What was done
The authors grew five aspen (Populus tremuloides) clones, with varying tolerance to ozone, for four 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.  In addition, the authors developed a "competitive stress index" based on the heights of the four nearest neighbors of each tree to study the influence of competition on the CO2 growth response of various clones as modified by ozone.

What was learned
Many complex interactions were discovered in this study.  Hence, we present but a brief summary of its key findings.  In general, elevated CO2 stimulated growth in young aspen trees with the relative growth enhancement being greater for competitively advantaged as opposed to competitively disadvantaged trees.  In contrast, elevated O3 typically reduced aspen growth independent of competitive status.  Interactions of elevated CO2 and O3 on growth were dependent upon clone and competitive status and were positive, negative or neutral in different situations.  Nevertheless, in the elevated CO2 and O3 treatment, the competitive performance between clones and their nearest neighbors was reduced.

What it means
As the air's CO2 concentration increases, aspen seedlings will likely exhibit increased growth, particularly when growing with a competitive edge over neighboring trees.  If the tropospheric ozone concentration continues to rise, it will likely impose a stress on regenerating aspen seedlings that will be modified by elevated CO2 in a clone- and competitive status-dependent manner.  Nonetheless, the authors note that "the apparent convergence of competitive performance responses in +CO2 +O3 conditions does suggest that stand diversity may be maintained at a higher level."  Thus, aspen trees will likely increase their biomass in the future while maintaining clonal diversity in regenerating stands.