How thinning and fuels treatment design influence outcomes
Image of post-treatment stand conditions in a ponderosa pine forest on the Arapaho Roosevelt National Forest in northern Colorado.
Land managers across the western United States need effective fuel treatment strategies to mitigate hazardous fuel loads and reduce the risks of high-severity fire in dry conifer forests. To address this need, scientists analyzed different treatment designs for forest thinning. They found that thinning – regardless of the method – reduced both the rate of fire spread and the forest canopy consumed by the fire, with greater reductions occurring for lower residual basal areas (i.e., in areas where the total cross-sectional area of the trees left after thinning is lower). Traditional fuel hazard reduction treatments emphasize reducing canopy fuel continuity and surface fuel loading using thin-from-below, while increasing spacing between tree crowns, and using pile or broadcast burning of surface fuels. However, such treatments can cause forest structures that differ from the historical conditions reinforced by frequent low to moderate-severity fires. In contrast, ecological restoration treatments emphasize enhancing structural heterogeneity but may produce less fire-resistant stands causing tradeoffs between fuel hazard reduction and restoration objectives.
Using maps of real forests, this study explored these treatment design tradeoffs by simulating thinning treatments on ponderosa pine sites, spanning several levels of basal areas, horizontal and vertical distributions of canopy fuels, surface fuel loads, fuel moistures, and wind speeds. One major finding was that all forest thinning treatments reduced fire behavior and severity. Fire weather, thinning design, and surface fuel load all influenced potential fire severity. While ecological restoration treatments did not reduce fire severity as much as traditional fuel hazard reduction treatments, the study found that these differences disappear when restoration treatments incorporate small tree removal. Additionally, overstory thinning combined with reductions in surface fuel loading, through prescribed fire or other means, were more effective than either overstory or surface fuel reductions alone. This work provides evidence that thinnings spanning a range of forest structural arrangements can reduce fire behavior and effects under moderate to high fire weather conditions. The team highlighted that regardless of thinning strategy and residual stocking levels, surface fuel loading had the greatest effect on fire hazard and highlights the need for treatments to directly address the accumulated fuels from a century of fire suppression.
