Wildfire-induced shifts in soil microbial communities affect growth and competitive ability of Coast Redwood and Douglas fir

Background/Question/Methods

Recent intensifying wildfires, spurred by colonial fire suppression and drought, are altering the composition of forests adapted to former climates and fire regimes, and there are gaps in our understanding of how wildfire-induced shifts in soil microbial communities may influence plant competition, especially in stressful environments. Microbes are particularly relevant in seedling recruitment, and fire may alter the presence of both mycorrhizal partners and fungal pathogens through physical and chemical changes to soils. In this study, we studied the soil microbial role in seedling competition to better understand the long-term successional outcomes in a regime of increasingly frequent wildfire. We conducted a field study to assess how wildfire alters soil microbial communities in Big Basin State Park, CA, on the traditional lands of the Amah Mutsun Tribal Band. We extracted DNA from field soil cores for sequencing using ITS2 and 16S primer sets for fungi and bacteria, respectively, using Illumina MiSeq. We used qPCR to assess changes in the relative abundance of microbial taxa after wildfire. We then used a greenhouse seedling experiment to ask how post-fire soil microbial communities impacted seedling competition dynamics between coast redwood (Sequoia sempervirens) and Douglas fir (Pseudotsuga menziesii).

Results/Conclusions

We found that wildfire significantly affected soil microbial composition and seedling growth across Big Basin. Fungal diversity declined by more than 40% and fungal communities at high burn severity sites were dominated by pyrophilic fungi. Bacterial diversity marginally increased with fire severity, likely due to the loss of dominant taxa. Similar to results from other systems, there was a distinct shift in soil saprotrophs from Mortierellomycota towards Mucoromycota as burn severity increased. Importantly, we observed that Agaricomycetes, which includes many fungal mutualists, were also much less abundant post-fire. As fungal mutualists are predicted to increase niche partitioning between different types of mycorrhizal hosts, a decline in mycorrhizal partners could lead to increased competition between plant hosts, a trend supported by our preliminary greenhouse study data. While Douglas fir growth was reduced by competitors, greater ectomycorrhizal colonization in live, unburned soils reduced competitive effects. For redwoods, in the absence of competitors, soil microbes increased growth in unburned soils, however, in sterile soils there was no significant difference in growth across burn severity. Our study suggests that the effects of wildfire on soil microbial communities may alter the outcome of seedling recruitment and competition.