Resilience of productivity and water use of montane grassland in response to drought intensity under current and future climate conditions

Multiple global change factors, such as elevated atmospheric CO2 concentrations, warming, and drought, are progressively affecting ecosystems worldwide. Future drought events will likely intensify, possibly leading to strongly non-linear threshold responses of ecosystem functioning. However, while of high ecological relevance, ecosystem responses to drought intensity have rarely been studied, and even less is known about whether future conditions involving a combination of elevated CO2 and warming can alter such responses. In an in-situ multifactor experiment in managed montane grassland, we studied the drought responses of ecosystem productivity, water use, and related canopy surface temperatures along with leaf-level stomatal conductance and photosystem II quantum yield. We analyzed whether and how resistance to and recovery from drought changed in response to drought intensity under current versus future (+300 ppm CO2, + 3° C) climate conditions. With increasing drought intensity, productivity and water use were increasingly reduced and were associated with increasing canopy temperatures, the effects being more pronounced in the future compared to current conditions. Our results suggest that the additional heat stress triggered by drought under future conditions can strongly reduce ecosystem resilience under scenarios of more extreme droughts.