High-severity wildfire reduces richness and alters composition of ectomycorrhizal fungi in low-severity adapted ponderosa pine forests

Ponderosa pine (Pinus ponderosa) forests are increasingly experiencing high-severity, stand-replacing fires.
Whereas alterations to aboveground ecosystems have been extensively studied, little is known about soil fungal
responses in fire-adapted ecosystems. We implement a chronosequence of four different fires that varied in time
since fire, 2 years (2015) to 11 years (2006) and contained stands of high severity burned P. ponderosa in eastern
Washington and compared their soil fungal communities to adjacent unburned plots. Using Illumina Miseq
(ITS1), we examined changes in soil nutrients, drivers of species richness for ectomycorrhizal (plant symbionts)
and saprobic (decomposers) fungi, community shifts, and post-fire fungal succession in burned and unburned
plots. Ectomycorrhizal richness was 43.4% and saprobic richness 12.2% lower in the burned plots, leading to
long-term alterations to the fungal communities that did not return to unburned levels, even after 11 years.
Differences in the post-fire fungal community were driven by pyrophilous, “fire-loving” fungi, including the
ectomycorrhizal Ascomycete genera Pustularia and Wilcoxina, and the saprobic Basidiomycete genus Geminibasidium.
Ectomycorrhizal and saprobic fungi were intimately linked to the soil environment: depth of the
organic matter, total carbon, total nitrogen, and their interaction with fire predicted ectomycorrhizal richness.
Whereas total carbon, time since fire, treatment, and the interaction between time since fire and treatment
predicted saprobic richness. We conclude that high-severity wildfires lead to lower ectomycorrhizal richness and
significantly altered ectomycorrhizal and saprobic communities in fire-adapted ecosystems, selecting resilient
and fire-adapted species, such as W. rehmii and Geminibasidium sp., thus initiating post-fire succession.