Numerical Simulation of Crown Fire Hazard Immediately after Bark Beetle-Caused Mortality in Lodgepole Pine Forests

Abstract

Quantifying the effects of mountain pine beetle (MPB)-caused tree mortality on potential crown fire hazard has been challenging partly because of limitations in current operational fire behavior models. Such models are not capable of accounting for fuel heterogeneity resulting from an outbreak. Further, the coupled interactions between fuel, fire, and atmosphere are not modeled. To overcome these limitations, we used the Wildland-Urban Interface Fire Dynamics Simulator (WFDS) to investigate the influences of tree spatial arrangement and magnitude of MPB-caused tree mortality on simulated fire hazard. Field-collected, tree-level data from 11 sites were used to populate WFDS simulation domains representing a range of lodgepole pine forest structures for the postoutbreak period of time when dead needles are still present in the tree crowns. We found increases in the amount of crown fuel consumption and the intensity of crown fires as the percentage of MPB-caused tree mortality increased. In addition, we found complex interactions between the level of mortality, stand structure, and spatial arrangement of trees. These results suggest that preoutbreak forest structure and percent tree mortality influence crown fire behavior while dead needles are in the crown, and that the effect varies with spatial heterogeneity among trees.