Fuel and Fire Behavior in High-Elevation Five-Needle Pines Affected by Mountain Pine Beetle

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Pre-print: Gen. Tech. Rep. RMRS-xx, USDA Forest Service. Rocky Mountain Research Station, Missoula, MT

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Abstract-- Bark beetle-caused tree mortality in conifer forests affects the quantity and quality of forestfuelsandhaslongbeenassumedtoincreasefirehazardandpotential firebehavior.In reality, bark beetles and their effects on fuel accumulation, and subsequent fire hazard have only recently been described. We have extensively sampled fuels in three conifer forest types (lodgepole pine, Engelmann spruce and Douglas-fir) and described bark beetle/fuels/fire interactions within the context of intermountain disturbance regimes. Our data sets were developed by measuring the forest biomass in stands with endemic, epidemic and post-epidemic bark beetle populations and comparing the quantity and quality of fuels present within each beetle population phase. Surface and canopy fuels data were used to create fuel models that are customized to represent the actual fuel conditions created by the bark beetles. Fire behavior predictions based on these custom fuel models showed that surface fire rate of spread and fireline intensities were higher in the current epidemic stands than in the endemic stands due to increased litter and fine fuel in all three forest types. Bark beetles selectively remove large diameter trees altering stand level canopy fuels and promoting release of herbaceous and shrub species which further affects fire potential. Bark beetle-caused tree mortality decreases vegetative sheltering which affects mid-flame wind speed and increases rate of fire spread. Passive crown fires are more likely in post-epidemic stands, but active crown fires are less likely due to decreased aerial fuel continuity. Intense surface fires are possible in post epidemic stands, but it is very much dependent on the rate at which dead trees fall. Our present research will utilize this information in addition to spatial data to describe the influence of mountain pine beetle (MPB) on fuels and fire behavior in stands of high- elevation five-needle pines, including whitebark, limber, foxtail, Rocky Mountain bristlecone, and Great Basin bristlecone pine.


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