April-May warmth had negative leaf-to-bloom slopes in four US shrub samples in 1961-2009

We tested whether April-May warmth was linked to a shorter interval from first leaf to first bloom after control for February-March temperature. Four US region and period fits used 9,833 paired reporting-unit years from 1961 to 2009. Models removed reporting-unit and calendar-year effects. All four slopes were negative, from -2.75 to -1.34 days per degree C. State and year cluster intervals were below zero after correction for four tests. The estimates stayed negative after each state omission. However, western plant-trend intervals included zero, and all four intervals included zero with a wider covariance check. The result is a repeated conditional association with model-dependent precision. Monthly weather can occur after an event. Missing visits, selected long records, different plant types and event rules, and few state clusters limit inference. This extends earlier tests on the same biological archive. It is not a causal climate-change or air-pollution test. Code, joined evidence, exact source hashes and reproduction steps are included.

phenology · lilac · honeysuckle · seasonal temperature · paired events · observation bias · uncertainty