Seasonal patterns of photosynthesis in Douglas fir seedlings during the third and fourth year of exposure to elevated CO2 and temperature

The interactive effects of elevated atmospheric CO2 and temperature on seas onal patterns of photosynthesis in Douglas fir (Psuedotsuga menziesii (Mirb .) France) seedlings were examined. Seedlings were grown in sunlit chambers controlled to track either ambient (similar to 400 p.p.m.) CO2 or ambient +200 p.p.m. CO2, and either ambient temperature or ambient +4 degreesC. Lig ht-saturated net photosynthetic rates were measured approximately monthly o ver a 21 month period. Elevated CO2 increased net photosynthetic rates by a n average of 21% across temperature treatments during both the 1996 hydrolo gic year, the third year of exposure, and the 1997 hydrologic year. Elevate d mean annual temperature increased net photosynthetic rates by an average of 33% across CO2 treatments during both years. Seasonal temperature change s also affected net photosynthetic rates. Across treatments, net photosynth etic rates were highest in the spring and autumn, and lowest in July, Augus t and December-January. Seasonal increases in temperature were not correlat ed with increases in the relative photosynthetic response to elevated CO2. Seasonal shifts in the photosynthetic temperature optimum reduced temperatu re effects on the relative response to elevated CO2. These results suggest that the effects of elevated CO2 on net photosynthetic rates in Douglas fir are largely independent of temperature.