Multiscale drivers of accidental epiphytism in temperate Europe

Accidental epiphytes — terrestrial plants that opportunistically establish in trees — occur across climatic regions, but the conditions that allow them to colonise arboreal habitats remain poorly understood. We combined field surveys along a coastal western European transect, from the Netherlands to northern Spain, with curated community-science records and used Bayesian models to test whether accidental epiphyte occurrence, abundance and richness are associated with habitat setting, host size and macroclimatic positioning within each species’ climatic envelope. In the field, accidental epiphytes were more frequent, abundant and species-rich in non-forest settings than in forests and larger trees were more likely to support accidental epiphytes in both habitat types. Across species’ climatic envelopes, Bayesian estimates indicated that accidental epiphytes tended to occur under cooler summer and milder winter conditions, whereas moisture and wind effects were weaker and less consistent. These results suggest that accidental epiphytism is shaped by hierarchical filtering: the availability and traits of terrestrial species determine the potential pool of accidental epiphytes, host size and associated structural complexity affect local arrival, retention and establishment probability, landscape structure determines broader opportunities for arboreal recruitment and macroclimatic context influences where terrestrial species most often appear in trees. By linking local host-level filters with species-level climatic-envelope positions, our study supports an open-landscape pathway into epiphytism, while leaving room for alternative routes in shaded, humid forests. Accidental epiphytes, therefore, provide a promising model for studying terrestrial-to-arboreal transitions across biogeographical contexts. First landscape-scale assessment of accidental epiphyte encounter probabilities. Proposal of a hierarchical-filtering framework in which species availability and traits, dispersal and diaspore retention, host traits and structure, microhabitat availability and microclimate, landscape setting and macroclimate interact to shape accidental epiphytism. Open-landscape trees may provide repeated canopy colonisation opportunities and potential refugia. Open-adapted and closed-forest species may follow complementary routes into epiphytism. Accidental epiphytes can be considered a tool to study early stages of terrestrial-to-arboreal plant transitions.