Oases in trees: patterns of aquatic hyphomycetes diversity in dendrotelmata and streams of a mediterranean forest

Freshwater ecosystems are vital oases for biodiversity across Mediterranean landscapes. Acquiring basic knowledge on how aquatic biodiversity is distributed across Mediterranean landscapes is pivotal to anticipate how aquatic ecosystems are responding to various stressors, such as drought intensification. Fungi – a very diverse group – are involved in important ecosystem functions and services, such as organic matter decomposition. Aquatic hyphomycetes (a polyphyletic group of fungi) are particularly relevant to forested aquatic ecosystems because of their capacity to decompose wood and leaf litter. So far, this group received little attention outside of their preferred habitat, known as oxygenated forest streams. In the Massane forest, an old-growth beech forest in southwestern France, we explored the patterns of alpha, beta and gamma diversity of aquatic hyphomycetes in a mosaic of two main freshwater habitats (water-filled tree holes – so called dendrotelmata –, and streams). Dendrotelmata harboured locally-rich communities of aquatic hyphomycetes (sometimes as much as adjacent streams), but their composition (beta diversity) was more heterogeneous in comparison to streams. The spatially discrete nature of dendrotelmata across a forested landscape (compared to vectorised, dendritic networks of permanent streams), and/or the environmental singularity of each dendrotelmata, might explain why beta diversity was higher in dendrotelmata than streams. Differences in community composition between stream and dendrotelmata habitats were as high as the differences between the two main types of dendrotelmata (pan vs. rot holes). A noticeable part of species co-occurred in both dendrotelmata and streams (29%), suggesting along with the observed strong environmental differences in both habitats, that a noticeable proportion of species we identified can have broad ecological niches. The high beta diversity of aquatic hyphomycetes among dendrotelmata could motivate fine-scale monitoring of dendrotelmata communities at the individual-tree level rather than at the forest plot level, giving new opportunities to improve current management and conservation of old-growth forests and their biodiversity. Our study suggests that we need to expand our knowledge on the distribution of aquatic hyphomycetes outside their preferred habitat to fully understand their biodiversity and ecological roles.