Effects of Mountain Pine Beetle on Fuels and Expected Fire Behavior in Lodgepole Pine Forests, Colorado, USA

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.

How drought-induced forest die-off alters microclimate and increases fuel loadings and fire potentials

“…More open canopies and high loads of large surface fuels due to tree fall have been shown to significantly increase surface fireline intensities (Schoennagel et al 2012). Our study shows that drought die-off and subsequent fire are potentially linked disturbances (in that one impacts the probability or severity of the next).…”

Recent mountain pine beetle outbreaks, wildfire severity, and postfire tree regeneration in the US Northern Rockies

“…Such variation allowed us to assess fire severity across the spectrum of recent prefire outbreak severity, including stands unaffected by the recent outbreaks (effectively serving as a control). Three fires burned forests where most attacked stands were in the red postoutbreak stage (0-2 y after beetle attack, ∼50% retention of largely red needles on beetle-killed trees) (12,14,15), considered to be most vulnerable to increased crown fire because canopy fuels are drier and more flammable (21,29). Three fires burned forests where most attacked stands were in the gray postoutbreak stage (3-10 y after beetle attack, <5% needle retention on beetle-killed trees, most beetle-killed trees still standing) (12,14,15).…”

“…Three fires burned forests where most attacked stands were in the red postoutbreak stage (0-2 y after beetle attack, ∼50% retention of largely red needles on beetle-killed trees) (12,14,15), considered to be most vulnerable to increased crown fire because canopy fuels are drier and more flammable (21,29). Three fires burned forests where most attacked stands were in the gray postoutbreak stage (3-10 y after beetle attack, <5% needle retention on beetle-killed trees, most beetle-killed trees still standing) (12,14,15). Gray-stage forests are considered less vulnerable to increased crown fire because canopy fuels are substantially reduced (14)(15)(16)30), although increased surface fuels from needle and branch fall could increase surface fire severity (15)(16)(17).…”

“…latifolia) (8). Severe MPB outbreaks can result in up to 90% mortality of tree basal area (14)(15)(16)(17)(18), which could compromise postfire resilience by increasing the severity of subsequent wildfires, decreasing seed sources (thus diminishing postfire tree regeneration), or both.…”

“…Tree mortality caused by MPB outbreaks alters the fuel structure of forests (i.e., the quantity, quality, and distribution of biomass) (14)(15)(16)(17) in ways that could affect fire severity (defined as the degree of short-term ecological change caused by a fire, typically measured by the proportion of biomass lost, or vegetation killed by fire) (19). Increases in dead and flammable fuels in postoutbreak forests can influence fire behavior (e.g., energy release and spread rate, see ref.…”