Warm and cool spring slopes had no clear difference in four US lilac samples in 1965-2008

We tested whether lilac first-bloom dates had different temperature slopes in warm and cool springs. We used 9,015 repeated plant records in separate eastern and western US samples in 1965-1986 and 1987-2008. Models used plant and year effects, winter temperature, and separate March-April temperature slopes above and below each site mean. Warm minus cool slopes were -0.72, -2.02, -1.42, and -4.35 days per degree C. All four state-cluster intervals, adjusted for four tests, included zero. Two-way state and year intervals also included zero. Removing flagged dates, changing record-length rules, giving equal plant weights, and trimming large anomalies did not give a clear common difference. The raw-data rebuild matched the reused panel byte for byte. Monthly weather, unknown visit delays, selected sites, different lilac types by region, and few clusters limit the result. We do not test daily heat waves, pollution, biodiversity loss, or causal climate effects.

phenology · lilac · temperature · nonlinearity · observation bias · regional replication