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Abstract
Despite comprising over half of all orchid species globally, mycorrhizal associations in tropical epiphytic orchids remain relatively understudied compared to their terrestrial counterparts. The greatest diversity of these orchids is found in tropical montane cloud forests, where they constitute more than 50% of all epiphytic plants. While the drivers of such high diversity remain uncertain, it has been suggested that mycobiont associations may play a role in this phenomenon. Previous studies on orchid mycorrhizal interactions have generally found little evidence of specialization, with limited research on tropical orchids also indicating generalist associations. However, epiphytic orchids often exhibit narrow distributions, high endemism, and dependency on both phorophyte and fungal partners, making them particularly vulnerable to environmental changes. This highlights the need to examine the effects of phorophyte identity and habitat disturbance on orchid mycobiont composition. In this study, we analyzed root-associated fungal communities in rare, closely related, and narrowly endemic epiphytic orchids from the rapidly diversifying genus Lepanthes within one of the world’s most biodiverse hotspots. Our objectives were to: (1) characterize the composition and diversity of mycorrhizal communities associated with four sympatric orchid species, (2) assess the degree of exclusiveness in these associations and the influence of keystone fungi on the structure of orchid-fungal symbioses, (3) determine whether orchids associate with closely related fungal partners by examining the degree of phylogenetic signal at different interaction frequency resolutions, and (4) investigate the impact of phorophyte identity and habitat disturbance on mycobiont composition in 22 orchid populations of a single species. Our findings revealed distinct mycorrhizal communities among the focal orchid species, with variation not uniformly distributed across species. Contrary to previous reports of generalism in tropical orchids, we detected a strong signal of specialization in species interaction networks. Opportunistic interactions influenced the phylogenetic signal, which was stronger in keystone fungal taxa. Additionally, mycobiont communities varied significantly among both phorophyte species and habitat types. To our knowledge, this is the first study demonstrating differing fungal communities in closely related orchid species, supporting phorophyte bias and highlighting the impact of environmental degradation on orchid fungal associates.