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

What was learned
Elevated ozone exposure caused visible foliar injuries in aspen clones simultaneously exposed to ambient atmospheric CO2 concentrations.  In contrast, aspen clones exposed to both elevated ozone and CO2 concentrations exhibited 40% fewer visible foliar injuries than clones exposed to elevated ozone and ambient CO2.

On another note, elevated ozone tended to increase the activities of several antioxidant enzymes, including catalase, glutathione reductase and ascorbate peroxidase; while elevated CO2 alone, and in combination with elevated ozone, generally decreased the activities of these enzymes.

What it means
As the tropospheric ozone concentration continues to rise, it will likely continue to impose an ever more severe stress on regenerating aspen clones that will likely cause reductions in their productivity and growth.  However, if the CO2 content of the air also continues to increase, the negative effects of elevated ozone on aspen growth likely will be reduced, thus allowing aspen trees to fare better under the stressful environmental conditions resulting from elevated ozone concentrations.  In addition, the CO2-induced closure of leaf stomata will likely reduce the indiscriminate uptake of ozone, thereby reducing oxidative damage and the need for elevated levels of antioxidant enzymes.  Thus, important resources can be allocated away from antioxidants and into other enzymes and materials needed to sustain enhanced plant growth under elevated atmospheric CO2 and ozone conditions.