RESPONSES OF FOLIAR GAS-EXCHANGE TO LONG-TERM ELEVATED CO2 CONCENTRATIONS IN MATURE LOBLOLLY-PINE TREES

Branches of field-grown mature loblolly pine (Pinus taeda L.) trees were exposed for 2 years (1992 and 1993) to ambient or elevated CO2 concentrations (ambient + 165 mu mol mol(-1) or ambient + 330 mu mol mol(-1) CO2). Exposure to elevated CO2 concentrations enhanced rates of net photosynthesis (P-n) by 53-111% compared to P-n of foliage exposed to ambient CO2. At the same CO2 measurement concentration, the ratio of intercellular to atmospheric CO2 concentration (C-i/C-a) and stomatal conductance to water vapor did not differ among foliage grown in an ambient or enriched CO2 concentration. Analysis of the relationship between P-n and C-i indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase during growth in elevated CO2 concentrations. Based on estimates derived from P-n/C-i curves, there were no apparent treatment differences in dark respiration, CO2 compensation point or P-n at the mean C-i. In 1992, foliage in the three CO2 treatments yielded similar estimates of CO2-saturated P-n (P-max), whereas in 1993, estimates of P-max were higher far branches grown in elevated CO2 than in ambient CO2. We conclude that field-grown loblolly pine trees do not exhibit downward acclimation of leaf-level photosynthesis in their long-term response to elevated CO2 concentrations.