With global habitat destruction progressing at an alarming rate, there is an urgent need to understand how species persist in dynamic habitats and how underlying processes influence their spatial distribution. Theories hinge on the principle that individuals of the same species are non-randomly distributed, and recent work emphasizes that community assembly is shaped by a dynamic interplay of deterministic and spatial processes. In epiphytic lichens and bryophytes, environmental filtering and spatial structure are known to shape community patterns, yet their roles at fine spatial scales remain poorly understood. Our study investigated the relative influence of fine-scale spatial structure and habitat quality on epiphyte diversity and composition. We assessed spatial structure by comparing mean pairwise similarity in species composition among conspecific and heterospecific tree pairs and by distance-based Moran's eigenvector maps (dbMEMs) to account for spatial structure across scales. To evaluate environmental filtering, we considered site-related factors (topography, microclimate) and tree-level traits (diameter, bark physico-chemical properties), introducing internal stem decay (ISD) as a novel structural variable. We studied 96 tree pairs of Fagus sylvatica, Abies alba and Picea abies in the southern Black Forest, Germany, across diameter at breast height categories along an elevation gradient of 700–1000 m a.s.l. Tree species identity, particularly via bark physico-chemical traits, was the strongest determinant of epiphyte diversity and composition. Spatial proximity increased community similarity in both bryophytes and lichens, but in lichens, environmental filtering via habitat-related factors, such as bark features and microclimate outweighed this effect. dbMEMs explained additional variation in species composition and richness, revealing spatial structure beyond measured environmental variables. Microclimatic and topographical variables had minor influences at mid-elevation sites. ISD significantly enhanced bryophyte cover. Synthesis: Fine-scale community assembly in epiphytic lichens and bryophytes is governed mainly by habitat quality at the bark surface, while spatial proximity differs between organism groups, reflecting the varying importance of niche-based filtering and habitat ranges. ISD emerged as a previously underrecognized driver of bryophyte abundance. By disentangling environmental and spatial drivers within a single framework, this study advances ecological understanding of community assembly in substrate-dependent forest organisms. Keywords: bark chemistry, community ecology, forest microclimate, internal stem decay, neighbourhood effects.