Wildfires are a necessary disturbance of chaparral ecosystems that affect both vegetation and fungal communities. Secondary successional dynamics have been extensively studied in vegetation, while the trends in fungal community succession remain unclear. Fungi are vital for biogeochemical cycling and decomposition, plant nutrient acquisition, and successful plant regeneration. Consequently, understanding the successional dynamics of fungi in an era where wildfires are becoming increasingly frequent can help elucidate the mechanisms underlying community assemblage and their role in post-fire ecosystem regeneration.
To quantify post-fire changes in fungal community composition and elucidate their succession, we took advantage of the 2018 Holy Fire, which burned 94km2 of chaparral in Southern California. Within 1 week of the fire, we established 9 plots (6 burned and 3 unburned), collecting 4 samples per plot in a 5m radius in the cardinal directions. We collected soils at 9 time points ranging from 1-week to 1-year post-fire, resulting in the most comprehensive fine-scale temporal sampling of fungi to date. We assessed fungal richness and composition with Illumina MiSeq sequencing of ITS2 amplicons. We also measured ash depth, plant cover and composition, soil texture and moisture, pH, microbial respiration and biomass C, nutrients, and extracellular enzyme potentials.
Results/Conclusions
Wildfire reduced fungal richness by 68% (P < 0.05), leading to significant alterations to the fungal community composition. The nearly complete removal of Manzanita (Arctostaphylos), an ectomycorrhizal host, resulted in a concurrent reduction in richness of ectomycorrhizal fungi in the burned plots, and a relative increase in saprobes. In accordance with other Mediterranean environments, wildfire also shifted fungi from basidiomycete- to ascomycete- dominated. In addition, pyrophilous fungi, which were undetectable pre-fire, significantly increased post-fire dominating up to 60% of the sequences at a single time-point. We observed clear patterns of fungal secondary succession, initiated by the dominance of Geminibasidium, a heat resistant and xerotolerant yeast, which dominated 27% of the sequences at 17 days post-fire. However, 25 days post-fire, dominance was seceded to Pyronema, an ectomycorrhizal pyrophilous fungus dominating on average 48% of sequences for up to four months. Over time, fast colonizing saprobic Ascomycetes in the genera Penicillium and Aspergillus increased in abundance. One-year post-fire was not enough time for fungal richness or composition to recover, as fungal richness was on average 62% lower and composition was 10% different than the unburned community. Continued sampling will elucidate the factors predicting fungal resilience and contribution to ecosystem regeneration.