Interacting effects of tree characteristics on the occurrence of rare epiphytes in a Swedish beech forest area

Many epiphytes in Swedish beech forests are associated with old and damaged trees. In this study we examined the impact of bark, soil and stemflow pH, water-holding capacity and bark structures on this association. We also analyzed whether the influence of these factors differed between species of conservation concern (red-listed and indicator species of woodland key habitats) and species not of conservation concern. One hundred and one age-determined living beech trees (range 58-277 yrs) in 13 beech-dominated stands were surveyed in a forest landscape in southern Sweden. We recorded 119 species in total (76 lichens, 43 bryophytes) of which 21 were red-listed and 17 indicator species (26 lichens, 12 bryophytes). NMS ordination showed that the species composition of lichens changed primarily with tree age and along the gradient of smooth bark versus moss cover. Bark pH and tree vitality were the most important gradients for bryophyte composition. The combination of old damaged beech trees and high bark pH resulted in the highest mean number of species of conservation concern for both lichens and bryophytes. The link between these factors is partly explained by a positive effect of tree age on the stemflow pH. Species number of bryophytes not of conservation concern increased with bark pH, whereas the corresponding group of lichens was favored primarily by increased light availability. Neither the water-holding capacity of bark nor soil pH affected patterns of species number. The results from this study show that old beech trees infected by fungi, with a higher bark pH, are the most valuable trees for epiphytes of conservation concern.