Exploring the Effects of Post-Wildfire Soil Microbe Communities on Tree Germination

Background/Question/Methods

Wildfires in California and the Western United States are increasing in size, frequency, and severity due to colonial fire suppression and climate change. As these catastrophic fires typically result in full-stand replacement, soil microbial communities, and specifically symbiotic fungi, may hold the key to understanding if and how these forests may recover. A high-profile megafire recently burned some of the last remaining old-growth coast redwood forests at the southern extent of their range. While fire is a naturally-occurring component of these ecosystems, there is an important gap in our understanding of the microbial component of post-fire seedling germination dynamics. Germination is a critical stage of forest recovery after a fire, as trees race to access newly available resources including nutrients, water, and sunlight. Elucidating how post-fire microbial communities contribute to dynamics of seedling germination and therefore fire recovery will be critical in understanding and predicting post-fire tree recruitment. We germinated both coast redwood and Douglas fir (a co-dominant competitor species) seeds in live and sterile soil collected across a gradient of burn severities in Big Basin State Park to test the role of post-fire microbial communities on potential for seedling germination.

Results/Conclusions

We found that the presence of soil microbes and burn severity affected coast redwood and Douglas fir seed germination in unique ways. Preliminary results indicate that germination rates of Douglas fir were 14% higher in live soils regardless of burn severity. Coast redwood seed germination improved by 6% with the presence of microbes and was also dependent on the severity of the burn. Coast redwood seed germination success increased as burn severity increased. Our results suggest that symbiotic fungi and pathogen load play an important role in the regeneration of multiple species in coast redwood forests after wildfire. Seed germination is often overlooked in greenhouse studies, and these results provide evidence that soil microbes and burn severity interact to influence early stage recruitment in coast redwood forests. This study contributes to our understanding of how wildfire may threaten the already shrinking range of coast redwoods and makes predictions about how soil microbes might affect the long-term trajectory of these forests in the wake of recent megafires.