Fuel Loads and Simulated Fire Behavior in “Old Stage” Beetle-Infested Ponderosa Pine of the Colorado Plateau

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Forest Science

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Recent bark beetle outbreaks in western North America have led to concerns regarding changes in fuel profiles and associated changes in fire behavior. Data are lacking for a range of infestation severities and time since outbreak, especially for relatively arid cover types. We surveyed fuel loads and simulated fire behavior for ponderosa pine stands of the Colorado Plateau 15–20 years after bark beetle infestation (i.e., “old-stage”). Increasing infestation severity resulted in reduced canopy bulk density, canopy base height, canopy cover, and litter loads whereas woody fuel loads were increased, especially among larger size classes. Using the Fire and Fuels Extension of the Forest Vegetation Simulator, torching index was predicted to decrease with infestation severity whereas crowning index was predicted to increase. Under modeled severe weather conditions, increasing infestation severity was predicted to shift the predominant fire type from surface fire to passive crown fire, whereas the probability of active crown fire was not significantly influenced by old-stage bark beetle-caused tree mortality. We also estimated snagfall rates of infested trees and found that the median time from infestation to snagfall was 9 –12 years, with larger trees taking longer to fall.