Résumé
This chapter is a synthesis of the results of three recent successive experiments, with characteristics imposed by the availability of CO2 enrichment facilities and by the hypotheses to be tested. The differential response of the species, the response of ecosystem processes, and the interactions between the ecosystem response and the diversity of the plant community are the topics addressed in the chapter. The findings indicate that the CO2 growth response of the individual species present in our microcosms is characterized by a high interspecific variability. A negative response is almost as frequent as a positive one, and large intrapopulation variability makes most differences insignificant. The vegetative and reproductive phenology of some species was strongly affected by CO2 and an interaction with soil fertility was found. In all the experiments, the variability at the species level was very high and resulted in very few species being significantly affected by CO2 despite a consequent average response for many of them. Photosynthesis and standing biomass were marginally stimulated by CO2 and often not significantly. The complexity of the response to CO2 is due to its indirect effect on plant growth and development, its interaction with other environmental factors, and its interaction with other ecosystem components.