Long-term monitoring of environmental change in German towns through the use of lichens as biological indicators: comparison between the surveys of 1970, 1980, 1985, 1995, 2005 and 2010 in Wetzlar and Giessen

In both towns, the pH of the tree bark has been increasing continuously, presently almost reaching pre-industrial values. The increase was stronger in Wetzlar than in Giessen. In 1970, the lichen vegetation showed a complete depletion. Since then the number of species has risen significantly. This development happened faster and more intensely in Wetzlar. The comparison of the lichen vegetation between 1970 and 2010 also shows an increased existence of species that are favored by hypertrophic air contaminants. In the 2010 survey, some species considered as being promoted by global warming were found for the first time.The increase of the bark pH can be explained by the decreasing acid pollution, primarily SO2. This is also assumed to be the main cause for the increasing number of lichen species. An explanation for Wetzlar's advantage is that the acid pollutants had been neutralized by a local industrial emittent of lime dust. With the reduction of dust emissions in Wetzlar due to the installation of filters, the bark pH in both towns steadily converged in the reference period. An examination of pre-industrial lichen studies shows that in both towns the original state of lichen vegetation has not been restored yet.The use of lichens as bioindicators to assess air pollution has been a long tradition in Europe. Nevertheless, there are rare long-term mappings of lichens under standardized conditions. Comparative assessments of the air pollution situation are also hardly possible; as in previous studies, different methods have been observed, and the transfer to modern, regulated and standardized methods [1] fails as the original data are no longer available. The towns of Central Hesse, Giessen and Wetzlar, represent a positive exception from these points: they have been mapped repeatedly at irregular intervals since 1970, and the original survey data of the respective investigations are still available. In our study areas, the surveys were not all conducted using the sam