Evaluating post-outbreak management effects on future fuel profiles and stand structure in bark beetle-impacted forests of Greater Yellowstone

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Forest Ecology and Management

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Large-scale bark beetle (Curculionidae: Scolytinae) outbreaks across western North America have prompted widespread concerns over changes to forest wildfire potentials. Management actions following outbreaks often include the harvest of beetle-killed trees and subsequent fuel treatments to mitigate expected changes to fuel profiles, but few data exist to inform these actions. In both lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca) forests of the Greater Yellowstone Ecosystem, Wyoming, USA, we used the Forest Vegetation Simulator to evaluate how fuel profiles, stand structure, and biomass carbon storage are influenced by various post-outbreak fuel treatments (removal of beetle-killed trees [‘salvage’] followed by either no treatment, prescribed burning, pile-and-burn, or whole-tree-removal). The model was initialized with field data from five unmanaged gray-stage stands in each forest type and projected over 50 years of post-treatment time. Across all treatment methods, the strongest projected effects relative to unharvested stands were reductions in coarse woody surface fuels (after 10–20 yr), fewer well-decayed standing snags (after 40 yr), and reduced biomass carbon storage (throughout all 50 years). The reduction in coarse woody surface fuels suggests reduced heat release and resistance to control in future fires. Projected effects on fine fuels, both in the canopy and surface layers, were surprisingly minor or short-lived; natural fall and decay of fine material in unharvested stands led to the convergence of most fuel variables between treated and untreated stands within about a decade, especially in Douglas-fir forests. Most follow-up treatment methods – whether unmerchantable tree parts were left in place, burned, piled, or removed entirely – had similar impacts on most aspects of fuel and stand structure in both lodgepole pine and Douglas-fir forests. However, the prescribed burning treatment was distinct and generally had the strongest effects, owing to greater consumption of forest floor mass and mortality of small trees, which had persistent influences on both the canopy and surface fuel layers. Treatment effectiveness in reducing fuels was mirrored by reductions in biomass carbon storage and recruitment of well-decayed snags, illustrating common tradeoffs involved in fuel treatments. Harvest of beetle-killed trees and subsequent treatments altered the fuel profile and structure of outbreak-impacted stands, but overall effects were similar among treatments, suggesting flexibility in management options in post-outbreak forests.