Wildfire severity reduces richness and alters composition of soil fungal communities in boreal forests of western Canada

Abstract

© 2019 John Wiley & Sons Ltd Wildfire is the dominant disturbance in boreal forests and fire activity is increasing in these regions. Soil fungal communities are important for plant growth and nutrient cycling postfire but there is little understanding of how fires impact fungal communities across landscapes, fire severity gradients, and stand types in boreal forests. Understanding relationships between fungal community composition, particularly mycorrhizas, and understory plant composition is therefore important in predicting how future fire regimes may affect vegetation. We used an extreme wildfire event in boreal forests of Canada's Northwest Territories to test drivers of fungal communities and assess relationships with plant communities. We sampled soils from 39 plots 1 year after fire and 8 unburned plots. High-throughput sequencing (MiSeq, ITS) revealed 2,034 fungal operational taxonomic units. We found soil pH and fire severity (proportion soil organic layer combusted), and interactions between these drivers were important for fungal community structure (composition, richness, diversity, functional groups). Where fire severity was low, samples with low pH had higher total fungal, mycorrhizal, and saprotroph richness compared to where severity was high. Increased fire severity caused declines in richness of total fungi, mycorrhizas, and saprotrophs, and declines in diversity of total fungi and mycorrhizas. The importance of stand age (a surrogate for fire return interval) for fungal composition suggests we could detect long-term successional patterns even after fire. Mycorrhizal and plant community composition, richness, and diversity were weakly but significantly correlated. These weak relationships and the distribution of fungi across plots suggest that the underlying driver of fungal community structure is pH, which is modified by fire severity. This study shows the importance of edaphic factors in determining fungal community structure at large scales, but suggests these patterns are mediated by interactions between fire and forest stand composition.

Distinct fungal successional trajectories following wildfire between soil horizons in a cold‐temperate forest

“…Here we found that burning effects on fungal richness were only pronounced for the EcM fungal guild and only so in the Oe horizon (Table 2), which was consistent with the findings in other recent studies (Salo & Kouki, 2018;Day et al, 2019;Owen et al, 2019). In the Oe horizon, the mean OTU richness of EcM fungi declined by > 40% in recently burned forests compared with that in unburned controls, and burning individually New Phytologist explained 37.8% of the variation in EcM fungal richness (Table 2).…”

“…Within the Eurotiomycetes class, five abundant OTUs assigned to Penicillium exhibited a significant positive response to fire occurrence and accounted for 7.56% of total sequences in the Oe horizon (Table S13). In a recent study on fungal communities and fire severity, Day et al (2019) found that the relative abundances of several OTUs identified as Penicillium significantly increased with increasing fire severity, and some Penicillium species have been reported to form sclerotia to resist harsh environments and survive high temperatures in laboratory conditions (McGee et al, 2006). In addition, we observed that the relative abundances in some pathogenic fungi OTUs increased by burning, and symbiotic fungi like dark septate endophytes and EcM fungi were suppressed in the Oe horizon (Table S13).…”

“…Existing studies suggest that, in the short-term (e.g. < 5 yr), soil fungal communities respond strongly to wildfire, with the most prominent changes being a transition from dominance of Basidiomycota to Ascomycota, and fire severity, stand age, pH, soil moisture and C : N ratio being considered the underlying drivers of these changes (Waldrop & Harden, 2008;Reazin et al, 2016;Day et al, 2019). In the long term (e.g.…”

Soil Water Content and Soil Respiration Rates Are Reduced for Years Following Wildfire in a Hot and Dry Climate

“…Conversely, however, lower pH, which we observed in soils burned in 2011, suppresses bacterial biomass (Bååth & Anderson, 2003). In addition, fungi can take longer to recover after a wildfire than bacteria, regardless of soil pH (Vázquez et al, 1993) and interactions between fire severity and increases in pH can reduce fungal richness and diversity (Day et al, 2019). Therefore, fire‐induced changes to pH can have profound, long‐lasting, effects on microbial community structure and, if the soils burned in 2011 now represent the chronosequence of soil development after a few years of regrowth, it is possible these mechanisms are influencing the microbial community and its ability to breakdown organic matter as bacterial activity is suppressed with decreasing pH and the fungal community has not had a chance to fully recover yet.…”

Patterns of Ecosystem Structure and Wildfire Carbon Combustion Across Six Ecoregions of the North American Boreal Forest

“…Warmer temperatures increase rates of organic soil decomposition, which release nutrients and increase aboveground C production and total net primary productivity (Chapin et al, 2002;Day et al, 2019). This leads to increased litter fall, root turnover, and an overall higher yield in C input per year in boreal forest soils (Deluca and Boisvenue, 2012).…”