Leaf and canopy responses to elevated CO in a pine forest under free-air CO enrichment.

Physiological responses to elevated CO at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO using a free-air CO enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO over an 8-day exposure period was similar to that of trees exposed to current ambient CO under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO-exposed trees showed minor (≤7%) but significant reductions in relative sap flux density compared to trees under ambient CO conditions. Short-term (minutes) direct stomatal responses to elevated CO were also relatively weak (≈5% reduction in stomatal aperture in response to high CO concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO for nearly 80 days compared to foliage grown under current ambient CO, so intrinsic leaf water-use efficiency at elevated CO was enhanced primarily by direct responses of photosynthesis to CO. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO (A -C curves) for Pinus taeda foliage grown under elevated CO (550 μmol mol) for 50-80 days compared to those for foliage grown under current ambient CO from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 μmol mol CO was enhanced by approximately 65% compared to the rate at ambient CO (350 μmol mol). A similar level of enhancement under elevated CO was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO during the study period appears to be primarily attributable to direct photosynthetic responses to CO in the pine forest, longer-term CO responses and feedbacks remain to be evaluated.