January-February warmth had positive links to the hazel-to-beech spring gap in western and eastern Switzerland in 1961-2020

We tested whether seasonal warmth had different links to hazel flowering and beech leaf unfolding. We used 3,753 matched station-years at 85 Swiss stations in 1961-2020. Four station-effect models split the records by region and period. Each year had equal total weight. January-February slopes for the time from flowering to leaf unfolding were +3.96 to +5.36 days per degree C. All eight-test adjusted intervals were above zero. March-April slopes were -0.57 to +0.15 days per degree C; all adjusted intervals included zero. Stricter station coverage and station trends kept the positive winter slopes. Bounded date errors of two days per event did not reverse the point slopes, but larger errors can do so. These are retrospective associations. They do not establish a causal climate effect, a long-term gap increase, or an ecological mismatch. Coarse weather, shared regional climate and missing visit records limit inference.

phenology · hazel · beech · seasonal temperature · matched stations · observation bias · Switzerland