Effects of Canopy Midstory Management and Fuel Moisture on Wildfire Behavior

Research Issue

Increasing trends in wildfire severity in recent years can partly be attributed to fire exclusion in the past century which led to higher fuel accumulation. Today, forest managers use tools such as mechanical thinning of stands and prescribed fires to reduce fuel loads. These fuel treatment strategies focus on reduction of surface fuels, increasing the height to the live crown, decreasing crown density and a species-selective approach.

The reduction of midstory and understory vegetation does not drive fire behavior in isolation. Depending upon the seasonality and fuel conditions, midstory vegetation can increase wind drag lowering wind speeds or increase fuel moisture. Both of these effects can slow fire spread or reduce intensity. To assess the impact of fuel reduction treatments on fire behavior outcomes, one must consider the types of fuels, such as dead and live fuels, litter, ladder (midstory) fuels and canopy fuels associated with larger trees and their effects on fire behavior under different conditions of moisture levels.

In this study, scientists sought to answer the following questions:

  • What are the driving factors governing fire behavior under different levels of midstory management and fuel moisture?
  • What are important factors leading to torching and crowning?
  • How do we characterize turbulent transport of momentum and energy to explain fundamental differences in fire behavior?

Our Research

To answer these questions, scientists conducted a suite of computer simulations (using the FIRETEC fire behavior model) with a simple fire line moving through a vegetative environment where the midstory (ladder fuel) has been removed by  thinning (prescribed fire or mechanical means) to a high, intermediate and low degree.  (Ladder fuels can facilitate transition of surface fires to the canopy, where they are much more difficult and expensive to suppress.)  Variations in moisture content of fuels were also specified resulting in six sets of simulations: dry no midstory (DN), dry sparse midstory (DS), dry dense midstory (DD), moist no midstory (MN), moist sparse midstory (MS) and moist dense midstory (MD), respectively

Simulated burn areas after 520 seconds from the time of ignition under different midstory vegetation and moisture conditions using the FIRETEC fire behavior model. The figures highlight the finding that complete removal of midstory vegetation may lead to enhanced fire spread and larger burn areas under both dry and moist vegetation conditions, due in part to stronger winds that can occur when no midstory vegetation is present.Scientists found that these six sets of simulations show widely different fire behavior, in terms of fire intensity, spread rate and consumption. The management operation of fuel load reduction and whether it gains the outcome of lowered fire intensity is highly sensitive to fine scale changes in the local wind conditions and fuel moisture. In dry conditions, too much thinning can introduce more windy conditions which can increase fire intensity. But doing nothing may also lead to higher fire intensity. A critical degree of thinning can reduce fire intensity and rate of spread.  In more moist conditions, more thinning may increase the rate of fire spread whereas a high density of moist fuels can result in low fire intensities.

Expected Outcomes

This work highlights the importance of considering fine scale fuel heterogeneity, seasonality, wind effects and the associated fire-canopy-atmosphere interactions when planning for and carrying out prescribed burns and forest management operations.

Research Results

Banerjee, T., Heilman, W. E., Goodrick, S., Hiers, J.K., and Linn, R.  2020.  Effects of canopy midstory management and fuel moisture on wildfire behavior.  Nature Scientific Reports 10:17312, DOI:10.1038/s41598-020-74338-9.

Research Participants

Principal Investigators

  • Tirtha Banerjee, University of California Irvine, Department of Civil and Environmental Engineering, Assistant Professor
  • Warren Heilman, USDA Forest Service, Northern Research Station, Research Meteorologist
  • Scott Goodrick, Southern Research Station, USDA Forest Service, Research Meteorologist
  • J. Kevin Hiers, Tall Timbers Research Station, Director of Fire Science Applications
  • Rod Linn, Earth and Environmental Sciences Division, Los Alamos National Laboratory, Team Leader, Atmospheric Modeling and Weapons Phenomenology Team 
  • Last modified: February 19, 2021