Topography, Fuels, and Fire Exclusion Drive Fire Severity of the Rim Fire in an Old-Growth Mixed-Conifer Forest, Yosemite National Park, USA

The number of large, high-severity fires has increased in the western United States over the past 30 years due to climate change and increasing tree density from fire suppression. Fuel quantity, topography, and weather during a burn control fire severity, and the relative contributions of these controls in mixed-severity fires in mountainous terrain are poorly understood. In 2013, the Rim Fire burned a previously studied 2125 ha area of mixed-conifer forest in Yosemite National Park. Data from 84 plots sampled in 2002 revealed increases in tree density, basal area, and fuel buildup since 1899 due to fire exclusion. A dendroecological fire history and reconstruction of forest structure in 1899 showed that this area historically experienced frequent, low-severity fire. In contrast with this region's historical fire regime, burn severity from Landsat imagery showed that this area burned at mixed-severity in the Rim Fire, with 13% of plots classified as unchanged, 31% low severity, 32% moderate severity, and 24% high severity. A random forest model was used to identify the controls of fire severity in this portion of the Rim Fire, using daily area burned, daily fire weather, and fuels and vegetation data for the surface and canopy. Topography, tree species composition, and cover of forbs and shrubs best explained the fire severity. As an example of a re-entry burn, this study demonstrates how fire exclusion alters fire-vegetation interactions, leading to uncharacteristically severe burns and potentially new fire-vegetation dynamics.