The lichen cover on trees as well as on dead respectively processed wood in the Nature Reserve "Siebengebirge“ near Bonn has been investigated from March to November 1997. 79 species have been identified. Out of the 59 species found by Klement (1959) 18 could not be found recently; 41 are confirmed. 38 taxa are recently found, among them several rare species. Apart from a few species called frequent by Klement but not found at present, more than a dozen species which at that time have been classified as rare or which have been found as stunted thalli, in the meantime have been found at many more localities. Some of them now had more numerous and larger thalli. An improvement of the state of air pollution and firstly the decrease in pollution by SO2 within the last decades is considered as a cause of the distinct increase of species number and a better growth condition of many species. Maps provide a survey on the places (i.e. the trees) where the species were found, the distribution of the species numbers within the nature reserve and the distribution of some particularly remarkable lichens. With the help of analysis of 864 vegetational surveys at 453 localities 16 communities of epiphytic lichens are proved. They are characterized separately and their distribution within the „Siebengebirge“ is explained. Ecological investigations showed that ash is the tree preferred by lichens, followed by oak (Quercus robur, Q. petraea) and sycamore. Horizontally oriented branches were preferred in comparison with vertically oriented trunks. Half of the species found are crustose lichens , somewhat more than a quarter is foliose and somewhat less than a quarter are fructicose lichens. About 40 % of the species have a moderate tolerance against air pollution . In 30 % this has to be considered as „middle“ respectively as „middle to moderately high“. The species-tumover between 1959 and 1997 has no fundamental influence upon changes within the distribution-spectrum of growth forms and tolerance against air pollution. Both the species which could not be found nowadays as well as the numerous new species of lichens are equally distributed among the different growth forms and levels of tolerance against air pollution. The need for protection for groups of trees, trees themselves, but also fence-posts which are characterized by the occurrence of rare lichens or a particularly species-richness is pointed out and proposals are made for their preservation. Keywords: species-tumover , lichen communities , airpollution , phorophytes , need for protection.