Accidental epiphytism in the Harz Mountains, Central Europe

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.