Paired Swiss tree records gave negative autumn-minus-spring species changes after 2000, but trend-adjusted shifts were uncertain

We tested a species and season contrast in 2,443 paired station-years at 84 Swiss stations. Every record had horse chestnut and beech leaf unfolding and autumn colour dates. Models compared 1981-1997 with 2001-2010 and 2011-2020 in separate western and eastern regions. Station and temperature-adjusted changes in the autumn species gap minus the spring species gap ranged from -8.38 to -9.41 days. Three of four Bonferroni family intervals excluded zero. The later western interval included zero. Stable-station checks gave similar values. A continuous year trend reduced the changes, and all marginal intervals then included zero. Thus these records show a species and season change, but do not clearly separate a late-1990s shift from a longer trend. Selected complete records, missing visits, coarse weather and no pest or pollution measurements limit interpretation. We make no causal claim about climate, pests or pollution. The package gives code, source hashes, aggregate results, numerical checks and an exact-input retrieval method.

phenology · observation bias · horse chestnut · beech · temperature · matched records · trend sensitivity