The influence of elevated CO₂ on community structure, biomass and carbon balance of mediterranean old‐fleld microcosms

Abstract

Abstract We studied the effects of a doubling of atmospheric CO 2 concentration on intact monoliths of Mediterranean grassland in growth chambers where climatic field conditions were simulated. During the six month growing season, changes in community structure were monitored by quantifying species richness and cover. The CO 2 exchange of microcosms was measured continuously and the resulting quantity and quality of biomass were evaluated. Species richness and cover did not respond to elevated CO 2 . After one month of treatment, CO 2 exchange measured during the day did not differ between CO 2 levels but the night respiration was two‐fold higher under elevated CO 2 . Stimulations of both day and night CO 2 flux by short‐term CO 2 enrichment were recorded several times during the growing season. These results suggest that despite some downward adjustment of photosynthesis, net canopy photosynthesis was stimulated by elevated CO 2 , but this stimulation was compensated for by an increased respiration. The 20% stimulation of final phytomass under elevated CO 2 was not significant: it resulted from unchanged live plant matter but a significant, 100% increase in litter accumulation. These results suggest that in low‐productivity Mediterranean herbaceous systems, the greatest effect of CO 2 is not on the storage of carbon in biomass but on the turnover of the carbon in the plants.