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.

Highlights

  • Native bark beetles (Dendroctonus spp.) are disturbance agents that drive stand development and dynamics across all the important coniferous forested systems of NorthAmerica [1]—and have probably existed in this capacity for millennia [2]

  • Landscapes have been implicated as factors contributing to the “age of mega-fires” [6], but may be a factor in the recent, unprecedented mortality caused by epidemic bark beetle populations [7,8]

  • We demonstrate the conceptual model for multiple, hierarchical spatial extents utilizing the Forest Inventory and Analysis (FIA) database

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Summary

Introduction

Native bark beetles (Dendroctonus spp.) are disturbance agents that drive stand development and dynamics across all the important coniferous forested systems of NorthAmerica [1]—and have probably existed in this capacity for millennia [2]. Native bark beetles (Dendroctonus spp.) are disturbance agents that drive stand development and dynamics across all the important coniferous forested systems of North. The simplification of structure (e.g., age structure; Figure 1) and species composition across western US landscapes have been implicated as factors contributing to the “age of mega-fires” [6], but may be a factor in the recent, unprecedented mortality caused by epidemic bark beetle populations [7,8]. The recent severity of mortality attributable to bark beetles is extraordinary; more than 6 million hectares in the last ten years in the conterminous western US (Figure 2), and the volume of mortality far exceeds that caused by fire in forests over the same time-period [9,10,11,12].

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