Model communities of trembling aspen ( Populus tremuloides ), paper birch ( Betula papyrifera ) and sugar maple ( Acer saccharum ) were grown for eight years in elevated CO 2 and O 3 using FACE technology. Annual growth measurements of tree height and diameter were used to calculate stem volume growth, that was then analyzed for potential changes in community composition. Given the nature of long-term changes in forest communities, we utilized trends in species' importance as indications of shifting community composition. Importance values were calculated as an index of volume growth and survival. In all communities, elevated CO 2 increased growth, elevated O 3 decreased it, and the combination treatment of elevated CO 2 + O 3 had no net effect on growth. The aspen-birch mixed community consisted of a relatively even canopy comprised of aspen and birch trees from 4-7 m tall. Elevated O 3 decreased aspen volume growth by 26% compared to controls whereas paper birch volume growth was not affected. In contrast, elevated CO 2 increased volume growth of aspen by 60%, and that of paper birch by 86%. In those same stands, aspen had 21-24% mortality whereas paper birch had 4-6% mortality. The result of these CO 2 and O 3 effects over eight years of the experiment was to change importance of aspen and birch in elevated O 3 by –27% and +87%, respectively, and in elevated CO 2 , by –10% and +45%, respectively. In the controls, importance of aspen and birch changed by -16% and +62%, respectively, after eight years. Thus, the presence of elevated O 3 hastens conversion of stands to paper birch, whereas the presence of elevated CO 2 delays it. The aspen-birch mixed community consisted of an open canopy of aspen aobut 4-7 m tall depending upon treatment, interspersed with a mid-story of sugar maple 2-4 m tall. Compared to control treatments, elevated O 3 decreased aspen volume growth after 8 years by 10%, and decreased sugar maple volume growth by 27%. Aspen and maple had 11-14% mortality in elevated O 3 . Interestingly, aspen volume growth was 72% greater and that of maple was 25% less in elevated CO 2 compared to control treatments. In elevated CO 2 , aspen had 9% mortality and sugar maple had 28% mortality. After 8 years in elevated O3, relative importance of aspen and maple changed by +13% and +33%, respectively. In elevated CO 2 , relative importance of aspen and maple changed by +19 and –44%, respectively. In the control treatments, relative importance of aspen and maple changed by +26% and +3%, respectively, after 8 years. Thus, elevated O 3 increases the rate of conversion of aspen stands to sugar maple, but maple is placed at a competitive disadvantage to aspen under elevated CO 2 .