Variation in grassland populations and species and the implications for community responses to elevated CO2

Variation in plant characteristics and potential responses to CO2 was measured in controlled environments for a set of different forage plant species and populations. The response of the plants to elevated CO2 was strongly determined by temperature. The greatest responses to elevated CO2 were observed at warm temperatures in C3 species with high potential growth rates at these temperatures. This suggests that the community composition could change most rapidly in response to CO2 in warm seasons, with the greatest positive responses to CO2 occurring in warm-season active species. This prediction was tested in a microcosm experiment. The prediction was confirmed under well-watered conditions but water stress resulted in an ingress of C4 species with low potential responses to CO2. The results suggest that variation between populations and species must be considered when predicting grassland community responses to CO2, and that it is inappropriate to ignore compositional changes in communities when modelling CO2 effects on pasture production. Given the importance of temperature in determining CO2 responsiveness, phenology may prove to be a useful attribute in plant functional type analyses of community responses to CO2.