Acacia, climate, and geochemistry in Australia

In anticipation of global climate change, the question of whether shifts in plant community composition (beta-diversity) are predictable from environmental variation is receiving considerable interest. Species strongly associated with local soil environments may be more vulnerable to climate change than species with a broad tolerance of soil conditions. Here we investigate relationships between climate, geochemistry and the distribution of Acacia over Australia. We use geostatistics to estimate total Ca, Mg, Na, Al, P, pH, and electrical conductivity at sites where Acacia species have been recorded in the Australian Virtual Herbarium database. We compare the median predicted geochemistry and reported substrate for individual species that appear associated with extreme conditions; this provides a partial evaluation of the predictions. We generate a site-by-species matrix by aggregating observations to the centroids of grid cells 100 km on edge, calculate diversity indices, and use numerical ecology methods (ordination, variation partitioning) to investigate the ecology of Acacia and its response to climatic and geochemical gradients. Many species that tolerate extreme geochemical conditions are range restricted. Species in the genus Acacia are widely distributed across Australia but strong associations exist between species turnover and climate and geochemistry. Climate, pH, P, Na, and EC account for much of the variation in Acacia distribution over the continent, especially across southern Australia. Climate and geochemistry together account for half of the variation in species turnover of Acacia across Australia and for about 60-80 % in areas of high species richness. The unique contribution of geochemistry to variation in species turnover of Acacia is smaller than that of climate except in the most species rich areas. Climate is more important than geochemistry in explaining Acacia species distribution and turnover across northern Australia. Geochemical variables are important in explaining the occurrence of Acacia species where species richness is high in southern Australia-it is important to investigate this further with other genera. Aridification, which has driven some the observed extremes in geochemical concentrations, is a key process in landscape evolution as well as biogeography. This study of Acacia diversity and environmental conditions underscores Australia's place as a natural laboratory for evolutionary ecology and biogeography.