Abstract: Aims How growth of wild and crop species responds to global environmental perturbations has both ecological and agricultural significance in a changing world. The primary aim of this synthesis was to quantitatively assess the interactive effects of intraspecific competition and elevated CO₂ on biomass production in herbaceous species.
Methods Using meta-analytical techniques, we synthesized data from publications before 2006 that reported biomass responses to elevated CO₂ in 321 herbaceous species grown in isolation or in competition with con-specific individuals.
Important findings Intraspecific competition differentially modified biomass responses to elevated CO₂ in wild and crop species. For example, competition reduced CO₂ stimulation of total biomass (W_T) from 27 to 23% in wild species, but by a much greater magnitude, i.e., from 43 to 32% in crops. Competition had no effect on responses of either above- (W_AG) or below-ground (W_BG) biomass to elevated CO₂ in wild species, but significantly diminished CO₂ enhancement of W_AG, although not of W_BG, in crops. Considerable variations were found among functional groups in the modification of growth responses to elevated CO₂ by intraspecific competition, which exerted greater depression on CO₂ enhancement in C₃ than in C₄ species and in non-legumes than in legumes. Elevated CO₂ affected leaf and stem growth of individually grown C₃ graminoids and forbs similarly, but increased leaf growth only in C₄ graminoids and stem growth only in C₄ forbs. Results from this synthesis demonstrated that intraspecific competition differentially affected growth responses to elevated CO₂ in wild and crop species. The wild–crop species differences will have important implications for understanding primary production by herbaceous species in both natural and agricultural ecosystems in the future when atmospheric CO₂ is significantly higher than the current level.

Key words: biomass production, CO₂ enrichment, functional groups, meta-analysis