Landscape Health In Australia
A rapid assessment of the relative condition of Australia's bioregions and subregions
Gethin Morgan
Environment Australia, 2000
ISBN 0 642 37119 9
2. Trend Attributes
Current rates of clearing of native vegetation
At the time of this assessment national maps of native vegetation clearing were only available for the period 1980 - 1995. They are the 1980 to 1990 Land Cover Change data (Graetz et al. 1995) and the 1990 - 1995 Agricultural Land Cover Change data set (Kitchin & Barson 1998). Clearing rates are readily available for all States and Territories up until 1995. Additional data is available for Tasmania until 1997 and for Queensland until the present.
This attribute is assessed only for the intensive use zone. Clearing rates were determined for each State where subregions extended across more than one jurisdiction.
- Victoria and South Australia have had only limited clearing since 1987.
- Broadscale clearing for agriculture in Western Australia decreased markedly during the 1990s and has now essentially ceased.
- Extensive clearing is now limited mainly to Queensland, New South Wales and Tasmania and parts of the Northern Territory.
| State | No. subregions >1000 ha/yr | No. subregions >10000 ha/yr | Total cleared ha/yr in intensive use zone | % total ha/yr cleared in intensive use zone |
|---|---|---|---|---|
| New South Wales | 3 | 0 | 19 483 | 5.5 |
| Queensland | 40 | 10 | 280 209 | 79.5 |
| South Australia | 0 | 0 | 285 | 0.1 |
| Tasmania | 1 | 0 | 4 345 | 1.2 |
| Victoria | 4 | 0 | 8 101 | 2.3 |
| Western Australia | 8 | 1 | 40 373 | 11.4 |
| Total cleared ha/pa | 325 997 | 192 072 | 352 798 |
Area of woody native vegetation cleared each year between 1990 and 1995
The Agricultural Land Cover Change (Kitchin & Barson 1998) study did not identify clearing in native vegetation with a projective foliage cover less than 20%, such as open woodlands and shrublands and hence underestimated clearing in most States (Figures 65, 66; Table 2).
- In New South Wales widespread clearing occurred in more open vegetation along the western parts of the wheat belt (e.g. between 1994 and 1996 there was an average of 11130ha cleared each year on the Moree 1:250000 map sheet alone [NSW NPWS 2000]).
- In Queensland the Agricultural Land Cover Change study recognised only 60% of the annual clearing identified by the subsequent Statewide Landcover and Trees Study (DNR 1999a), a difference of almost 111000ha/yr over the State.
- Approximately 2770 ha was permitted for clearing each year in South Australia over the 1990 - 95 period (DEH 2000), compared with 5238 ha assessed by the Agricultural Land Cover Change study.
- Between 1990 and 1995 broadscale clearing was occurring at a rate greater than 1000ha/yr in 56 (27%) of the subregions (by jurisdiction) within the intensive use zone. The most extensive clearing identified occurred in Queensland and Western Australia, with annual clearing exceeding 10 000 ha in 10 subregions in Queensland and one in Western Australia. In Queensland clearing was focused on subregions within the eastern part of the Great Artesian Basin (including subregions of the Brigalow Belt South and the Mulga Lands bioregions) and the Brigalow Belt North bioregion. These 56 subregions contributed 92% of the total annual clearing (352798 ha/yr) in the intensive use zone between 1990 and 1995. The 11 subregions with annual clearing exceeding 10000ha contributed 54% of the total clearing.
* Queensland data is from the Statewide Landcover and Trees Study (DNR 1999a). This revised Queensland data covers the period 1991 to 1995.
Area of woody native vegetation cleared each year between 1995 and 1997
Mapped information on broadscale clearing between 1995 and 1997 is only readily available for Queensland (DNR 1999b) and Tasmania (Kirkpatrick pers. comm.) (Figures 67, 68). Queensland Wet Tropics data on clearing is for the bioregion as a whole, and is not readily available by subregion.
New South Wales has data on clearing of vegetation with a projective foliage cover (density of tree crowns or what satellites can see/distinguish readily) greater than 20% for this period but the data was not available for this project. An estimate is possible for South Australia based on the area given under permit, but similar data was not readily available for Western Australia and Victoria. In South Australia an average of 1310ha clearing each year was permitted across the intensive use zone for this period (DEH 2000). Clearing in Western Australia, Victoria and the Northern Territory appears to have been of a similar magnitude. In New South Wales between 1996 and 1998 an average of 6280ha was cleared each year on the Moree 1:250000 map sheet alone (NSW NPWS 2000).
- In Queensland, an average of 339662ha was cleared each year between 1995 and 1997.
