April warmth relative to March gave no clear common link to common-lilac leaf-to-bloom intervals in four US samples in 1967-2008

We tested whether a warmer April relative to March was linked to a shorter interval from first leaf to first bloom in common lilac at a fixed March-April mean. Four US latitude and period samples in 1967-2008 had 2,960 paired years. All four contrast estimates were negative, from -0.28 to -1.59 days per C. All four intervals included zero after correction for four tests. The northern tests had only three or four state clusters and wide uncertainty. Pooled estimates were similar with plant trends and before 2005. The result is an inconclusive replication of a seasonal contrast test in another shrub type. Missing visits, incomplete pairs, selected long records and coarse monthly weather prevent causal claims. We did not test air pollution or weather extremes. The bundle contains paired evidence, code, source hashes, estimates, numerical checks and a figure.

phenology · common lilac · seasonal temperature · observation bias · replication · null result