Global synchronous increase in light-saturated and peak vegetation productivity

Terrestrial gross primary productivity (GPP), which refers to the photosynthetic uptake of CO₂ by leaves, increases with incident irradiance and plateaus when leaves become light-saturated (GPP_sat). While evidence suggests enhanced peak vegetation productivity (GPP_max) since the 1980s, the existence of a light-saturation constraint on this enhancement remains unclear. Here, we combine the FLUXNET network's 1269 site-years measurements with a flux-based gridded dataset to construct the first observationally derived light-response curves for global vegetation productivity. Our results show a synchronous increase of GPP_max and GPP_sat during the study period. There is convergence to a common ratio between GPP_max and GPP_sat, with averaged ratio values of about 80% across global biomes. In particular, GPP_max was less light-saturated in evergreen broadleaved forests, suggesting significant potential for further productivity. The atmospheric CO₂ fertilization effect was the primary driver of the synchronous increase in GPP_sat and GPP_max, followed by temperature. Future projections by CMIP6 models indicate a continuing increase in GPP_max through the end of the 21st century. These findings illuminate the crucial role that rising peak and light-saturated vegetation productivity will play in sequestering atmospheric CO₂ under future climate change scenarios.