Increasing concentrations of CO2 in the atmosphere are likely to affect the ecological dynamics of plant populations and communities worldwide, yet little is known about potential evolutionary consequences of high CO2. We employed a quantitative genetic framework to examine how the expression of genetic variation and covariation in fitness-related traits, and thus, the evolutionary potential of a species, is influenced by CO2. In two field experiments, genotypes of the dominant grassland perennial Bromus erectus were grown for several years in plots maintained at present-day or at elevated CO2 levels. Under noncompetitive conditions (experiment 1), elevated CO2 had little impact on plant survival, growth, and reproduction. Under competitive conditions in plots with diverse plant communities (experiment 2), performance of B. erectus was reduced by elevated CO2. This suggests that the effect of CO2 was largely indirect, intensifying competitive interactions. Elevated CO2 had significant effects on the expression of genetic variation in both the competitive and noncompetitive environment, but the effects were in opposite direction. Heritability of plant size was generally higher at elevated than at ambient CO2 in the noncompetitive environment, but lower in the competitive environment. Selection analysis revealed a positive genotypic selection differential for plant size at ambient CO2, indicating selection favoring genotypes with high growth rate. At elevated CO2, the corresponding selection differential was nonsignificant and slightly negative. This suggests that elevated CO2 is unlikely to stimulate the evolution of high biomass productivity in this species.
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