Area burned in the western United States is unaffected by recent mountain pine beetle outbreaks

Abstract

In the western United States, mountain pine beetles (MPBs) have killed pine trees across 71,000 km 2 of forest since the mid-1990s, leading to widespread concern that abundant dead fuels may increase area burned and exacerbate fire behavior. Although standlevel fire behavior models suggest that bark beetle-induced tree mortality increases flammability of stands by changing canopy and forest floor fuels, the actual effect of an MPB outbreak on subsequent wildfire activity remains widely debated. To address this knowledge gap, we superimposed areas burned on areas infested by MPBs for the three peak years of wildfire activity since 2002 across the western United States. Here, we show that the observed effect of MPB infestation on the area burned in years of extreme fire appears negligible at broad spatial extents. Contrary to the expectation of increased wildfire activity in recently infested red-stage stands, we found no difference between observed area and expected area burned in red-stage or subsequent gray-stage stands during three peak years of wildfire activity, which account for 46% of area burned during the 2002-2013 period. Although MPB infestation and fire activity both independently increased in conjunction with recent warming, our results demonstrate that the annual area burned in the western United States has not increased in direct response to bark beetle activity. Therefore, policy discussions should focus on societal adaptation to the effects of recent increases in wildfire activity related to increased drought severity.wildfire | bark beetle | disturbance interactions | lodgepole pine | Dendrocotnus ponderosae

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

“…Similarly, Simard et al [86] reported that the MPB outbreak in Greater Yellowstone may reduce the probability of active crown fires in the short term (up to 35 years after a beetle outbreak) by thinning lodgepole pine canopies. Likewise, a recent study in western USA claimed that the observed effect of MPB infestation on the area burned in 2002-2013 appears negligible at broad spatial extents [36]. Meigs et al [89] claimed that following MPB outbreak, fire likelihood is neither higher nor lower than in non-MPB-affected forests in the Pacific Northwest of USA.…”

“…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]. Based on an extensive literature review, Jenkins et al [80] concluded that both rates of fire spread and fire-line intensities were higher in the current outbreak's stands than in endemic stands.…”

“…The impacts of this colossal outbreak, coupled (compounded) with large-scale salvage operations will likely lead to conditions outside the natural range of disturbances that these ecosystems have experienced historically [14,31,33]. Information regarding the response of pine-dominated ecosystems to disturbances such as fire and harvesting is available [2,[34][35][36]; however, it is still unclear whether stand dynamics will respond similarly with respect to a large-scale beetle epidemic. Therefore, understanding how MPB-attacked stands will develop and characterize the future landscape is essential for assessing the impending impacts on ecological and socioeconomic conditions.…”

Impact of anthropogenic climate change on wildfire across western US forests

“…Nor do we account for the influence of climate change on lightning activity, which may increase with warming (40). We also do not account for how fire risk may be affected by changes in biomass/fuel due to increases in atmospheric CO 2 (41), drought-induced vegetation mortality (42), or insect outbreaks (43).…”

A Geospatial Analysis of Bark Beetle-Induced Wildfire Risk Zones in the Okanogan Wenatchee National Forest

“…Detection rates also increased with fire size. Possibly this is because heavy fuels in western fires [13] may continue to combust after the fire front has passed. …”