Integrated monitoring of the effects of airborne nitrogen and sulfur in the Austrian limestone Alps

Many forested areas in industrial countries are exposed to excess deposition of airborne nitrogen (N) and sulfur (S). Biodiversity decline is one of many observed effects. Unlike N deposition, S loads have been decreasing continuously over the last 25 years. In this paper, we evaluated the use of species diversity in four taxonomic groups (vascular plants, lichens, bryophytes, birds) as indicators of N and S deposition. Long-term monitoring data from 1992 to 2005 were taken from an intensively monitored site in Austria. Both temporal changes and determinants of diversity were explored. On average, diversity declined from the beginning of the 1990s until the year 2005, but there was a considerable variation among the organisms and the diversity indicators. Few changes in diversity were statistically significant. Strongest changes occurred at the level of single species, as an increase or decrease of their abundance. Factors other than N and S deposition?particularly historical forest management and natural disturbances ? were significant, complicating interpretation of the observed diversity changes. The authors conclude that species diversity alone is not a reliable indicator of N and S impacts on forests, particularly if few indicator species groups are used and the observation period is short.