Elevated concentrations of atmospheric CO2 and tropospheric O3 will profoundly influence future forest productivity, but our understanding of these influences over the long-term is poor. Leaves are key indicators of productivity and we measured the mass, area, and nitrogen concentration of leaves collected in litter traps from 2002 to 2008 in three young northern temperate forest communities exposed to elevated CO2 and/or elevated O3 since 1998. On average, the overall effect of elevated CO2 (+CO2 and +CO2+O3 versus ambient and +O3) was to increase leaf mass by 36% whereas the overall effect of elevated O3 was to decrease leaf mass by 13%, with similar effects on stand leaf area. However, there were important CO2 × O3 × year interactions wherein some treatment effects on leaf mass changed dramatically relative to ambient from 2002 to 2008. For example, stimulation by the +CO2 treatment decreased (from +52 to +25%), whereas the deleterious effects of the +O3 treatment increased (from −5 to −18%). In comparison, leaf mass in the +CO2+O3 treatment was similar to ambient throughout the study. Forest composition influenced these responses: effects of the +O3 treatment on community-level leaf mass ranged from +2 to −19%. These findings are evidence that community composition, stand development processes, CO2, and O3 strongly interact. Changes in leaf nitrogen concentration were inconsistent, but leaf nitrogen mass (g m−2) was increased by elevated CO2 (+30%) and reduced by elevated O3 (−16%), consistent with observations that nitrogen cycling is accelerated by elevated CO2 but retarded by elevated O3.