Abstract
BackgroundFuel reduction treatments have been widely implemented across the western US in recent decades for both fire protection and restoration. Although research has demonstrated that combined thinning and burning effectively reduces crown fire potential in the few years immediately following treatment, little research has identified effectiveness of thinning and burning treatments beyond a decade. Furthermore, it is unclear how post-treatment disturbances such as a bark beetle outbreak affect fuel treatment effectiveness.ResultsWe evaluated differences in surface and canopy fuel characteristics and potential fire behavior metrics between fuel reduction treatments (no-action or control, burn-only, thin-only, thin+burn) implemented in ponderosa pine (Pinus ponderosa Lawson & C. Lawson)−Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco)-dominated forests that were subsequently affected by a mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak after treatment. Experimental units were measured in 2002 (immediately following fuel treatment) and in 2016 (14 years after treatment and at least 4 years following a beetle outbreak). We found that beetle-altered thinning treatments (thin-only and thin+burn combined) had less fuel (i.e., 34% and 83% lower fine and coarse woody debris loading, respectively) and lower crown fire potential (i.e., 47% lower probability of torching and 42% greater crowning index) than corresponding unthinned treatments (control and burn-only). There was no post-beetle-outbreak effect of burning treatments (burn-only and thin+burn combined) on surface fuel loading, but burning reduced crown fire potential (i.e., 37% greater crowning index) over unburned units (control and thin-only) 14 years after treatment. Additionally, we determined the relative impacts of fuel treatments and the bark beetle outbreak on fuel and crown fire potential differences and found that bark beetle-caused tree mortality inflated differences between controls and thinned treatments (thin-only and thin+burn) for surface fuel loading and probability of torching, but diminished differences between these treatments for canopy fuel loading, canopy bulk density, and crowning index.ConclusionsDespite the differential effects of bark beetle-caused tree mortality in the treatments, our study suggests that the effects of fuel treatments on mitigating crown fire potential persist even after a stand-transforming insect outbreak, especially when thinning and burning are combined.
Highlights
Fuel reduction treatments have been widely implemented across the western US in recent decades for both fire protection and restoration
Differences between thinned and unthinned units in stem density, basal area, and stand density index abated over time by 33%, 63%, and 54%, respectively, the contrast between thinned and unthinned quadratic mean diameters increased by 161% over the measurement period as large trees in the unthinned units were killed by mountain pine beetle (MPB)
Burning reduced fine woody debris (FWD) loading by 63% and thinning increased FWD loading by 250%
Summary
Fuel reduction treatments have been widely implemented across the western US in recent decades for both fire protection and restoration. Research has demonstrated that combined thinning and burning effectively reduces crown fire potential in the few years immediately following treatment, little research has identified effectiveness of thinning and burning treatments beyond a decade. It is unclear how posttreatment disturbances such as a bark beetle outbreak affect fuel treatment effectiveness. Fuel treatments are typically designed to reduce surface fuel loading and canopy densities; increase heights to canopy base; and retain large, fire resistant trees (Agee and Skinner 2005, Hessburg et al 2015) These goals can be attained with various silvicultural techniques, thinning and burning are the most typical means of fuel reduction. The relative effectiveness of thinning and burning to reduce crown fire behavior has been thoroughly studied immediately after treatment (Stephens and Moghaddas 2005, Harrington et al 2007, Stephens et al 2009, Fulé et al 2012, McIver et al 2012), generally highlighting that burning reduces surface fuels, thinning improves forest structure, and the combination of the two best reduces crown fire potential
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