Do tree-related factors mediate the response of lichen functional groups to eutrophication?

In the last decades, the pollution regime has been drastically changed in most industrialized countries, with a considerable decrease in sulphur dioxide (SO2) emissions and an increasing relevance of eutrophication compounds, such as nitrogen compounds and particulate matter. This situation hampers the interpretation of data in biomonitoring surveys, as high lichen diversity is not always associated with good air quality. The objective of this study was to test whether the effects of eutrophication on the abundance of different lichen functional groups varies according to some tree-related factors. We analysed the relationships between epiphytic lichen diversity, emissions of main atmospheric pollutants and tree characteristics (circumference and bark pH, light transmitted through the canopy). Hierarchical partitioning of variance and Generalized Linear Mixed Models (GLMM) confirmed that lichen functional groups with different nitrogen tolerances responded to several atmospheric pollutants, with both independent and joint effects, whereas they did not show significant differences depending on main tree-related factors. We demonstrated that, under high eutrophication levels, differences in bark pH did not significantly differentiated the composition of epiphytic lichen communities.