The interval from leaf unfolding to colouring was 8 days longer at 63 Swiss beech stations in 2001–2025 than in 1961–1985

Did the leaf development interval of beech increase more at higher Swiss stations? We used open MeteoSwiss dates for European beech at the same 63 stations in 1961–1985 and 2001–2025. Each station had at least 12 years with both 50% unfolding and 50% colouring in each period. The selected elevations were 200–1350 m. We calculated the change in each station mean and gave all stations equal weight. We used 4999 joint station and shared five-year block bootstrap draws. The interval was 8.09 days longer in the later period (95% interval 4.01 to 12.18). Earlier unfolding contributed 7.16 days. Later colouring contributed 0.94 day, with an interval from −2.84 to +4.64. The elevation slope of interval change was −3.69 days per 1000 m (−17.11 to +9.98). Thus the data did not resolve whether higher stations gained more days. Coverage rules, other periods, leaf fall, median values, and geographic group resampling retained the mean interval increase. Equal year weights gave an 8.11-day increase. The result extends a known Swiss seasonal pattern through 2025. It does not establish a uniform mountain response or increased carbon uptake. Observer changes, tree changes, missing years, and the limited elevation range remain concerns. The package includes the unchanged source files, source URLs and hashes, all code, figures, results, and independent validation.

plant phenology · European beech · Switzerland · leaf unfolding · leaf colouring · elevation · paired records · season length · open data · reproducibility