Co-occurring epiphytic orchids have specialized mycorrhizal fungal niches that are also linked to phenology

Abstract Mycorrhizal symbiosis has been related to the coexistence and community assembly of coexisting orchids in few studies despite their obligate dependence on mycorrhizal partners to establish and survive. In hyper-diverse environments like tropical rain forests, coexistence of epiphytic orchids may be facilitated through mycorrhizal fungal specialization. However, information on the role of orchid mycorrhizal fungi (OMF) in niche differentiation and coexistence of epiphytic orchids is still scarce. In this study, we sought to identify the variation in fungal preferences of four co-occurring epiphytic orchids in a tropical rainforest in Costa Rica by addressing the identity and structure of their endophytic fungal and OMF communities across species and life stages. We show that the endophytic fungal communities are formed mainly of previously-recognized OMF taxa, and that the coexisting orchid species display distinct (OMF) communities while keeping a base of shared fungi. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time, a strategy that may serve as a complementary mechanism to fulfill the nutritional needs associated with reproduction. This study provides evidence for niche partitioning in coexisting tropical epiphytic orchids through the utilization of specific OMF, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.