The value of alien roadside trees for epiphytic lichen species along an urban pollution gradient

Abstract Alien tree species have been introduced to Europe and often used as roadside trees. Currently, preference is given that are adapted urban climate drier conditions. Native epiphytic lichens local species; however, little known whether a shift toward more alien trees would affect the diversity of within cities. We considered three genera common in Hamburg (Germany) Acer, Tilia, Quercus, each with native an per genus. Species numbers, frequency were assessed compared. Tree locations grouped according classes increasing traffic volume proxy for air pollution. Mean bark pH values recorded individual. The pairs Quercus robur/rubra Acer platanoides/saccharinum showed significant differences abundance lichens, showing lower values. pair Tilia cordata/tomentosa did not show any differences. number nitrogen-tolerant lichen differed among all species. Lichen richness decreased pairs, thus demonstrating inhibiting effect on occurrence lichens. Bark between yet high resulted increased pH. In conclusion, two out had negative effects diversity. Further should be better estimate