Questions: Differences among host trees in chemical and physical bark features strongly influence the preferences of epiphytic lichens and bryophytes. Element contents can enhance, limit, or impair performance, with bark pH further modulating bioavailability. Establishment is closely linked to functional traits that define habitat thresholds, particularly under acidic conditions. Yet such traits are often overlooked in community analyses. We asked how host-specific bark chemistry filters epiphytes and how functional traits mediate these responses. Location: Temperate mountain forests of the Southern Black Forest, southwestern Germany. Methods: We surveyed 96 trees of Abies alba, Picea abies, and Fagus sylvatica across 24 plots. Bark physico-chemical features (pH, element concentrations, water-holding capacity) were linked to epiphyte communities using double-constrained canonical analyses, considering functional traits such as bryophyte growth habit, lichen growth form, photobiont type, and lichen substances. Threshold Indicator Taxa Analysis identified community and species thresholds along bark chemical gradients, and functional diversity metrics quantified community-level responses. Results: Fir occupied an intermediate position between beech and spruce in bark chemistry and community structure. Lichen substances were key correlates of composition across hosts. On spruce, strong associations between transition metals (Fe, Zn, Cu, Mn) and lichen substances indicated metal homeostasis as a central tolerance strategy, accompanied by a marked Asterochloris signature. Bryophyte communities segregated along C/N, Zn, and Mn ratio gradients, with acrocarpous and pleurocarpous groups showing contrasting responses. Functional divergence of lichens increased along Mn and pH gradients and declined with higher bark P. Conclusions: Epiphyte assembly is governed by interactions between bark chemistry and species traits. Host identity and substrate chemistry create functional thresholds that drive community turnover, supporting a trait-based filtering perspective that links tree-scale habitat conditions with the structure and functional diversity of epiphytic communities. Keywords: Asterochloris | bark chemistry | functional diversity | functional traits | lichen substances | metal homeostasis | species thresholds | tree species identity.