Does a warmer climate with frequent mild water shortages protect grassland communities against a prolonged drought?

Global warming accelerates land surface drying, increasing the incidence of extreme climatic events such as severe droughts with detrimental effects on ecosystem functions and structure. We investigated the effects of an imposed severe drought (24 days) on fully established synthesized grassland communities with three species richness (S) levels (one, three or nine species), grown for 3 years at either ambient air temperatures (unheated) or ambient +3 degrees C (heated). Since water supply during these 3 years was equal in all treatments, heated communities experienced more frequent, short mild droughts, but it was unknown whether this conferred greater or smaller resistance for facing prolonged droughts. During the 24-day drought period, soil matric potential decreased in a similar way over time in both temperature treatments and was lower at increasing S-levels. Although green vegetation cover was significantly reduced by the drought in both temperature treatments, the decrease was higher in heated than unheated communities indicating a lower resistance of heated communities to the drought. After only 13 days of recovery, green vegetation cover of both temperature treatments approached values similar to those observed before the imposed drought, suggesting similar resilience in both treatments. Above-ground biomass was reduced by elevated temperature, consistently in all S-levels, showing that the drought period did not change the biomass production patterns observed in the treatments before the imposed drought. Our results suggest that, regardless of the continuous exposure to elevated temperatures and associated short mild droughts, heated communities had not developed clear mechanisms to better cope with extended summer droughts.