Résumé
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.