Evidence for widespread and strong canopy photosynthetic acclimation to diffuse light

Increased diffuse radiation is known to enhance plant photosynthesis instantaneously, yet its role in regulating photosynthetic light acclimation over long-term remains elusive. Using global eddy-covariance observations and the accompanied diffuse and direct radiation measurements, we investigated how diffuse radiation controls canopy-scale light acclimation rates across diverse ecosystems. Our results showed that the maximum photosynthetic assimilation rate (A_max) is on average 40% higher under diffuse than direct radiation, consistent with the instantaneous diffuse radiation fertilization effect. As for light acclimation, we found the acclimation rate driven by diffuse light (1.8 μmol m⁻² s⁻¹ per mol photon m⁻² d⁻¹) is more than twice those under direct light (0.8 μmol m⁻² s⁻¹ per mol photon m⁻² d⁻¹). Statistical analysis showed that diffuse radiation fraction is important in determining canopy-scale light acclimation rate. The benefits from diffuse light on light acclimation weakened under high air temperature and elevated vapor pressure deficit but increased strongly with absorbed light. These findings demonstrate that diffuse radiation enhances ecosystem photosynthesis not only instantaneously but also by accelerating long-term light acclimation. Given ongoing changes in atmospheric aerosol loading and cloud cover, accounting for this photosynthetic acclimation effect of diffuse light is essential for improving predictions of terrestrial carbon uptake under changing atmospheric conditions.

Keywords: Light acclimation rate; FLUXNET; diffuse fertilization effects