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Vegetation

Interpreting the data on Australian vegetation requires an understanding of the difference between native and non-native vegetation and between woody and non-woody vegetation.

For the purposes of this commentary, native vegetation  is broadly defined as plant life present in Australia prior to European settlement. Non-native is plant life, such as cereal crops, many garden plants and introduced pasture species that have been introduced since European settlement. It is generally accepted that native vegetation is more likely to provide a better habitat for native fauna and flora, and that native species are likely to be better adapted to local conditions.

The status of native vegetation is a function of:

• initial vegetation
• vegetation loss
• vegetation gain from growth and thickening of existing plants, from unassisted growth of new plants and from plantings.

(Vegetation loss can be a result of fire, flood, drought, natural competition and native grazing, from clearing—broadscale, selective or thinning—or harvesting, from ground or nutrient disturbance including erosion, harvesting of biomass, and salinity, for example as a result of grazing pressure, from other ecosystem disturbance—such as loss of predatory or pollinating insects or invasion by introduced species—and, potentially, from poisoning as a result of pollution.)

The Australian Terrestrial Biodiversity Assessment, carried out in 2002 as part of the National Land and Water Resources Audit, assessed the condition of ecosystems and species in bioregions across Australia. Each state and territory was asked to identify threatened ecosystems, which were described as ‘integrated units of vegetation including the dominant species in the principal stratum and structural formation, geology or soils and landform’ (NLWRA 2002). The results show more than a third of Australian bioregions (39 per cent) have more than 30 per cent of their ecosystems threatened. Of these, the most dominant types of threatened ecosystems are eucalypt forests and woodlands. Vegetation data for SoE2006 are primarily derived from remote sensing of woody vegetation . Woody vegetation is defined (in the National Carbon Accounting System dataset) as vegetation, native or non-native that can grow at least two metres high and has 20 per cent canopy cover. Non-woody vegetation includes most introduced annual crops, as well as both native and introduced grasses, low shrubs and heath vegetation.

Despite woody vegetation potentially being a mix of native and introduced species it does play an important role. Woody vegetation generally provides better habitat and it is perennial, which also means it provides this habitat for the entire year. It is also high in biomass, providing good greenhouse sink value, and deep-rooted, with benefits for the maintenance of surface water absorption and groundwater levels.

The extent of woody vegetative cover in the Intensive Landuse Zone (ILZ) is still decreasing. In 2002 the extent of woody vegetation in the ILZ in Queensland was approximately 36 million hectares. This is almost half of the woody (perennial) vegetation cover in Australia. This was significantly reduced from the 1995 level of almost 38.5 million hectares (NLWRA 2001a).

The impacts of all modes of native vegetation loss are both immediate and long term. The impacts of high intensity pressures, such as broadscale clearing , are more visible and more easily quantified than are the impacts of less intensive pressures such as grazing of livestock, intentional clearing of dead wood by fire and firewood collection, increasing soil acidity, changed fire regimes and selective timber removal. Grazing pressure , for instance, is known to select against delicate palatable native species, whilst changing fire regimes can result in less frequent, hotter burning bushfires that favour quickly establishing invasive weed species. Such negative impacts could be further exacerbated if palatability declines and woody species growth increases as an outcome of greenhouse gas increases .

Regardless of the intensity of the various pressures, Australian native vegetation species and communities continue to decline in their distribution and diversity and, in many cases, introduced vegetation has extensively replaced native vegetation; for instance, temperate pasture and cropping species in southern Australia and Buffel Grass (Genus spp.) in the Queensland brigalow woodlands.

Whatever the value of native vegetation versus that of non-native vegetation, the reality is that non-native vegetation is now an integral part of many Australian ecosystems. Given this mix of native and non-native components, vegetation is increasingly likely to be measured and assigned values according to the objectives of vegetation management, for example greenhouse gas abatement, protection of endangered species, protection of biodiversity, protection of surface water catchments for human water supply, protection of groundwater to prevent salinity, protection of soil from water or wind erosion, exclusion of non-native species, or any combination of these. Good examples of this are the use of more productive non-native species for the production of food and fibre and the use of particular introduced species to colonise bare burnt ground precisely because they recolonise more quickly than native species, thereby protecting and restoring the soil to the point where native plants can also recolonise.

Remote sensing data do not distinguish between native and non-native vegetation or provide a measure of the condition of the vegetation in biodiversity terms. Consequently, as the criteria used to assess the status of vegetation may vary considerably according to the objectives of vegetation management, it is necessary to interpret remote sensing data together with data on species composition, landuse and land management practices at lower spatial scales. In fact, given the increased focus on catchment level planning and monitoring it may well be timely to consider intergovernmental, catchment-level state of the environment reporting.

Broadscale clearing of native trees is the most visible and controversial aspect of ecosystem modification and hence it attracts considerable attention; but the potential impact of broadscale and selective tree clearing is not evenly distributed between ecosystems. In regions where past tree clearing has dramatically reduced the level of native vegetation, particularly in southern Australia, the most critical factor influencing future vegetation status will be the management of remnant trees to allow for and even encourage regeneration of more sustainable lots of woody vegetation.

There are many drivers of broadscale tree clearing and or the prevention of regeneration of woody vegetation including financial, aesthetic and social factors and their relative importance varies between places and over time.

Broad scale clearing of woody vegetation is prevented principally by legislation, whereas in areas already largely cleared, regeneration of woody vegetation is supported by financial incentives of varying forms. These different policy instruments have markedly different impacts on landholders, particularly as there is a clear financial benefit in many areas from the clearing of woody vegetation. A 2002 report by CSIRO found that restricting access to cleared riparian areas on four farms in Queensland reduced net profit by between 29 and 77 per cent across the four farms (Pyper 2002).

A prime driver of the destruction of many communities of native species, particularly communities of non-woody native species and of measures to prevent regeneration of these communities, is the belief, rightly or wrongly, that native species are less agriculturally productive than introduced species. At the margin this belief is being increasingly questioned as more effort is being directed towards designing agricultural systems that more closely mimic natural systems. For instance, native eucalyptus species now dominate forestry plantings  and there is an increasing interest in mixed species plantations which may provide better habitat than monocultures of, for instance, Eucalyptus globulus (DAFF 2001).

This new way of looking at the value of native vegetation will assist in the prevention of clearing of woody vegetation and in the enabling of regeneration of woody vegetation. In fact, the amount of regrowth in the ILZ has been increasing (AGO 2004). Nevertheless, the political conditions and policies governing the status of woody vegetation are broadly unstable.

We conclude that further and more effective responses will be required to arrest the decline in the extent and diversity of Australian native vegetation species and communities.

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