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Native vegetation is a key element contributing to Australia's biodiversity. Across Australia, 23 major native vegetation groups have been identified, which collectively comprise 'tens of thousands of plant species, thousands of vegetation communities and assemblages, and provide habitat to myriads of microorganisms and animal species' (NLWRA 2002b). In general, the extent and distribution of native vegetation across Australia is determined by climatic variation and the physical landscape (i.e. landform, geology and soils). Rainforests and eucalyptus forests are limited to the higher rainfall areas across the tropical north, around eastern and south-western coastal regions, and across Tasmania. Australia's arid interior is dominated by grasslands and forblands. Regions between these climatic extremes are occupied primarily by woodlands, shrublands and Acacia forests.
Clearing of native vegetation
Since European settlement, large tracts of Australia's native vegetation have been cleared to facilitate human settlement and the expansion of agriculture. Extensive broadscale clearing continues to take place. Clearance of vegetation reduces the natural range of ecosystems as well as the diversity of habitats and ecological processes occurring within them. Consequently, native vegetation clearance has been identified as one of the most threatening processes for biodiversity loss and species extinction in Australia (SoE 2001a). Broadscale vegetation clearance has other important implications for the state of the environment through its effect on dryland salinity, carbon cycling and changes in hydrological cycles.
According to the Australian Native Vegetation Assessment 2001, approximately 982,000 square kilometres, or 13% of Australia's native vegetation, has been cleared or substantially modified since European settlement (table 14.18). Clearing has been concentrated in the higher rainfall areas and where there are more fertile soils, generally excluding the arid interior and the tropical far north. In the intensively used areas of Australia (primarily the agricultural and urban zones), about 33% of native vegetation has been cleared (NLWRA 2002b).
The extent of clearing of major vegetation groups since European settlement is shown in table 14.17. The most affected groups include: 'eucalypt woodlands' and 'eucalypt open woodlands', where 31% and 25% of pre-European extent has been cleared, accounting for 32% and 13% respectively of all clearing; and 'inland acacia forests' and 'woodlands, and mallee woodlands and shrublands', where approximately 15% and 35% of pre-European extent has been cleared, accounting for 10% and 14% respectively of all clearing. The extensive clearing of low closed forests, rainforest and heath communities is particularly important given that they were already highly restricted in their natural, pre-European distribution.
Recent estimates of annual native vegetation clearing rates in Australia vary markedly and are highly uncertain (table 14.19). Nonetheless, they are indicative of a relatively high rate of clearance. The extent of land clearance in the intensively used regions of Australia (38% of the continent) from 1990 to 1995 was estimated at 1.2 million hectares (Barson, Randall & Boardas 2000). The most recent estimates of annual native vegetation clearing in Australia include 468,844 hectares for 1999 (AGO 2001b) and 564,000 for 2000 (ACF 2001). The latter figure is exceeded by only four other countries: Brazil, Indonesia, the Democratic Republic of Congo and Bolivia (ACF 2001, cited in Hamblin 2001). Most of Australia's recent vegetation clearance has been conducted in Queensland.
Greenhouse gas emissions from land clearing
Land clearing makes a significant contribution to Australia's greenhouse gas emissions and consequently has important implications for global climate change, global warming and associated policy mechanisms. Where vegetation is cleared for a different land use, the cleared vegetation is usually burned, leading to emissions of carbon dioxide (CO2) and other greenhouse gases into the atmosphere. CO2 is also released from the soil and from decay of unburned aboveground biomass. Although significant quantities of CO2 are sequestered from the atmosphere during vegetation regrowth, land clearing is a net emitter of CO2 in Australia (AGO 2001b).
Australia provides greenhouse gas emission estimates to the United Nations Framework Convention on Climate Change (UNFCCC), including land use change, which is defined as the deliberate removal of forest cover by humans and replacement of it with pasture, crops, urban development or other land uses. Emissions from the clearing of other vegetation types (such as grasslands and shrublands) are excluded from the analysis of land use change emissions, unless it was reclearing (AGO 2002a).
The National Carbon Accounting System (NCAS) assists in monitoring land use change and its impact on Australia’s emissions. According to NCAS, emissions from land use change were estimated to be 61 megatonnes (Mt) CO2 in 2000, contributing 11% of Australia’s total emissions in that year (AGO 2002b). However, all estimates of emissions from land clearing are subject currently to high degrees of uncertainty and are likely to change in the future.