Plant traits and soil organic matter quality drive fungal functional groups in shrub-encroached subalpine grasslands

Shrub encroachment is a global process driven by land use and climate changes. Although the effects of shrub encroachment on ecosystem functions have been increasingly researched, effects on aboveground-belowground interactions are still not fully understood. In this study, we aim to elucidate the ecosystem-level effects of plant community traits and soil properties on key fungal functional groups, arbuscular mycorrhizal fungi (AMF), ericoid mycorrhiza (ErM) and ligninolytic fungi. Across two subalpine grassland sites in the Alps, community weighted means (CWM) of plant traits and soil properties measured along gradients of increasing shrub density varied along two leading axes representing plant woodiness and accumulation of soil organic matter (SOM). Woodiness was associated with CWM of plant traits representing tissue quality, while SOM accumulation was mainly associated with SOM, soil carbon, soil phosphorus and pH. Both of these functional gradients had significant, linear effects on the three fungal functional groups. Plot-level relative abundances of AMF decreased, ErM increased and ligninolytic fungi increased in response to increasing woodiness and SOM accumulation. These responses were consistent across plant communities with different dominant shrub species, although for mycorrhizal fungal groups shrub identity modulated the magnitude of effects. This study demonstrates that both the amount of woody tissue, and the accumulation of organic material impact soil fungal functional groups, reflecting the direct and indirect effects of shrub encroachment. These gradual effects are consistent across plant communities, showing general mechanisms that underpin aboveground-belowground interactions and thereby ecosystem functions central to nutrient cycling and carbon sequestration. As such, soil fungal functional groups deserve specific attention as key players of ecosystem transformation under global changes.