Crown-fire severity is more important than ground-fire severity in determining soil fungal community development in the boreal forest

Wildfire shapes the structure, dynamic and functioning of boreal forests. With predicted warmer and drier summers, increased incidence and intensity of crown-fires may affect plant-soil interactions with consequences for post-fire fertility and forest productivity. We assessed how severity of crown- and ground-fire in boreal pine forests affected post-fire responses of soil fungal communities and their associated enzyme activities, and how variation in fire severity interacts with salvage (post-fire) logging in impacting soil fungi. Crown fire-induced tree mortality had a stronger impact on fungal biomass and community composition than did ground-fire-induced loss of soil organic matter. Severe crown-fire led to replacement of ectomycorrhizal- and litter-associated fungi by stress-tolerant ascomycetes. Elevated activities of hydrolytic enzymes in burned areas were correlated with root-associated ascomycetes and moulds, suggesting opportunistic exploitation of labile organic substrates. Fire did not, however, increase the abundance of more potent basidiomycete decomposers in the organic layer, nor did it enhance organic matter oxidation by fungal peroxidases, indicating that the potential for major post-fire losses of carbon due to stimulated decomposition is limited. Rather, peroxidase activity was low in burned areas, likely reflecting the absence of ectomycorrhizal fungi. Post-fire salvage logging induced larger shifts in fungal communities in areas with low crown-fire severity. Synthesis. Historically, boreal pine forests have been shaped by low-severity ground-fires. Our study highlights a risk that increasing occurrence of high-severity crown-fire as climate warms will have detrimental effects on mycorrhizal-mediated functions that are pivotal for maintaining organic matter turnover, soil fertility and forest resilience.

Automated summary

Wildfires in boreal pine forests affect soil fungal communities and enzyme activities differently depending on the severity of crown and ground fires. Post-fire salvage logging induces larger shifts in fungal communities in areas with low crown-fire severity. Climate warming and increasing occurrence of high-severity crown fires may have detrimental effects on mycorrhizal-mediated functions essential for maintaining soil fertility and forest resilience.