Genotype × elevated CO₂ interaction and allocation in calcareous grassland species

•  Biomass of genotypes of Carex flacca and Bromus erectus under CO₂ enrichment was measured to test for 'winners' and 'losers' within the populations. Allocation was studied to identify reasons for varying biomass responses of the genotypes. •  In a first experiment, genotypes of both species were grown separately and mown five times. In a second experiment, introducing species interaction, species were grown together (using nine genotypes of each) and harvested destructively during that time. •  The first experiment showed that growth stimulation across harvests (Bromus+15%; Carex+11%) and genotype × CO₂ treatment interactions in single harvests was significant, but repeated measures ANOVA was not (Carex) or only marginally significant (Bromus). Destructive harvests in the competition experiment indicated a significant growth stimulation (Bromus+28%, Carex+35%), but no CO₂  × genotype interaction was found. Similarly, ANOVA of dry matter allocation never indicated a significant CO₂  × genotype interaction. •  The lack of a sustained CO₂  × genotype interaction in isolated and competitive growth conditions with good moisture supply suggests, that if there is such an interaction, it must be very small.