Observed increasing light-use efficiency of terrestrial gross primary productivity

Abstract

Widespread global greening driven by CO₂ fertilization implies a denser canopy structure, and more leaves could be used to collect light from the atmosphere for plant photosynthesis. Whether this increase in leaf quantity could enhance the capacity of vegetation to convert absorbed light to photosynthate remains unclear. In this study, we investigate the spatial-temporal variations of canopy light-use efficiency (LUE), an indicator of leaf photosynthesis capacity, with FLUXNET recordings of 540 site-years and seven satellite-derived proxies. We find that flux tower measurements identify an increasing trend of LUE, and the temporal variations of cross-site LUE are mainly caused by nitrogen fertilization (18.09 %), temperature (17.06 %), and CO₂ fertilization (16.59 %). Globally, satellite-derived datasets also show widespread increasing LUE over the past two decades, most attributed to the nitrogen deposition and CO₂ fertilization effects, especially in evergreen broadleaf forests. Future projections of terrestrial LUE by CMIP6 Earth system models further suggest an overall increasing trend of LUE to the end of the 21st century. Our findings highlight the importance of vegetation physiology such as LUE in understanding of enhancement on terrestrial plant photosynthesis and carbon sink under climate change.