DARLING RIVERINE PLAINS BIOREGION Background Report

16/08/02 Darling Riverine Plains Bioregion Background Report
soils of swampy stream channels, occurs only in the extreme south-west of this province.
About 40% of this ecosystem has been cleared and the remaining 60% is subject to total
grazing pressure and ringbarking. The other ecosystem of concern is also in the south west of
this province and is extensively grazed. It is low open woodland of Flindersia maculosa
(leopardwood) and Atalaya hemiglauca (whitewood) with Acacia omalophylla, Acacia
cambagei (gidgee) and Tripogon loliiformis (five-minute grass).
2.6.2 Land systems
Land systems are an area or group of areas throughout which there is a recurring pattern of
topography, soil and vegetation (Walker 1991). Ninety-one of the land systems defined and
mapped by Walker cover 46% of DRP. These land systems are listed in Appendix B, which
includes a brief description of topography and dominant vegetation. The percentage of the
DRP covered by each land system ranges from 4.5% (Nelyambo - floodplains with
Eucalyptus coolabah), less than one precent (45 landsystems each cover less than one percent
of the DRP) (Appendix B). Several land systems occur almost entirely within the DRP and
thirty-five have less than 10% of their total area within the DRP. The distribution of the land
systems which each cover more than 1% of the area is illustrated on Map 22. Approximately
90% of the mapped area contains land systems described as playas and basins or alluvial
plains.
2.7 SUMMARY AND PROPOSED FURTHER WORK
2.7.1 Summary
The DRP lies almost entirely in a semi-arid climatic zone, with only a small portion in the
temperate zone in the east and the south-western arm of the bioregion in an arid zone. There
is a substantial variation in conditions throughout the bioregion. Rainfall varies both spatially
and temporally with annual averages being of only limited value for understanding the
requirements and responses of the vegetation and fauna. In the north there is a summer
dominance in mean monthly rainfall but in the south it is evenly distributed throughout the
year and the mean monthly temperatures, both maxima and minima, are lower than in the
north.
The native geology, topography and soils of the DRP demonstrates that this is a derived,
water driven environment. The geological substrate is of alluvial origin, the topography is
generally low in relief with most areas of higher elevation being the product of deposits from
prior streams, and the soils are predominantly clays typical of alluvial fans and floodplains.
The Collarenebri Interfluve is the most obvious exception to this. Rivers and wetlands are
significant features of the DRP as it consists of the lower fans and floodplains of six
catchments. Wetlands and riverine corridors are critical components of this bioregion.
Morgan and Terrey (1992) have subdivided the DRP into smaller units (provinces) and
Walker (1991) has mapped land systems and land units for the western third of the bioregion.
These two classifications need to be compared and the boundaries of the provinces refined.
Since European settlement the DRP has been subjected to high levels of disturbance with the
impact of clearing and introduced pests combining with extreme climatic conditions in the
1890s and the middle of this century to destroy much of the original vegetation. The extent
and rate of land degradation must be taken into account if effective biodiversity conservation
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