Widespread pre-noon photosynthesis peak driven by afternoon photoprotection

Diurnal patterns of photosynthesis of ecosystems are theoretically expected to mimic those of incoming solar radiation (SW) and peaks at noon. By examining global ecosystem eddy covariance observations, however, we found ecosystem photosynthesis often peaks before noon, indicating widespread midday or afternoon photosynthesis depression. While some studies have attributed this depression to stomatal closure, a strategy that limits water loss under high atmospheric vapor pressure deficit (VPD), leaf-level studies suggest that excess light can trigger photoprotective responses and also cause the depression. Following the hypothesis, we studied the gaps between ecosystem carbon uptake peak and that of SW (0.48 ± 0.26 h), and found that the gaps advance increases with SW even on site-days characterized by the lowest VPD. Biomes receiving the highest SW, such as savannas and evergreen broadleaf forests, exhibit the largest gap between carbon uptake peak and SW peak. Together, these findings indicate that excess light is a key yet underappreciated driver of ecosystem-scale midday depression. Incorporating light-driven photoprotective processes into terrestrial carbon models may improve simulations of diurnal carbon fluxes.