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

The responses to CO2 of perennial grasses (Danthonia richardsonii and Phala ris aquatica) and legumes (Lotus pedunculatus and Trifolium repens) were co mpared under controlled conditions for isolated plants, monoculture stands and mixed-species stands along a N gradient to test whether: plant-plant in teractions between species in mixed stands changed with concentration of CO 2; responses to CO2 of species in mixtures could be related to their respon ses as single stands; responses of mixtures to CO2 could be related to the responses of individual species to CO2 and to competition. Plants were grow n for 60 d in sand, using nutrient solutions (six nitrate concentrations fr om 0.25 to 16 mM NO3), at ambient (c. 357 mu l l(-1)) or elevated CO2 (c. 7 12 mu l l(-1)). Species dominance in the mixtures depended more on the rang e of N than of CO2 concentration provided: T. repens and L. pedunculatus do minated at low concentrations of N; L. pedunculatus and P. aquatica perform ed better at high concentrations. Responses of species in mixtures to CO2 w ere related to their responses in monocultures but not to those of isolated plants. Species biomass proportions in mixtures under ambient CO2 determin ed the outcome of mixture responses to CO2 more than of individual species responses to CO2. These results emphasize the influence of plant-plant inte ractions on community responses to CO2, since mixture behaviour under eleva ted CO2 could not be scaled-up from responses by isolated plants in this ex periment.