A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland

The effects of elevated CO2 on plant biomass and community structure have b een studied for four seasons in a calcareous grassland in northwest Switzer land. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain specie s composition. Plots of 1.3 m(2) were exposed to ambient or elevated CO2 co ncentrations (n = 8) using a novel CO2 exposure technique, screen-aided CO2 control (SACC) starting in March 1994. In the 1st year of treatment, the a nnual harvested biomass (sum of aboveground biomass from mowings in June an d October) was not significantly affected by elevated CO2. However, biomass increased significantly at elevated CO2 in the 2nd (+20%, P = 0.05), 3rd ( +21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable di fferences in root biomass in the top 8 cm of soil between CO2 treatments on eight out of nine sampling dates. There were significant differences in CO 2 responsiveness between functional groups (legumes, non-leguminous forbs, graminoids) in the 2nd (P = 0.07) and 3rd (P < 0.001) years of the study. T he order of CO2 responsiveness among functional groups changed substantiall y from the 2nd to the 3rd year; for example, non-leguminous forbs had the s mallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO2 exposure, large species-specific differences in CO2 response had developed. For five important species or genera the order of r esponsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+ 33%), Sanguisorba minor (no significant CO2 effect), and six Tri folium species (a negative response that was not significant). The positive CO2 responses in Bromus and Carex were most closely related to increases i n tiller number. Species richness was not affected by CO2 treatment, but sp ecies evenness increased under elevated CO2 (modified Hill ratio; P = 0.03) in June of the 3rd year, resulting in a marginally significant increase in species diversity (Simpson's index; P = 0.09). This and other experiments with calcareous grassland plants show that elevated atmospheric CO2 concent rations can substantially alter the structure of calcareous grassland commu nities and may increase plant community biomass.