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

Abstract

Widespread tree mortality caused by outbreaks of native bark beetles (Circulionidae: Scolytinae) in recent decades has raised concern among scientists and forest managers about whether beetle outbreaks fuel more ecologically severe forest fires and impair postfire resilience. To investigate this question, we collected extensive field data following multiple fires that burned subalpine forests in 2011 throughout the Northern Rocky Mountains across a spectrum of prefire beetle outbreak severity, primarily from mountain pine beetle (Dendroctonus ponderosae). We found that recent (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010) beetle outbreak severity was unrelated to most field measures of subsequent fire severity, which was instead driven primarily by extreme burning conditions (weather) and topography. In the red stage (0-2 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity with few effects detected only under extreme burning conditions. In the gray stage (3-10 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity under moderate conditions, but several measures related to surface fire severity increased with outbreak severity under extreme conditions. Initial postfire tree regeneration of the primary beetle host tree [lodgepole pine (Pinus contorta var. latifolia)] was not directly affected by prefire outbreak severity but was instead driven by the presence of a canopy seedbank and by fire severity. Recent beetle outbreaks in subalpine forests affected few measures of wildfire severity and did not hinder the ability of lodgepole pine forests to regenerate after fire, suggesting that resilience in subalpine forests is not necessarily impaired by recent mountain pine beetle outbreaks. disturbance interactions | forest resilience | fire ecology | serotiny | conifer forest

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“…The increase in fire severity and synergism predicted in our study for red-phase stands under low-to moderate-wind speeds appears to contradict the conclusion by Harvey et al (2014a) that fire severity in lodgepole pine stands was largely unaffected by pre-fire bark beetle-outbreak severity. The differences between the two studies could be because Harvey et al studied a mid-red-phase lodgepole pine forest, with only ~50 % of the dead needles retained in the canopy, and our study addressed the early red phase, before any of the dead needles had fallen off the trees.…”

“…The differences between the two studies could be because Harvey et al studied a mid-red-phase lodgepole pine forest, with only ~50 % of the dead needles retained in the canopy, and our study addressed the early red phase, before any of the dead needles had fallen off the trees. In addition, Harvey et al (2014a) studied mountain pine beetle-caused mortality that resulted in needle loss over multiple years, whereas the abrupt and widespread mortality that we studied in Southwestern ponderosa pine forests was incited by a "global change-type drought" (Breshears et al 2009) that increased susceptibility to multiple Ips and Dendroctonus bark beetle species, killing trees within one to two years (Negrón et al 2009). Finally, if the "moderate" and "extreme" burning conditions approximated by Harvey et al (2014a) by relative humidity and temperature also included higher wind speeds, their results are in line with our simulations that suggested that increasing levels of mortality during the red phase under moderate-and high-wind speeds led to minor and sometimes non-significant increases in fire severity.…”

“…Theoretical frameworks and some studies predict higher fire severity in the red phase and lower in the gray phase (Jenkins et al 2008, Hicke et al 2012, Prichard and Kennedy 2014, Meigs et al 2016. However, other studies have suggested that fires during the gray phase may be more severe than fires in green-phase forests, or they may be less severe or have little effect on fire severity in either phase (Hoffman 2011;Simard et al 2011;Donato et al 2013;Harvey et al 2014aHarvey et al , 2014b. These varying fuel effects across the temporal phases thus can potentially alter both the severity of a subsequent fire as well as type and magnitude of the bark beetle−fire interaction.…”

“…For example, several studies have suggested that the severity and rate of bark beetle-caused mortality plays a pivotal role in the severity of a subsequent fire (DeRose and Long 2009;Hoffman et al 2012b;Harvey et al 2014aHarvey et al , 2014bHoffman et al 2015). Thus, varying levels and rates of mortality may alter subsequent fire severity, whether the disturbances are linked or not, and even the type and magnitude of the interaction across different phases.…”

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

“…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.…”