Rising atmospheric CO₂ concentrations are expected to boost terrestrial plant productivity, reducing the airborne fraction of anthropogenic CO₂ emissions. However, this effect may be limited by nitrogen (N) availability, which constrains plant growth. A 11-year free-air CO₂ enrichment (FACE) experiment in a sweetgum forest in Tennessee showed that net primary productivity (NPP) increased under elevated CO₂ for the first 6 years but declined over time. The initial 24% increase in NPP under elevated CO₂ dropped to 9% by 2008, indicating a diminishing response. This decline was attributed to decreasing N availability, which became more pronounced as the forest matured. N limitation was confirmed by adding nitrogen fertilizer, which significantly increased NPP and reduced the relative allocation to fine roots. Leaf and stand-level observations showed that declining N availability constrained the tree response to elevated CO₂. These findings challenge previous assumptions that the CO₂ fertilization effect is sustained and highlight the importance of N cycling in ecosystem models. The study underscores the need for models to incorporate N limitations and feedbacks to accurately predict future climate change impacts. The results suggest that long-term N availability is critical for sustaining productivity in forests, and that without adequate N inputs, the positive effects of elevated CO₂ may be offset by N limitation. The study also highlights the importance of long-term experiments in understanding the complex interactions between CO₂, N cycling, and forest productivity.Rising atmospheric CO₂ concentrations are expected to boost terrestrial plant productivity, reducing the airborne fraction of anthropogenic CO₂ emissions. However, this effect may be limited by nitrogen (N) availability, which constrains plant growth. A 11-year free-air CO₂ enrichment (FACE) experiment in a sweetgum forest in Tennessee showed that net primary productivity (NPP) increased under elevated CO₂ for the first 6 years but declined over time. The initial 24% increase in NPP under elevated CO₂ dropped to 9% by 2008, indicating a diminishing response. This decline was attributed to decreasing N availability, which became more pronounced as the forest matured. N limitation was confirmed by adding nitrogen fertilizer, which significantly increased NPP and reduced the relative allocation to fine roots. Leaf and stand-level observations showed that declining N availability constrained the tree response to elevated CO₂. These findings challenge previous assumptions that the CO₂ fertilization effect is sustained and highlight the importance of N cycling in ecosystem models. The study underscores the need for models to incorporate N limitations and feedbacks to accurately predict future climate change impacts. The results suggest that long-term N availability is critical for sustaining productivity in forests, and that without adequate N inputs, the positive effects of elevated CO₂ may be offset by N limitation. The study also highlights the importance of long-term experiments in understanding the complex interactions between CO₂, N cycling, and forest productivity.