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Carbon Dioxide vs. Ozone Effects in Birch and Aspen Trees
Reference
Riikonen, J., Kets, K., Darbah, J., Oksanen, E., Sober, A., Vapaavuori, E., Kubiske, M.E., Nelson, N. and Karnosky, D.F. 2008. Carbon gain and bud physiology in Populus tremuloides and Betula papyrifera grown under long-term exposure to elevated concentrationsa of CO2 and O3. Tree Physiology 28: 243-254.

What was done
The authors studied various physiological consequences of increases in the atmospheric concentrations of CO2 (+36%) and O3 (+39%) -- alone and in combination -- in paper birch (Betula papyrifera Marsh.) and trembling aspen (Populus tremuloides Michx.) trees during the 8th-9th years of growing-season CO2 enrichment at the Aspen FACE site near Rhinelander, Wisconsin (USA).

What was learned
Riikonen et al. report that "elevated O3 decreased net photosynthesis in birch short-shoot leaves by 27%, aspen Clone 42E by 30% and Clone 271 by 13%, averaged over the growing season, and in birch long-shoot leaves by 23% and aspen Clone 216 by 42% in the late-season," while "elevated CO2 increased net photosynthesis in birch short-shoot leaves by 49% and in aspen Clones 42E and 271 by 73 and 52%, respectively, averaged over the growing season," and that "in birch long-shoot leaves and aspen Clone 216, measured in the late-season only, elevated CO2 enhanced net photosynthesis by 42%." They also observed that "elevated CO2 delayed, and elevated O3 tended to accelerate, leaf abscission in autumn." And when both treatments were applied together, they found that "elevated CO2 generally ameliorated the effects of elevated O3," noting that "in both species, leaf stomatal conductance was usually lowest in the combination treatment, which probably caused a reduction in O3 uptake."

What it means
Much has been written about the negative effects of ozone pollution on plant growth and development. When near-equal percentage increases in both the air's ozone and CO2 concentrations occur concurrently, however, the negative effects of ozone pollution are typically more than overpowered by the aerial fertilization and transpiration-reducing effects of atmospheric CO2 enrichment, as they were in this study.

Reviewed 14 May 2008