- In Tasmania, clearing averaged 78316ha each year (Figure 72).
- Broadacre clearing continued in almost all of the 83 subregions for which data is available within the intensive use zone of Queensland and Tasmania.
- Annual clearing rates exceeded 1000ha/yr in 56 subregions, and exceeded 10000ha in 14 subregions, four of which are in Tasmania.
- Between the 1990 - 95 and 1995 - 97 periods, average annual clearing rates increased in 50 of the 84 subregions for which data is available.
As with the 1990 to 1995 period, clearing in Queensland was mostly in subregions of the Great Artesian Basin (including subregions of the Brigalow Belt South, Mulga Lands and Desert Uplands bioregions), and the Brigalow Belt North. In Tasmania clearing was most extensive in the South East, and in the Northern Midlands bioregions.
Area of woody native vegetation cleared each year (1997 - 1999)
Broadacre clearing had largely ceased in most jurisdictions, except for Queensland, New South Wales, Tasmania and small areas in the Northern Territory between 1997 and 1999. In South Australia an average of 613 ha/yr was permitted to be cleared across the intensive use zone for this period (DEH 2000) and a similar order of magnitude would apply to Victoria and Western Australia. Mapped information on clearing for this period is largely limited to Queensland (DNR 2000) (Figures 69, 70).
- Broadacre clearing continued in 73 of 74 Queensland subregions for which data is available.
- Annual clearing rates exceeded 1000ha each year in 54 subregions, and exceeded 10000ha in 14 subregions.
- 445683ha was cleared on average annually in Queensland between 1997 and 1999, an increase of 106021ha (31%) annually on the 1995 to 1997 period.
- Clearing rates increased in 45 of the 74 Queensland subregions.
As with the 1990 - 95 and 1995 - 97 periods, this clearing was mostly in subregions of the Great Artesian Basin and the Brigalow Belt North.
Change in annual rate of clearing between 1995-97 and 1997-99
The change in annual rate of clearing during this period can only be determined for Queensland, where regular and consistent mapping of the extent of native vegetation is available from the Statewide Landcover and Trees Study (DNR 1999a, 1999b, 2000). This attribute was derived by comparing the average annual clearing rates of the two periods 1995-97 and 1997-99 (Figures 71, 72).
Clearing occurred in 70 of 73 subregions for which data is available. The rate of clearing was increasing in 40 of these, including almost all of the subregions in the Queensland part of the Murray - Darling basin, the southern subregions of the Brigalow Belt North bioregion, the Desert Uplands bioregion, and the acacia woodlands along the eastern margin of the Mitchell Grass Downs bioregion. Clearing was also increasing in the Cape River Hills and Townsville Coastal Plains subregions in the far north of the Brigalow Belt North bioregion.
Trends in dryland salinity
Predicted area of subregion affected by dryland salinity in 2050
The national assessment of dryland salinity extent compiled by the Audit (see extent of dryland salinity risk or hazard p. 21) produced predictions of the extent of high dryland salinity risk or hazard for 2050. This coverage was intersected by subregions to examine the implications of 2050 predictions for specific subregional landscapes. (Figures 73, 74, 75, 76, 77, 78, 79, 80).
- 32 subregions (18%) are expected to have a high risk or hazard of dryland salinity over more than 10% of their area by 2050. Ten subregions (5%) are currently in that condition.
- 13 subregions (7%) are expected to have a high risk or hazard of dryland salinity over more than 30% of their area by 2050. One subregion (0.5%) is currently in that condition.
The major part of this predicted increase in extent will be in south-west Western Australia. where
- eight of the 13 Western Australian subregions in the intensive use zone are predicted to have greater than 30% of their area affected by a high risk of dryland salinity, and another three will have greater than 10% affected.
- Recherche will be the worst affected subregion, which is predicted to have a high risk of dryland salinity over 67% of its area by 2050.
- Four subregions in south-west Western Australia will have a high risk of dryland salinity over more than 40% of their area. These are the Dandarragan Plateau, the northern and southern subregions of the Jarrah Forest bioregion, and the Perth subregion.
By 2050 almost 30% of the total area of the intensive use zone in Western Australia is predicted to be at high risk of dryland salinity.
Victoria is predicted to be the State next worst affected, with five of its subregions expected to have a high risk of dryland salinity over more than 30% of their area.
- The Dundas Tablelands and the Otway Plain will be the most extensively affected, with 66% and 40% respectively of their areas predicted to be at high risk of dryland salinity by 2050.
- By 2050 almost 14% of the total area of Victoria will be affected by a high risk of dryland salinity.
