Forest Soil Carbon Dynamics 20 Years after Catastrophic Wildfire

Although extensive fire suppression throughout the Western United States (WUS) has helped to retain carbon on the landscape over the past 70 years, it also predisposes forests to high-severity wildfire. The frequency and severity of wildfires is now expected to increase across the WUS due to a combination of accumulated fuels, declining snowmelt, and extreme weather events, such as drought, due to climatic change. Increasing fire severity is projected to continue but its long-term effect on forest carbon sequestration remains difficult to quantify. To enhance understanding of the dynamics of climatic change, extreme weather events, and forest management on soil carbon stability and preservation, I sampled fire-affected landscapes across the Rogue River-Siskiyou National Forest where nearly 500 thousand acres burned in the 2002 Biscuit Fire. This region is characterized by wet winters with snow present at higher elevations and dry hot summers and diverse geology which contributes to abrupt vegetative community shifts with high biodiversity and variation in carbon pools. To understand the effect of fire on soil carbon, I collected nearly 150 samples from two parent materials (i.e., metamorphic and ultramafic serpentine rock) in fire-affected areas and in unburned areas. Metamorphic sites were dominated by intermixed pine and fir forests and serpentine sites dominated by grasslands. Based on a hillslope topography model I used a systematic design to compare varying fire severities, soil types, post-fire management, and landscape position. Our preliminary data show a surprising trend of increasing soil carbon accumulation in areas affected by high-severity fire with respect to the low-severity and control sites. The highest carbon stocks were found at the valley topography between high-severity sites, which suggests a post-fire erosion effect. Significant variation in bulk density across the landscape, generally lower in control than in burned sites, also contributed to differences in carbon stock values. Our next steps will include quantification of different carbon pools (i.e., mineral-associated, roots and fungal biomass) to elucidate the mechanisms behind differences in stock and stability of carbon after fire.