Winter warmth gave no clear common change in spring leaf-to-bloom slopes in four US shrub samples in 1961-2004

We tested whether January-February warmth changes the March-April temperature association with the time from first leaf to first bloom. The historical US shrub panel contains 10,114 pairs from 647 reporting units. Four region and period fits used unit and year effects, separate March and April main terms, and a winter by spring anomaly product. The four interaction estimates did not show a clear common result. Missing visits, pair selection, mixed plant types and phase rules, weather after events, and approximate cluster intervals limit inference. Monthly means do not measure chilling. These results do not establish a zero biological interaction or a causal climate effect. The bundle includes a derived panel, source hashes, code, a figure and numerical checks.

phenology · winter temperature · seasonal interaction · paired events · observation bias · regional comparison