Nature Climate Change, Published online: 02 February 2026; doi:10.1038/s41558-025-02528-2The authors consider the changing sensitivity of the leaf-onset date to temperature (ST) for boreal deciduous broadleaf forests. ST increased between 1982–1996 and 1998–2012—potentially linked to enhanced chilling accumulation—but this increase is underestimated in phenology models.