In addition to the Forest Service (USFS) BAER team that began last month evaluating post-fire effects of the Dixie fire on National Forest System (NFS) lands, a US Department of Interior (DOI) BAER team is focused specifically on the portion of the Dixie fire that burned in Lassen Volcanic National Park (LAVO) and was assembled by the National Park Service (NPS) on September 9, 2021. This DOI BAER team is evaluating post-fire values-at-risk and will recommend stabilization and rehabilitation treatments specific to the LAVO. The DOI BAER team is currently conducting field reconnaissance of fire impacted recreation facilities, infrastructure, cultural resources, watershed and hydrology, fish and wildlife habitat, rare plants, and invasive species within the LAVO. For information about the DOI BAER assessment, please go to the LAVO Dixie Fire webpage found at: https://www.nps.gov/lavo/learn/nature/dixie-fire.htm.
Due to the large size and continual active burning of the Dixie Fire, the USFS BAER team divided the burned area into three phases for their assessment and analysis. The BAER specialists recently completed their data gathering and analysis of the Dixie burned area to produce a Phase 1 soil burn severity (SBS) map on August 25—analyzing 365,678 acres, and a Phase 2 SBS map on September 15—analyzing 404,473 acres. The map and the data display SBS categories of unburned/very low, low, moderate, and high. For Phase 2, approximately 39% of the 403,473 acres are either unburned/very low and/or low soil burn severity, while 57% sustained a moderate soil burn severity and only about 3% identified as high soil burn severity.
The Phase 2 SBS map also shows the acreage for each of the landowners for the 403,473 acres in the Phase 2 assessment to be: 140,206 acres for the Plumas National Forest; 125,199 acres for the Lassen National Forest; 79,364 acres of private/forestry industry lands; 33,585 acres for the National Park Service; 23,366 acres of private/unknown lands; 911 acres for the State of California-Department of Fish and Game; and 839 acres for the DOI Bureau of Land Management.
The low category of soil burn severity indicate that there was only partial consumption of fine fuels and litter coverage remains relatively intact on the soil surface. Residence time at the soil surface was short, leaving root systems and structure undamaged. Recovery time in the low category will vary based on ecological community but is expected to recovery in the short-term.
A moderate category of soil burn severity indicates consumption of litter and fine fuels at the soil surface. In forested communities, the heat from moderate severity fire will result in water repellant conditions at the mineral soil surface. The canopy in the forest is browning and it is expected that trees will drop needles and leaves that can provide some litter cover at the soil surface. In these systems, recovery can take longer for tree species to re-establish.
The moderate soil burn severity category in lower-elevation communities indicates complete consumption of shrub cover that can but does not necessarily result in water repellant conditions at the soil surface. Several shrub species in the lower-elevation communities do re-sprout after fire and recovery time will be variable. Moderate soil burn severity category in the lower-elevation shrub communities that did not express water repellant behavior can still result in a runoff potential category of high as a result of the soils inherent qualities and the removal of effective vegetative cover.
A high soil burn severity category is the result of higher intensity fire behavior or longer residence time at the soil surface. This category is found in forested or dense woodland communities and the litter and fuels, including an overstory canopy, was consumed. The soil structure is weakened, roots are charred and water repellant soil conditions persist through the upper horizon of mineral soil. Recovery time in the conifer systems can be significant.
The Dixie BAER assessment team used initial remote sensing imagery with its field validated soils data, to develop and produce a map showing soil burn severity levels for the burned area. The BAER team and the US Geological Survey (USGS) both use the soil burn severity maps as an analysis tool to estimate post-fire flows and debris flow probability.
The BAER team relied on its refined soil burn severity map to produce data used in its subsequent modeling and determination of post-fire runoff and sedimentation. In specific areas that experienced moderate-to-high burn severity, there could be increased runoff from steep hillslopes and resultant increases in post-fire soil erosion and potential debris flows.
The Dixie Fire soil burn severity map can be downloaded at the “Dixie Post-Fire BAER” InciWeb site (https://inciweb.nwcg.gov/incident/7811/) as a JPEG or PDF version under the “Maps” tab.