Effect of competition on the responses of grasses and legumes to elevated atmospheric CO₂ along a nitrogen gradient: differences between isolated plants, monocultures and multi-species mixtures

Abstract

The responses to CO₂ of perennial grasses (Danthonia richardsonii and Phalaris aquatica) and legumes (Lotus pedunculatus and Trifolium repens) were compared under controlled conditions for isolated plants, monoculture stands and mixed-species stands along a N gradient to test whether: plant–plant interactions between species in mixed stands changed with concentration of CO₂; responses to CO₂ of species in mixtures could be related to their responses as single stands; responses of mixtures to CO₂ could be related to the responses of individual species to CO₂ and to competition. Plants were grown for 60 d in sand, using nutrient solutions (six nitrate concentrations from 0.25 to 16 mM NO₃), at ambient (c. 357 μl l⁻¹) or elevated CO₂ (c. 712 μl l⁻¹). Species dominance in the mixtures depended more on the range of N than of CO₂ concentration provided: T. repens and L. pedunculatus dominated at low concentrations of N; L. pedunculatus and P. aquatica performed better at high concentrations. Responses of species in mixtures to CO₂ were related to their responses in monocultures but not to those of isolated plants. Species biomass proportions in mixtures under ambient CO₂ determined the outcome of mixture responses to CO₂ more than of individual species responses to CO₂. These results emphasize the influence of plant–plant interactions on community responses to CO₂, since mixture behaviour under elevated CO₂ could not be scaled-up from responses by isolated plants in this experiment.