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

Anet, 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 ε, 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.