Burn severity explained

Shovel Creek: an example of burn severity assessment and ecology in interior Alaska

Zav Grabinski, Alaska Fire Science Consortium | https://storymaps.arcgis.com/ |Chris Smith, Geographic Information Network of Alaska

Wildfire is a natural process that shapes the landscape of Alaska

In 2019, the Shovel Creek Fire grew rapidly and threatened nearby neighborhoods north of Fairbanks. The fire was started by lighting on June 21. After 39 days of burning, and $25 million spent on suppression the fire was put out and no homes or lives were lost.

The resulting burn scar is easily accessed and provides opportunities for investigating wildfire effects. Fire severity is a central topic of post-fire research and has implications for how a landscape changes after a fire, as well as for fire suppression operations.

Using the Shovel Creek Fire as an example of wildfire in interior Alaska, this story map aims to explain:

  • What is burn severity?
  • What are the drivers of burn severity?
  • How is burn severity assessed?

For more information or comments contact  Zav.Grabinski@Alaska.edu 

Shovel Creek Fire

Murphy Dome, Fairbanks, Alaska

20 miles northwest of Fairbanks, the 2019 Shovel Creek Fire was started by lightning strikes on a dry 84 degree day in late June.

The hot and dry weather continued through July, and the fire grew to over 22,000 acres across the black-spruce boreal forest.

The initial suppression attack involved water-scooping aircraft, air retardant tankers, and smoke jumpers. An existing fuel break completed in 2009 adjacent to Murphy Dome Road was an important tactical asset to fire managers.

Shovel Creek provides a notable opportunity to explore burn severity due to the road-side accessibility and the amount of post-fire research that occurred.

In the past, the Murphy Dome area has had other fires including the 2009 Hardluck Fire and 2011 Hastings Fire.

Past fires can create natural fuel breaks and provide tactical opportunities for fire suppression operations.

The wildland urban interface, or WUI, is where houses are in or near wildland vegetation.

These areas are the highest priority for fire managers because of the potential for fires to threaten human life and infrastructure.

114 residents living near the Shovel Creek fire were evacuated with many more prepared to evacuate.

What is burn severity?

Burn severity is the effect of fire on the environment. It is a way to measure changes in vegetation and soils after a fire.

Burn severity is different from, but related to, fire intensity, which describes the amount of energy released from the fire. Because this is difficult to measure, flame length is often used as a proxy to assess intensity.

Use the slide bar to compare low and high burn severity.

Burn severity in Alaska is largely driven by the degree that the fire burns into the ground.

Low severity fire does not burn deep into the ground and vegetation will recolonize more quickly.

High severity fire burns deep into the ground and may expose the underlying mineral soil, making it easier for deciduous trees to recolonize the area.

Burn severity is driven by a large number of environmental factors including, slope, aspect, vegetation, moisture, and weather. Because of this, burn severity is patchy within the fire perimeter, with a mix of sites ranging from unburned to high severity.

The degree of burn severity can have long-lasting implications for what types of vegetation regenerate after fire and, therefore, the wildlife species that will use an area post-fire.

Burn severity is best assessed the summer following a fire.

If severity is assessed too early the damage caused by fire, such as plant mortality, will not be observable. If it is assessed too late, the fire’s impact will not be visible due to vegetation regrowth.

This video was taken one year after the fire. Note that most of the spruce trees have died, but there are patches where they, and other vegetation, have survived.

(Click and drag to see the research plot, use the keyboard down arrow to move to the next slide)

Research plots such as this one examine the burn severity on the landscape. This information helps us understand the ecological change on the landscape.

Drivers of burn severity

Burn severity affects and is affected by the landscape:

  • Weather and landscape characteristics such as vegetation and topography are key drivers of fire behavior and burn severity.
  • Burn severity impacts the landscape and can alter vegetation type, soil properties, and recreational/subsistence opportunities.

Vegetation provides fuel

Duff is a subsurface layer of slowly decomposing moss, lichen, and litter. In boreal forests, duff is often about a foot deep and is a primary driver of burn severity. Deep burning of duff that exposes the underlying mineral soil is a key indicator of high burn severity.

Fine fuels such as grasses and twigs can lead to fast spreading fires. Fires involving mostly fine fuels tend to be low severity.

Woody fuels are thicker, burn longer, and release more heat than fine fuels.

When the fire is hot enough to reach the entirety of the trees as well as the surface fuels it is called crowning or torching.  At this point, the fire intensity and it’s potential to grow rapidly and spread embers across long distances is dramatically increased. The fire can become a crown fire, where fires spreads from tree top to tree top.

