Elevated carbon dioxide and ozone alter productivity and ecosystem carbon content in northern temperate forests

Three young northern temperate forest communities in the north‐central United States were exposed to factorial combinations of elevated carbon dioxide (CO₂) and tropospheric ozone (O₃) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO₂enhanced ecosystem C content by 11%, whereas elevated O₃decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO₂and O₃. Treatment effects on ecosystem C content resulted primarily from changes in the near‐surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r² = 0.96). Elevated CO₂enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m⁻²) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O₃lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆NPP/∆N) decreased through time with further canopy development, the O₃effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O₃and less soil C from 0.1 to 0.2 m in depth under elevated CO₂. Overall, these results suggest that elevated CO₂may create a sustained increase in NPP, whereas the long‐term effect of elevated O₃on NPP will be smaller than expected. However, changes in soil C are not well‐understood and limit our ability to predict changes in ecosystem C content.