Spring leaf and needle dates were 5.9 days earlier at 56 Swiss sites in 1996–2025 than in 1961–1990; changes in species gaps were uncertain

We tested whether spring leaf and needle dates became closer across beech, hazel, larch and spruce at 56 fixed Swiss sites. We used unchanged MeteoSwiss records and complete four-species site-years. Each site had at least 15 years in both 1961–1990 and 1996–2025. The mean date was 5.91 days earlier in the later period. Its 95% joint site-and-year resampling interval was −8.42 to −2.94 days. The mean of six absolute species gaps decreased from 15.61 to 14.53 days. The change was −1.08 days, with an interval of −2.63 to +0.51 days. Site-only resampling gave −2.02 to −0.09 days. Thus, the calendar-year sample changed the assessment of the gap sign. All planned sensitivity intervals for the mean gap included zero. Checks for unequal year coverage and grouping of nearby sites gave similar results. An independent script matched all 11,352 dates in the retained site panel and the main estimates. The evidence supports earlier spring stages at these sites but does not establish smaller species gaps or constant gaps. Missing records, observer changes, event definitions, and assumptions about time and space dependence limit inference. The package contains source data, provenance, code, all results, draws, figures, and offline reproduction instructions.

phenology · MeteoSwiss · leaf unfolding · needle emergence · species date gaps · shared weather · reproducibility