Multi-scale habitat filtering of epiphytic lichens and bryophytes in temperate mountain forests - host identity, species traits, and physiological thresholds

Epiphytic lichens and bryophytes are integral components of forest biodiversity and contribute substantially to ecosystem functioning through their roles in water retention, nutrient cycling, and habitat provision. As poikilohydric organisms, their physiological activity is tightly coupled to near-surface environmental conditions, making them highly sensitive to microhabitat variation at the bark surface. They provide a mechanistic link between forest structure, substrate properties, and organismal performance. Framed within the conceptual perspective of the Research Training Group ConFoBi (Conservation of Forest Biodiversity in Multiple-Use Landscapes), this dissertation adopts a scale-explicit approach to understand how environmental filters across hierarchical spatial levels shape biodiversity patterns and functional responses in managed temperate forests. This dissertation investigates how environmental filters operating across tree, stand, and landscape scales shape the diversity, community composition, functional traits, and physiological responses of epiphytic cryptogams in temperate mountain forests. Field surveys across 24 forest plots in the Southern Black Forest combined multiscale environmental data, spatial modelling, trait-based analyses, and physiological experiments. Host tree identity and bark physico-chemical properties emerged as the dominant drivers of epiphyte richness, composition, and functional structure, while stand heterogeneity and landscape context exerted secondary but significant influences. Large trees increased species richness and habitat availability. At neighbourhood scales, environmental filtering outweighed spatial proximity, particularly for lichens. Trait- and threshold-based analyses revealed species-specific responses along bark physico-chemical gradients, highlighting the relation of bark chemical composition and functional traits in shaping community assembly of different host tree species. Physiological experiments on the epiphytic lichen Hypogymnia physodes indicated intraspecific acclimation of osmotic properties and dehydration dynamics to tree-scale microhabitat variations such as tree size, linking community patterns to mechanistic thresholds in dehydration and photophysiology. Overall, the dissertation provides an integrated, scale-explicit framework connecting environmental filtering, functional traits, and physiological thresholds. The findings emphasize the central role of host tree characteristics and bark properties in structuring epiphytic communities and offer implications for biodiversity conservation in temperate forests.