title: Leaf and canopy responses to elevated CO2in a pine forest under free-air CO2enrichment.
abstract: Physiological responses to elevated CO2at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO2using a free-air CO2enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO2over an 8-day exposure period was similar to that of trees exposed to current ambient CO2under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO2-exposed trees showed minor (≤7%) but significant reductions in relative sap flux density compared to trees under ambient CO2conditions. Short-term (minutes) direct stomatal responses to elevated CO2were also relatively weak (≈5% reduction in stomatal aperture in response to high CO2concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO2for nearly 80 days compared to foliage grown under current ambient CO2, so intrinsic leaf water-use efficiency at elevated CO2was enhanced primarily by direct responses of photosynthesis to CO2. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO2(Anet-Cicurves) for Pinus taeda foliage grown under elevated CO2(550 μmol mol-1) for 50-80 days compared to those for foliage grown under current ambient CO2from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 μmol mol-1CO2was enhanced by approximately 65% compared to the rate at ambient CO2(350 μmol mol-1). A similar level of enhancement under elevated CO2was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO2during the study period appears to be primarily attributable to direct photosynthetic responses to CO2in the pine forest, longer-term CO2responses and feedbacks remain to be evaluated.
authors.affiliation: Biosystems and Process Sciences Division, Department of Applied Science, Brookhaven National Laboratory, 11973, Upton, N.Y., USA.
authors.forename: David S
authors.initials: DS
authors.lastname: Ellsworth
authors.affiliation: School of the Environment, Duke University, 27708, Durham, N.C., USA.
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.affiliation: School of the Environment, Duke University, 27708, Durham, N.C., USA.
authors.forename: Ce
authors.initials: C
authors.lastname: Huang
authors.affiliation: School of the Environment, Duke University, 27708, Durham, N.C., USA.
authors.forename: Nathan
authors.initials: N
authors.lastname: Phillips
authors.affiliation: Biosystems and Process Sciences Division, Department of Applied Science, Brookhaven National Laboratory, 11973, Upton, N.Y., USA.
authors.forename: George R
authors.initials: GR
authors.lastname: Hendrey
keywords: Elevated CO2; Forest ecosystem; Photosynthesis; Pinus taeda; Stomata