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Effects of Elevated CO2 and Ozone on Aspen Clones
Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment. Water, Air, and Soil Pollution 116: 311-322.

What was done
The authors grew O3-sensitive and O3-tolerant aspen (Populus tremuloides) clones for one year in 30-m diameter FACE plots located in Wisconsin, USA, at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient ozone concentration) to study the interactive effects of these parameters on this deciduous tree species.

What was learned
After one year of growth at ambient CO2, elevated O3 had caused visible injury to leaves of both types of aspen, with the average percent damage in O3-sensitive clones being more than three times as great as that observed in O3-tolerant clones (55 vs. 17%, respectively).  In combination with atmospheric CO2 enrichment, however, O3-induced damage to leaves of these same clones was only 38 and 3%, respectively.  Thus, elevated CO2 ameliorated much of the physical foliar damage induced by high O3 concentrations.

Elevated CO2, alone, significantly increased net rates of light-saturated photosynthesis in trembling aspen by an average of 43%.  In addition, elevated CO2 decreased dark respiration rates by 24% and lowered the light compensation point by 36%.  Thus, atmospheric CO2 enrichment not only increased the amount of light-saturated carbon uptake in these trees, it also enabled carbon to be assimilated at significantly lower light intensities, while retaining greater proportions of that carbon by reducing dark respiration rates.

What it means
As the tropospheric ozone concentration continues to rise, it will likely continue to impose a stress upon plants that physically damages their tissues and negatively impacts their photosynthetic rates and other physiological processes.  Fortunately, because the CO2 content of the air is also on a rising trajectory, it should provide plants, including trembling aspen, with an increasing degree of protection from this harmful aerial pollutant.  Moreover, with increasing photosynthetic rates, and greater carbon sequestering abilities, trees like these aspen clones will likely increase their growth rates and biomass production as the air's CO2 concentration continues to rise.

Reviewed 15 March 2000