Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, United States of America.
PLoS One. 2012;7(1):e30002. doi: 10.1371/journal.pone.0030002. Epub 2012 Jan 17.
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.
在科罗拉多州和怀俄明州南部，山松甲虫（MPB）已经影响了超过 160 万公顷的主要是黑云杉林，这引起了人们对 MPB 引起的死亡率对随后野火风险和行为的影响的关注。使用经验数据，我们模拟了在风速和湿度梯度下绿林与 MPB 攻击后三个阶段（红期[1-3 年]、灰期[4-10 年]和老 MPB 期[约 30 年]）的潜在火灾行为。MPB 导致红期和灰期的树木死亡率达到 50%，林分断面积死亡率达到 70%。在所有湿度情景下，与绿林相比，红期和灰期的林冠燃料湿度降低了三分之一，这使得在较低的风速和较少极端湿度条件下就有可能发生活跃树冠火。由于灰期和老 MPB 期的树木倒伏，树冠更加开阔，大表面可燃物负荷增加，这显著提高了地表火线强度，使得在所有湿度情景下，较低风速（>30-55 公里/小时）就有可能发生活跃树冠火。如果不考虑红期和灰期的低叶湿度和灰期和老 MPB 期的大表面可燃物，就会低估活跃树冠火的发生。在极端燃烧条件下，与绿林相比，红期、灰期和老 MPB 期活跃树冠火的最小风速降低了 25-35 公里/小时。然而，如果发生树冠火转变（在林外，或通过梯级可燃物或风速超过 65 公里/小时在林内），则活跃树冠火将在相似的风速下持续，这表明在极端燃烧条件下，观察到的火灾行为在 MPB 阶段之间可能没有质的不同。总的来说，活跃树冠火的风险（概率）在受 MPB 影响的林分中似乎较高，但主要的火灾危险（树冠火）在所有 MPB 阶段都相似，这是黑云杉林的特征，在这种林分中，极其干燥、阵风天气是决定火灾行为的关键因素。
