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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 Tair) or ambient Tair + 3°C (elevated Tair), 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 Tair 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 (gs) and transpiration (Tr) were higher at elevated than ambient Tair, indicating acclimation to lower soil water content. Despite the stomatal acclimation observed, plants in elevated Tair showed a lower resistance to the drought extreme as indicated by their lower photosynthetic rate (Amax), gs and Tr during the entire duration of the drought extreme. Lower values for Amax, Tr and gs 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 (Amax/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.