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In 2017/2018, diversity and quantity of lichens were studied on 53 trees in Rostock city. The results were compared with the results of a mapping project in 1994/1995. Humidity conditions and degree of eutrophication at trees of the genera Tilia and Acer were interpreted with reference to the known indicator characteristcs of the lichen species concerning air humidity/precipitation, eutrophication and pH conditions. In 2017/2018, 79 lichen species and 14 lichenicolous fungus species were found. The total number of lichens increased to 80 species. Most species were found on Tilia spec. (60), Acer pseudoplatanus (43) and A. platanoides (34). The most common species are Phaeophyscia orbicularis, Physcia adscendens, Ph. tenella and Xanthoria parietina. Candelariella xanthostigmoides, Flavoparmelia soredians, Hyperphyscia adglutinata, Intralichen lichenum, Lecanora subcarpinea, Parmelia serrana, Parmelina quercina and Taeniolella delicata are new to Mecklenburg-Western Pomerania. The distribution and characteristics of the collection sites allow for deviding the species into three groups: 1. species also present along streets with heavy traffic or in densely populated residential areas with a lack of green areas and in the districts Kröpeliner Tor-Vorstadt or City Centre, 2. species still present in sparse residential areas and smaller green areas but not in the districts Kröpeliner Tor-Vorstadt or City Centre, 3. species only colonizing trees close to the open landscape. Twenty-two species were assigned to the first, 32 species to the second and 25 species to the latter group. According to the proportion between the sum of frequencies of the strongly nitrophytic lichens and the sum of frequencies of the anitrophytic and moderately nitrophytic lichens on trees, air quality was indicated to be good at 15 trees of the genera Acer and Tilia, moderate at 14 trees and bad at 13 trees. Based on the proportion between the number of basiphytic lichen species and the sum of species colonizing acidophytic and subneutrophytic bark, six locations were categorized as little, 21 locations as moderately and 15 locations as strongly polluted. Mesophytic or hygrophytic lichens were present with only one or two species at ten locations, three to five species at 21 locations and over five species at 11 locations. Within the period under consideration, the epiphytic lichen flora of the Rostock urban area changed considerably. The populations of 69 lichen species increased, with 52 lichens newly establishing after 1995. In 32 lichen species a slight, in 19 species a moderate, and in 18 species a strong population increase was recorded. Lecanora conizaeoides disappeared and the populations of four species decreased. [ABSTRACT FROM AUTHOR]