Expanding forests in alpine regions lead to a corresponding shift in belowground fungal communities

Climate change causes upward shift of forest lines worldwide, with
consequences on soil biota and carbon sequestration (C). We here analyse
compositional changes in the soil biota across the forest line ecotone, an
important transition zone between different ecosystems. We collected soil
samples along transects stretching from subalpine mountain birch forests
to low-alpine vegetation. Soil fungi and micro-eukaryotes were surveyed
using DNA metabarcoding of the 18S and ITS2 markers, while ergosterol was
used to quantify fungal biomass. We observed a strong shift in the soil
biota across the forest line ecotone: Below the forest line, there were
higher proportions of basidiomycetes and mucoromycetes, including
ectomycorrhizal and saprotrophic fungi. Above, we observed relatively more
root-associated ascomycetes, including Archaeorhizomycetes, ericoid
mycorrhizal fungi and dark septate endophytes. Ergosterol and percentage C
content in soil strongly and positively correlated with the abundance of
root-associated ascomycetes. The predominance of ectomycorrhizal and
saprotrophic fungi below the forest line likely promote high C turnover,
while root-associated ascomycetes above the forest line may enhance C
sequestration. With further rise in forest lines, there will be a
corresponding shift in the belowground biota, likely leading to enhanced
release of soil C.