Seasonal patterns of photosynthetic light response in Douglas-fir seedlings subjected to elevated atmospheric CO(2) and temperature.

Increases in atmospheric CO(2) concentration and temperature are predicted to increase the light response of photosynthesis by increasing light-saturated photosynthetic rates and apparent quantum yields. We examined the interactive effects of elevated atmospheric CO(2) concentration and temperature on the light response of photosynthesis in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Seedlings were grown in sunlit chambers controlled to track either ambient (~400 ppm) CO(2) or ambient + 200 ppm CO(2), at ambient temperature or ambient + 4 degrees C. Photosynthetic light response curves were measured over an 18-month period beginning 32 months after treatments were initiated. Light-response curves were measured at the growth CO(2) concentration, and were used to calculate the light-saturated rate of photosynthesis, light compensation point, quantum yield and respiration rate. Elevated CO(2) increased apparent quantum yields during two of five measurement periods, but did not significantly affect light-saturated net photosynthetic rates, light compensation points or respiration rates. Elevated temperature increased all parameters. There were no significant interactions between CO(2) concentration and temperature. We conclude that down-regulation of photosynthesis occurred in the elevated CO(2) treatments such that carbon uptake at a given irradiance was similar across CO(2) treatments. In contrast, increasing temperature may substantially increase carbon uptake rates in Douglas-fir, assuming other environmental factors do not limit photosynthesis; however, it is not clear whether the increased carbon uptake will increase growth rates or be offset by increased carbon efflux through respiration.