Elevational range expansion of Betula pubescens may be slowed by lack of suitable mycorrhizal symbionts in alpine soils

The elevational limit for trees is increasing worldwide due to global warming. The elevational range expansion of obligate ectomycorrhizal trees, such as Betula pubescens, may however be limited by lack of fungal symbionts above the treeline since the vegetation here is often dominated by plants forming other mycorrhizal types. We set up a cross-factorial experiment cultivating Betula pubescens seedlings in alpine and lowland soils, at simulated alpine and lowland temperatures, to test whether the fungal community may limit seedling establishment. We found that alpine soils contained fewer ectomycorrhizal fungi in general and that seedlings cultivated in alpine soil were smaller and recruited fewer ectomycorrhizal fungi compared to seedlings cultivated in lowland soil. Our results indicate a link between seedling success, soil fungal community and mycorrhizal colonization. We suggest that unsuccessful mycorrhization of seedlings above the treeline may be limiting for treeline expansion under increased alpine temperatures.