Consistent sorting but contrasting transition zones in plant communities along bioclimatic gradients

Correlations between plant species occurrences and climate are used as evidence of ecological sorting and climate sensitivity. However, because observed patterns may be spatially and historically contingent, interpretations of compositional responses to spatial differences in climate should ideally consider past climatic fluctuations, edaphic factors, nonlinear turnover rates and transition zones (ecotones) in environmental and geographic space. We tested whether drivers of plant community compositional turnover and transition zones are consistent, rather than spatially contingent, across spatially isolated and biologically contrasting (independent) ecosystems. We used four broad-scale transects in Australia, which are spatially isolated and climatically and ecologically diverse: NATT (Northern Territory); BATS (New South Wales); SWATT (Western Australia) and TREND (South Australia). We used Generalised Dissimilarity Modelling to ascertain the relative contributions of space (geographic distance) and environmental variables to compositional turnover. We mapped transition zones using modelled nonlinear responses. Purely spatial contributions were estimated using variance partitioning. Models explained 26-52% of deviance in compositional dissimilarity. Mean annual rainfall and geographic distance were the most important explanatory variables, although topographic relief outranked rainfall for the topographically complex BATS transect. Purely spatial structure accounted for only 7-12% of explained deviance, pointing to the shared prevalence of species sorting along spatially structured rainfall gradients. Zones of rapid compositional turnover across bioclimatic gradients were evident across the contrasting ecosystems. These transitions zones tended to occur at the arid extremes of rainfall gradients and as plains transitioned into complex topography. Responses to other variables were inconsistent among transects. Transition zones were spatially contingent, influenced by local topography, and generally occurred within different subintervals (value ranges) along gradients. The somewhat idiosyncratic compositional responses suggest that assessment of broadscale compositional drivers including climate change should incorporate regional variation. However, the consistently strong species sorting along rainfall gradients demonstrates a common key response to moisture limitation.