Spring leaf-out keeps pace with warming across the Northern Hemisphere

Advances in spring leaf-out are among the most visible biological responses to climate warming, with far-reaching consequences for ecosystem functioning. Recent reports of a slowdown in these advances have been widely interpreted as evidence that trees are becoming less responsive to rising spring temperatures, yet this possibility has not been evaluated at broad spatial and temporal scales. Using six decades of ground observations across Europe together with satellite-derived phenology from Northern Hemisphere deciduous broadleaf forests, we show that the apparent slowdown reflects changes in warming rates and the statistical properties of commonly used sensitivity metrics rather than a decline in biological temperature responsiveness. Across species and regions, spring temperature remains the dominant and temporally stable driver of leaf-out timing, with an effect size at least three times larger than that of any other climatic or plant-internal factor. Warmer winter temperatures exert a modest delaying effect, likely through reduced chilling accumulation, that has increased in recent decades and shows strong biogeographic structure, but has not measurably weakened the advancing influence of spring warming. Simulations with fixed thermal requirements reproduce apparent declines in temperature sensitivity (in days per degree), demonstrating that such declines can arise mathematically from advancing phenology and increasing temperature variability, without changes in biological responsiveness. Together, these results demonstrate that trees have not lost sensitivity to warming and suggest that future spring phenology will remain tightly coupled to temperature.