A single range‐expanding species reshapes alpine ecosystems and their belowground diversity

Species around the globe are shifting their ranges into new territories at an unprecedented rate. In particular, the spread of foundation species can transform recipient communities and ecosystems, however, the effects on belowground processes and diversity remain poorly documented. Belowground fungi are well suited for taking the 'pulse' of changing ecosystems given their rapid turnover and implication in a wide variety of ecosystem processes. To better understand the belowground effects of range‐expanding species, we leveraged an ongoing invasion of a foundation tree species Pinus contorta into alpine tundra to study the impacts on belowground abiotic conditions and the fungal communities associating with the roots of resident plants. We found that individual range‐expanding trees create distinct abiotic 'islands' with wetter soils and altered soil nutrients compared to the surrounding alpine tundra ecosystem. Potentially driven by these abiotic changes, we observed a decrease in the α diversity of mutualistic fungi and an increase in the α‐diversity of pathogenic fungi during later stages of range expansion. Changes in γ‐diversity mirrored patterns of α diversity while β‐diversity was only minorly affected by range‐expanding trees, suggesting that local habitat amelioration/deterioration rather than changes in among‐patch heterogeneity underpin trends in belowground diversity. In sum, our results show that range‐expanding foundation species can modify ecosystems by altering belowground abiotic conditions and diversity across scales. These impacts begin only a few years after initial range‐expansion and establishment and scale rapidly over time, indicating the need for preventative or swift conservation action to prevent long‐term consequences.