The lichen monitoring programme included in the Dutch National Air Quality Survey was used to explore the utility of these organisms as indicators for atmospheric ammonia. Over the period 1977-1990 the “nitrophytic” species (assumed to occur optimally in N-rich habitats) strongly increased at the 150 monitoring stations of the network. Furthermore, a positive correlation was found between the occurrence of these species and local NH3 concentrations. Earlier reports therefore proposed the use of nitrophytic lichens as bioindicators for NH3 and considered their increase as an indication for increasing NH3 concentrations. However, a more careful statistical analysis of the available data shows a strong impact of decreasing SO2 levels on all epiphytic lichens, including the nitrophytic species. It is now clear that the “nitrophytic” species do respond to atmospheric NH3, but their response to SO2 is far stronger. Furthermore, chemical analysis of tree bark shows that nitrophytic lichen species do not respond directly to N levels, but are rather favoured by the high bark pH associated with high NH3 levels. Three mechanisms are presented to explain the strong response of the nitrophytic lichens to decreasing SO2 levels.
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