PROCEEDINGS: Index of Abstracts

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

J. G. Isebrands-1 and D. F. Karnosky-2

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.