Effects of High-Severity Wildfires on the Ectomycorrhizal Fungal Community of Ponderosa Pine Ecosystems in Eastern Washington

Abstract

Ectomycorrhizal (EcM) fungi have the ability to mediate ecosystem responses to post-fire environmental changes, yet we have limited understanding of how high-severity wildfires influence the EcM community, especially in systems adapted to low-severity fires. To elucidate the effects of high-severity fires on the EcM community of ponderosa pine (Pinus ponderosa Dougl. ex. Laws) ecosystems, I performed a chronosequence study of the post-fire period from 2006 to 2015, with the objective of identifying the changes to species richness and community composition. Additionally, we addressed if an eleven-year span was sufficient time for species richness to reach pre-fire levels. Microsite variability has been shown to affect the EcM community, thus, we also tested for differences in soil nutrients, total carbon (TC), phosphorus (TP), nitrogen (TN) and their respective ratios, to determine if they play a role in the EcM community assemblage. Comparison of the EcM communities between burned and neighboring unburned areas revealed distinct differences between these communities. Both species richness and community composition were significantly lower in the high-severity sites, leading to a complete turnover in species composition. The new environment became dominated by taxa that respond positively to high-severity fires. Most of these taxa were members of the Ascomycota phylum, and include the genera Cennococum and Pustularia, which increased on average by 9-fold (adj-p<0.005) in the post-fire environment. Of importance to this community was the presence of Wilcoxina rehmii, a known PIPO symbiont and pyrophytic fungi. We found no relationship between species richness and increased time since fire, possibly due to the small scale of the chronosequence. However, the significant differences in community composition between the burned and unburned sites suggest that an 11-year recovery period is not sufficient for the EcM fungi communities to return to pre-fire levels. The changes to the post-fire communities were not solely dependent on fire disturbance. We found significant associations between the EcM communities and physicochemical site characteristics, such as elevation and slope, as well as TC and TC: TP. High-severity fires significantly affected the soil nutrient composition, but the alterations were shown to be ephemeral. Total carbon, TN, TC: TN and TC: TP decreased with increase time since fire. In contrast, TP and OM depth increased with increase time since fire.