The epiphytic bird's nest ferns (Asplenium nidus) in the forest canopy resemble distinctive island habitats for microbes due to their abundant suspended leaf litter and soil. Although research has shown that plants can actively structure the surrounding microbiome by root exudates and species-specific litter chemistry, it is unclear whether A. nidus plays a similar role. This study aims to understand whether A. nidus acts as passive islands or active drivers of microbiomes in their suspended soil. We examined 24 A. nidus individuals of varying sizes (small, medium, and large) in a Cryptomeria japonica plantation in Wulai, Taiwan. Soil samples were collected from small individuals and different layers in medium and large individuals. The Fungal community was characterized using the rDNA ITS1 region. In line with the theory of island biogeography, we showed that the fungal OTU richness significantly increased with the size of A. nidus. Moreover, the pH range (i.e., differences in pH value between the top and bottom of A. nidus individuals) increased with the size of A. nidus and is positively correlation with the mean dissimilarity between fungal community composition within the A. nidus individual. However, we found no convergence of fungal community composition with the size of A. nidus, which may imply that A. nidus will not actively structure the fungal community through time. Overall, our results suggest that A. nidus function as islands, with their growth providing diverse niches that foster fungal diversity.