What was done
Aspen (Populus tremuloides) clones with varying degrees of tolerance to ozone were grown together in randomized proportions with sugar maple and paper birch (Betula papyrifera) trees for three years in 30-m diameter FACE plots subjected to atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated ozone (1.5 times ambient ozone concentration) to study the interactive effects of these trace gases on leaf ultrastructure.

What was learned
In the aspen clones, ozone exposure caused significant structural injuries to thylakoid membranes and the stromal compartment within chloroplasts; but these injuries were largely ameliorated by atmospheric CO2 enrichment.  Likewise, leaf thickness, mesophyll tissue thickness, the amount of chloroplasts per unit cell area, and the amount of starch in chloroplasts were all decreased in the high ozone treatment; but in the case of these leaf characteristics, the simultaneous exposure of the ozone-stressed trees to elevated CO2 more than compensated for the ozone-induced reductions.  In birch, the competing responses of elevated CO2 and ozone on leaf ultrastructure were hardly evident, except for total leaf and mesophyll tissue thickness, where ozone-induced injuries in thylakoid membranes were partially ameliorated by concurrent atmospheric CO2 enrichment.

What it means
As the tropospheric ozone concentration continues to rise, it will likely pose a problem for regenerating aspen and birch trees by negatively affecting chloroplast ultrastructure at the site of carbon fixation, which will likely decrease productivity and growth.  However, if the atmospheric CO2 concentration also continues to rise, the various negative effects of elevated ozone will be either partly, completely or more than completely ameliorated, thus stimulating productivity and growth to varying degrees within these species.