Greater impact of extreme drought on photosynthesis of grasslands exposed to a warmer climate in spite of acclimation

In view of the projected increase in the frequency of extreme events during this century, we investigated the impact of a drought extreme on leaf ecophysiological parameters and carbon isotope composition (δ¹³C) of grassland communities with species richness (S) of one, three or nine species. The communities, grown for 3 years at either ambient air temperatures (ambient T_air) or ambient T_air + 3°C (elevated T_air), were additionally subjected to an imposed drought by withholding water for 24 days. During the previous 3 years equal precipitation was applied in both temperature treatments, thus communities at elevated T_air had experienced more frequent, mild droughts. However, it was unknown whether this resulted in a higher resistance for facing extreme droughts. At similar soil matric potentials stomatal conductance (g_s) and transpiration (Tr) were higher at elevated than ambient T_air, indicating acclimation to lower soil water content. Despite the stomatal acclimation observed, plants in elevated T_air showed a lower resistance to the drought extreme as indicated by their lower photosynthetic rate (A_max), g_s and Tr during the entire duration of the drought extreme. Lower values for A_max, Tr and g_s were also recorded in species at S = 3 as compared with species at S = 1 for both temperature treatments, but no further differences with S = 9 suggesting that stress was not alleviated at higher S‐levels. The discrimination of ¹³C was poorly correlated with measurements of instantaneous leaf water‐use efficiency (A_max/Tr) and, with this time scale and sampling method, it was not possible to detect any potential change in plant water‐use efficiency using leaf δ¹³C.