Mechanical mastication and prescribed burning reduce forest fuels and alter stand structure in dry coniferous forests

Mechanical thinning is often prescribed in dry coniferous forests to reduce stand density, ladder fuels, and canopy fuels before using prescribed burning to manage surface fuels. Mechanical mastication is a tool for thinning forests where commercial thinning is not viable. We evaluated the effects of mastication-based thinning – with and without subsequent prescribed burning – on forest structure and fuels in dry coniferous forests of the Pacific Northwest, USA. We thinned stands by masticating small-diameter trees and depositing the resulting slash on the forest floor. We then used prescribed burning to reduce surface fuels in half of the masticated stands. Mastication reduced overstory tree density, tree regeneration, stand basal area, canopy bulk density, and total canopy fuels, while increasing surface woody fuel loads for most fuel size classes. Increases in 100-hour and 1000-hour sound woody surface fuels were positively correlated with pretreatment stand density and local thinning intensity. Mastication reduced 1000-hour rotten woody fuels, however. Prescribed burning following mastication reduced surface woody fuels across all size classes, often as a proportion of available fuels. Surface fuel reductions from prescribed burning generally exceeded fuel inputs from mastication, so the combined treatments reduced stand density, canopy fuels, ladder fuels, and most surface fuels relative to pretreatment conditions. Mastication increased modeled surface wildfire intensity but reduced crown fire potential. Prescribed burning after mastication reduced fuels and modeled surface wildfire intensity to pretreatment or lower levels. This study shows that mastication-based thinning followed by prescribed burning can be effective for reducing fuels and wildfire hazards and for modifying overstory stand structure in forest stands with small-diameter trees.