Questions: Vascular epiphytes make up about 9% of all vascular plants globally but are clearly underrepresented in the temperate zones. The accidental epiphytic occurrence of terrestrial species, in contrast, is common at these latitudes and can provide important insights in the evolution of obligate epiphytes. Here, we present the results of the first two annual censuses of a planned long-term study on accidental epiphytes. We particularly aim to identify: (a) the abundance and species richness of accidental epiphytes; (b) the dynamics of accidental epiphytism; (c) occupied substrates and microsites; and (d) suitable host tree species. Location: Harz Mountains, Germany, Central Europe. Methods: We surveyed more than 1,200 trees in a low mountain range in two consecutive years for epiphytic individuals of vascular plants, considering host tree species and occupied microsites. Results: About one quarter of the surveyed trees hosted epiphytic plants, totalling 1 450 (2016) and 1 350 (2017) individuals, respectively. These belonged to more than 100 species and 39 different families. The majority of epiphytic individuals newly emerged in 2017, whereas one-third persisted since 2016 (or earlier) and a smaller proportion even reproduced in the epiphytic habitat. Accidental epiphytes were mostly restricted to host tree species providing water-storing substrates such as extensive moss pads or arboreal soil accumulated in crotches. Compared to tropical epiphyte communities, accidental epiphytes in the temperate zones show a higher turnover, as they are less consistent in species abundance and composition in time.Conclusion: For the majority of the observed species, epiphytism indeed is an accidental phenomenon. Undoubtedly, abiotic conditions limit the occurrence of obligate epiphytes at higher latitudes, but the presence and persistence of numerous epiphytic individuals illustrate that abiotic conditions do not per se preclude epiphytic occurrence of vascular plants in the north-temperate zone. Besides water shortage, the availability of suitable host trees is a decisive environmental factor that contributes to limit epiphytism of vascular plants in Central Europe.

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Vascular epiphytes are an important component of many ecosystems and constitute a substantial part of global plant diversity. In this context, accidental epiphytism, that is, the opportunistic epiphytic growth of typically terrestrial species, deserves special attention because it provides crucial insights into the global distribution of vascular epiphytes and the initial evolution of epiphytic lineages. Even though accidental epiphytes have been mentioned in the literature for more than a century, they have been neglected in most epiphyte studies. Only recently has accidental epiphytism been investigated more thoroughly. Therefore, the aim of this article is to provide a comprehensive review of the ecological basis and evolutionary relevance of this common but largely neglected phenomenon and to highlight open questions and promising research directions. Our central statement—that any species has the potential to grow epiphytically given the availability of suitable microhabitats and successful dispersal—is backed up by a compilation of observations of accidental epiphytes from numerous ecosystems with diverse climates, even including semiarid Mediterranean ones. A variety of arboreal microhabitats and environmental conditions conform to the ecological niche of typical terrestrial species, with the availability of such microhabitats depending on the interaction of local climate conditions, host tree age, and host species identity. Whenever suitable microhabitats are available in tree crowns, accidental epiphytism is limited primarily by dispersal. In an evolutionary context, the conquest of forest canopy represents an ecological opportunity where accidental epiphytes act as links between terrestrial and epiphytic life forms. We discuss two fundamental scenarios with sympatric speciation, selective pressure, autopolyploidy, and allopatric speciation as underlying mechanisms in the transition from terrestrial to epiphytic growth. In conclusion, we argue that accidental epiphytism is a substrate and dispersal‐dependent phenomenon and that, both from an individual perspective and an evolutionary perspective, epiphytism reflects the occupation of suitable but previously unexploited arboreal microhabitats. Acknowledging the fundamental principles that plant growth is opportunistic and that dispersal is a stochastic process can decisively improve our understanding of species distributions and other ecological patterns, as in the case of accidental epiphytism.

Table S1. Calculated water potential, concentration of PEG 6000 and measured water potential of the solutions used for the germination experiment Calculated Ψ (MPa) PEG 6000 (g/kg H 2 O) Measured Ψ (MPa) 0 0 --0.25 127 -0.34 -0.5 192 -0.69 -0.75 243 -0.82 -1.0 284 -1.05Figure S5 (part 1 of 3). Above-ground and below-ground RGR of the tested species. Analysis of variance (ANOVA) was applied seperately for above-ground and below-ground RGR and each species. Different letters indicate significant differences according to the Tukey's HSD (on a confidence level of 95 %). In case of a significant ANOVA, but no significant differences in Tukey's HSD, groups are marked with *. Groups with P > 0.05 (according to the ANOVA) are marked with n.s.

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