Duff is a unique fuel bed where wildfire can burn deep below the surface and smolder for days or weeks, reigniting fuels at the surface when weather conditions become favorable. 

It is possible to have high burn severity where the duff is burned to mineral soil but flames do not get into the trees.

This can happen when the fire smolders in the duff over time, consuming it without producing tall flames. The trees can fall, sometimes even with intact green needles and leaves, because the fire burns the roots and the top layer of soil supporting the trees.

Fires can smolder in the duff belowground and can remerge, sometimes after an entire winter!

While the fire’s consumption of soil layers is a primary component of burn severity, especially in boreal ecosystems, fire’s effect on the trees themselves is also a factor.

Black spruce trees tend to be killed even by low intensity fires, but how high the fire burns into tree canopy is considered when assessing burn severity.

Other times, trees are not killed by direct contact with flames but rather due to the heat generated by the fire. This results in trees with brown, dead needles still attached.

Black spruce are incredibly flammable due to their lower moisture content, abundant resin production, and low branches that carry fire upward.

On the other hand, hardwoods such as birch and quaking aspen contain more moisture making them less flammable, although they will burn under the right conditions.

Needles and branches of coniferous species are flammable in part due to the presence of aromatic resin that permeate the whole tree and allow fires to quickly spread to the top of the tree.

Black spruce are semi-serotinous, which means the seeds are released from the cone due to the heat of the fire, but they do not require fire to release their seeds.

 Click here to read more about boreal fire ecology from the National Park Service  

Regeneration and succession

The depth of the burn has major influence on subsequent regeneration (short-term regrowth) and succession (species composition) of vegetation.

Regeneration and succession

Fireweed and other herbaceous plants (grasses) often resprout from below ground roots in a low or moderate severity fire. In high severity fires regeneration tends to be from seed dispersal.

The new organic layer allows shrub species and young trees to grow in the area.

Regeneration and succession

The young trees will continue to grow until the area transitions into a forest with large trees that are able to outcompete shrub species.

Over time, vegetation will continue to grow until enough fuel accumulates to support a fire and restart the regeneration process.

Topography is the shape of the land described by aspect, slope, and elevation.

Studying burned areas helps researchers determine which topographic features contributed to burn severity.

Topography can then to be compared to other factors to determine the relative importance of each driver of burn severity.

Aspect is the direction that land faces. South-facing slopes are warmer and get more sunlight while north-facing slopes are cooler and receive less sunlight. Certain slope directions allow for growth of vegetation that can favor or suppress fire.

Slope is the steepness of the land. Steeper slopes can cause fires to move more quickly upslope as a result of hot air rising and drying fuel sources up the hill from the fire. Conversely, fires move down steeper slopes more slowly than more gradual slope.

(Use the slidebar to compare the burn severity of a flat area and a steep slope)

Terrain can create natural barriers that can act as a natural fire break. Examples of natural fire breaks include lowlands, areas along bodies of water that have a high amount of moisture, or rocky areas that don’t contain fuel to allow a fire to spread.

Elevation is the height above sea level and can affect how fire behaves. Areas of low elevation are generally warmer but contain more moisture while areas of higher elevation tend to be cooler but drier. 

At Shovel Creek researchers did not find a relationship between burn severity and topography.

Fire suppression efforts affect the level of burn severity. This can make it more difficult to attribute burn severity to landscape properties in sites that had active fire suppression.


The Shovel Creek fire was started by lightning on June 21, 2019, on a dry day with a high temperature of 84 degrees.

For the rest of June and most of July, the weather remained hot and dry.

The first day with significant precipitation was July 29. With the help of favorable weather and firefighting practices, Shovel Creek was contained on July 30.

Weather is a key driver of fire severity, although the effects can be difficult to quantify. Hot, dry, and windy weather creates conditions where fire can spread rapidly, burn intensely, and be difficult or impossible to suppress.

The next screen is an animation of the progression of the Shovel Creek fire perimeter. Pay attention to how the fire spreads (or stops spreading) in context of each day’s weather.

Animation of Shovel Creek fire day-by-day progression.

As weather becomes hotter and fuels become drier there is an increase in fire danger and greater effect of fire on the landscape.

Slide the bar to the left to see how burn severity changes with hot and dry conditions.

Assessment of burn severity

Burn severity is measured on a continuous scale by field surveys or by evaluating changes in satellite imagery.

Experts then create discrete classes of severity based on the ecological impacts of these continuous values.

Use the sidebar to explore what a satellite image of the Shovel Creek fire scar looks like and see how the severity was classified.

Fire severity is typically divided into unburned, low severity, moderate severity, and high severity classes.

