Environmental DNA reveals geologic and climatic isolation effects on plant and fungal Communities in the Hengduan Mountains

Species range limits are typically constrained by their tolerance to abiotic factors such as climate, as well as by dispersal limitations due to geographic barriers like mountain ridges and river valleys. Montane regions, which are hyperdiverse in many different clades, and characterised by high turnover, and complex topography, provide ideal systems for investigating the drivers of range limits. In this study, we collected 30 environmental DNA (eDNA) samples from the tributaries of the Salween, Mekong, and Yangtze rivers and employed ITS2 primers to analyse the species composition of plant and fungal communities. We then quantified and partitioned the relative contributions of geographic and climatic isolation to the assembly of these communities. Our analyses revealed a congruent distribution pattern between plant and fungal groups, with distinct segregation across different river valleys and along latitudes. Habitat preference analyses indicated that our eDNA samples predominantly captured mid-to-low elevation species. The taxonomic composition of plant and fungal communities was jointly influenced by geographic and climatic isolation, although the strength of these influences varied across different groups. Fungal communities were more strongly affected by geographic isolation, while plant communities were more influenced by the combined constraints of geography and climate. These findings underscore the joint influence of topographic and climatic isolation in shaping community composition, highlighting the importance of environmental filtering and dispersal limitation processes. We conclude that the high biodiversity in montane regions depends on both complex climate and topography and, secondarily, note that riverine environmental DNA provides an efficient approach to understand geological and climatic mechanisms in driving biodiversity patterns in terrestrial clades.