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 (delta(13)C) 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 degrees 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 (13)C 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 delta(13)C.

How do climate warming and species richness affect CO2 fluxes in experimental grasslands?

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.

Effects of climate warming and species richness on photochemistry of grasslands.

In view of the projected climatic changes and the global decrease in plant species diversity, it is critical to understand the effects of elevated air temperature (T(air)) and species richness (S) on physiological processes in plant communities. Therefore, an experiment of artificially assembled grassland ecosystems, with different S (one, three or nine species), growing in sunlit climate-controlled chambers at ambient T(air) and ambient T(air) + 3 degrees C was established. We investigated whether grassland species would be more affected by midday high-temperature stress during summer in a warmer climate scenario. The effect of elevated T(air) was expected to differ with S. This was tested in the second and third experimental years by means of chlorophyll a fluorescence. Because acclimation to elevated T(air) would affect the plant's stress response, the hypothesis of photosynthetic acclimation to elevated T(air) was tested in the third year by gas exchange measurements in the monocultures. Plants in the elevated T(air) chambers suffered more from midday stress on warm summer days than those in ambient chambers. In absence of severe drought, the quantum yield of PSII was not affected by elevated T(air). Our results further indicate that species had not photosynthetically acclimated to a temperature increase of 3 degrees C after 3 years exposure to a warmer climate. Although effects of S and T(air) x S interactions were mostly not significant in our study, we expect that combined effects of T(air) and S would be important in conditions of severe drought events.