Spring dates had positive but uncertain links to autumn colour in four Swiss beech samples in 1961-2020

We tested whether later spring leaf unfolding was linked to later autumn leaf colour after station, year and seasonal weather controls. We used 3,428 paired station-years at the same 72 Swiss beech stations in 1961-1990 and 1991-2020. Four separate models split the records into western and eastern regions. Spring-date slopes were positive, from 0.043 to 0.178 days per day. All four intervals included zero after correction for four tests. The later eastern slope had a marginal 95 percent interval above zero, but its corrected interval was -0.035 to +0.325. Current-minus-next-spring slopes also had corrected intervals that included zero in all four samples. Seven planned checks did not establish a common positive link. The estimates remain consistent with small seasonal effects. They do not show that those effects are absent. One observation network, unknown observer and tree changes, coarse weather, and selected long records limit inference. Year controls test local differences and remove shared annual responses. We do not measure productivity or air pollution. External source and joined record tables stay outside the package. Code, hashes, aggregate results and figures permit inspection and reproduction.

phenology · European beech · spring leaf unfolding · autumn leaf colour · MeteoSwiss · E-OBS · observation bias · regional tests · year controls · uncertainty