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 CO 2 , ambient + 165 μmol mol −1 CO 2 or ambient + 330 μmol mol −1 CO 2 concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO 2 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 CO 2 treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO 2 treatment, rates of net assimilation were ∼1·6 times greater in the ambient + 165 μmol mol −1 CO 2 treatment and 2·2 times greater in the ambient + 330 μmol mol −1 CO 2 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°C. This indicated that the effect of elevated CO 2 concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO 2 (+ 165 or + 330 μmol mol −1 ) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.