March warmth was linked to longer leaf-to-flower gaps and fewer date pairs in selected US common lilac records in 1967-2008

We tested March and April temperature links to paired US lilac dates with site and year effects. The common-lilac model used 2,947 paired site-years at 187 sites in 1967-2008. March warmth was linked to a 0.95 day longer leaf-to-flower gap per C (nominal 95% interval 0.20 to 1.70). April gave no clear link. Among 3,130 leaf-report site-years at the same sites, March warmth was linked to 1.58 percentage points fewer valid pairs per C (interval -2.85 to -0.32). Thus record selection was related to weather, but its effect on the gap slope remains unknown. Later-period and cloned-lilac intervals included zero. Only one retained common-lilac site was eastern, so a same-type eastern test was not feasible. We did not establish a common response across regions and periods. Missing visits, selected cultivated plants, coarse monthly weather and unmeasured local changes limit inference. The package gives paired evidence, source hashes, code, all estimates, figures and an independent check. It does not test pollution, daily extremes or causal climate-change effects.

phenology · lilac · observation bias · monthly temperature · paired events · missing records