Geographical variation in the importance of water and energy for oak diversity

Aim The water–energy dynamics hypothesis posits that species diversity is cor- related with water availability and temperatures; diversity is lowest when water availability is reduced at low temperatures because few species can persist under such conditions. However, the relationship between water and energy availability and diversity likely varies geographically along environmental gradi- ents. Here, we examined the drivers of such variability, using a global-scale data set on oaks. Location Global. Method To quantify spatial variation in both species diversity–water relation- ships and species diversity–energy relationships, we performed geographically weighted regressions (GWR) of (1) species diversity against energy availability, measured as potential evapotranspiration and temperature, and (2) species diversity against water availability, measured by precipitation and an aridity index. We then regressed the local slopes of these GWR models against water and energy separately, and water 9 energy combined to assess whether the regional-scale relationships among oak diversity, water and energy depended on climatic conditions at larger spatial extents. Results We found that the effects of low water availability and cold tempera- tures on oak diversity varied geographically. In particular, the effects of water and energy availability on oak diversity increased as water and energy availabil- ity decreased. Furthermore, the interaction between water and energy availabil- ity in?uenced regional-scale oak diversity–energy relationships but not oak diversity–water relationships. Main conclusion Our results suggest that the relationship between climate (and potentially climatic change) and oak diversity varies geographically, and that the effects of limiting water and temperature are likely to be most severe in arid environments.