TREE MORTALITY AND ITS IMPACT ON HILLSLOPE PROCESSES AND LANDSLIDE SUSCEPTIBILITY IN THE KERN RIVER WATERSHED OF CALIFORNIA

The recent continuous drought in California (2012-2015) has left the trees in the Sierra Nevada especially susceptible to the bark beetle infestation and disease. The drought conditions and its residual effects have led to the mortality of 147 million tree across 9.7 million acres of federal, state, and local land in California since 2010-2018 (USDA Forest Services, 2019). High levels of tree mortality can create an environment prone to high intensity wildfires and pose an increased risk for post-fire debris-flows during subsequent rainfall events. Steep slopes that experience moderate and high burn severity are particularly susceptible to debris flow and soil erosion (USGS, 2018). Even without the occurrence of wildfires, significant levels of tree mortality are likely to result in lower effective cohesion from roots, increased soil erosion due to higher rates of surface runoff, and elevated groundwater levels due to an overall decrease in evapotranspiration. While these factors are expected to impact hillslope processes, current United States Geological Survey (USGS) models for landslide susceptibility do not consider the direct impact of tree mortality on hillslope stability. In this study we will assess the impact of tree mortality on the hillslopes within the Kern River watershed. We will map the spatial distribution and intensity of tree mortality and present preliminary results of our investigation into the direct and indirect roles that it plays on hillslope processes and landslide susceptibility.