Canopy photosynthesis of crops and native plant communities exposed to long‐term elevated CO₂

There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO 2 : three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO 2 increased canopy photosynthesis in all cases. The relative effect of CO 2 was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO 2 were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO 2 treatment to date, monospecific stands of a C 3 sedge, Scirpus olneyi (Grey), and a C 4 grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO 2 concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO 2 exchange per unit green biomass (NCE b ) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO 2 increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.