Impacts of Sudden Oak Death Treatment and Wildfire on the Soil Microbial Communities of Douglas-fir/Tanoak Forests

Biotic and abiotic forest disturbances can significantly alter soil microbial communities. Phytophthora ramorum, the causal agent of Sudden Oak Death (SOD), is a plant pathogen invading western United States forests that has been subject to eradication efforts in Oregon since 2001. In this study, a metabarcoding approach targeting bacteria (16S), fungi (ITS), and oomycete (rps10) markers were used to characterize soil microbial communities in P. ramorum-infested sites that underwent treatment and those that did not. SOD management was associated with an initial increase in bacterial alpha diversity and significant shifts in bacterial and fungal beta diversity. Oomycete communities showed no significant changes in richness or composition in response to SOD management. Across a 1-12 year chronosequence, bacterial communities exhibited early signs of recovery in diversity and composition by 10-12 years post SOD treatment, while fungal communities did not show evidence of recovery. Oomycete communities fluctuated in diversity and composition throughout the chronosequence without a clear recovery trend. Conversely, subsequent wildfire disturbances, specifically the 2017 Chetco Bar Fire, had more pronounced effects on soil microbial communities. Wildfire-burned sites exhibited a slight reduction in richness for bacteria and distinct shifts in community composition for both bacteria and fungi. Oomycete communities declined in both richness and community composition only following wildfire. These findings highlight the differential impacts of invasive disease management and wildfire on soil microbial dynamics and underscore the importance of long-term monitoring to understand ecosystem recovery.