Lichens of the rare and predominantly epiphytic old forest community Lobarion were attached to Picea abies twigs with significantly higher bark pH (KCl) than members of the common and ubiquitous Pseudevernion community. The Lobarion species of spruce twigs, mainly members of the order Peltigerales, had distincly higher thallus pH than substratum pH, while the greenalgal Pseudevernion species, mainly belonging to the Lecanorales, were consistently more acidic than their substratum. pH of lichens and bark of P. abies twigs responded to a forest vegetation gradient reflecting the soil nutrient condition at the forest floor. However, the two groups of epiphytes seem to modify the bark pH in a way that enlarges a difference originally determined by soils in the root zone of the phorophyte. The lack of the Lobarion on spruce twigs in eastern Norway is probably a result of acid rain, as the bark and thallus pH in spruce canopies of eastern Norway appeared too low to support the Lobarion, even in stands with a species-rich epiphytic assemblage of alectorioid species.