Soil microbial communities in two reindeer grazing regimes in subarctic mountain birch forests recovering from severe moth herbivory

In subarctic mountain birch forests, reindeer grazing, and moth outbreaks act as important biotic drivers of ecosystem functioning. These drivers cause foliage loss of trees and shrubs, reallocation of assimilated carbon, altered nutrient availability and vegetation composition, which may impact soil microbial communities. We investigated how a long-standing contrast in reindeer grazing regime (year-round vs. winter) and short-term ungulate exclusion affected soil microbial communities in birch forests recovering from a recent moth outbreak. Differences in microbial communities were mainly explained by grazing regime and soil layer whereas ungulate exclusion had little effect. Abundance of ectomycorrhizal fungi in winter-grazing regime suggests that long-term ungulate grazing may cause cascading impact in mycorrhizal fungal communities. Our results also indicate that belowground fungal communities of mountain birch forests are relatively resilient towards cyclic/predictable cascades such as moth and rodent outbreaks.