Understanding the effects of soil exposure in fuels treatments that balance fuel reduction and erosion control in the Tahoe basin

The 2007 Angora Fire served as a stark reminder of the need for fuel reduction treatments in the Lake Tahoe Basin. Concerns exist regarding the potential for fuels treatments to increase erosion rates and negatively affect the clarity of Lake Tahoe. In my study snowmelt runoff simulation was conducted within the Lake Tahoe Basin in 16 sites treated with mechanical mastication or prescribed fire. Erosivity was measured in order to determine if a level of litter, duff, and woody fuel cover could be established that minimized fire spread and fireline intensity, while simultaneously trapping sediment and increasing infiltration. In this study, as little as 25% of the ground surface covered with masticated fuels over a duff layer was sufficient to mitigate erosion. Prescribed fires characterized by heterogeneous patches of severe fire exposure resulted in the formation of unburned or less severely burned patches of surface fuel and duff, which mitigated erosion by increasing infiltration. Considerable increases in sediment yield were observed in plots whose area was 54% burned, with the highest total sediment yields occurring in the 66 to 100% range. The efficacy of the Watershed Erosion Prediction Program model (WEPP) was tested in predicting erosion due to snowmelt runoff in forested areas treated with mechanical mastication and prescribed fire. Agreement between observed measurements of erosion and WEPP model predictions became stronger as the potential for erosion in modeling scenarios (hillslopes that contained large areas of bare soil exposure) increased. Field results and those derived through WEPP modeling suggest that erosion and wildfire severity can be simultaneously mitigated in the Lake Tahoe Basin through the use of masticated fuel reduction treatments that result in minimal, yet sufficient, amounts of surface cover to trap sediment and prescribed fires that reduce fuel loading while limiting areas of extreme burn severity.