Bark beetles, fuels and future fire hazard in contrasting conifer forests of Greater Yellowstone

Bark beetles, fuels and future fire hazard in contrasting conifer forests of Greater Yellowstone" (2013). ABSTRACT The extent and severity of bark beetle (Curculionidae: Scolytinae) epidemics and the frequency of large, severe fires have reached unprecedented levels in recent decades, and these trends are expected to continue with ongoing climate change. Insects and fire have tremendous ecological and economic effects in western forests, yet their interactions are poorly understood. We combined field studies and simulation modeling to understand how bark beetle infestation and post-outbreak management affect fire hazard in two widespread but contrasting forest types, lodgepole pine (Pinus contorta) and Douglas-fir (Pseudotsuga menziesii) in the Greater Yellowstone Ecosystem (GYE) in northwestern Wyoming. We directly addressed key bark-beetle research priorities identified by US Forest Service scientists for the western US, via three primary questions and several supplemental studies. (1) How do effects of bark beetle outbreaks on fuel profiles and subsequent fire hazard differ between lodgepole pine and Douglas-fir forests? A 20-year " time-since-beetle-outbreak " chronosequence was sampled in Douglas-fir forests of the GYE to quantify changes in fuel profiles and to contrast these fuel profiles with those previously developed for lodgepole pine. The strongest effects of beetles in Douglas-fir were canopy thinning and increased spatial heterogeneity of fuels. Significant reductions were seen in available canopy fuel load and canopy bulk density, whereas effects on surface fuels were minor after accounting for pre-beetle variation in stand structure. Coarse fuel accumulation was lower in Douglas-fir forests (2x higher than in green stands) than reported for higher-elevation forest types like lodgepole pine (3-8x higher). Post-outbreak seedling regeneration was sparse in Douglas-fir stands, suggesting that beetle-caused reductions in stand densities will persist for a long time. (2) How was the severity of recent fire in lodgepole pine and Douglas-fir forests affected by prior bark beetle infestation, and does the combination of beetle infestation and fire compromise forest recovery? Field studies were conducted in recently burned forests that had been affected by beetle outbreaks prior to the fire. Fire severity in gray-stage Douglas-fir forests was related primarily to local topography and weather conditions at the time of burning; the effect of pre-fire beetle outbreak severity on fire severity was negligible. However, greater beetle-caused tree mortality was associated with lower post-fire tree seedling densities because of loss of the Douglas-fir seed source. Fire severity in lodgepole pine forests was greatest in stands having high proportions of …