Lichen vegetation on oak was studied along two transects from a nature reserve with a low ammonia emission into an intensive animal husbandry area with high ammonia emission. Acidophytic species were dominant in the nature reserve, whereas nitrophytic species dominated in the agricultural area. Bark analysis showed the pH to be more important for species composition than the ammonium concentration. Ammonia emission favours nitrophytic species by raising the pH of oak bark, which is normally in the range 3-5-5 0. This phenomenon has been observed in many sites in The Netherlands, where it replaces earlier acidification by sulphur dioxide.