Single-year removal kept negative spring warmth slopes in four US lilac samples in 1965-2008

We tested whether one observed year could account for the negative link between spring warmth and lilac bloom date. We used 9,015 records in eastern and western US samples in 1965-1986 and 1987-2008. Models used reporting-unit and year effects and winter temperature. All 84 single-year removal fits kept negative spring slopes. Full slopes were -2.48 to -3.74 days per degree C. The largest change after year removal was 0.62 day per degree C. Removing the warm tail of observed site years also kept negative slopes. Unknown visit times, selected sites, monthly temperature and different lilac types limit inference. This is a check on fitted associations, not a causal test of climate change, daily extremes or pollution.

phenology · climate · lilac · observation bias · influence analysis