Climatic drivers of plant-plant interactions and diversity in alpine communities

Alpine plant communities are particularly amenable to experimentally test the stress-gradient hypothesis (SGH), which predicts that competitive interactions will be more frequent in relatively productive environments, whereas facilitation will be more common in severe systems. Experimental testing of the SGH along latitudinal and elevation gradients within and across continents indicated that particular climatic variables act as drivers of plant-plant interactions and community structure. However, the SGH in its current form remains a general framework that does not link explicitly climate variables such as temperature and precipitation to plant interactions or diversity. Here, we re-analyse our published data in order to explore whether climate can regulate biotic interactions and species diversity in alpine communities. We applied PCA to meteorological data, introduced latitude as a variable, and also used specially developed composite variables that combine temperature and precipitation during the growing season. The intensity of competitive interactions at low elevations decreased with increasing latitude, whereas the intensity of facilitative interactions at higher elevations did not vary with latitude. Micro-scale spatial patterns followed the same trend indicating that plant-plant interactions could generate these patterns. These findings specify the role of temperature in shifting the balance of plant interactions and can be readily incorporated in the SGH. We also found that species richness correlated positively with a composite climate variable, which is the product of maximum temperature and precipitation. Inclusion of this finding into the SGH will, however, need further studies focusing on the importance of water-energy relations for the dynamic balance of facilitation and competition.