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Elevated CO2 Reduces the Negative Effects of Ozone on Aspen Clones
Isebrands, J.G., McDonald, E.P., Kruger, E., Hendrey, G., Percy, K., Pregitzer, K., Sober, J. and Karnosky, D.F.  2001.  Growth responses of Populus tremuloides clones to interacting elevated carbon dioxide and tropospheric ozone.  Environmental Pollution 115: 359-371.

What was done
The authors grew five aspen (Populus tremuloides) clones with varying tolerance to ozone for three years in 30-m diameter FACE plots located in Wisconsin, USA, receiving 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 treatment exposure, there were no significant effects of the extra 200 ppm of CO2 or ozone on apparent tree volume.  After two and three years, however, elevated CO2 increased apparent tree volume by 22 and 28%, respectively, while elevated ozone reduced it by 26% in each of those years.  Although elevated CO2 did not completely alleviate the negative affects of ozone on apparent tree volume, CO2-enriched trees that were simultaneously exposed to elevated ozone concentrations exhibited apparent volumes that were still 6 and 19% greater than volumes displayed by trees subjected to ambient CO2 and elevated ozone concentrations in the second and third years of the study, respectively.

What it means
If the tropospheric ozone concentration rises in the years ahead, it will likely impose a significant stress on regenerating aspen clones that will reduce their growth.  However, if the atmospheric CO2 concentration continues to rise, the negative impact of elevated ozone on aspen growth will be greatly reduced.