A field study of the effects of elevated CO 2 on plant biomass and community structure in a calcareous grassland

Abstract

The effects of elevated CO(2) on plant biomass and community structure have been studied for four seasons in a calcareous grassland in northwest Switzerland. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain species composition. Plots of 1.3 m(2) were exposed to ambient or elevated CO(2) concentrations (n = 8) using a novel CO(2) exposure technique, screen-aided CO(2) control (SACC) starting in March 1994. In the 1st year of treatment, the annual harvested biomass (sum of aboveground biomass from mowings in June and October) was not significantly affected by elevated CO(2). However, biomass increased significantly at elevated CO(2) in the 2nd (+20%, P = 0.05), 3rd (+21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable differences in root biomass in the top 8 cm of soil between CO(2) 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. The order of CO(2) responsiveness among functional groups changed substantially from the 2nd to the 3rd year; for example, non-leguminous forbs had the smallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO(2) exposure, large species-specific differences in CO(2) response had developed. For five important species or genera the order of responsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+33%), Sanguisorba minor (no significant CO(2) effect), and six Trifolium species (a negative response that was not significant). The positive CO(2) responses in Bromus and Carex were most closely related to increases in tiller number. Species richness was not affected by CO(2) treatment, but species evenness increased under elevated CO(2) (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 CO(2) concentrations can substantially alter the structure of calcareous grassland communities and may increase plant community biomass.