Recolonization of epiphytic bryophytes after decades of air pollution in forest ecosystems in the Erzgebirge (Ore Mountains) shows the importance of deciduous trees for the diversity of this species group

In recent decades, great efforts have been made in Central Europe to convert homogeneous conifer stands into mixed stands. One major goal of this measure is to promote the biodiversity of forest ecosystems. Research on forest biodiversity is mostly limited to the spatial scale of the stand (alpha diversity), and only few studies focus on the landscape level (gamma diversity). In our study, we investigate the recolonization of epiphytic bryophytes after decades of air pollution on different host tree species. The study area is the Erzgebirge (Ore Mountains, eastern Germany), a low mountain range in which very high SO₂ inputs from industrial emissions caused severe damage to the vegetation up to the end of the 20th century. We selected twelve plots each of the tree species Picea abies, Fagus sylvatica, Acer pseudoplatanus and Sorbus aucuparia and recorded the occurrence of all epiphytic bryophytes on trunk and branches at a height between approx. 0.5 and 2.0 m. We recorded a total of 84 species, finding a significantly higher species richness and number of species of conservation concern as well as a larger regional epiphytic species pool (RESP) on F. sylvatica, A. pseudoplatanus and S. aucuparia compared to P. abies. In contrast, there were only minor differences in epiphytic species diversity and composition between the studied deciduous tree species. The dominance of colonists in the current species composition shows that these epiphyte communities are still in the early stages of succession, supported by the observation that species with small range sizes are still very rare. Compared to other studies, the species pool of the examined tree species is relatively large. This is an indication of the much improved environmental conditions for epiphytes in the Erzgebirge. We also recorded Ulota drummondii in the stands, providing its first record in Germany since 1886. In general, we conclude that changing tree species composition from monospecific coniferous to broadleaved stands will lead to an increase in the epiphytic bryophyte diversity of forest ecosystems.