Evaluating fuel treatment efficacy on Arizona’s largest wildfire

Background/Question/Methods

In 2011 Arizona’s largest recorded wildfire, the Wallow fire, burned 215,000 hectares in the Apache Sitgreaves national forest, USA. The fire burned through many units that had been treated prior to the fire in order to reduce wildfire hazard around the wildland urban interface (WUI) communities of Alpine, Eager, Greer, and Nutrioso, AZ.  The overlap of wildfire with fuel treatments has created a rare opportunity to evaluate the effectiveness of fuel treatments in the WUI under extreme fire weather conditions.  We installed linear transects across untreated/burned and treated/burned stands. On each transect, permanent sample plots were installed to measure stand structural characteristics (e.g., tree density, basal area, canopy base height, canopy bulk density) and fire effects (severity and mortality). To evaluate treatment effectiveness, we compared fire severity between treated and untreated areas.  To identify the characteristics of the treated area associated with treatment effectiveness we compared stand structural characteristics associated with treated areas to those in the untreated areas, and we evaluated how stand structural characteristics relate to fire severity within the treated area.

Results/Conclusions

All plots in the untreated area had 100% crown scorch, implying a very severe wildfire event. Within the treated area plots near the border between the untreated and treated area also had very high crown scorch, but moving from the treatment edge deeper into the treated area crown scorch decreased.  We found an inflection point in crown scorch at approximately 485 m from the treatment edge.  Closer than 485 m to the treatment edge trees experienced high severity fire, further than 485 m from the treatment edge trees experienced more low severity fire.  Treated areas had significantly lower basal area, stem density, canopy bulk density, and a significantly higher quadratic mean diameter than untreated areas.  We found that crown scorch in the treated area increases with increasing basal area, increasing stem density, and increasing crown bulk density. Crown scorch in the treated area decreases with increasing crown base height. These results will help fire and resource managers develop effective thinning treatment prescriptions and treatment size required to reduce wildfire effects and to buffer WUI communities from severe fire events.