Foliar gas exchange responses of two deciduous hardwoods during 3 years of growth in elevated CO₂: no loss of photosynthetic enhancement

Responses of photosynthesis and stomatal conductance were monitored throughout a 3‐year field exposure of Liriodendron tulipifera (yellow‐poplar) and Quercus alba (white oak) to elevated concentrations of atmospheric CO 2 . Exposure to atmospheres enriched with +150 and +300 umol mol ‐1 CO 2 increased net photosynthesis by 12–144% over the course of the study. Net photosynthesis was consistently higher at +300 than at +150 umol mol ‐1 CO 2 . The effect of CO 2 enrichment on stomatal conductance was limited, but instantaneous leaf‐level water use efficiency increased significantly. No decrease in the responsiveness of photosynthesis to CO 2 enrichment over time was detected, and the responses were consistent throughout the canopy and across successive growth flushes and seasons. The relationships between internal CO 2 concentration and photosynthesis (e.g. photosynthetic capacity and carboxylation efficiency) were not altered by growth at elevated concentrations of CO 2 . No alteration in the timing of leaf senescence or abscission was detected, suggesting that the seasonal duration of effective gas‐exchange was unaffected by CO 2 treatment. These results are consistent with data previously reported for these species in controlled‐environment studies, and suggest that leaf‐level photosynthesis does not down‐regulate in these species as a result of acclimation to CO 2 enrichment in the field. This sustained enhancement of photosynthesis provides the opportunity for increased growth and carbon storage by trees as the atmospheric concentration of CO 2 rises, but many additional factors interact in determining whole‐plant and forest responses to global change.