Notice that in the image unburned vegetation can be easily identified while the different burn severity classes can be hard to differentiate.

Low Severity:

Trees have been scorched while shrubs are mostly alive. Patches of duff have been burned.

Plot notes:

  • Variable shrub damage
  • Patchy surface fire
  • Little slope

Moderate severity:

Many trees are fallen, browned, and dead. Some shrubs have survived. The duff layer has burned evenly across, but not deeply. Herbaceous species like fireweed are already growing, and some new vegetation species may grow in the area moving forward.

Plot notes:

•Burned in an area of deciduous trees, near a valley bottom

•Some trees have fallen but were not torched. Some trees are still alive

•Mineral soil has been exposed by the falling trees and burned surface, allowing fireweed to grow

High severity:

Most of the shrubs have died, and many trees have been torched. A large amount of duff has burned deeply, exposing mineral soil in parts. Species such as grass and fireweed can easily grow in this area.

Plot notes:

•This south-facing slope receives more sunlight increasing the chance of high severity

•Areas around trees have been burned to mineral soil

•A majority of the shrubs have been killed

•The ground has been so disturbed that one year after the fire no new vegetation is growing in the area

Tools of burn severity assessment:

  • Composite Burn Index (field surveys)
  • Spectral indices (changes in ground cover pre- and post-fire detected by satellite imagery)
  • Machine Learning (combination of field surveys and satellite imagery to improve classification accuracy)

Composite Burn Index

CBI measures burn severity with on the ground field surveys.

It is time intensive and regarded as the most accurate assessment.

The Composite Burn Index classifies burn severity by assessing fire effects across vegetation strata and creating a composite score.

Fire effects include:

  • Species mortality
  • Char height 
  • Changes to soil and duff layers
  • Changes in species composition

Vegetation strata include:

  • Dominant trees
  • Intermediate trees
  • Tall shrubs
  • Herbs
  • Substrate (duff)

Fire effects on each strata are scored on a continuous scale from 0 to 3 and then averaged to create the overall Composite Burn Index.

Thresholds for the values are later assigned so that burn severity classes can be explained based on ecological impact.

Spectral indices

Spectral indices calculate burn severity using satellite imagery on a continuous scale. Like CBI, threshold values are assigned to differentiate burn severity classes by ecological effect.

One method is to look at the change in ground coloration in satellite imagery using a measure called the Normalized Burn Ratio (NBR). The greater the burn severity the darker the ground will appear.

A drawback of spectral indices is that deep burning into the duff and soils is not well detected. Deep duff burning is characteristic of high burn severity in Alaska.

Use the slide bar to explore the difference in satellite imagery of a burn scar before the fire and about one year after the fire.

Spectral indices utilize satellites to measure fire severity by looking at changes in the landscape pre- and post-fire.

A larger amount of changes between the images means that the area was more severely burned.

Machine learning

Machine learning classifiers can classify burn severity using the entire electromagnetic spectrum and can utilize other factors.

Machine learning at Shovel Creek created burn severity maps that were 10% more accurate than spectral indices products. 

Training data (CBI field plots) from other fires can be used to create a burn severity map shortly after a fire allowing land and fire managers to understand the impacts the fire had on the landscape.

For detailed information see the 2021 publication  Assessing Wildfire Burn Severity and Its Relationship with Environmental Factors: A Case Study in Interior Alaska Boreal Forest 

Latest advancements in fire severity assessment

Recently a  hybrid method   that uses a cloud based system has been used to assess fire severity of fires in the Alaskan Boreal forest.  

This hybrid method combines images starting immediately after a fire when first order effects can be observed, including vegetation mortality, scorching, and charring, through the early growing season of the following year.

The average spectral signature of the burned area is compared to the average spectral signature collected prefire (May 20-Aug 31 of year before the fire) and can thus create maps that adequately map burn severity.

The advantage of the hybrid method is the processing is done in a cloud environment where large temporal datasets can be compiled and analyzed to assess burn severity in a quick fashion that will be useful to fire managers.

In this rapidly changing Alaskan environment, fires burn more acres, are more frequent, burn throughout more areas of the state, and start earlier and later in the season.

Improvements in burn severity assessment in Alaska will aid understanding of the environmental impacts of wildfires, which may benefit science-based decision making to better protect resources, lives, and property, and achieve ecological objectives.

The following reviewers made this StoryMap possible: Jennifer Barnes, Elizabeth Fernandez, Randi Jandt, Santosh Panda, Lisa Saperstein, and Alison York. Additional thanks to Chris Waigl and Jennifer Delamere.