Temperate forests regrowing from historical land use and land cover change in the eastern U.S. serve as carbon (C) sinks. Environmental drivers have been significantly altered (e.g., rising atmospheric CO₂ concentration, warmer temperature, and elevated nitrogen (N) deposition) and will have a wide range of impacts on future forest C sinks. However, the interactions among these environmental drivers are unclear and their effects are subject to uncertainty. We assessed the combined and interactive effects of rising CO₂ concentration, climate change (temperature, precipitation), and N deposition on forest aboveground net primary production (ANPP) and their relative contribution to ANPP changes of a temperate forest in the eastern U.S. We used a process-based ecosystem model PnET-Day to simulate coupled cycles of C, water, and N of forest ecosystems. We found that 1) climate change exerted negative effects on ANPP (-0.250 kg C m^-2 year^-1) whereas rising CO₂ and N deposition enhanced ANPP (+0.253, +0.014 kg C m^-2 year^-1); 2) climate change interacted with rising CO₂ and N deposition to decrease ANPP (-0.032, -0.018 kg C m^-2 year^-1); rising CO₂ and N deposition acted in synergy to increase ANPP (+0.014 kg C m^-2 year^-1); 3) changes in ANPP were mainly attributed to rising CO₂ and climate change whereas N deposition effects and any two- or three-factor interactive effects were relatively small. Our results suggest that the total negative effect sizes will not be offset by the total positive effect sizes, thus resulting in reductions in forest ANPP over the 21st century.

Temperate forests regrowing from historical land use and land cover change in the eastern U.S. serve as carbon (C) sinks. Environmental drivers have been significantly altered (e.g., rising atmospheric CO₂ concentration, warmer temperature, and elevated nitrogen (N) deposition) and will have a wide range of impacts on future forest C sinks. However, the interactions among these environmental drivers are unclear and their effects are subject to uncertainty. We assessed the combined and interactive effects of rising CO₂ concentration, climate change (temperature, precipitation), and N deposition on forest aboveground net primary production (ANPP) and their relative contribution to ANPP changes of a temperate forest in the eastern U.S. We used a process-based ecosystem model PnET-Day to simulate coupled cycles of C, water, and N of forest ecosystems. We found that 1) climate change exerted negative effects on ANPP (-0.250 kg C m^-2 year^-1) whereas rising CO₂ and N deposition enhanced ANPP (+0.253, +0.014 kg C m^-2 year^-1); 2) climate change interacted with rising CO₂ and N deposition to decrease ANPP (-0.032, -0.018 kg C m^-2 year^-1); rising CO₂ and N deposition acted in synergy to increase ANPP (+0.014 kg C m^-2 year^-1); 3) changes in ANPP were mainly attributed to rising CO₂ and climate change whereas N deposition effects and any two- or three-factor interactive effects were relatively small. Our results suggest that the total negative effect sizes will not be offset by the total positive effect sizes, thus resulting in reductions in forest ANPP over the 21st century.