Soil Fire Ecology: Assessing Wildfire Severity Impacts on Pacific Northwest Forest Soil Carbon

Pacific Northwest temperate rainforests are anticipated to experience increasingly frequent and severe wildfires in the foreseeable future due to climate change, increased fuel loads, and deforestation. Soil, underlying these forests, is the largest terrestrial carbon (“C”) reservoir. Fire disturbance influences soil C persistence, the timeframe and magnitude of which remain unclear. Understanding post-fire soil C dynamics may prove helpful for creating predictive models that incorporate the role of terrestrial ecosystems in climate change mitigation. In this study, we measured mean soil C content in density-separated soil fractions in HJ Andrews Experimental Forest one year after the 2020 Holiday Farm Fire. These include light fractions (“LF”, representing particulate organic matter) and heavy fractions (“HF”, representing mineral-associated organic matter). Four burn severity classes were investigated: high, moderate, low, and no burn. Significant differences between unburned and burned mean soil C content were determined through Welch’s t-Tests. We identified that the LF mean soil C content in the high severity burn class significantly increased, which is likely due to the fire-induced influx of low-density charcoal. HF post-fire mean soil C content was not significantly different in any burn class. These findings offer insight into the timeframe of soil C fluctuations following natural disturbances in PNW forest ecosystems. This may inform the creation of predictive models that incorporate the role of terrestrial ecosystems in climate change mitigation.