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 commun ities in northwestern Switzerland. The experimental communities were compos ed of 5, 12, and 31 species assembled from the native species pool. The stu dy aimed at testing whether the CO2 responses of ecosystems change when spe cific sets of species are lost from plant communities. Species were selecte d so that the proportion of grasses, legumes, and non-legume forb individua ls remained constant across levels of diversity. The most diverse plant com munity had approximately the same diversity as the surrounding grassland, a nd species occurring in less diverse communities were subsets of the specie s in the more diverse communities. The factorial atmospheric-CO2 treatment was applied using 50-cm-tall, open-bottom, open-top wind screens. Plant com munity-level responses and the responses of the individual species were ass essed over a period of five years. A significant positive correlation betwe en 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 l argest in the communities with the highest number of plant species and were primarily due to the presence of responsive species not present in the les s diverse communities. The time dependency of community responses to elevat ed CO2 and species diversity was related to shifts in community structure o f the experimental plots. Community responses at the beginning of the exper iment were dominated by the response of species with a less competitive/str ess-tolerant life history, These species were successively lost from experi mental plots as the experiment proceeded, and the observed community-level effects became smaller. Changes in species composition over the experimenta l duration were affected by elevated CO2 in the way that species loss was r educed (i.e., coexistence of species performing well at the beginning and a t the end of the five-year period increased) and the way that community eve nness was increased (i.e., dominance was reduced). Based on these results o ur main conclusions are that (1) community-level responses to CO2 enrichmen t depend on the species present; (2) the positive correlation between produ ctivity and species numbers was caused by different species at the beginnin g and at the end of the experiment; (3) therefore, a large, redundant speci es pool is important in assuring high productivity under altering environme ntal 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 misleadin g when attempting to predict longer-term effects.