The April-March temperature difference reduced later errors for birch and beech leaf date gaps in two Swiss regions, 2011-2025

We tested whether the April-minus-March temperature difference adds information about the gap between birch and beech leaf dates beyond the March-April mean. We used 2,615 paired station-years at 104 Swiss stations. Four separate region and period fits gave negative gap slopes of -0.86 to -1.86 days per C of the temperature difference. All four station and year cluster family intervals were below zero. Three-year block intervals included zero in three of the four family tests. Fits trained on 1996-2010 reduced 2011-2025 root mean square error by 13.6% in both regions when the monthly difference was added. Two-region family intervals for this gain were 4.2-19.6% in the East and 3.3-21.5% in the West, conditional on the frozen training fits. Without a time trend, gains were 7.3% and 5.7%. Seven-day reporting-delay bounds permit either slope sign in all four samples. The regions share much of their weather. April weather often follows leaf unfolding. Thus the result supports a monthly term in later-year estimates with known weather; it does not establish a causal climate effect, a real-time forecast, or ecological harm. Missing visits, selected records, coarse weather and one observation network limit wider use. Code, aggregate evidence, source hashes, a retrieval method and independent numerical checks are included.

phenology · birch · beech · monthly temperature · seasonal mean · time transfer · observation bias · reporting delay · regional replication