Congruent responses of epiphytic bryophyte communities to air pollution on two species of trees differing in bark chemistry

Epiphytic bryophytes are susceptible to air pollution. The disappearance of sensitive species from highly polluted areas and their recovery after a decrease in pollution were recorded in the second half of the 20th century in Europe. However, the effect of current air pollution on the composition of epiphytic communities and the associated role of host-tree bark chemistry have not been sufficiently studied. Here, the effects of acidifying air pollution on the structure of epiphytic bryophyte communities hosted by tree species with different bark pH are assessed. Due to the higher acid-buffering capacity of basic substrates, a smaller difference between communities on host trees with high bark pH in areas with different pollution loads was expected. Epiphytic bryophytes were studied at 50 sites with similar climate but contrasting levels of SO2 and NOx air pollution in central Europe. As a proxy for the current pollution load at each site, in addition to SO2 and NOx atmospheric concentrations, tissue N concentration was measured in Hypnum cupressiforme. Abundances of species of bryophytes were recorded on trunks of oaks (Quercus robur and Q. petraea, low bark pH expected) and ash (Fraxinus excelsior, high pH expected). Ninety species of bryophytes were recorded. Acidifying air pollution still influences the structure of epiphytic bryophyte communities in Europe, despite the lower levels of pollutants than previously. Atmospheric concentration of SO2 was found to be a significant environmental variable affecting structure of epiphytic bryophyte communities. Ash with high bark pH hosted more diverse communities, including sensitive species, but were just as affected by SO2 pollution as oaks. Species richness, occurrence of epiphytic specialists, and diversity of epiphytic bryophyte communities decrease with increasing SO2 pollution, both on oak and ash. However, it is likely that acidifying air pollution is not the sole driver of the structure of current epiphytic communities in central Europe.