Fire severity drives shifts in ectomycorrhizal fungal communities and Pseudotsuga menziesii seedling performance

Abstract Background and Aims High-severity wildfires are increasing in western North America, disrupting soil conditions and mutualisms between plants and ectomycorrhizal fungi (EMF), which support seedling nutrient uptake, stress tolerance, and survival. This study investigates how fire severity and recency affect fungal communities and Pseudotsuga menziesii (Douglas-fir) seedling performance, and whether live-soil inoculation or stress priming improve post-fire outcomes. Methods In a greenhouse experiment, Douglas-fir seedlings were grown in soils from high-severity burns, recent and historic low-severity burns, and unburned sites in the Sierra Nevada. Each soil was applied as a live or autoclaved inoculum. Drought stress and abscisic acid priming treatments were imposed before a final 3-week dry-down. We measured seedling biomass, chlorophyll fluorescence (Fv/Fm), EMF colonization, and fungal community composition using ITS amplicon sequencing. Results High-severity soils reduced seedling growth, EMF colonization, and fungal richness. Low-severity burns supported more rich, diverse fungal communities and greater seedling performance. Stress priming treatments had minimal effects. Fungal community composition varied significantly by burn severity. Conclusion Fire severity drives changes in fungal community structure that influence seedling outcomes. Low-severity soils retain beneficial microbial legacies, supporting stronger mycorrhizal associations that result in seedling performance comparable to unburned soils. Targeted microbial restoration may enhance post-fire forest regeneration in high-severity burn areas