Elevated CO2 alters carbon fluxes in early successional Mediterranean ecosystems

Annual carbon budgets of ecosystems are central to our understanding of the biotic control of atmospheric composition, but they are not available unde r elevated CO2 for most vegetation types. Using gas exchange techniques, we assessed carbon fluxes of four early successional Mediterranean model comm unities, consisting of grasses, legumes and composites. The assemblages wer e grown on the same monoliths for three consecutive years in greenhouses tr acking field conditions except for COP maintained at ambient (370 mu mol mo l(-1)) or elevated (700 mu mol mol(-1)) concentration. During the third year of study, CO2 enrichment consistently shifted the ann ual carbon balance towards lower efflux, with displacements between 4.3 and 26.2 mol m(-2) y(-1) (one assemblage became a net CO2 sink, another just r eached equilibrium, and the remaining two remained as a CO2 source). At lea st 50% of the shift under elevated CO2 originated from a decrease in belowg round respiration. This indicates that, during this year, CO2 enrichment di d not predominantly enhance C-cycling, but on the contrary inhibited root r espiration or microbial C-utilization. Although elevated-CO2-grown systems acted as a net CO2 sink during a longer period of the year (4-7 months) compared with ambient-CO2-grown systems (3 -3.5 months), gross canopy photosynthesis was modified only to a limited ex tent (between -5.9 and +14.8%). Interaction between the carbon and the wate r cycle was apparently responsible for this weak stimulation. In particular , reduced evapotranspiration under elevated CO2 coincided with inhibited ca nopy photosynthesis in early spring, most likely resulting from water satur ation of the soil. In addition, only the earliest-planted assemblages had a n increased gross canopy photosynthesis during late autumn and early winter . This suggests that a longer summer drought, by delaying the establishment of such an annual type of vegetation, would reduce the positive impact of elevated CO2 on productivity. Water regime appears to strongly govern the i nfluence of CO2 on the carbon fluxes in Mediterranean ecosystems with annua l herbaceous vegetation.