How cushion plant communities structure nival soil biodiversity: A metabarcoding study in the French Alps

In the challenging environmental conditions of high elevation ecosystems, cushion plants create micro-climatic and fertile shelters which host a vast diversity of organisms. Yet, the taxonomic diversity of these hosts remains poorly described, and to what extent cushion plants structure these communities remains unclear. We sampled soils beneath six different species of cushion plants, along with bare-ground controls, across two different elevation gradients in the French Alps. We used environmental DNA metabarcoding to investigate the effect of different species of cushion plants on the $\alpha$ and $\beta$ diversity of fungi, bacteria, eukaryotes, and for the first time in these ecosystems, unicellular eukaryotes and soil worms. Cushion plants hosted a surprisingly large diversity of organisms, from bacteria to mites and collembolans, forming rich and complex ecosystems. $\alpha$-diversity between cushion plant and bare soil samples differed only for fungi, with communities partly structured by the cushion plant species’ identity. The effect of cushion plant species on composition and $\beta$-diversity of eukaryotic and fungal communities surpassed the environmental effect, while it equaled the site effect for bacterial communities. These results highlight the key role of biotic interactions in shaping the composition of high elevation communities, and clarify the role of cushion plants as engineer and foundation species in these harsh environments. By sheltering highly diverse communities at such high elevation, cushion plants may play a prominent role in the ecological assembly of these diverse, yet poorly known, ecosystems.