Biomass responses to intraspecific competition differ between wild species and CO2

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 WAG, although not Of WBG, 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 C-3 than in C-4 species and in non-legumes than in legumes. Elevated CO2 affected leaf and stem growth of individually grown C-3 graminoids and forbs similarly, but increased leaf growth only in C-4 graminoids and stem growth only in C-4 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.