An overview of recent native vegetation clearance in Australia and its implications for biodiversity
Biodiversity Series, Paper No. 6
Andreas Glanznig, Biodiversity Unit
Department of the Environment, Sport and Territories, June 1995
Impact of native vegetation clearance on ecological processes
A major impact of inappropriate native vegetation clearance on ecological processes is the alteration of water cycles and the resultant rise in water tables and salinity problems. While the effects of altered ecological processes on biodiversity may not be immediately apparent, recent research has highlighted the serious implications of current and future impacts.
In a recent examination of the consequences of a changing hydrologic environment for native vegetation in south western Australia, George et al (forthcoming) calculated that as a result of native vegetation clearance and replacement by crop species which use less water, watertables are rising beneath cleared lands and remnant vegetation at rates of between 0.1 and 1.0 m/yr. They conclude that most of the remnants that have survived to date will become severely degraded within a generation and that nearly all the remaining remnants will be permanently modified (see example below). Numerous significant wetlands, for example Lake Toolibin, are also being affected.
The effects of recent clearance on Capercup Nature Reserve
The Capercup Nature Reserve is a Eucalyptus wandoo woodland located 5 km west of Duranillin, about 200km south-south-east of Perth. Recent drilling has shown the reserve has very saline groundwater (2000 to 6000 mS/m) within 2 m of the surface on the western side of the reserve. Groundwater is almost as close, but much fresher (300 mS/m) on the eastern side.
Most of the catchment adjoining the reserve has been cleared within the last 15 to 30 years and as a result salinity has just begun to develop on farmland immediately upslope from the reserve. With rates of watertable rise of between 0.3 and 0.5m per year recorded in the area, it is likely the western side of the reserve will begin dying in the next two or three years and between 40 and 50 per cent of the reserve will probably be affected within the next decade.
Source: George et al (forthcoming)
The productivity of land used for agricultural purposes has been also significantly affected by inappropriate native vegetation clearance. The situation in the Murray-Darling Basin illustrates some of the linkages between native vegetation clearance and land degradation and is discussed in the box below.
Broadscale native vegetation clearance and land degradation: the case of the Murray-Darling Basin
As part of the agricultural development of the Murray-Darling Basin, about 500 000 km2 of native vegetation has been cleared (Murray-Darling Basin Ministerial Council 1987, p.203). Subsequent analysis equated this loss to approximately 12 to 15 billion trees (Walker et al 1993). A study of dryland salinity in the Murray-Darling Basin by the Dryland Salinity Management Working Group:
established that changed land use, particularly the broadscale clearing of native vegetation and its replacement with systems which use less water, is the principal cause of secondary dryland salinity (DSMWG 1993, pp.11-23,24).
The rates of encroachment of salinity affected land vary. It has been estimated that the rate varies between two and five per cent in Victoria depending on site conditions. In New South Wales the Working Group considers that the current experience indicates that a rate in the order of 10 to 15 per cent seems appropriate but in the longer term it is anticipated that the rate will probably decline to five to seven per cent.
These encroachment rates, however, may not necessarily give an accurate measure of future costs. The Working Group provides the Liverpool Plains as a cogent example:
The current area of severe salting (total production loss) is about 500 ha, with a rate of change of 15 per cent. However, about 30 000 ha of highly productive land has saline groundwater within 1.5 m of the soil surface. It is estimated that if nothing were done to manage the situation, the entire area could suffer total production losses within 10 years (DSMWG 1993, p.49).
The economic impacts of land degradation are significant. One study provides estimates of the yearly production losses in Australia resulting from various land degradation problems: $180m for waterlogging, $80m for erosion, $300m for soil acidity and $200m for both loss of soil structure and salinity (Price 1993).
Forms of land degradation, such as dryland salinity, often impose costs on other landholders and the community. These can be very significant. A study of salinity problems in the Kyeamba Valley estimated that 96 per cent of the costs are borne by non-farmers through flow-on effects such as road damage and deterioration of downstream water quality (Wilson 1993). Greening Australia reports that the Young and Cootamundra councils in New South Wales are spending half their road maintenance budgets on repairing road surfaces damaged by rising saline water tables, and that maintenance costs are increasing (Greening Australia 1995, p.10).
Native vegetation clearance can also affect regional rainfall patterns. Recent research indicates that regional rainfall and atmospheric energy patterns have been changing in certain areas which have been extensively cleared. In a major review of rainfall patterns and atmospheric conditions in south-western Australia between 1946 and 1988, Smith et al (1992) concluded that the meteorological records indicate that annual rainfalls were being reduced by 1.5 mm/yr and that the mean summer (surface) temperatures had increased by one to five degrees centigrade. George et al (forthcoming) believe that:
the present 14 per cent reduction in mean annual rainfall is likely to have a significant impact on native flora and fauna, and will compound the problems faced by our decaying remnant ecosystems.
Of particular interest is that, while annual rainfall decreased from 1947 to 1988 over the agricultural area of south-western Australia, meteorological records indicate it increased by a similar amount over uncleared native vegetation further to the east (Williams 1991). A later study observed that convective clouds may form earlier in the day over extensive areas of native vegetation than over similar land that has been cleared for agriculture (Smith 1994, pp.61-62).
Native vegetation clearance is a major contributor to greenhouse gas emissions. The net total of greenhouse gases emitted in 1990 was the equivalent of 572 million tonnes of carbon dioxide. Of this total, the National Greenhouse Gas Inventory provides estimates that during 1990 carbon dioxide emissions from forest clearing for agriculture totalled 156 million tonnes, which is some 27.3 per cent of Australia's net emissions in carbon dioxide equivalent. This estimate is subject to the caveat that there is much uncertainty associated with emissions from native vegetation clearance due to the lack both of statistics on land clearing and of data on the carbon content of the vegetation cleared and of the soils involved (NGGIC 1994, pp.3,8,16,129).