Effects of host size and substrate types on the distribution of accidental and obligate epiphytes: a case study in a temperate forest of Japan

In forests, the distribution of vascular epiphytes is controlled by various environmental factors such as host tree size and substrate type. However, a few studies focused on temperate forests, despite the growing interest in temperate forests with high diversity of accidental epiphytes. This study examined the distribution of accidental and obligate epiphytes on 8 Cercidiphyllum japonicum (Cercidiphyllaceae) trees in a Japanese temperate forest based on observations of 1,301 epiphyte individuals belonging to 49 species. As predicted, the number of species and individuals of accidental and obligate epiphytes increased with host tree size. While the percentage of accidental epiphyte species increased with the host tree size, the percentage of obligate epiphyte species decreased; however, these changes were not significant for the number of individuals. Consistent with our hypothesis, the major rooting substrates of accidental and obligate epiphytes was different. Accidental epiphytes primarily depend on arboreal soil and, to a lesser extent, on moss, but are absent on tree bark. Contrastingly, obligate epiphytes were most frequently found on moss, followed by bark and arboreal soil. These results highlight the impact of host tree size on the diversity of accidental epiphytes compared to obligate epiphytes, suggesting that the host tree size effects are different between accidental and obligate epiphytes in temperate forests. Additionally, a thick accumulation of arboreal soil is crucial for the establishment of accidental epiphytes.