Field Response of Photosynthesis to CO"2 Enhancement in Ponderosa Pine

Net photosynthesis was determined by CO2 analysis for Pinus ponderosa Laws. in situ in the San Bernardino Mountains of southern California. Leafy shoots were maintained in cuvettes under ambient and enhanced levels of CO2 on successive days. Carbon dioxide enhancement was up to 150 ppm above ambient levels. Mean response of net photosynthesis increased 84%. Responsiveness to CO2 enhancement was not altered by high temperature or decreasing soil water potential during the months June—October. The added CO2 compensated for reduction of 50% in light intensity. Severe damage from oxidant air pollution reduced but did not eliminate the response to CO2 enhancement. The evidence suggests that net photosynthesis will increase in linear relation to rising atmospheric CO2 during the next few decades.