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

Abstract In a forest, the distribution of vascular epiphytes depends on host tree size and substrate type. However, previous studies have mainly focused on tropical forests, despite the growing interest in temperate forests with a high diversity of epiphytes and the frequent occurrence of accidental epiphytism in terrestrial species. This study examined the distribution of accidental and obligate epiphytes on 8 Cercidiphyllum japonicum (Cercidiphyllaceae) trees in a Japanese temperate forest based on the observation of 1,301 epiphyte individuals belonging to 49 species. The hypothesis that the number of total epiphyte species and individuals increased significantly and exponentially with increasing tree size were supported. As predicted, the proportion of accidental epiphyte species would increase with tree size but that of obligate epiphytes would decrease. In agreement with our hypothesis, the major rooting substrate types of accidental and obligate epiphyte assemblages were different. Accidental epiphytes depended mainly on arboreal soil and, to a lesser extent, on moss; however, they were absent on the bark. By contrast, obligate epiphytes were most frequently found on moss, followed by bark and arboreal soil. These results highlight the greater impact of host tree size on the diversity of accidental epiphytes compared to obligate epiphytes, suggesting that there are differential effects of host tree size on accidental and obligate epiphytes in temperate forests. Additionally, the thick accumulation of organic matter is crucial for the establishment and growth of accidental epiphytes.