LEAF AND CANOPY RESPONSES TO ELEVATED CO2 IN A PINE FOREST UNDER FREE-AIR CO2 ENRICHMENT

Physiological responses to elevated CO2 at the leaf and canopy-level w ere studied in an intact pine (Pinus taeda) forest ecosystem exposed t o elevated CO2 using a free-air CO2 enrichment (FACE) technique. Norma lized canopy water-use of trees exposed to elevated CO2 over an 8-day exposure period was similar to that of trees exposed to current ambien t CO2 under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO2-exposed trees showed minor (less than or equal to 7%) but significant reductions in relative sap flux d ensity compared to trees under ambient CO2 conditions. Short-term (min utes) direct stomatal responses to elevated CO2 were also relatively w eak (approximate to 5% reduction in stomatal aperture in response to h igh CO2 concentrations). We observed no evidence of adjustment in stom atal conductance in foliage grown under elevated CO2 for nearly 80 day s compared to foliage grown under current ambient CO2 so intrinsic lea f water-use efficiency at elevated CO2 was enhanced primarily by direc t responses of photosynthesis to CO2. We did not detect statistical di fferences in parameters from photosynthetic responses to intercellular CO2 (A(net)-C-i curves) for Pinus taeda foliage grown under elevated CO2 (550 mu mol mol(-1)) for 50-80 days compared to those for foliage grown under current ambient CO2 from similar-sized reference trees nea rby. In both cases, leaf net photosynthetic rate at 550 mu mol mol(-1) CO2 was enhanced by approximately 65% compared to the rate at ambient CO2 (350 mu mol mol(-1)). A similar level of enhancement under elevat ed CO2 was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO2 duri ng the study period appears to be primarily attributable to direct pho tosynthetic responses to CO2 in the pine forest, longer-term CO2 respo nses and feedbacks remain to be evaluated.