Epiphytes account for approximately 10% of global plant diversity and grow on various substrates such as trees. Among them, epiphytic orchids are the most species-rich and ecologically significant group, playing a crucial role in nutrient cycling at the canopy level while they are highly vulnerable to anthropogenic disturbance. Despite their ecological importance, data on epiphytic orchids remain scarce, particularly in seasonally dry forests of the African tropics.Ankarafantsika National Park (ANP) in Northwestern Madagascar harbors intact seasonally dry forests. Within ANP, a 15-ha permanent tree census plot has been established, where approximately 9,000 tree trunks of 150 species over 1 cm in DBH have been individually identified. Additionally, Komada and Noyori et al. (in submission) compiled a checklist of epiphytic orchids within the 15-ha plot, confirming the presence of eight genera, and eleven species. However, the relationship between epiphytic orchids and their host trees remains unexplored. This study aimed to reveal two fundamental interactions between epiphytic orchids and host trees: (1) the influence of host tree size on the abundance of epiphytic orchids and (2) host specificity among epiphytic orchids.
Within the 15-ha plot, we randomly selected 100 quadrats (10m x 10m each) and recorded the presence, species, and ramet abundance of epiphytic orchids through direct observation. We analyzed the effects of host tree size and host specificity patterns using the tree census data.
 Our results demonstrated that the probability of epiphytic orchid presence increased with host tree size, except for hemi-epiphytic species. Furthermore, among the two holo-epiphytic orchid species analyzed, Bulbophyllum rubrum exhibited host specificity and increased their abundance with host tree size. In contrast, Microcoelia perrieri did not exhibit host specificity and their abundance did not increase with the host tree size even though its possibility did not. The hemi-epiphytic Vanilla species did not exhibit host specificity but showed increased abundance with the tree host size. The host preference of B. rubrum may be attributed to its relatively large body size (up to 20 cm), which may require stable support from large host trees. In contrast, the smaller M. perrieri (up to 10 cm) lacks leaves and may not require the same level of structural support. Vanilla species may be attributed to its hemi-epiphytic growth habit, which allows it to establish on various substrates without requiring host-associated mycorrhizal symbionts.