Responses of individual grassland species to climate warming and species richness: Ecophysiological processes, development and biomass production

In this post-Kyoto era many research has focused on the effects of the rising CO2 concentrations. However, two other important aspects of global change, climate warming and declining species richness were less-studied. We therefore imposed two temperature treatments (unheated and heated chambers) crossed with three species richness treatments (1, 3 and 9 species) on an experimental platform of 288 artificially assembled grassland model ecosystems. The unheated chambers were kept at the ambient air temperature, while the heated chambers were continuously warmed 3°C above fluctuating ambient values. This experimental platform enabled us to determine both the single-factor influence and possible interactions of these two global changes on several aspects of individual grassland species. To account for the summer drought concomitant with warming (as predicted by the IPCC), identical amounts of precipitation were administrated to the two temperature treatments, leading to slightly drier soil conditions in the heated chambers. During the four years of our experiment, several plant characteristics as light-saturated photosynthetic rates, and carbon and nitrogen concentrations of the leaves, were enhanced by the warming. Based upon phenological observations, a prolonged growing season (earlier spring onset and postponed senescence) was found. The plants also acclimated physiologically to their warmer environment. Despite all these positive warming effects, the plants were not more productive in the heated chambers, neither at the individual plant level nor at the community level. This was mainly caused by the drier soil conditions of the heated chambers. Although the species richness effect was overall present on community biomass (caused by a positive complementarity effect), hardly any effect was found at the individual plant level, due to opposite individual plant responses. In addition the few effects found (i.e. on specific leaf area and chlorophyll content of the leaves) were mainly driven by the drier soil conditions of the mixed communities compared to the single-species ones. These drier soil conditions may also be the reason why hardly any interaction between warming and species richness was found at individual plant level.