Other subregions predicted to have a high risk of dryland salinity will be all those of the Naracoorte Coastal Plain bioregion near the mouth of the Murray River, and the Upper Slopes subregion of the New South Wales South Western Slopes bioregion.
Predicted area of remnant vegetation affected by dryland salinity in 2050
The predicted extent of dryland salinity risk or hazard can also be used with the current extent of native vegetation to predict the extent of native vegetation likely to be affected by increasing dryland soil salinity. Analysis for this attribute assumes there will be no significant changes in land use or in the extent of native vegetation between now and 2050.
It is predicted that by 2050:
- twenty-two subregions will have more than 10% of their native vegetation threatened by high dryland salinity risk, compared with nine at present;
- half of the 22 are in south-west Western Australia, while South Australia and Victoria both have four, and three are in New South Wales;
- eight subregions will have greater than 30% of their remaining native vegetation affected by a high risk of dryland salinity, six of which are in Western Australia. The other two are the Lucindale and Tintinara subregions of the Naracoorte Coastal Plain bioregion near the mouth of the Murray River.
The greatest areas of native vegetation at risk from high dryland salinity by 2050 are in south-west Western Australia.
- Over 22% of the total remaining native vegetation in the intensive use zone in Western Australia is likely to be affected by a high risk of dryland salinity by 2050.
- The Perth subregion is predicted to have the greatest proportion of native vegetation affected by a high risk of dryland salinity by 2050, with 47% of its remaining native vegetation affected.
- The southern subregion of the Avon Wheatbelt is the next most threatened, with almost 42% of its native vegetation threatened by high dryland salinity risk by 2050.
Salinity trends in subregions and remnant vegetation
The trend in high dryland salinity risk or hazard between 2000 and 2050 for subregions as a whole, and for the remaining native vegetation, is similar.
- High dryland salinity risk or hazard is expected to increase in extent in 160 (88%) subregions in the intensive use zone, and the extent of native vegetation affected is also expected to increase in 159 subregions (87%).
- Dryland salinity will remain constant in remaining 22 subregions (12%), with no subregions expected to show decreasing dryland salinity.
Inappropriate fire regimes
Fire is clearly an issue of national significance for biodiversity. Analysis is required at the scale of individual tenures, ecosystems and species. Recent studies in northern Australia indicate the urgent need for this analysis (Russel-Smith 2001).
Perceptions of change in landscape health due to altered fire regimes differ greatly between experts. Species-level information is required as an indicator, but is lacking for much of Australia. Expert assessment alone was considered to be too variable for spatial representation. Some general observations about the potential effects of fire are provided to give insight into the major issues.
Northern Australia
In the dry and wet/dry tropics, issues largely relate to:
- intensity and frequency of fires; and
- the area that single fires can cover.
Many fires occur in the late dry season and are consequently very hot, killing young perennials. In better watered areas, grass density increases at the expense of woody and fire sensitive speciescompounded by fires occurring annually or biennially. In drier or sandier areas, a net loss of organic matter results in an associated decline in ecosystem productivity. Where ecosystems are steadily invaded by introduced pasture species (e.g. buffel grass) a spiralling loss of biodiversity due to increasing fire intensities and species competition can occur.
In the more heavily grazed parts of the tropics (e.g. the northern part of the intensive use zone in Queensland and in the arid pastoral zone of Western Australia) climatic variation, the absence of sufficient ground cover to carry a fire and a move away from using fire in land management is enabling extensive regeneration of woody species. The denser shrub and lower tree stories further reduce the amount of grasses, compounding the degradation. In parts of the intensive use zone in Queensland the increasing density of woody species is a significant factor stimulating tree clearing.
Southern Australia
In the main cropping and grazing areas, fire is rarely used intentionally as a landscape management tool. Fire dependent species, and ecosystem health in general, are being adversely affected. Where the native vegetation remains as small and isolated remnants, fire cannot readily be used as a management tool due to weed invasion and potential effects on small and often stressed populations of plants and animals. Where fire is intentionally used for management (most commonly in conservation reserves and forestry reserves) there is often disagreement over appropriate regimes. In many cases a risk reduction objective for fire management requires repetitive and frequent cool burns, although some ecosystems (e.g. heaths) reach maximum biodiversity after at least a decade without fire. In some ecosystems periodic crown fire is desirable (e.g. to control mistletoe and facilitate the creation of tree hollows through branch dieback). Responses of individual species vary greatly too (e.g. some plant species depend on seedling production to maintain populations; where fires occur at rates more frequent than the period they require to reach maturity, these plants face a high probability of being loss from the community).
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