REMOTE-SENSING ASSESSMENT OF POST-FIRE VEGETATION REGROWTH IN THE LITTLE NORTH SANTIAM WATERSHED, OREGON

Wildfire is a natural part of forest ecology, with well-established fire regime records for different ecosystems. However, the increase in fire frequency and burn severity across the Western U.S. has altered fire regimes and forest succession patterns, making it harder for scientists to predict overall ecosystem response. Severe burns have shown to increase sediment erosion and stormwater runoff, mobilizing high concentrations of ash and sediment. This increased flux of sediment within a burned watershed can lead to nutrient and contaminant loading to downstream reservoirs, ultimately affecting drinking water. Post-fire sediment runoff is often triggered by precipitation events and is influenced by the spatial pattern of soil hydrophobicity, as well as the presence or absence of vegetative cover. Numerous studies identify vegetation cover percentage as one of the primary factors controlling post-fire erosion rates. Here we examine the Little North Santiam watershed within the Willamette River Basin in Oregon, which was 95% burned in the Beachie Creek Fire of August 2020, with 60% of the burned area classified as high severity. With multiple sources of remotely sensed imagery available, at varying spatial and temporal resolutions, we track post-fire vegetation regrowth from 2019 through 2023. We use a multi-sensor approach with “pre and post” imagery from PlanetScope, Sentinel-2, Landsat-8/9, and MODIS. We analyzed the imagery using the Normalized Difference Vegetation Index (NDVI), a spectral index often used to describe vegetation properties such as photosynthetic activity and canopy structure. Average pre-fire NDVI values in the high burn severity area in July of 2019 and 2020 were 0.84 and 0.88, respectively. One month after the fire, on October 2020, average NDVI values dropped to 0.24. In July of 2021, 2022 and 2023, NDVI values showed a steady increase at 0.28, 0.39, and 0.47. When these NDVI values are normalized with a fractional vegetation cover equation, the four post-fire NDVI values translate to 6%, 12%, 27%, and 39% vegetation cover in the high burn severity area. Combined with water quality gage data following large precipitation events, the spatially explicit results produced in this study can be used as a model parameter to further evaluate sediment erosion rates and post-fire regrowth in the Little North Santiam watershed.