DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
3.5 SUMMARY AND PROPOSED FURTHER WORK
3.5.1 Summary
The vegetation of the DRP has been described in a variety of ways and at several levels of
detail. There have been some attempts to describe the pre-European vegetation at both a
landscape level and for smaller portions of the bioregion. However, detailed survey and
mapping has been very limited. Broadscale mapping has mostly been from aerial photo
analysis, which is limited to a crude structural analysis and naming of the dominant tree
species, but this has not been extensively ground truthed. There has also been variation in the
scale of vegetation mapping between the east and the western portions of the bioregion with
smaller quadrat sizes being used in the east.
In broad terms the original vegetation of the DRP is thought to have been open woodland on
alluvial fans and riverine corridors. The height and density of the overstorey decreased from
east to west and the grassy ground cover was dominated by Mitchell grasses (Astrebla
species). Since European settlement the proportion of grassland has increased and is
dominated by introduced species. Description of the pre-European vegetation is of most value
in determining the rate and extent of change that may have occurred as a result of European
settlement of the bioregion. The 1750 mapping by Carnahan (1976) indicates that the lower
reaches of the Darling floodplains within the bioregion were Astrebla grasslands. This region
then suffered from a period of extreme conditions as a result of drought in association with
rabbit infestation and sheep grazing resulting in almost total denudation of the landscape.
This area has now recovered and is dominated by black bluebush communities. Carnahan’s
mapping indicates that there has been a change in the dominant plant community after the
recovery of the landscape or that the area is at a seral stage in the re-establishment of the
vegetation. Any attempts to map pre-European vegetation based on existing remnants should
consider these possibilities and ensure that detailed analyses of historical documents are also
incorporated.
The existing broad vegetation communities within the DRP have been defined by several
authors. These descriptions show a high degree of consistency and fifteen communities can
be described. These include seven forest/woodland communities, tall Acacia shrublands,
saltbush shrublands, grasslands with varying dominant genera (Astrebla, Triodia or
Austrostipa) and wetland, swamp and marsh communities. While these community
definitions are not based on systematic floristic surveys and could be considered subjective,
their consistency indicates that they provide a sound broad definition of the plant
communities of the bioregion. The fundamental importance of riverine and alluvial landforms
is clearly demonstrated by the dominance of water tolerant communities.
Variation within these broad plant communities has not been mapped across the bioregion but
the mapping of land units by Walker (1991) in the Western Region and the relationship
between topographic variation and vegetation associations indicated by Morgan and Terrey
(1990) for all provinces provide an indication and could be analysed further. The more
detailed vegetation descriptions for small areas within the bioregion could be used to verify
these descriptions.
Of the plant communities occurring within the DRP, the Corymbia tessellaris open forest,
Acacia loderi shrublands, and the Artesian Springs community are listed under the TSC Act.
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