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PROCEEDINGS: Index of Abstracts

INTERACTING EFFECTS OF OZONE AND CO2 ON GROWTH AND PHYSIOLOGICAL PROCESSES IN NORTHERN FOREST TREES

1-USDA Forest Service, North Central Forest Experiment Station, Rhinelander, WI 54501. 2-School of Forestry, Michigan Tech University, Houghton, MI 49931.

Globally, surface-level concentrations of both CO2 and ozone (O3) are increasing annually. Because many studies have shown beneficial effects of increasing CO2, predictions have been made that elevated levels of CO2 would compensate for growth decreases caused by O3. For the past two years, we have been examining the interaction of O3 and CO2 on trembling aspen (Populus tremuloides) and eastern white pine (Pinus strobus) in open-top chamber studies involving both plants in pots and plants growing in the ground.

After two seasons of exposure to elevated ozone, alone or in combination with elevated CO2 (ambient plus 150 ppm), soil-grown aspen and eastern white pine trees are exhibiting different response. While neither of the two pine seed sources has been negatively affected by ozone, significant negative effects of O3 have been found for two aspen clones differing in O3 tolerance: The negative impact of ozone was not compensated by CO2 and for some physiological responses such as photosynthesis, stomatal conductance, chlorophyll content and leaf abscission, a significant negative interaction has been demonstrated for O3 plus CO2 treatment. Second-year growth and biomass measurements appear to be following our physiological measurements. Crown architecture has also been altered by the O3 and CO2 combination.

In addition, elevated CO2 appears to alter the sensitivity of the tolerant aspen clone, making it more sensitive to O3, as determined both by gas exchange and biomass.