Does wildfire likelihood increase following insect outbreaks in conifer forests?

Abstract

Abstract. Although there is acute concern that insect-caused tree mortality increases the likelihood or severity of subsequent wildfire, previous studies have been mixed, with findings typically based on standscale simulations or individual events. This study investigates landscape-and regional-scale wildfire likelihood following outbreaks of the two most prevalent native insect pests in the US Pacific Northwest (PNW): mountain pine beetle (MPB; Dendroctonus ponderosae) and western spruce budworm (WSB; Choristoneura freemani). We leverage seamless census data across numerous insect and fire events to (1) summarize the interannual dynamics of insects and wildfires across forested ecoregions of the PNW; (2) identify potential linked disturbance interactions with an empirical wildfire likelihood index; (3) quantify this insect-fire likelihood across different insect agents, time lags, ecoregions, and fire sizes. All three disturbance agents have occurred primarily in the drier, interior conifer forests east of the Cascade Range. In general, WSB extent exceeds MPB extent, which in turn exceeds wildfire extent, and each disturbance typically affects less than 2% annually of a given ecoregion. In recent decades across the PNW, wildfire likelihood does not consistently increase or decrease following insect outbreaks. There is evidence, however, of linked interactions that vary across insect agent (MPB, WSB), space (ecoregion), and time (interval since insect onset). Specifically, in most cases following MPB activity, fire likelihood is neither higher nor lower than in non-MPB-affected forests. In contrast, fire likelihood is lower following WSB activity across multiple ecoregions and time lags. In addition, insect-fire likelihood is not consistently associated with interannual fire extent, suggesting that other factors (e.g., climate) control the disproportionately large fire years accounting for regional fire dynamics. Thus, although both bark beetles and defoliators alter fuels and associated fire potential, the windows of opportunity for increased or decreased fire likelihood are too narrow-or the phenomena themselves too rare-for a consistent signal to emerge across PNW conifer forests. These findings suggest that strategic plans should recognize (1) the relative rarity of insect-fire interactions and (2) the potential ecosystem restoration benefits of native insect outbreaks, when they do occur.

Near‐future forest vulnerability to drought and fire varies across the western United States

“…Therefore, our metrics may identify forested areas vulnerable to beetle attack due to the presence of drought-stressed trees (Boone, Aukema, Bohlmann, Carroll, & Raffa, 2011;Breshears et al, 2005), but do not necessarily represent forest vulnerability to aggressive bark beetle outbreaks in which healthy trees may be killed (Raffa et al, 2008). This is unlikely to have a strong impact on estimated fire vulnerability because wildlife likelihood does not consistently increase or decrease following insect outbreaks (Hicke, Johnson, Jane, & Preisler, 2012;Meigs et al, 2015). This is unlikely to have a strong impact on estimated fire vulnerability because wildlife likelihood does not consistently increase or decrease following insect outbreaks (Hicke, Johnson, Jane, & Preisler, 2012;Meigs et al, 2015).…”

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

“…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.…”

“…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. Many studies emphasize the importance of climate rather than fuels as the primary driver of wild fires [34,35,87,90].…”

Human-caused climate change is now a key driver of forest fire activity in the western United States

“…Trends of increasing area burned in the western United States have been blamed on various factors, but rigorous attribution has yet to materialize. For example, outbreaks of native bark beetles (which are also triggered by warm/dry climate conditions) are routinely blamed for many recent forest fires, although scientific evidence has found weak to nonexistent links between beetle outbreaks and subsequent fire occurrence (13), area burned (14), and burn severity (15). A second common assertion is that past forest management (particularly on public lands) is to blame for recent increases in fire activity.…”