Climate-driven divergence between leaf and stem phenology constrains carbon dynamics in trees

Leaves and stems jointly regulate carbon assimilation, water transport, and carbohydrate storage, forming the foundation of tree growth and forest carbon cycling. Coordinated phenology between these organs enables efficient allocation of photosynthates to structural biomass and long-term reserves, yet the resource allocation strategies that maintain the coordination under climate change remain elusive. By integrating paired leaf-stem phenological observations across the Northern Hemisphere with controlled warming, drought, and shading experiments, we find that, across sampled sites, autumn leaf phenology is on average more responsive to warming and drought than autumn stem phenology. This source–sink divergence in climate sensitivity is amplified in warm compared with cold regions and more pronounced in conifers than in broadleaves. Experimental evidence reveals that water stress, rather than carbon assimilation, drives this divergence, further constraining carbon dynamics and ultimately limiting tree growth. Model simulations project that trees may increasingly advance leaf senescence to reduce water loss while maintaining stem hydraulic function to ensure survival under intensifying future climate stress. Together, our findings reveal how divergent source–sink phenology under water stress constrains growth and carbon dynamics, underscoring the necessity of organ-level coordination for sustaining forest productivity and carbon sequestration under climate change.