Abstract
Abstract 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.
Highlights
Wildfire is a natural disturbance that shapes the structure, dynamics and functioning of boreal forests worldwide (Barnekow et al, 2008; Engelmark, 1999; Rogers et al, 2015; Wardle et al, 2003)
Pinus sylvestris has evolved particular traits, such as thick and non-flammable bark, that increase survival through low-intensity ground-fires (Rogers et al, 2015), and grows high and self-prunes low branches, creating a gap between ground surface fuels and the canopy that reduces the onset of crown-fires (Keeley, 2012)
We found that survival of P. sylvestris after fire played a key role in maintaining fungal biomass, diversity and community composition
Summary
Wildfire is a natural disturbance that shapes the structure, dynamics and functioning of boreal forests worldwide (Barnekow et al, 2008; Engelmark, 1999; Rogers et al, 2015; Wardle et al, 2003). Changes in fire type (crown- vs ground- or surface-fire), intensity (the energy output from a fire), severity (the impact of fire measured as tree mortality and/or organic matter loss) and frequency may, potentially, alter the structure, productivity and successional dynamics of boreal forest ecosystems by shifting feedbacks between plants, microbes and the abiotic soil environment (Clemmensen et al, 2013, 2015; Johnstone et al, 2010). By disrupting the competitive dominance of ectomycorrhizal fungi in the mor layer, it is possible that severe crown-fires as well as salvage logging may trigger proliferation of saprotrophs and thereby accelerate decomposition of the remaining organic matter that was not directly combusted by the fire—the so-called ‘Gadgil effect’ (Averill & Hawkes, 2016; Fernandez & Kennedy, 2016; Gadgil & Gadgil, 1971, 1975; Kyaschenko, Clemmensen, Hagenbo, et al, 2017). In exploring these three hypotheses, our overarching goal was to provide insights into short-term changes of fungal communities with fire behaviour, crown-fire versus ground-fire, and its interaction with post-fire logging, to enable better prediction of ecosystem responses to future fire scenarios in Fennoscandian boreal forest subjected to climate change and post-fire management
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