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°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°C after 3 years exposure to a warmer climate. Although effects of S and T_air × 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.