A long-term field study on biodiversity x elevated CO2 interactions in grassland

Interactive effects of increases in atmospheric CO2 and reductions in plant species diversity were investigated in planted calcareous grassland communities in northwestern Switzerland. The experimental communities were composed of 5, 12, and 31 species assembled from the native species pool. The study aimed at testing whether the CO2 responses of ecosystems change when specific sets of species are lost from plant communities. Species were selected so that the proportion of grasses, legumes, and non-legume forb individuals remained constant across levels of diversity. The most diverse plant community had approximately the same diversity as the surrounding grassland, and species occurring in less diverse communities were subsets of the species in the more diverse communities. The factorial atmospheric-CO2 treatment was applied using 50-cm-tall, open-bottom, open-top wind screens. Plant community-level responses and the responses of the individual species were assessed over a period of five years. A significant positive correlation between plant community diversity and biomass was detected, but this effect was not present on all dates. Significant effects of elevated CO2 on community biomass were only found in the first years of treatment. CO2 effects were largest in the communities with the highest number of plant species and were primarily due to the presence of responsive species not present in the less diverse communities. The time dependency of community responses to elevated CO2 and species diversity was related to shifts in community structure of the experimental plots. Community responses at the beginning of the experiment were dominated by the response of species with a less competitive/stress-tolerant life history, These species were successively lost from experimental plots as the experiment proceeded, and the observed community-level effects became smaller. Changes in species composition over the experimental duration were affected by elevated CO2 in the way that species loss was reduced (i.e., coexistence of species performing well at the beginning and at the end of the five-year period increased) and the way that community evenness was increased (i.e., dominance was reduced). Based on these results our main conclusions are that (1) community-level responses to CO2 enrichment depend on the species present; (2) the positive correlation between productivity and species numbers was caused by different species at the beginning and at the end of the experiment; (3) therefore, a large, redundant species pool is important in assuring high productivity under altering environmental conditions; (4) elevated CO2 has the potential to substantially alter the structure of grassland communities, even if community productivity does not increase; and (5) a short-term effect of elevated CO2 may be misleading when attempting to predict longer-term effects.