Do fuel treatments reduce fire severity? Evaluating treatment effectiveness in the 2006 Tripod Complex fires

The 2006 Tripod Complex fires burned over 70,000 ha of dry mixed conifer forests in north-central Washington State. Recent fuel treatments burned in the wildfire offered an opportunity to quantitatively evaluate if fuel treatment effectively mitigated fire severity. We quantified the relative effect of two common fuel treatments: mechanical thinning only (thin) and mechanical thinning followed by prescribed burning (thinRx). Fire severity was markedly different between the two treatments. Over 57% of trees survived in thinRx units versus 19% in thin and 14% in control units. Considering only large-diameter trees (\u3e 20 cm dbh), 73% survived in thinRx units versus 36% in thin and 29% in control units. Logistic regression models demonstrate significant reductions in the log-odds probability of tree mortality under both treatments with a much greater reduction in thinRx units. They also suggest that three years following the fire, largediameter trees are at greater risk of mortality in either thin or control units than in thinRx units. Other severity measures, including maximum bole char, percentage crown scorch, and burn severity index, are significantly lower in thinRx units than thin and control units. There were no significant differences in fire severity measures between thin and control units. This study provides strong quantitative evidence that without treatment of surface fuels, thinning alone is not a viable surrogate for prescribed fire in these dry, mixed conifer forests. In contrast, thinning followed by prescribed burning to reduce surface fuels appears to be an effective strategy for mitigating wildfire severity. Given the similar findings to other studies, our results should be applicable to many dry forests with low to mixed-severity fire regimes in the western United States