119 investigated Quercus trunks and 67 species of lichens, 23 bryophytes and 3 non-lichenized fungi are arranged by means of the Minimum Spanning Tree analysis, resulting in four Lobarion communities and one Parmelion community. Lobarion is restricted to bark with pH > 5.0 in old forests. pH and Ca-content in the bark are strongly correlated; Ca seems to originate from Ca-uptake through the roots of the phorophyte. Within a single site the soil also seems to be responsible for the variation in the Mg-content of the bark, being correlated with Ca. The dominating source of the regionalvariationinMg, however, seems to be marine aerosols. Parmelion caperatae occurs on acid bark of young trees. The bark is, however, rich in Mg. Soluble tannins in the bark decrease with increasing age of the phorophyte, but are probably of minor significance for the epiphytic vegetation.