Diversity and assembly of lichens and bryophytes on tree trunks along a temperate to boreal elevation gradient

Based on hypotheses related to environmental filtering vs. stochastic community assembly, we tested taxon-specific predictions regarding the relationships of alpha diversity, beta diversity and species composition of epiphytic macrolichens and bryophytes with elevation and the lateral gradient on trees (the different sides of the tree bole related to aspect and trunk inclination) at Parc national du Mont-Mégantic in Southeastern Québec, Canada. For lichens on firs, increasing elevation was associated with increasing alpha diversity, and a marked shift in community composition, at the scale of whole trees. In contrast, for bryophytes on maples, tree inclination and the lateral gradient had the strongest effects: more inclined trees had greater whole-tree alpha diversity and stronger within-tree contrasts in composition between the upper and lower bole surfaces. For lichens on maples, whole-tree alpha diversity showed a weak, negative relationship with inclination, and beta diversity increased slightly with elevation. Our results are consistent with theories predicting greater alpha diversity in more favorable environments (for lichens: high elevation with high relative air humidity and lower temperatures; for bryophytes: upper surfaces of tree boles with liquid water available), but support was weak for the prediction of greater beta diversity in more favorable environments. Overall, the important predictors of epiphytic cryptogam diversity vary more among the species of tree host (maple vs. fir) than focal taxa (lichens vs. bryophytes), with patterns likely related to different effects of water, temperature, and competition between lichens and bryophytes.