Ecotones between forests, dominated by ectomycorrhizal trees, and heathlands, dominated by ericoid mycorrhizal dwarf shrubs, are common in natural and cultural landscapes. Such ecotones are naturally found in transitions towards arctic and alpine zones, and ericaceous shrubs comprise an increasing share of primary production in aging boreal forests. In cultural heathlands, tree regrowth has traditionally been restricted by grazing or biomass harvest. Recently, we showed that belowground carbon sequestration was lowest when ectomycorrhizal fungi dominated soil processes and increased when ericoid mycorrhizal fungi increased in dominance in old-growth boreal forests. This suggests that increasing forest cover or production with changes in climate or management practices could have drastic effects on soil carbon sequestration. To investigate the generality of these findings, we here present results from a subarctic-to-alpine ecotone from mountain birch forest to heath tundra. We combined high-throughput bar-code sequencing to characterize fungal community composition in fine-scaled soil profiles with ecosystem-level measurements of plant and fungal biomass and production and soil carbon, nitrogen and stable isotope pools.
We found a strong positive coupling between tree production and ectomycorrhizal fungal growth (as assessed by ingrowth bags), both of which were negatively coupled with carbon sequestration along the ecotone. In the fungal community we identified a shift in dominance from root-associated ascomycetes (mostly ericoid mycorrhizal) in the heath to cord-forming ectomycorrhizal fungi (mostly Cortinarius and Leccinum spp.) in the forest. High carbon/nitrogen-ratios and low inorganic nitrogen levels in the forest humus suggested a more efficient nitrogen mobilization linked to higher activities of these ectomycorrhizal fungi. Together, our data suggest that a lower carbon sequestration rate in forests, despite higher plant and mycelial litter inputs, is a consequence of more efficient ectomycorrhizal nutrient foraging from organic pools. In contrast, when soil processes are dominated by stress-tolerant ericoid mycorrhizal plants and fungi, as in heaths, more carbon accumulates. We propose that a general trade-off between rapid growth and turnover versus tolerance to exogenous stresses in mycorrhizal fungal communities is a main driver of soil carbon balance across ecto- and ericoid mycorrhizal dominated ecosystems.