Elevated shortwave radiation enhances the limitation of atmospheric dryness on forest carbon sequestration

Since the 1980s, both incoming shortwave radiation (SW) and atmospheric vapor pressure deficit (VPD) have increased significantly across Europe and are projected to continue rising in the coming decades, potentially altering forest photosynthesis, carbon uptake, and carbon storage. However, the joint impacts of SW and VPD on forest carbon sequestration remain poorly understood. Here, using half-hourly flux tower observations combined with remotely sensed vegetation indices, we show that SW and VPD are the dominant energy- and water-related drivers of forest net ecosystem exchange (NEE; negative values indicate net carbon uptake) at the half-hourly scale in Europe. We identify a distinct threshold in the VPD–NEE relationship at approximately 7 hPa, beyond which influence of VPD shifts to positive values and strengthens sharply. The influence of VPD on NEE is strongly mediated by SW, increasing gradually under low SW conditions but intensifying sharply under high SW conditions. This pattern arises because strong solar radiation amplifies VPD-induced stomatal closure, thereby suppressing photosynthesis and altering ecosystem carbon exchange. Together, these findings suggest that future increases in atmospheric dryness—potentially reinforced by continued brightening—may substantially constrain forest productivity and carbon sequestration. Our results underscore the importance of accounting for SW–VPD interactions in climate impact assessments and forest management strategies.