Mean date correlation among nine tree species was higher in spring than in autumn at 110 Swiss stations in 1996–2025

Do tree species at one station vary together more in spring than in autumn? We tested MeteoSwiss records for nine species in 1996–2025. We selected 1,889 species pairs at 110 stations. Each pair had at least 20 years with all four dates. Both seasons used the same years. We removed a linear trend from each series and gave each station equal weight. Mean correlation was 0.548 for spring unfolding and 0.361 for autumn colouring. The difference was 0.188, with a 95% interval of 0.090–0.289 from common five-year blocks across the network. Rank dates, adjacent-year changes, removal of exact ties, and stricter coverage gave positive mean differences. Species-pair means varied from −0.052 to +0.483. A different sample selected within 1961–2025 gave an uncertain difference of 0.052 [−0.029, 0.137]. The evidence supports a recent network mean contrast. It does not show a change over time, a climate cause, or a difference for every species pair. Source files, provenance, all results, figures and reproduction code are preserved.

phenology · MeteoSwiss · tree species · spring · autumn · correlation · matched years · open data