Increasing ozone levels (0, 50, 100, 150, 200, 300 ppb) coupled with 80% shading and 50% defoliation were examined for two year-old sugar maple seedlings for one growing season (June-September). Biomass, soluble root carbohydrates and mycorrhizal colonization progressively decreased with increasing ozone stress. Shading had a negative effect on vesicles and coils at certain sampling periods. While defoliation negatively affected biomass and mycorrhizal colonization at the beginning of this study, towards the end, it increased biomass and mycorrhizal colonization and appeared to make those defoliated seedlings more tolerant to ozone. This experiment also showed how sensitive mycorrhizal fungi are to plant stress.  In a second experiment, low/high CO 2 (350/650 ppm) coupled with low/high O 3 (10/200 ppb) was examined for biomass, total soluble carbohydrates and mycorrhizal fungi of one month-old sugar maple seedlings for 61 days. By day 61, high CO 2 seedlings had the highest biomass, followed by control, high CO 2 - high O 3 and high O 3 though none of these results were significant. The same trends could be seen for the individual plant parts (second leaves, stems and roots) yet only high ozone had a significant effect on second leaves. For the carbohydrate analysis, high O 3 decreased soluble carbohydrate concentrations for the first leaves and stems though these results were not significant. No mycorrhizal colonization was found for any of the treatments. In both experiments, root carbohydrates did not always give expected results pointing towards examining new factors that may affect mycorrhizal colonization within a host plant