (Williams et al. 2002). Triodia species may be present in localised stony sites but is rarely dominant over the tussock grasses.

  • Species composition varies considerably from east to west and with the rainfall gradient from north to south (Fox et al. 2001).

  • Potential subgroups differentiating lowland savannah eucalypt woodlands from rocky upland savannah eucalypt woodlands warrant further investigation. The latter includes hummock grasses (Triodia species) and more conspicuous sclerophyll shrub component with species of (Calytrix, Jacksonia, Acacia and Grevillea, lower abundance of mesic shrubs and palms, and less continuous layer of C4 tussock grasses.

Environment

  • Tropical climates with highly seasonal, monsoonal, summer rainfall with mean annual rainfall generally exceeds 600 mm (Russell-Smith et al. 2014).

  • Where mean annual rainfall exceeds about 900 mm, MVG 12 intergrades with more mesic forested savannas assigned to MVG 4. To the south, where mean annual rainfall is less than 900 mm MVG 12 is gradually replaced by more open semi-arid savannas (MVG 11), that lack many of the annual grasses and characteristics eucalypt species (Williams et al. 1996).

  • Along with water availability, nutrients and herbivory, fire regimes play a critical role in regulating the floristic composition, vegetation structure, function and dynamics of savanna systems (Russell-Smith et al. 2014). Vegetation responses vary with the timing of fires in early, mid or late dry season determines fire intensity due to increasing levels of grass curing and flammability, which increases as the dry season progresses.

  • Occurs on flat lowland plains, escarpments, slopes and plateaux of lateritic stony country and outwash at the base of slopes and plateaux (Williams et al. 2002).

  • Substrates vary from deep sandy alluvial and colluvial loams, basalt, residual sandstone and metasediments with shallow rudisols (Williams et al. 2002; Beard et al. 2013; Neldner et al. 2014).

Geography

  • Covers the majority of northern Australia, stretching from the Kimberley in Western Australia through the Top End of Northern Territory to Cape York and the north Queensland coast (Fox et al. 2001; Russell-Smith et al. 2014).

  • The major distribution is in the top end of the Northern Territory (79 839 km2) and the Kimberley region of Western Australia (32 642 km2).

Change

  • Approximately 2.6% of the estimated pre-1750 extent cleared accounting for 0.3% of total clearing in Australia. Clearing has accelerated in recent years with expansion of tropical agriculture, irrigated cropping and plantations.

  • Approximately 3 000 km2 cleared since European settlement.

  • Contemporary land uses and changed fire regimes are having significant regional impacts on the biodiversity of Australian savannas (Woinarski et al. 2011; Russell-Smith et al. 2012).

  • Invasive plants, such as Andropogon gayanus (Gamba grass) are excluding native species and changing savanna fire regimes.

  • Introduction of large grazing animals has led to profound changes in vegetation structure, abundance and distribution with flow on effects to native fauna (Kutt and Gordon 2012).

  • Increased frequency of intense late-season fires is modifying woodland structure and composition (Russell-Smith et al. 2014).

  • Invasive feral cats and cane toads are implicated in recent declines of native vertebrate fauna through trophic interactions (Woinarski et al, 2011).

  • Threats include fragmentation, weed infestation, inappropriate fire regimes (e.g. fires too regular and/or too intense) and over-grazing.

  • There are issues associated with understanding and managing these areas for multiple values. Part of this is providing support to Indigenous groups and developing among these groups an understanding of methods of feral animal and weed control.

Tenure

Tropical Eucalypt Woodlands/Grasslands occur largely on leasehold land.



Northern Territory:

leasehold land, freehold land, protected areas and some other crown land

Queensland:

leasehold land, protected areas, freehold land, some state forests

Western Australia:

leasehold land, reserved crown land, other crown land, protected areas, some freehold land

Key values

  • Biodiversity including a unique mixture of biota with arid and wet tropical origins.

  • Globally one of the most extensive continuous tracts of woodland and the only savanna dominated by eucalypts.

