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.
The 2006 Tripod Complex fires burned moer than 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.
Fire severity was markedly different between the two treatments. Over 57 percent of trees survived in units thinned and then burned under a prescription, versus 19 percent in thinned and 14 percent in control units. When considering only large-diameter trees (> 20 cm dbh), 73 percent survived in units that were thinned and then burned under a prescription, 36 percent in prescribed thinning units, and 29 percent 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 prescribed thinning units. They also suggest that three years following the fire, large diameter trees are at greater risk of death in either thinnned or control units, in comparison to prescribed thinning units. Other severity measures, including maximum bole char, percentage crown scorch, and burn severity index, are significantly lower in prescribed thinning units than thin and control units. There were no significant differences in fire severity measures between thinned 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.
We acknowledge funding provided by the Joint Fire Science Program under Project JFSP 07-1-2-13.
Treatment units were located within the southwestern section of the Tripod Complex fires, approximately 10 km north of Winthrop, Washington. Of the 19 treatment units potentially available for this study, several were excluded because they were located along the wildfire perimeter or were surrounded by unburned forest.
We limited treatment units to those that had been harvested or prescribed burned within the past 15 years. Eight control areas with no record of harvesting or burning were randomly selected within the matrix of treatment units. An additional paired sampling design was used to evaluate differences in fire severity between treated units and adjacent untreated control units that had similar topography and likely experienced similar fire weather at the time of the wildfire. Units were sampled with circular plots along systematic grids. Plots were marked with a permanent center stake and numbered metal tag. At each plot, we collected general plot information including site description, aspect, slope gradient, and slope position (i.e., lower slope, mid slope, upper slope, ridgetop).
A variety of fuel treatments are done on public lands each year and yet, with the exception of national parks, managers often do not have the direction, time, or training to adequately monitor the effectiveness of their fuel treatments. A plan to monitor the effects of fuel treatments on public lands would be difficult and expensive to follow, but there are a lot of potential benefits.
Preburn data on forest structure and surface fuel loading at all sites provided datasets needed to carefully evaluate treatment effectiveness following wildfires and allowed us to address some of the key management questions: