COS 153-2 - Small fire refugia maintain unique bacterial and fungal diversity nine years after wildfire

Abstract: Microbial communities are key mediators of ecosystem function and may be altered by wildfire partially or fully sterilizing soil and altering the abiotic environment. However, small-scale variation in fuels, topography, and weather may result in unburned refugia where fire-sensitive microbes may persist through fire. Microbes that inhabit these refugia may have notably different composition and function relative to neighboring disturbed communities and may disperse into burned soils in the months and years following wildfire. Importantly, we do not know the extent to which communities differ between burned soils and unburned refugia and if differences persist over time scales commensurate with forest and fuel recovery.

To address this gap, we sought to 1) identify the compositional and functional differences in bacterial and fungal communities between burned and refugia soils, and 2) assess how microbial communities differentially correspond to fire-altered environmental features in burned and refugia soils. We assessed post-fire microbial communities in the Yosemite Forest Dynamics Plot, California, USA which burned in the 2013 Rim Fire. We mapped small fire refugia (1.0 – 172.0 m²) in summer of 2014, and in 2021 we sampled burned and refugia soils. We used MiSeq sequencing to assess bacterial and fungal community composition and characterized bacterial and fungal function by assigning functional traits to amplicon sequence variants.

Nine years after wildfire, bacterial richness was elevated in burned soils relative to refugia soils. In contrast, fungal richness was lower in burned soils relative to refugia. Symbiotrophic and saprotrophic richness were greater in refugia relative to burned soils. Litter and duff biomass were similar across sample types and did not explain differences between samples. Microbial communities varied strongly with soil nutrients and slightly with initial burn severity and topography.

Our results show that microbial communities in unburned refugia are notably different than those in burned patches, nine years after fire. Thus, small-scale refugia may help preserve fire-sensitive microbes and promote belowground heterogeneity, thereby potentially improving resilience of the belowground ecosystem and its role in post-fire forest recovery. Unburned refugia may become increasingly ecologically important as wildfire size and severity continue to increase in many dry western forest types. The stark differences in microbial richness highlight the enduring legacy of wildfire nearly a decade after burning.