Epiphytic species grow (almost) exclusively on a living substrate, typically a tree, but epiphytic growth is not restricted to them. Individuals of normally lithophytic or terrestrial species may occasionally be found on a tree as so-called accidental epiphytes. Species of the focal group of this study, Crassulaceae, are typically found on rocks and in rock fissures. While there is a small proportion of true epiphytes globally, the propensity of the other family members to occur as accidental epiphytes is largely unexplored. Here, I investigated this question for 29 European members of the family with the use of the participatory science data platform iNaturalist. Umbilicus rupestris stands out in regard to epiphytic occurrences, although the incidence of epiphytic growth is still rather low with c. 1% of c. 14,000 observations. For all other species, epiphytic growth has not been reported or was exceptional. As expected, epiphytic individuals of U. rupestris were limited to regions without frost, while a predicted limitation to the wettest parts of the species’ geographic range was not supported by the data. Arguably, Umbilicus rupestris could be a promising model to study the early steps of epiphyte evolution by comparing epiphytic and terrestrial individuals in regard to differential germination success, ease of establishment, differences in morphological and physiological traits and general population dynamics. The results of such studies should be highly instructive for our understanding of the challenges that terrestrial species face when conquering tree crowns.
