Bark nitrophyte lichens increased in British records in 1986-2025, but not more at high NHx deposition in 10-km squares; SO2-tolerant acidophytes decreased most where SOx decreased most

We tested two explanations for the shift from acid-bark lichens (acidophytes) to nitrogen-loving lichens (nitrophytes) in Great Britain: high ammonia-type nitrogen (NHx) deposition, or the decrease of sulphur (SOx). We used 77,729 British Lichen Society recording visits from GBIF (1960-2025) and the van Herk indicator lists. Poisson models counted indicator species per visit, with list length as the effort control, and compared changes from 1986-1995 across 2,582 10-km squares with CBED deposition. Bark nitrophytes increased to about 1.5 times the 1987-1989 level in England and in Scotland and Wales. SO2-tolerant acidophytes decreased to 0.30 and 0.45. The nitrophyte gain did not follow NHx: +0.035 log units per SD (95% CI -0.029 to 0.099) in 2005-2012, against a national gain of about 0.29. Acidophytes decreased less, not more, at high NHx. SO2-tolerant acidophytes decreased most where SOx decreased most (-0.158 per SD; repeated in both regions and in 2013-2025). SO2-sensitive acidophytes and nitrophytes had no clear SOx term; common non-indicator bark lichens had the largest SOx gain. The planned model gave a negative NHx term for nitrophytes, but this came from fewer churchyard surveys after 1995; models that compare visits of the same survey type remove it. Limitations: opportunistic records without substrate, 10-km exposure, correlated NHx, NOx and SOx, CBED ends in 2012, and indicator-only download after 1991 because of GBIF rate limits.

lichens · nitrogen deposition · ammonia · sulphur dioxide · bioindicators · observation bias · GBIF · British Lichen Society · air pollution · Great Britain