Condition
Vegetation condition is effectively a subjective assessment of the health of an ecosystem, and so takes into account a suite of factors operating at different spatial and temporal scales. The most important factors are the extent of a community relative to its former extent, the extent of invasion by invasive species, and the degree of fragmentation within that remnant extent. Climatic impacts, such as the effects of drought, flood and wind storms, have been affecting vegetation for millennia, but these effects can be magnified if communities are limited in extent or highly fragmented. Much of Australia’s remaining forest, shrubland, grassland and open woodland ecosystems are now degraded or fragmented (Tulloch et al. 2015, Evans 2016). A result of fragmentation is that smaller patches of habitat are now a common feature in many landscapes and represent an increasingly large component of remaining habitat for many ecosystems (Tulloch et al. 2015). Approximately 22 per cent of major vegetation communities in Australia have more than 50 per cent of their remaining extent in patches less than 1000 hectares. The contribution of patches less than 5000 hectares has increased in almost all areas of Australia, and significantly so along the east coast and in the south-west (Figure LAN6b). Other anthropogenic impacts, such as weeds, feral animal grazing and altered fire regimes, decrease vegetation condition.
Historically, vegetation condition has been assessed at a range of scales and using a variety of approaches. Progress has recently been made towards developing a nationally consistent approach to assessing vegetation condition; however, national-level results from this work are not yet available. In the interim, related parameters that provide insights into native vegetation condition at a continental scale are:
- the degree of fragmentation of native vegetation (Figure LAN6)
- annual and seasonal variation in green vegetation cover (mean annual greenness fraction—the fraction of land surface covered by photosynthesising green vegetation, which reflects variation in net primary productivity as a proxy for vegetation condition and indicates risk of erosion; see Figure LAN22)
- the degree of vegetation modification, as assessed under the ‘vegetation assets, states and transitions’ (VAST) framework developed by the Bureau of Rural Sciences.
The degree of modification of Australia’s native vegetation across Australia’s land area as assessed by VAST is illustrated in Figure LAN29. This classification is provided by continental-scale remotely sensed data, and is most useful for broad regional assessments rather than fine detail.
Again, the continental pattern of vegetation modification reflects Australia’s history of European settlement, land clearing and agricultural land uses, and—perhaps less obviously—the legacy of 50,000 years of Indigenous land management practices. The greatest extent of least-modified vegetation is in the north and centre of the continent, along the eastern and south-western ranges of mainland Australia, and in the eastern ranges and south-west of Tasmania. In these zones, an average of 80 per cent (range 70–96 per cent) of vegetation is classified as VAST category I or II (residual or modified; for definitions, see Table LAN6). Conversely, the greatest extent of most-modified or replaced vegetation is in the intensive-use zones of the eastern and southern mainland, and in the midlands and north of Tasmania. In these zones, an average of only 40 per cent (range 15–69 per cent) of vegetation is classified as VAST category I or II.
Figure LAN30 illustrates the extent of modification of each of the major vegetation groups, as assessed by VAST.
Table LAN6 Vegetation assets, states and transitions (VAST) classification framework
Increasing modification →
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Native vegetation cover Dominant plant species indigenous to the locality and spontaneous in occurrence (i.e. a vegetation community described using definitive vegetation types relative to estimated pre-1750 types)
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Non-native vegetation cover Dominant structuring plant species indigenous to the locality but cultivated, alien to the locality and cultivated, or alien to the locality and spontaneous
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Vegetation cover class
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Class 0: residual bare
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Class I: residual
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Class II: modified
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Class III: transformed
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Class IV: replaced—adventive
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Class V: replaced—managed
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Class VI: removed
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Criteria
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Areas where native vegetation does not naturally persist
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Native vegetation community structure, composition and regenerative capacity intact—no significant perturbation from land use or land management practice. Class I forms the benchmark for classes II to VI
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Native vegetation community structure, composition and regenerative capacity intact—perturbed by land use or land management practice
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Native vegetation community structure, composition and regenerative capacity significantly altered by land use or land management practice
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Native vegetation replaced with species alien to the locality and spontaneous in occurrence
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Native vegetation replaced with cultivated vegetation
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Vegetation removed
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Source: Thackway & Leslie (2008)