Recent bark beetle outbreaks influence wildfire severity in mixed‐conifer forests of the Sierra Nevada, California, USA

Abstract

In temperate forests, elevated frequency of drought related disturbances will likely increase the incidence of interactions between disturbances such as bark beetle epidemics and wildfires. Our understanding of the influence of recent drought and insect-induced tree mortality on wildfire severity has largely lacked information from forests adapted to frequent fire. A recent unprecedented tree mortality event in California's Sierra Nevada provides an opportunity to examine this disturbance interaction in historically frequent-fire forests. Using field data collected within areas of recent tree mortality that subsequently burned in wildfire, we examined whether and under what conditions wildfire severity relates to severity of prefire tree mortality in Sierra Nevada mixed-conifer forests. We collected data on 180 plots within the 2015 Rough Fire and 2016 Cedar Fire footprints (California, USA). Our analyses identified prefire tree mortality as influential on all measures of wildfire severity (basal area killed by fire, RdNBR, and canopy torch) on the Cedar Fire, although it was less influential than fire weather (relative humidity). Prefire tree mortality was influential on two of three fire-severity measures on the Rough Fire, and was the most important predictor of basal area killed by fire; topographic position was influential on two metrics. On the Cedar Fire, the influence of prefire mortality on basal area killed by fire was greater under milder weather conditions. All measures of fire severity increased as prefire mortality increased up to prefire mortality levels of approximately 30-40%; further increases did not result in greater fire severity. The interacting disturbances shifted a pine-dominated system (Rough Fire) to a cedar-pine-fir system, while the pre-disturbance fir-cedar system (Cedar Fire) saw its dominant species unchanged. Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems. This study contributes to a growing body of evidence that the influence of prefire tree mortality on wildfire severity in temperate coniferous forests may depend on other conditions capable of driving extreme wildfire behavior, such as weather.

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

“…Warmer temperatures and increasing aridity have been associated both with increasing drought frequency and intensity (Diffenbaugh et al 2015) and greater frequency and size of wildfires (Westerling et al 2006;Littell et al 2016;Abatzoglou and Williams 2016). Furthermore, drought often triggers bark beetle (Coleoptera: Curculionidae, Scolytinae) outbreaks in conifer forests across the northern hemisphere (Kolb et al 2016;Young et al 2017), and when this combination causes extensive tree mortality, it may alter wildfire hazard, behavior, or resulting fire severity through changes in fuel characteristics (Kane et al 2017;Gray et al 2021;Wayman and Safford 2021).…”

ECOSTRESS reveals pre‐fire vegetation controls on burn severity for Southern California wildfires of 2020

“…In contrast, slower‐growing vegetation, including evergreen needleleaf forests, are less responsive to variability of precipitation in the growing season, and flammability is more controlled by longer‐term periods of dry conditions and drought. Drought also promotes tree death by bark beetles, which can influence burn severity, especially when coupled with additional weather stressors (Wayman & Safford, 2021), although the effect of bark beetles also remains contested (Hicke et al, 2012; Jolly et al, 2012). Nuances in terms of short‐ and long‐term predictors are also expected to produce varying patterns of fire burn severity.…”

Bark Beetle Effects on Fire Regimes Depend on Underlying Fuel Modifications in Semiarid Systems

“…We found that surface fire probability and severity only responded to beetle outbreaks when mortality was higher than 10% and stopped increasing at around 50% during the old phase. Similarly, based on field observations in mixed-conifer forests in the Sierra Nevada, Wayman and Safford (2021) reported that fire severity increased most substantially when beetle-caused mortality surpassed 15% and was below 30%-40%. In this study, fire severity may have stopped increasing with mortality above 50% because plant water demand decreased, leading to decreases in fuel aridity.…”

“…While the conceptual framework outlined above is useful for understanding temporal wildfire responses to beetle outbreaks, several uncertainties still remain (Halofsky et al., 2020; Hicke et al., 2012). For example, in the gray and old phases, many field observations (Bebi et al., 2003; Berg et al., 2006; Lynch et al., 2006) and modeling studies (Ager et al., 2007; Lundquist, 2007; Meigs et al., 2016) have documented decreases or no change of fire probability and fire severity, while others have found increases (Bigler et al., 2005; Turner et al., 1999; Wayman & Safford, 2021). These increases in crown fire severity during the gray phase may have occurred because once the canopy opened, more radiation was able to penetrate, and competition for resources decreased.…”

“…While climate change is expected to continue to increase the severity and frequency of these disturbances, it is less clear how they will interact with one another (Bennett et al., 2018). Bark beetle outbreaks can change fuel conditions and corresponding wildfire characteristics by altering ecohydrological processes and forest fuel structure (Goeking & Tarboton, 2020; Wayman & Safford, 2021). However, the direction of ecohydrological responses to beetle outbreaks can vary over space and time within watersheds (Ren et al., 2021), which can in turn influence fuel loading, fuel moisture, and fire regimes.…”