Interactions among the Mountain Pine Beetle, Fires, and Fuels

Abstract

Bark beetle outbreaks and wildfires are principal drivers of change in western North American forests, and both have increased in severity and extent in recent years. These two agents of disturbance interact in complex ways to shape forest structure and composition. For example, mountain pine beetle, Dendroctonus ponderosae Hopkins, epidemics alter forest fuels with consequences for the frequency and intensity of wildfires and, conversely, fire injury to trees can promote bark beetle attack and increase beetle populations. Given the great influence these processes have on forest ecosystems, a better understanding of how bark beetles and fires interact is needed to better manage forests and to predict and manage wildfires. Here we review current knowledge on how fire and bark beetles interact, focusing on the mountain pine beetle. We highlight research on how fuel reduction treatments and wildfires affect bark beetles and how bark beetles can affect wildfires by modifying the moisture content, chemistry, and structure of fuels. Last, we discuss the implications these findings have for fire management and firefighter safety.

Aftermath of Mountain Pine Beetle Outbreak in British Columbia: Stand Dynamics, Management Response and Ecosystem Resilience

“…Generally, it is hypothesized that MPB-induced tree mortality affects fire behavior by altering the flammability, continuity, and structure of fuels [79][80][81][82]. Moreover, the fuel profile such as surface, ladder, and crown fuels are expected to change with time since outbreak, potentially altering fire behavior and fire risk.…”

“…After tree death, needles fade to red within a year of attack (red stage) and risks of ignition, torching, and canopy fire are expected to increase due to lower leaf moisture content (10 times lower in foliar moisture content compared to green needles), non-fiber carbohydrates and fats, which increase flammability [83,84]. Approximately 3 to 10 years (gray stage) after the beetle attack, trees drop their needles and twigs and become exposed in the upper crown [81], which likely increases the forest floor fuels [81,82]. Therefore, it is expected that surface fires will be more likely to spread into the canopy during the gray stage [85] and salvage logging is a recommended method of forest management to reduce the fire hazard [80][81][82].…”

“…Approximately 3 to 10 years (gray stage) after the beetle attack, trees drop their needles and twigs and become exposed in the upper crown [81], which likely increases the forest floor fuels [81,82]. Therefore, it is expected that surface fires will be more likely to spread into the canopy during the gray stage [85] and salvage logging is a recommended method of forest management to reduce the fire hazard [80][81][82]. However, MPB and its impacts on fuel accumulation and subsequent fire hazard are poorly understood: some studies have suggested higher probability or severity of fire following beetle outbreak [66,79,80], while others suggest a decreased fire probability [2,86], or no evidence of any relationship between MPB and active fire [34][35][36][87][88][89].…”

Wildfire’s resistance to control in mountain pine beetle-attacked lodgepole pine forests

“…The changes in the characteristics of fire behaviour in MPB-attacked lodgepole pine stands over the course of an outbreak have previously been discussed in detail by Jenkins et al (2008Jenkins et al ( , 2012Jenkins et al ( , 2014 and Hicke et al (2012). Based on their conclusions, in addition to the endemic or pre-attack condition (Fig.…”

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“…species-specific beetle-host combination, and the fire weather (see reviews in Jenkins et al 2008Jenkins et al , 2012Jenkins et al , 2014Hicke et al 2012). How bark beetle-caused mortality alters the severity of a subsequent fire as well as the type and magnitude of the interaction between fires following bark beetle-caused mortality remains poorly understood across the gradient of these controlling factors, especially in dry forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson).…”