Climate Variability Drives Phenological Decoupling of Dryland Photosynthesis from Canopy Greenness and Water Content

Drylands strongly modulate interannual variability of the global land carbon sink, yet photosynthetic seasonality is often inferred from vegetation greenness and canopy water content under the assumption that canopy dynamics and carbon uptake remain tightly coupled. Here we show that this assumption is widely violated. Across global drylands during 2001–2020, satellite-derived photosynthesis becomes increasingly decoupled from both greenness and canopy water content in the timing of growing-season onset and senescence. Approximately 35% of drylands exhibit significant decoupling at both the start and end of the growing season, with pronounced hotspots in the Sahel, Central Asia and Australia, where the correlation between photosynthetic phenology (SIF-derived) and canopy dynamic phenology (EVI2/VOD) declined by 0.42–0.48 from 2001–2010 to 2011–2020. Spatial attribution indicates that higher precipitation seasonality drives start-of-season decoupling, whereas higher temperature seasonality drives end-of-season decoupling, with both strengthened under elevated CO₂. However, state-of-the-art process-based models fail to reproduce either the emergent decoupling patterns or their inferred controls, suggesting that key nonlinear responses of dryland vegetation to hydroclimatic variability and CO₂ are misrepresented. This widespread decoupling suggests that changes in canopy condition no longer provide a consistent proxy for when carbon uptake begins or ends, potentially biasing estimates of terrestrial carbon sequestration under ongoing climate change. By pinpointing where and why dryland productivity decouples from canopy dynamics, our analysis reveals key model limitations and provides new constraints for predicting dryland carbon uptake and carbon–climate feedbacks under ongoing climate change.