Freeze-Thaw Dynamics Reshape Climatic Control of Spring Phenology Across Northern Ecosystems

Under global warming, spring phenology in northern ecosystems (tundra, boreal forests, temperate grasslands, coniferous forests) has advanced, yet the causes of regional disparities and nonlinear responses remain unclear. While temperature and precipitation are recognized primary drivers, the regulatory role of freeze-thaw cycles (FTCs) on the start of growing season (SOS) is largely overlooked. Integrating 20 years (2003-2022) of satellite phenology and climate reanalysis data, this study assesses FTCs frequency impacts on SOS dynamics across the pan-Northern Hemisphere. Despite SOS advancing 1.9 days/decade on average, over 28% of vegetated areas show stable or delayed trends, particularly in boreal forests, alpine regions, and tundra. This pattern is linked to the heterogeneous increase in FTCs frequency, which modulates SOS in biome-specific and nonlinear ways. Frequent FTCs advanced SOS in boreal forests by up to 7 days, likely due to cumulative thermal pulses that reduce dormancy depth. In contrast, desert and temperate forest systems experienced delays exceeding 20 days, associated with repeated low-temperature stress. Sensitivity analyses using ridge regression and generalized additive models revealed that FTCs accounted for up to 14.6% of SOS variability in temperate broadleaf forests-comparable to precipitation and radiation. However, SOS sensitivity to FTCs has declined over time, coinciding with shorter frozen seasons, while sensitivity to shortwave radiation increased. These shifts indicate a reorganization of climatic constraints on phenology. Biomes exhibited divergent sensitivity trajectories: forests and grasslands became more responsive to temperature and radiation, whereas FTCs influence declined in boreal systems but intensified in coniferous and shrub-dominated landscapes. Our findings highlight FTCs as dynamic regulators of phenology that can offset warming-driven advancement.