Environmental determinants of vascular plant species richness in the Austrian Alps

Aim To test predictions of different large-scale biodiversity hypotheses by analysing species richness patterns of vascular plants in the Austrian Alps. Location The Austrian part of the Alps (c. 53,500 km(2)). Methods Within the floristic inventory of Central Europe the Austrian part of the Alps were systematically mapped for vascular plants. Data collection was based on a rectangular grid of 5 x 3 arc minutes (34-35 km(2)). Emerging species richness patterns were correlated with several environmental factors using generalized linear models. Primary environmental variables like temperature, precipitation and evapotranspiration were used to test climate-related hypotheses of species richness. Additionally, spatial and temporal variations in climatic conditions were considered. Bedrock geology, particularly the amount of calcareous substrates, the proximity to rivers and lakes and secondary variables like topographic, edaphic and land-use heterogeneity were used as additional predictors. Model results were evaluated by correlating modelled and observed species numbers. Result Our final multiple regression model explains c. 50% of the variance in species richness patterns. Model evaluation results in a correlation coefficient of 0.64 between modelled and observed species numbers in an independent test data set. Climatic variables like temperature and potential evapotranspiration (PET) proved to be by far the most important predictors. In general, variables indicating climatic favourableness like the maxima of temperature and PET performed better than those indicating stress, like the respective minima. Bedrock mineralogy, especially the amount of calcareous substrate, had some additional explanatory power but was less influential than suggested by comparable studies. The amount of precipitation does not have any effect on species richness regionally. Among the descriptors of heterogeneity, edaphic and land-use heterogeneity are more closely correlated with species numbers than topographic heterogeneity. Main conclusions The results support energy-driven processes as primary determinants of vascular plant species richness in temperate mountains. Stressful conditions obviously decrease species numbers, but presence of favourable habitats has higher predictive power in the context of species richness modelling. The importance of precipitation for driving global species diversity patterns is not necessarily reflected regionally. Annual range of temperature, an indicator of short-term climatic stability, proved to be of minor importance for the determination of regional species richness patterns. In general, our study suggests environmental heterogeneity to be of rather low predictive value for species richness patterns regionally. However, it may gain importance at more local scales.