Assessing Fuel Treatment Effectiveness After the Tripod Complex Fires

Over the past 50 years, wildfire frequency and area burned have increased in the dry forests of western North America. To help reduce high surface fuel loads and potential wildfire severity, a variety of fuel treatments are applied. In spite of the common use of these management practices, there have been relatively few opportunities to quantitatively measure their efficacy in wildfires. That changed with the 2006 Tripod Complex fires in the Okanogan-Wenatchee National Forest in Washington—one of the largest fire events in Washington state over the past five decades. A serendipitous involvement of recent fuel treatments and the availability of pre-wildfire data provided a rare chance to study the effects of different types of fuel treatments on wildfire severity. In this project, tree mortality, and tree damage were assessed and differences in wildfire severity were evaluated in units with thin-only treatments, thinning followed by prescribed burning treatments, and no treatment. With this study, researchers aimed to provide resource managers with the definitive evidence and specific scientific information needed to determine which fuel treatment methods will be the most successful at reducing fuels and mitigating wildfire severity.Key Findings Compared to thin-only and untreated units, units treated with thinning followed by prescribed burning had the lowest wildfire severity. Percent change in tree mortality did not differ significantly by treatment. However, tree mortality did differ considerably by species, with western larch and ponderosa pine experiencing the lowest mortality and lodgepole pine and Engelmann spruce experiencing the highest. Three years post fire, over 57 percent of trees survived in thin/prescribed burn units compared to 19 percent in thin-only units and 14 percent in untreated units. There was no significant difference in treatment results between thin-only units and untreated units. Treatment unit size does not appear to affect treatment effectiveness. In the study, even small treated units (less than 20 acres) successfully reduced fire severity.