Single Publication

FRENCH POST-FIRE BAER SOIL BURN SEVERITY MAP RELEASED

USDA Forest Service Burned Area Emergency Response (BAER) assessment teams are multi-disciplinary teams sent to federal lands following significant wildfires to characterize the fire effects to watersheds, identify imminent post-fire threats to human life, safety, property, infrastructure, critical natural and cultural resources. Once the assessment is complete, the team develops BAER emergency treatment recommendations to mitigate identified risks. After the assessment, a different BAER team implements the assessment team’s recommended treatments and action stabilization measures.

For complex fires such as the French Fire, assessments are done as an inter-agency effort which includes a California State Watershed Emergency Response Team (WERT) (Watershed Emergency Response Team (acwi.gov). The CalFire WERT assessment team is currently evaluating burned private and state lands from the French Fire. Both assessment teams share information and data as they complete their burned area evaluations, analysis and subsequent reports.  The teams also collaborate with other agency, utility, and private entities.

These teams begin their assessment immediately after the fire threat passes. The team focuses on the direct damage caused by the fire rather than from fire suppression activities. Post-fire conditions such as loss of vegetation and the changes in the soils will increase the likelihood of floods and may cause potential debris and sediment flow impacts.

BAER specialists recently completed their data gathering and analysis of the French burned area to produce a Soil Burn Severity (SBS) map of the 27,285-acre fire. The map and the data display SBS categories of burn severity including Unburned/Very Low, Low, Moderate, and High. Approximately 54% of the 27,285 acres are either Unburned/Very Low and/or Low SBS, while 40% sustained a Moderate SBS and about 6% High SBS.

It is important to note that the BAER team assesses the effects on soils and not the effects to vegetation. SBS characterizes the soil surface and below-ground impact, whereas vegetation effects are determined based on mortality and vegetation canopy changes.

Changes in soil cover, water repellency, and soil physical/biological changes determine the severity level of soil burn severity. Changes in water repellency are a much-discussed fire effect.  Water repellency is a natural soil property. Fire can increase the severity and the thickness of the water repellent soil, significantly affects post-fire water runoff.

Low SBS indicates only partial consumption of fine fuels while litter coverage remains relatively intact on the soil surface. Burning time at the soil surface was short, leaving root systems and root structure undamaged. Vegetative recovery time in the low category will vary based on ecological community but is expected to recover in the short-term.

Moderate SBS indicates nearly all soil cover of vegetative litter and fine fuels was consumed or converted to ash. Because soil cover is significantly reduced, accelerated water runoff is expected. Charring of the mineral soil occurs in Moderate SBS as well as shallow root burning. The extent of the burning of the leaves and needles on the trees (aka tree canopy) can be unpredictable and can range from high to relatively low mortality. Water repellency is often found at the surface and is increased for both in severity and thickness of the water repellent soils which reduces the ability of precipitation to infiltrate the soil surface.

High SBS is the result of higher intensity fire behavior or longer burning time at the soil surface. As a result of the high heat, nearly all the soil cover of vegetative litter and fuels has been consumed leaving bare soil prone to the impacts of precipitation and resulting water runoff. The surface mineral soil has been reduced to powder (single grain) and often several inches thick. This single grain soil is very easily transported or moved during rain events resulting in excessive soil erosion and sediment loading in rivers, streams, and creeks. The roots in the High SBS areas tend to be completely consumed by the resulting heat of the fire above the soil surface. Water repellency does not exist at the surface because water repellent compounds have been vaporized and tend to be found below the powdered soil surface, but the repellency thickness and more severe burning tend to be much greater than a Moderate SBS soil. Generally, there is 100% tree mortality in High SBS soils. Fire-adapted shrubs and vegetation such as bear clover, manzanita, and deer brush tend to come back with vigor because of root sprouts. However, because seed sources are consumed in these High SBS areas, conifers may take many years to re-establish without tree planting.

The French BAER assessment team used initial remote sensing imagery with field-validated soils data, to develop and produce the final SBS map. The BAER team and the US Geological Survey (USGS) both use the SBS map as an analysis tool to estimate post-fire flows, debris flow probability, erosion, and sedimentation flow rates.

The French Fire soil burn severity map can be downloaded at the “French Post-Fire BAER” InciWeb site (https://inciweb.nwcg.gov/incident/7854/) as a JPEG or PDF version under the “Maps” tab.