Site, weather, and forest characteristics associated with Rim Fire burn severity in forests with restored fire regimes
Previous work has characterized forest structure in stands of Sierra Nevada mixed-conifer forest following the reintroduction of an active fire regime. Areas with restored frequent fire are expected to have greater resilience to wildfire and other environmental stressors. However, with increasing incidence of wildfires occurring under high- to extreme-fire weather conditions and the presence of high fuel loads on adjacent lands, there is question as to how resistant these areas are to extreme wildfire behavior. The Rim Fire in 2013 burned into stands incorporating a diverse recent fire history, allowing for analysis of wildfire effects on forests with an active fire regime. We used random forests and regression tree analysis to examine the influence of forest structure, topography, fire history, and weather on satellite-derived fire severity in the Rim Fire, using field data from 53 0.1 ha plots collected 3-4 years prior to burning. The majority of plots were located in Yosemite National Park, with 12 unique fire histories represented and a variety of forest structure, topographic, and fuel load conditions present prior to the Rim Fire.
According to database satellite derived fire severity estimates, 23% of plots burned at high severity, 32% burned at moderate severity, and the remaining 45% of plots were classified as unchanged or low severity. Most plots that burned at high severity were associated with plume-dominated fire behavior, suggesting that extreme fire behavior may overwhelm the ability of a restored fire regime to moderate fire behavior under some conditions. Elevation was also highly associated with fire severity, with lower severity observed in plots over 1700 m. Burning index (a composite index of fire weather), time since the last fire, and shrub cover also had strong positive associations with fire severity. Plots that had experienced fire within the last 14 years burned mainly at low severity in the Rim Fire, while plots that exceeded that time since last fire tended to burn at moderate or high severity. This was even more pronounced on days when the burning index was high. These results suggest that some sites with a restored frequent fire regime may still be subject to stand replacing fire under extreme fire conditions, particularly following longer fire-free intervals and in lower elevation forests.