  • Product of recent global evolutionary processes associated with the spread of C4 grasses.

  • Critical habitat for populations of a wide range of tropical vertebrate and invertebrate species.

  • Globally significant, growing carbon pool.

  • Ecotourism and scenic landscapes.

  • Beef cattle production.

List of key management issues

  • Total grazing pressure management.

  • Feral animal impacts e.g. cats, and cane toads, and associated decline in mammal populations (Russell-Smith 2014).

  • Fire management particularly mitigating fire regimes with a high frequency of severe fire brought about by the fuel loads of introduced grasses and ignitions associated with grazing land management practices.

  • Control of invasive grasses and other weeds.

  • Expansion and intensification of mining and agriculture.

  • 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 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

Brock J (2001) Native Plants of Northern Australia pp. 21 – 23. Reed New Holland, Sydney

Brooker M.I.H. and Kleinig D.A. (1994) Field guide to Eucalypts, Volume 3, Northern Australia. Inkata Press, Sydney, 383pp.

Dyer R., Jacklyn P., Partridge I., Russell-Smith J., Williams R.J. (eds) (2001) Savanna Burning: Understanding and using Fire in northern Australia. Tropical Savanna CRC, Darwin.

Fox I.D., Neldner V.J., Wilson G.W., et al. (2001) The Vegetation of the Australian Tropical Savannas. Env. Prot. Agency, Qld and Tropical Savannas CRC, 2 map sheets and 1 legend; online at http://savanna.ntu.edu.au/information/.

Kutt A.S., Gordon I.J. (2012) Variation in terrestrial mammal abundance on pastoral and conservation land tenures in north-eastern Australian tropical savannas Animal Conservation 15, 416 – 425.

Lehmann C.E.R., Archibald S.A., Hoffmann W.A., Bond W.J. (2001). Deciphering the distribution of the savannah biome. New Phytologist 191, 197 – 209.

Murphy B.P., Bowman D.M.S.J. (2012) What controls the distribution of tropical forest and savannah? Ecology Letters 15, 748 - 758

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.

Russell-Smith J., Edwards A.C., Price O.F. (2012). Simplifying the savannah: the trajectory of fire-sensitive vegetation mosaics in northern Australia. Journal of Biogeography 39, 1303 – 1317.

Russell-Smith J., Edwards A., Woinarski J., Fisher A., Murphy B., Lawes M., Crase B. and Thurgate N. (2014) Northern Australian tropical savannas: the Three Parks Savanna Fire Effects Plot Network. In. Biodiversity and Environmental Change Monitoring, Challenges and Direction (ed. Lindenmayer D, Burns E, Thurgate N and Lowe A.) pp. 335 - 378. CSIRO, Victoria.

Williams R.J., Duff G.A., Bowman D.M.S.J., Cook G.D. (1996) Variation in composition and structure of tropical savannas as a function of rainfall and soil texture along a large-scale climatic gradient in the Northern Territory, Australia. Journal of Biogeography 23, 747 – 756.

Williams R.J., Griffiths A.D., Allan G.E. (2002). Fire regimes and biodiversity in the wet-dry tropical landscapes of northern Australia. In Flammable Australia. The fire regimes and biodiversity of a continent. (ed Bradstock R.A., Williams J.E and Gill A.M.) pp. 281 - 304. Cambridge University Pres, Cambridge.

Wilson B.A., Brocklehurst P.S., Clarke M.J., Dickinson K.J.M. (1990) Vegetation Survey of the Northern Territory, Australia. Conservation Commission of the Northern Territory, Technical Report no. 49, Darwin.

Woinarski J.C.Z., Fitzsimons J.A., Traill B.J., Burbidge A.A., Fisher A. (2011) The disappearing mammal fauna of Australia: context, cause and response. Conservation Letters 4, 192 – 201.

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

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



Tropical eucalypt woodlands on rocky sandstone uplands, Kakadu National Park, NT (Photo: B. Pellow).



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