A positive nitrogen contrast in Meadow Brown warmth slopes did not persist with geographic controls in southern England, 1999-2012

We tested whether spring-temperature slopes of Meadow Brown mean flight dates differed with nitrogen deposition. We joined UK butterfly flight records to a published site and temperature panel. Four tests used 9,247 records in two British regions in 1986-1998 and 1999-2012. The main model gave a positive contrast only in later southern England: 1.04 days per degree C per 5 kg N/ha/year, with a four-test interval of 0.35 to 1.74. With geographic temperature slopes, this contrast was 0.38, with an interval of -0.37 to 1.13. Peak-count checks retained the main positive contrast. A fixed site set gave an interval that included zero. All other main test intervals included zero. Thus the positive contrast did not persist across region, period and geographic controls. Static deposition, coarse weather, selected sites, missing visit records and habitat differences limit inference. This is an association test, not a causal pollution effect or a full test of larval microclimate.

butterfly phenology · nitrogen deposition · temperature · geographic confounding · observation bias · Meadow Brown