Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO2, ambient + 165 mu mol mol(-1) CO2 or ambient + 330 mu mol mol(-1) CO2 concentrations in combination with ambient or ambient + 2 degrees C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO2 treatments in all seasons. On the basis of A/C-i curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO2 treatments. The imposed 2 degrees C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO2 treatment, rates of net assimilation were approximate to 1.6 times greater in the ambient + 165 mu mol mol(-1) CO2 treatment and 2.2 times greater in the ambient + 330 mu mol mol(-1) CO2 treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12.6 to 28.2 degrees C. This indicated that the effect of elevated CO2 concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO2 (+ 165 or + 330 mu mol mol(-1)) was much greater than the effect of a 2 degrees C increase in air temperature on net assimilation and growth in this species.