Climate constrains the enhancement of CO₂ fertilization on forest gross primary productivity

Forest gross primary production (GPP) is influenced by the interplay between climate conditions and atmospheric CO₂ levels, which interact in complex ways, generating both compensating and amplifying effects. In this study, eddy covariance flux measurements from 50 forest ecosystems were integrated with simulations from 14 terrestrial biosphere models to investigate how climate conditions and atmospheric CO₂ concentrations regulate forest GPP. This approach bridges site-level observations with biome-scale model estimates to develop a global understanding. Our findings suggest that in boreal and cold temperate regions, temperature primarily constrains the enhancement of the CO₂ fertilization on forest GPP; however, warming and higher atmospheric CO₂ levels are projected to alleviate these limitations. In tropical forests, CO₂ fertilization strongly enhances GPP, but this benefit will be counterbalanced by the adverse impacts of projected climate warming. Consequently, the interplay between climate and atmospheric CO₂ in affecting forest GPP is dynamic and subject to continual change.