Divergent climate–phenology–carbon mechanisms in deciduous needleleaf and broadleaf forests across the northern hemisphere

Global climate is shifting the phenological rhythms of deciduous forests, yet how phenology affects subsequent carbon sequestration, and whether these dynamics vary by leaf functional type, remains unclear. Using satellite records, we investigated the spatial-temporal variations in the start of the growing season (SOS) and net primary productivity (NPP), along with their responses to climate variations across the Northern Hemisphere, and further the differences in their relationships between deciduous needleleaf forests (DNF) and deciduous broadleaf forests (DBF). From 2001 to 2021, DNF advanced its SOS 3.8 times faster than DBF (−0.34 vs. −0.09 d·yr −1 ), but this pattern largely reflects the concentration of DNF in rapidly warming high-latitude regions. After controlling for spatial and climatic background, the intrinsic SOS advancement was stronger in DBF (LMM coefficient: −0.605). DBF and DNF showed comparable and significant NPP increases (2.28 and 2.17 g C·m −2 ·yr −1 , respectively). These similar productivity gains were achieved through fundamentally different pathways: In DBF, NPP was decoupled from SOS and driven directly by current and prior-year precipitation increases. In DNF, earlier SOS and increased precipitation were jointly associated with productivity. Specifically, prior-year and winter precipitation can influence the SOS of DNF; increased winter precipitation was associated with DNF NPP directly through its influence on physiological performance and indirectly by mediating spring leaf-out. With each day of advancement yielding ∼3.505 g C·m −2 of additional NPP (non-standardized coefficient), ∼45.3% of winter precipitation's effects on NPP were mediated through this phenological pathway. Interestingly, spring temperature was the common SOS driver for both DNF and DBF, but legacy precipitation effects uniquely amplified DNF's phenological sensitivity. These results highlight a fundamental divergence in climate–phenology–carbon sequestration potential coupling between leaf forms, underscoring the need for leaf-form-specific approaches in carbon sink projections under future climate scenarios.