Landscape-Scale Drivers of Resistance and Resilience to Bark Beetles: A Conceptual Susceptibility Model

Abstract

Bark beetle (Dendroctonus spp.) outbreaks in the middle latitudes of western North America cause large amounts of tree mortality, outstripping wildfire by an order of magnitude. While temperatures play an important, and direct role in the population dynamics of ectothermic bark beetles, an equally important influence is the nature of the host substrate—the structure and composition of forested communities. For many of the dominant tree species in the western United States, “hazard” indices have been developed for specific bark beetles, which generally include three key variables—host tree size, absolute or relative density of the stand, and percentage of host composition. We provide a conceptual model to apply these three variables across forest ecosystems and bark beetles that shifts the thinking from a species–specific model to a model which focuses on the underlying ecological factors related to bark beetle outbreak susceptibility. We explored the use of our model across multiple scales using the Forest Inventory and Analysis database: Interior West, USA; the states of Colorado and Arizona; and specific national forests within Arizona that are implementing a large-scale restoration effort. We demonstrated that across the Interior West and Colorado, the vast majority of forests have moderate to high susceptibility to bark beetles. Our conceptual model maintains the simplicity of previous “hazard” models but acknowledges the need to consider scale when managing bark beetles. It also shifts the management approach from resistance thinking to the development of “associational resilience”, where the focus is not on any one individual stand or area but the longer-term perspective of forest persistence across the landscape.

Rocky Mountain forests are poised to recover following bark beetle outbreaks but with altered composition

“…Although rare in our study, other research has demonstrated that outbreak-driven declines in tree density or conversions to non-forest communities are most likely in monospecific stands composed of larger trees (Andrus, Hart, & Veblen, 2020), which are most susceptible to severe outbreaks (Hart et al, 2015;Windmuller-Campione et al, 2021). Indeed, species diversity, structural variability and spatial heterogeneity have long been theorized to promote stability and adaptive capacity in ecological systems (Koontz et al, 2020;Tilman & Downing, 1994;Turner, 2010).…”

Overlapping outbreaks of multiple bark beetle species are rarely more severe than single‐species outbreaks

“…480 m) to minimize the impact of fine‐scale spatial autocorrelation in our analyses. For each sampled pixel, we extracted the BA loss due to bark beetles, presence or absence of each bark beetle species, and stand structure and composition (species‐specific BA, SDI, and relative abundance; Windmuller‐Campione et al, 2021).…”

“…These forests are characterized by an infrequent (200–500 years), high‐severity fire regime (Buechling & Baker, 2004; Sibold et al, 2006) that can create extensive even‐aged stands that are consequently more susceptible to bark beetle outbreaks when they reach maturity. Through the same mechanism, widespread fire and logging activity in the 1800s may have increased landscape‐scale susceptibility to bark beetle outbreaks in recent decades (Veblen & Donnegan, 2005; Windmuller‐Campione et al, 2021). In these outbreaks, tree mortality was mainly driven by three beetle species: the spruce beetle (SB; Dendroctonus rufipennis Kirby), which affects Engelmann spruce; the mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins), which predominantly affects lodgepole pine; and the western balsam bark beetle (WBBB; Dryocoetes confusus Swaine), which affects subalpine fir.…”

“…(3) In areas susceptible to multispecies outbreaks, is cumulative tree mortality greater in stands affected by overlapping outbreaks of different bark beetle species when compared with stands affected by outbreak of only one bark beetle species? We expected that most forests were susceptible to at least one bark beetle species prior to the recent outbreaks due to their existing size and age structures (USDA, 2019b; Windmuller‐Campione et al, 2021). Because most forests are composed of more than one host species, we expected that stands susceptible to multiple bark beetle species would be similarly widespread, as would the occurrence of overlapping outbreaks (Guo et al, 2019).…”