The effect of elevated carbon dioxide and ozone on leaf- and branch-level photosynthesis and potential plant-level carbon gain in aspen

Two aspen (Populus tremuloides Michx.) clones, differing in O-3 tolerance, were grown in a free-air CO2 enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, elevated CO2, elevated O-3 and elevated CO2+O-3. Leaf instantaneous light-saturated photosynthesis (P-S) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in Pa and A. For example, in the O-3-sensitive clone (259), inhibition of P-S in elevated O-3 (at both ambient and elevated CO2) was partially ameliorated by an increase in total A. For the O-3-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO2 was nullified by decreased total A.