CO2 enrichment in a maturing pine forest: are CO2 exchange and water status in the canopy affected?

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Abstract
A net, leaf net CO2 assimilation ca, CO2 concentration of air surrounding a leaf ci, leaf intercellular CO2 concentration Δ, 13C isotope discrimination δ13C, relative stable carbon isotope content ɛ, ratio of Anet at ca = 560μmol mol–1 to Anet at ca = 360 μmol mol–1 FACE, free‐air CO2 enrichment gw, stomatal conductance to water vapour Πi, initial leaf osmotic potential Rt, relative water content at incipient turgor loss Ψl, xylem water potential of leaves Ψm, soil matric potential Elevated CO2 is expected to reduce forest water use as a result of CO2‐induced stomatal closure, which has implications for ecosystem‐scale phenomena controlled by water availability. Leaf‐level CO2 and H2O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free‐air CO2 enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO2. Current‐year foliage in the canopy was continuously exposed to elevated CO2 (ambient CO2+200μmol mol–1) in free‐air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO2 compared with ambient controls. This enhancement was similar in current‐year, ambient‐grown foliage temporarily measured at elevated CO2 compared with long‐term elevated CO2 grown foliage. Significant photosynthetic enhancement by CO2 was maintained over a range of conditions except during peak drought.There was no evidence of water savings in elevated CO2 plots in FACE compared to ambient plots under drought and non‐drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO2 versus ambient trees of P. taeda. Calculations of time‐integrated ci/ca ratios from analysis of foliar δ13C showed that these ratios were maintained in foliage under elevated CO2. Second, soil moisture was not significantly different between ambient and elevated CO2 plots during drought. Third, pre‐dawn and mid‐day leaf water potentials were also unaffected by the seasonal CO2 exposure, as were tissue osmotic potentials and turgor loss points. Together the results strongly support the hypothesis that maturing P. taeda trees have low stomatal responsiveness to elevated CO2. Elevated CO2 effects on water relations in loblolly pine‐dominated forest ecosystems may be absent or small apart from those mediated by leaf area. Large photosynthetic enhancements in the upper canopy of P. taeda by elevated CO2 indicate that this maturing forest may have a large carbon sequestration capacity with limiting water supply.

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