Deciphering the stability of grassland productivity in response to rainfall manipulation experiments

Aim: Rainfall manipulation experiments are essential tools for deciphering the mechanisms leading to variation in ecosystem stability across sites. Here, we gathered articles reporting results of experimental droughts on the above-ground biomass of grasslands to identify which indices have been used to assess stability, to evaluate the overall grassland responses to drought and to quantify the relative importance of drought characteristics and climatic conditions for explaining variation in stability. Location: Global. Time period: 1989-2018. Major taxa studied: Grasslands. Methods: We used meta-analytical approaches to evaluate overall grassland stability in terms of resistance, recovery and resilience, and multi-model inference to assess the relative importance of different moderators on explaining the variability of those three stability properties. Results: Numerous indices of stability have been used, but they are inadequate for comparisons across sites. After applying standardized indices, we found that grasslands were resilient (biomass remained unchanged 1 year after drought) and exhibited a trade-off between low resistance (biomass was lost during drought) and high recovery (new biomass was produced after drought). Overall, climatic conditions and drought characteristics (intensity, duration and frequency) were not important to explain the differences in stability observed across grasslands. Main conclusions: Grasslands are resilient, but if drought events last > 1 year, there might be long-term declines of biomass production owing to incomplete recovery. Despite the hundreds of experiments conducted in grasslands across the globe, the results are still inconclusive because of four important shortcomings: 50% of the studies have failed to create drought; 81% have not included recovery and resilience, assessing only resistance; 87% have not applied quantitative indices to assess stability; and < 1% of the studies were conducted on tropical grasslands. We discuss how to overcome those limitations to improve our ability to ensure stable grassland productivity under climate change.