FOLIAR GAS-EXCHANGE RESPONSES OF 2 DECIDUOUS HARDWOODS DURING 3 YEARSOF GROWTH IN ELEVATED CO2 - NO LOSS OF PHOTOSYNTHETIC ENHANCEMENT

Responses of photosynthesis and stomatal conductance were monitored th roughout a 3-year field exposure of Liriodendron tulipifera (yellow-po plar) and Quercus alba (white oak) to elevated concentrations of atmos pheric CO2. Exposure to atmospheres enriched with +150 and +300 mumol mol-1 CO2 increased net photosynthesis by 12-144% over the course of t he study. Net photosynthesis was consistently higher at +300 than at 150 mumol mol-1 CO2. The effect Of CO2 enrichment on stomatal conducta nce was limited, but instantaneous leaf-level water use efficiency inc reased significantly. No decrease in the responsiveness of photosynthe sis to CO2 enrichment over time was detected, and the responses were c onsistent throughout the canopy and across successive growth flushes a nd seasons. The relationships between internal CO2 concentration and p hotosynthesis (e.g. photosynthetic capacity and carboxylation efficien cy) were not altered by growth at elevated concentrations of CO2. No a lteration in the timing of leaf senescence or abscission was detected, suggesting that the seasonal duration of effective gas-exchange was u naffected by CO2 treatment. These results are consistent with data pre viously reported for these species in controlled-environment studies, and suggest that leaf-level photosynthesis does not down-regulate in t hese species as a result of acclimation to CO2 enrichment in the field . This sustained enhancement of photosynthesis provides the opportunit y for increased growth and carbon storage by trees as the atmospheric concentration of CO2 rises, but many additional factors interact in de termining whole-plant and forest responses to global change.