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Seasonal and long-term effects of CO2 and O3 and their interaction with climate and soil moisture on water loss in ponderosa pine
Citation:
LEE, E., D. T. TINGEY, R. S. WASCHMANN, D. L. PHILLIPS, D. M. OLSZYK, M. G. JOHNSON, AND W. E. HOGSETT. Seasonal and long-term effects of CO2 and O3 and their interaction with climate and soil moisture on water loss in ponderosa pine. Presented at 23rd IUFRO Conference for Specialists in Air Pollution and Climate Change Effects on Forest Ecosystems, Murten, SWITZERLAND, September 07 - 12, 2008.
Impact/Purpose:
Evapotranspiration (ET) is driven by evaporative demand, available solar energy and soil moisture as well as by physiological plant activity which may be substantially affected by elevated CO2 and O3.
Description:
Evapotranspiration (ET) is driven by evaporative demand, available solar energy and soil moisture as well as by physiological plant activity which may be substantially affected by elevated CO2 and O3. A multi-year study was conducted in outdoor sun-lit controlled-environment chambers containing ponderosa pine seedlings growing in a reconstructed soil-litter system. The study used a 2 x 2 factorial design with two concentrations of CO2 (ambient, elevated) and two levels of O3 (low, high) and three replicates of each treatment. The objective of our study was to assess the effects of chronic exposure to elevated CO2 and O3, singly and in combination, on daily ET. The study evaluated three hypotheses: (1) because elevated CO2 stimulates stomata closure, O3 effects on ET will be less under elevated CO2 than ambient CO2; (2) elevated CO2 will ameliorate the long-term effects of O3 on ET; and (3) because stomatal conductance (g) decreases with decreasing soil moisture, the impacts of elevated CO2 and O3, alone and in combination, on water loss via g will be greater in mid-season when soil moisture is not limiting than other times of the year. A mixed-model covariance analysis was used to adjust daily ET for seasonality and the effects of soil moisture when testing for the effects of CO2 and O3 on ET via the vapor pressure deficit (VPD) gradient. Empirical results indicated that the interactive stresses of elevated CO2 and O3 resulted in a greater reduction in ET via reduced canopy stomatal conductance than the sum of the individual effects of each gas. O3-induced reductions in ET and stomatal conductance under elevated CO2 were more pronounced when trees were physiologically most active and increased over time. The persistence and carry-over of O3 effects on ET and stomatal conductance under elevated CO2 was likely due to impaired stomatal behavior over time because needle area and root biomass were not affected by earlier exposures to elevated O3 in our study.