Impact of Thinning Strategy, Surface Fuel Loading and Burning Conditions on Fuel Treatment Efficacy in Ponderosa Pine Dominated Forests of the Southern Rocky Mountains

Managers across the western US seek effective fuel treatment strategies to mitigate hazardous fuel loads and risks of high severity fire in dry conifer forests. Conventional fuel hazard reduction treatments emphasis reducing canopy fuel continuity and surface fuel loading using an even spaced, thin-from-below approach, with pile or broadcast burning of residual surface fuels. Such treatments often result in forest structures that differ from the historical conditions. Ecological restoration treatments emphasize enhancing structural heterogeneity but may produce less fire-resistant stands causing tradeoffs between fuel hazard reduction and restoration objectives. This study explored these 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. All types of thinning reduced fire behavior and severity relative to untreated forests. Fire rate of spread was slightly increased following variable retention harvests or treatment that included thinning from below. Fire weather, cutting methods, and surface fuel load all influenced potential fire severity. Variable retention thinnings did not reduce severity as much as treatments including a thin-from-below, regardless of the horizontal arrangement of trees. Our results suggest tradeoffs between ecological restoration and hazard reduction could be overcome if restoration treatments incorporate small tree removal. Overstory removal combined with reductions in surface fuel loading, through prescribed fire or other means, were more effective than either overstory or surface fuel reductions alone.