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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. [ABSTRACT FROM AUTHOR]

• This paper presents the results of 2 yr of CO2 flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. • We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3°C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO2 fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. • While CO2 outputs through respiration were largely unchanged under warming, CO2 inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO2 fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. • This study shows that in a future warmer climate the CO2 sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness. [ABSTRACT FROM AUTHOR]

This paper presents the results of 2 yr of CO(2) flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 degrees C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO(2) fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. While CO(2) outputs through respiration were largely unchanged under warming, CO(2) inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO(2) fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. This study shows that in a future warmer climate the CO(2) sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness.