Bacterial, fungal, and mycorrhizal communities in the soil differ between clearcuts and insect outbreaks in the boreal forest 50 years after disturbance

Abstract

Soil microorganisms influence the functions and processes of forest ecosystems, and their composition is affected by natural and anthropogenic disturbances. Timber harvesting disturbs boreal soil microbiomes, most notably ectomycorrhizal communities which are reportedly less diverse in the first decade following a clearcut. However, the long-term impact of harvesting on forest soil microorganism communities have rarely been investigated nor compared with natural disturbances. Our objective was to compare the composition and diversity of bacterial, fungal, and mycorrhizal communities between boreal old-growth and nearby 50–year–old stands regenerating after either an insect outbreak or a clearcut. Our main hypothesis was that the nature of the stand–replacing disturbance influences the composition of the soil microbiome, and that the effect is still detectable 50 years later. We collected 90 samples from 30 plots across six forest stands dominated by Abies balsamea. We sequenced the genome regions 16S rRNA v3v4 for bacteria and ITS1 for fungi and we constructed distance matrices to evaluate changes in community composition with permutational analyses of variance. Results show that 10.2 % to 12.4 % of the variability in community composition can be explained by stand type alone for bacteria, fungi, and mycorrhizae. The composition of soil microbiomes did not vary with soil physicochemical properties. Stands regenerating after a clearcut had a greater alpha diversity (H’) of fungi and mycorrhizae than stands regenerating after an insect outbreak, while old–growth stands were intermediate. Our data indicate that soil microbiomes associated with natural disturbance dynamics differ from those of clearcutted stands, although the mechanisms underlying this pattern remain unclear. Therefore, we suggest that forest managers spare the largest possible tracts of unmanaged forests across the harvested landscape, including areas affected by natural disturbances, so that benchmark soil communities remain available for future studies.