January warmth did not show a common change in spring leaf-out sensitivity in four US samples from 1965 to 2008

We tested whether a warmer January weakened the link between February-March warmth and earlier first leaf. We used 15,705 repeated plant-years from public US lilac and honeysuckle records. Four models had plant and year effects, a January by spring interaction, and separate curved temperature terms. Eastern interactions were +0.050 and +0.056 days per C squared. Western interactions were -0.078 and -0.234. The planned state and year cluster covariance failed in the early West. The other three adjusted intervals included zero. A disclosed check that summed state and year covariance gave intervals that included zero in all four samples. These records did not establish a common weakening after a warm January. Monthly temperature is not a chilling measure. Selected plants, different taxon mixtures, missing visit schedules, exposure overlap with leaf-out, and few clusters limit inference. This is not a causal climate-change or pollution test. The package includes code, the selected panel, source hashes, all estimates, numerical checks, and a figure.

phenology · January temperature · leaf-out · winter chilling · temperature interaction · observation bias · regional replication · uncertainty