Atmospheric Ammonia pp 71-86
Estimation of the Ammonia Critical Level for Epiphytic Lichens Based on Observations at Farm, Landscape and National Scales
In past decades, a huge amount of information was collected on the sensitivity of lichens to atmospheric sulphur dioxide. As the concentrations of sulphur dioxide have decreased following emission control measures, much more attention is now focusing on the possibility of direct effects of ammonia. In the Netherlands, a change in lichen populations through the 1980s and 1990s was originally attributed to an increase in ammonia emissions, although subsequently it was found difficult to separate these changes from the effects of parallel decreases in sulphur dioxide emissions (van Dobben and Ter Braak 1998). However, several subsequent studies across Europe have shown that ammonia is having substantial and unambiguous effects on epiphytic lichen populations (van Herk 1999; Sutton et al. 2004a, b; Wolseley et al. 2004, 2006; Frati et al. 2007; Pinho et al. 2008, this volume).
Our field measurements in the UK have trialled a number of indices to summarize the response of lichens to excess nitrogen deposition and specifically ammonia (Sutton et al. 2004a, b; Leith et al. 2005; Wolseley et al. 2004, 2006). Of these, we have found the approach of distinguishing two functional species groups, ‘nitro-phytes’ and ‘acidophytes’, to be most successful. This approach was originally developed by van Herk (1999) and consists of establishing lists of the known species which prefer a high supply of reactive nitrogen, the nitrophytes, and the known species which avoid a high supply of reactive nitrogen, the acidophytes. The name for the latter group reflects the fact that, as gaseous ammonia is typically the driving variable, high nitrogen supply tends to increase bark pH, which is naturally acidic under clean conditions. The fact that ammonia tends to increase bark pH in the field indicates that the effect of NH3 as a base dominates over any nitrification on bark surfaces (which could potentially acidify the surface, cf. Sutton et al. 1993).
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