Influence of tree species on the epilithic bryophyte flora in deciduous forests of Sweden

Bryophytes growing on siliceous boulders were studied in two forests in east-central Sweden to investigate the influence of different overstorey tree species on bryophyte species composition. Granite boulders, 0.5–2.0 m across, lying below the crowns of Ulmus glabra, Fraxinus excelsior, Acer platanoides, Quercus robur, Betula pendula, and Picea abies were studied in mixed stands. The presence, abundance and sporophyte frequency of all bryophyte species were noted. Tree species was the factor explaining most of the variation in bryophyte species composition. Boulders below tree species with a high bark pH (Ulmus, Fraxinus and Acer) had a similar species composition, with many bryophytes present indicating high pH conditions such as Brachythecium populeum, Homomallium incurvatum and Pseudoleskeella nervosa. On boulders below Picea the bryophyte composition was very different, with species indicating low pH conditions such as Ptilidium pulcherrimum, Dicranum montanum and Platygyrium repens. Below Quercus and Betula the bryophyte composition was intermediate between these two groups, with species indicating both high and low pH conditions such as Brachythecium reflexum and Hedwigia ciliata. The amount of litter on the boulders was another important factor explaining much of the variation. Species like H. incurvatum and Orthotrichum urnigerum were more common on boulders with much litter, while P. pulcherrimum was more common on boulders without litter. The size of the boulder was also important for bryophyte species composition, primarily because of the larger number of species on large boulders. Trunk diameter, canopy cover and distance from the tree had little or no influence on the bryophytes. The conclusion is that throughfall chemistry and chemical composition of leaf litter are the two most important factors explaining bryophyte species composition on boulders in these forests.