Hydroclimate shapes photosynthetic sensitivity to cloud cover across global terrestrial ecosystems.

Vegetation photosynthesis primarily depends on surface energy and water availability, both of which are simultaneously regulated by clouds through radiation and precipitation, respectively. However, the net impact of cloud-induced changes in surface solar radiation and precipitation on photosynthesis remains elusive. Here, using observational- and model-based datasets spanning the past few decades, we show that, consistently across scales from site-level eddy covariance measurements to global-scale gridded datasets, the sensitivity of photosynthesis to cloud cover is spatially shaped by the hydroclimate, as quantified by the humidity index (mean annual precipitation-to-evapotranspiration ratio). Specifically, we find that in water-limited arid regions, clouds promote photosynthesis through increased precipitation, with a delayed effect typically within one month, whereas in energy-limited humid regions, they inhibit photosynthesis almost instantaneously by blocking sunlight. An annual scale spatially resolved sensitivity metric of photosynthesis to cloud cover is further examined to estimate potential changes in vegetation productivity driven by clouds. The findings indicate that, under a warming climate, particularly in the Coupled Model Intercomparison Project Phase 6 "ssp585" scenario (2015-2099), gross primary productivity is projected to decline in arid regions and increase in humid regions due to changes in cloud cover, suggesting an exacerbation of regional disparities in ecosystem functions.