Abstract
In Colorado and southern Wyoming, mountain pine beetle (MPB) has affected over 1.6 million ha of predominantly lodgepole pine forests, raising concerns about effects of MPB-caused mortality on subsequent wildfire risk and behavior. Using empirical data we modeled potential fire behavior across a gradient of wind speeds and moisture scenarios in Green stands compared three stages since MPB attack (Red [1–3 yrs], Grey [4–10 yrs], and Old-MPB [∼30 yrs]). MPB killed 50% of the trees and 70% of the basal area in Red and Grey stages. Across moisture scenarios, canopy fuel moisture was one-third lower in Red and Grey stages compared to the Green stage, making active crown fire possible at lower wind speeds and less extreme moisture conditions. More-open canopies and high loads of large surface fuels due to treefall in Grey and Old-MPB stages significantly increased surface fireline intensities, facilitating active crown fire at lower wind speeds (>30–55 km/hr) across all moisture scenarios. Not accounting for low foliar moistures in Red and Grey stages, and large surface fuels in Grey and Old-MPB stages, underestimates the occurrence of active crown fire. Under extreme burning conditions, minimum wind speeds for active crown fire were 25–35 km/hr lower for Red, Grey and Old-MPB stands compared to Green. However, if transition to crown fire occurs (outside the stand, or within the stand via ladder fuels or wind gusts >65 km/hr), active crown fire would be sustained at similar wind speeds, suggesting observed fire behavior may not be qualitatively different among MPB stages under extreme burning conditions. Overall, the risk (probability) of active crown fire appears elevated in MPB-affected stands, but the predominant fire hazard (crown fire) is similar across MPB stages and is characteristic of lodgepole pine forests where extremely dry, gusty weather conditions are key factors in determining fire behavior.
Highlights
Epidemic outbreaks of native mountain pine beetle (Dendroctonus ponderosae; MPB) populations have affected over 1.6 million ha of predominantly lodgepole pine (Pinus contorta var. latifolia) forests in Colorado and southern Wyoming since 1996
While total basal area was not significantly different across the MPB stages, live basal area was significantly higher in the Green stands compared to other stands (45 vs. 15 m2/ha; p,0.001)
Basal area of grey trees with no needles and no 1-hr fuels was highest in Old-MPB stands, but not significantly different from other MPB stages (p = 0.159)
Summary
Epidemic outbreaks of native mountain pine beetle (Dendroctonus ponderosae; MPB) populations have affected over 1.6 million ha of predominantly lodgepole pine (Pinus contorta var. latifolia) forests in Colorado and southern Wyoming since 1996. Mountain pine beetle and wildfire are the two primary disturbance agents in lodgepole pine forests Both have increased significantly in recent years, especially in mid- to high-elevation forests of the central and northern Rockies [1,2]. The most recent MPB outbreaks have been linked to warmer and drier conditions and to past history of fire or land use that have promoted an abundance of older, large-diameter lodgepole pine trees, which increase susceptibility to the insect [6,7,8]. Drought conditions conducive to large wildfires in these high-elevation forests in Colorado are rare, typically recurring at .100- year intervals within a stand, and historically coinciding with the negative phases of the El Nino Southern Oscillation (La Nina) and Pacific Decadal Oscillation, and the positive phase of the Atlantic Multidecadal Oscillation [9]
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