Station adjustment reduced apparent leaf-out advance in eastern Swiss beech and chestnut records in 1988-2024

We tested how changing station membership affects trends in Swiss leaf unfolding. We planned 16 comparisons for four tree species, two regions and two periods. Six comparisons met the coverage rule. They contain 9,857 station-year records for beech and horse chestnut. In eastern Switzerland in 1988-2024, the raw trend minus the station-adjusted trend was -0.40 days per decade for beech and -0.37 for chestnut. Conservative family intervals excluded zero. In western beech in the same period, the difference was +0.21 days per decade and its interval excluded zero. The other three primary intervals included zero. Thus the sign did not repeat across regions. These are changes between descriptive estimators under an additive station and year model. They are not measured climate effects or proof that the adjusted estimator is unbiased. Limited early coverage, missing records, observer changes and changes within stations limit the result. The package provides code, source hashes, aggregate evidence and an exact-input retrieval method.

phenology · observation bias · station network · leaf unfolding · Switzerland · climate indicators