The epiphytic lichen flora of 332 trees standing at roadsides or in churchyards in non–urban areas of Denmark and Schleswig–Holstein, Federal Republic of Germany, was investigated. The condition of the lichen vegetation in each of 107 subareas was evaluated by means of a nine–point scale largely based upon presence, fertility and vitality of Anaptychia ciliaris and Physconia pulverulacea. A marked depauperation of the lichen communities of the alliance Xanthorion parietinae is demonstrated in Schleswig–Holstein and is attributed to long–range SO₂ air pollution. No significant pattern of adverse effects on the corresponding communities can be seen in Danish rural areas, but the reduction in frequency and size of Ramalina species in recent times is believed to be the result of background air pollution. A list of 111 recorded taxa is given and selected species are commented on. Candelariella reflexa (Nyl.) Lettau is reported as new to Denmark. The effects of agricultural spraying and the decrease in the numbers of phorophytes in recent decades are discussed.