Small-scale fire refugia increase soil bacterial and fungal richness and increase community cohesion nine years after fire

Small-scale variation in wildfire behavior may cause large differences in belowground bacterial and fungal communities with consequences for belowground microbial diversity, community assembly, and function. Here we combine pre-fire, active-fire, and post-wildfire measurements in a mixed-conifer forest to identify how fine-scale wildfire behavior, unburned refugia, and aboveground forest structure are associated with belowground bacterial and fungal communities nine years after wildfire. We used fine-scale mapping of small (0.9–172.6 m²) refugia to sample soil-associated burned and refugial microbial communities. Richness was higher in refugia for bacteria (+19 %) and fungi (+31 %) and in all functional guilds relative to burned soils. Refugial communities had greater proportions of saprotrophic and lower proportions of pathogenic fungi relative to burned soils. Composition differed in burned areas and refugia and was most strongly associated with small-scale fire behavior, aboveground live tree basal area, and tree mortality. Refugial communities had more connected association networks and fewer facilitative interactions relative to burned soils – supporting both the stress-gradient hypothesis and the conclusion that refugial communities may have greater resistance to future disturbance. Small-scale differences in wildfire behavior and effects can have long-term impacts on belowground microbes, highlighting the need to assess neighborhood effects at spatial scales that influence microbes.