ASSESSMENT OF AIR-POLLUTION IN GIESSEN AN D WETZLAR ON THE BASIS OF EPIPHYTIC LICHEN GROWTH

The sensitivity of epiphytic lichens to air pollutants - especially ac id ones - make them useful instruments for ecological studies. In 1995 lichen mappings of the Hessian towns Giessen and Wetzlar were carried out. The investigations followed a new standardized guideline for lic hen mapping provided by VDI. In addition the surveys of 1995 were comp ared with lichenological studies of previous centuries and with three mapping projects of the last 25 years. Today in both towns we can find five different classes of air quality. In Giessen the rate of immissi on stress reaches from ''extremely high'' over ''very high'' to ''high ''. In Wetzlar better conditions can be found (from ''very high'' over ''high'' to ''moderate''). The air quality classes ''low'' to ''very low'' can neither be found in Giessen nor in Wetzlar. In comparison wi th lichen investigations of the time before industrialization the epip hytic lichen vegetation has been impoverished. However, comparing the current surveys with investigations carried out 25 years ago, an incre ase of species diversity and individual frequency can be recorded in b oth towns. The recolonization is caused by a decrease in air pollution load. The reasons for the different rates of recolonization in Giesse n and Wetzlar are discussed.