WWETAC Projects

Project Title:  Evaluating fuel treatment efficacy on Arizona’s largest recorded wildfire, 2011 Wallow Fire
Principal Investigator:
Morris C. Johnson, US Forest Service, PNW Research Station, Fire and Environmental Research Applications (FERA), Seattle, WA


Collaborators:   Maureen Kennedy (University of Washington), Robert E. Vihananek (PNW, FERA), Russell Bigelow, AFMO Springerville Ranger District (Apache-Sitgreaves National Forest)

Key Issues/Problem Addressed:                              

Fire scientists and fire managers do not have the opportunity to experimentally evaluate fuel treatment efficacy in North America dry forest ecosystems (Agee and Skinner 2005). The opportunity to thin stands to variable densities and then intentionally subject them to a worst-case wildfire is non-existent (Agee and Skinner 2005). The scientific guidelines for designing effective fuel treatments are limited to testing on fire simulation models (Johnson et. al 2011). On May 29, 2011, Arizona’s largest recorded wildfire, Wallow Fire, burned 215,000 ha and burned over more than 21,000 ha of fuel treatment within several wildland urban interface (WUI) communities. Given that wildfire encounters with fuel treatments are infrequent, we must capture each opportunity to evaluate and validate treatment effects on fire behavior.

Setting and Approach:

Opportunities to empirically assess the efficacy of fuel treatments during extreme fire behavior and weather conditions have been few. We assessed the performance of a set of fuel treatment areas that encountered a wind-driven crown fire. We sampled three areas previously treated for fuels reduction plus adjacent untreated areas which burned on the days the Wallow Fire made major runs and where fire suppression did not occur. In each permanent plot, we collected the following measurements for each sampled tree: DBH (cm), tree height (m), and species. We used a laser range finder to calculate crown base height (m), height to live crown (m), crown scorch (%), maximum height of crown scorch (m), tree height (m), and minimum and maximum bole char (m).  To evaluate treatment effectiveness, we will characterize stand structural characteristics (e.g., tree density, basal area, canopy base height, canopy bulk density) and fire effects (crown scorch and bole char) associated with treated areas and compare those to untreated areas. We also are fitting complementary Weibull non-linear curves to two metrics of fire severity (crown scorch proportion, maximum bole char height ratio) along the path of the Wallow fire as it moved from the untreated are through the treated for each of three treatment units.

Progress to Date:  

We installed 273 permanent plots on 14 linear transects in three treatment areas. We will test for differences using t-test for normal data (or transformed to normal data) and Wilcoxon rank-sum tests for non-normal data. Preliminary results indicate that average tree density (trees per hectare), basal area, stand density index, canopy closure, crown competition factor, and canopy bulk density were higher in the control units compared to the treated units. Canopy base heights were lower in untreated units compare to the treated units. The average canopy base heights were lower in untreated units compared to the treated units. Crown scorch was highest in untreated units and varied from 100 to 0 percent in the treated units. The location parameter for the complementary Weibull indicates the distance from the treatment edge the severity metric begins to approach zero and provides an estimate of the depth of fuel treatment buffer necessary to sufficiently reduce fire severity.


Results will validate the scientific basis for fuel treatments in reducing fire behavior and fire severity (Johnson et al. 2007; Johnson et al. 2011; Prichard et al. 2011) and will help educate the public on the value of managing dense forests across the western United States. Study results provides empirical data on the effectiveness of fuel treatments in reducing fire behavior and fire effects on large wildfires, and documents the scientific basis for fuel treatments in dry forest ecosystems.