Understanding How Fire History, Topography, and Vegetation Affect Drought Resistance and Resilience in the Sierra Nevada

Drought is an existential risk for water supply and ecosystem health, and a severe drought from 2013 to 2015 across the US caused widespread tree mortality. In this study, we focus on the unique Illilouette Creek Basin (ICB) in Yosemite National Park, which has been allowed to experience regular low-intensity wildfire since the 1970s, to understand how natural wildfire allows vegetation to resist water stress during the drought and show resilience in their recovery following the drought. We use historical fire extent, more recent NASA Airborne Visible Infrared Imaging Spectrometer (AVIRIS), and Light Detection and Ranging (LiDAR) from the Airborne Snow Observatory to measure forest health over a six-year period of drought, fire, and recovery. Examining the relationships between tree mortality and variables including elevation, aspect, and topographic wetness index, helps to form process-based hypotheses. We then use photosynthetic change via the normalized difference vegetation index (NDVI) and chlorophyll carotenoid index (CCI) to track conifer phenology and LiDAR to estimate canopy structure. By tracking photosynthetic activity and combining this with structural information over the ICB from 2013 to 2018, we analyze the distribution and fluctuation of forest health throughout the drought and recovery period. Further, we apply these time series and other remote-sensed climate data to an existing dynamic linear model to predict early warning signals of the tree mortality. We expect that these analyses will be useful tools to better predict and manage tree mortality throughout the Sierra Nevada forests.