Terrestrial flora and vegetation of the Western Australian wheatbelt

WA Museum Records and Supplements | Updated 8 years ago

ABSTRACT – Six hundred and eighty-two quadrats were located across the Western Australian wheatbelt and adjacent regions to cover as much of the geographical, edaphic and geomorphological variation of the terrestrial plant communities as possible. The study area covered 230,000 km' or 70% of the South West Botanical Province, one of the world's 25 biodiversity hotspots. It included all or part of the six biogeographical regions centred on the wheatbelt. The native vegetation in the study area is highly fragmented with 74% having been cleared for agriculture. Clearing has not been uniform; more extensive areas of bushland remain along the southern and western margins.

A total of 2609 taxa of vascular plants in 103 families was recorded from the quadrats. The species frequency pattern followed a lognormal distribution. At a scale of 400 m2, there was little difference in species richness between woodland, mallee and shrubland formations but those on duplex soils had lower species richness than those on deep sand, Iaterite and granite soils.

Over 60% of taxa were recorded in fewer than five quadrats. These uncommon taxa were not randomly distributed but concentrated at the periphery of the study area, particularly in the west and south. At least eight previously unrecognized taxa were collected for the first time. Further collections of 15 taxa listed as Declared Rare under the Wildlife Conservation Act, and 161 taxa on the Department of Conservation and Land Management's priority flora list were made.

There was little congruence in phytogeographic patterning between lifeforms or taxonomic levels. This implies that analyses of only some components of the flora, or analyses at higher taxonomic levels will not elucidate the full phytogeographic pattern. Phytogeographic patterning in the quadrat classification, which included the 1022 most common species, was strongIy related to climatic gradients and edaphic factors.

Twenty-six assemblages were defined from the species classification, four were later combined with the total number of assemblages analyzed reduced to 23. Of these 20 formed natural groups that were consistent with the known distribution and habitat preferences of the component taxa. Species richness in these assemblages was modeled as a surrogate for composition using climatic and soil parameters. Explanatory power of the models ranged from 43.2 to 84.2%.

The present biogeographical regions and subregions defined for the study area received little support from the analyses of either the quadrat classification or the assemblage patterning. Available structural vegetation mapping also showed little correlation with composition. Species patterning across the study area was primarily gradational along a northeast-southwest rainfall gradient and a north-south temperature gradient, as well as a widespread diffuse pattern associated with specific ganite and saline associations.