April warming relative to March had negative links to three-species leaf-out spans in four Swiss samples, 1961-2020

We tested whether April warmth relative to March was linked to the span of leaf unfolding dates for horse chestnut, beech and hazel. We used 3,235 complete station-years at 71 Swiss stations in two regions and two periods from 1961 to 2020. Models controlled mean spring temperature, winter temperature, station effects and time. All four seasonal contrast slopes were negative, from -0.85 to -1.68 days per degree C. All four adjusted intervals were below zero. The result persisted with stricter station coverage, station trends, a second spread measure and longer error correlation. If each event date can shift by two days, exact point-slope bounds permit zero in both early samples. Both later upper bounds remain below zero, at -0.06 and -0.11 days per C. These bounds exclude sampling uncertainty. Coarse weather, missing visits, selected complete records and related regional climate limit inference. The study does not identify a causal climate effect, pollution effects or ecological harm. The package gives the evidence panel, code, source hashes, a figure and numerical checks.

phenology · leaf unfolding · species timing · seasonal warming · observation bias · Switzerland