Megafire in Redwood Tanoak Forest Reduces Bacterial and Fungal Richness and Selects for Pyrophilous Taxa that are Phylogenetically Conserved

Large, uncontrolled mega-fires of unprecedented size, intensity, and socio-economic impact have surged globally in recent decades due to climate change, fire suppression, and development. How these mega-fires impact the soil microbiome, which drives all major ecosystem processes, remains unclear. We had a serendipitous opportunity to obtain rare pre- and post-fire soil samples from the same sampling locations because the 2016 Soberanes Fire burned down several of our established redwood tanoak plots in Big Sur, CA. Within two months of the mega-fire and before first rain, we re-sampled soils from two burned plots and one unburned plot to compare the effect of time versus fire on soil microbial communities. We used Illumina MiSeq of 16S and ITS1 to determine that both bacterial and fungal richness were reduced in burned plots, with richness unchanged in the unburned plot. Fire altered composition for bacteria and fungi, whereas the unburned plots experienced no change in fungal and negligible change in bacterial composition. Most exciting, we found that pyrophilous taxa that positively or negatively responded to fire were phylogenetically conserved. For bacteria, fire selected for increased Firmicutes and Actinobacteria. For fungi, fire selected for the Ascomycete classes Pezizomycetes and Eurotiomycetes and for a Basidiomycete class of heat-resistant Geminibasidiomycete yeasts. This is the first report of bacterial and fungal responses to megafires from pre- and post-fire samples with both burned and unburned controls, the first report of microbial response in redwood tanoak forests, and the first to demonstrate that pyrophilous microbial taxa are phylogenetically conserved. Comparing to other recent studies, we propose a trait-based conceptual model of microbial response to wildfire analogous to plants.