What Drives Wildfire Behavior? Insights from a 20-year Dataset

Scientists analyzed an extensive set of data collected by the U.S. Forest Service Fire Behavior Assessment Team (FBAT) to identify key determinants of wildfire energy.  The Fire Behavior Assessment Team has been monitoring and collecting data on wildfires and prescribed fires since 2003 to develop fire behavior and fire effects decision tools and to foster a better understanding of wildland fire dynamics.  

Key Findings   

• Wildfire energy and smoke emissions are primarily influenced by pre-fire fuel loads and how the fire spreads. 
• Fuels management, particularly prescribed fire, can reduce wildfire energy and smoke emissions while promoting restoration. 
• Fire management tactics that promote backing and flanking fires are important tools for controlling wildfires and reducing future wildfire risk while minimizing ecological effects where desired. 

Photo Credit

FBAT Team, Matt Dickinson

Transect photos taken before and after fire in western Cascades old growth sampled by the FBAT.  Intensities were low, but tree mortality was substantial because of heavy consumption of duff, including old, rotten nurse logs.  A tree which started its life on a nurse log is shown.

 

The analysis found that pre-fire fuel loading plays a pivotal role in shaping wildfire energy, while fire weather is less important, likely because large wildfires monitored by FBAT tend to burn on the severe end of the fire weather spectrum. 

The study also revealed that the mode of fire spread—whether a fire was spreading into the wind or downslope (a backing fire), to the sides (flanking), or spreading with the wind (a heading fire)—was a significant determinant of wildfire energy.  Fires moving downslope or against the wind consumed less litter and duff fuels compared to heading fires.

Both fuel loading and mode of fire spread are under the influence of management practices. Fuel treatments, especially those involving prescribed fires, offer the potential to reduce fuel loading and diminish wildfire energy.  

Rather than directly attacking a fire, indirect tactics such as backfiring (firing inside the control line to slow, knock down, or contain a rapidly spreading fire) or burnout operations (firing to consume fuels between the fire’s edge and a control line) are used not only to contain fires but they can also serve to mitigate risks of future wildfires while moderating the ecological effects of the wildfire.