Doctoral Dissertation Research: Climate Variability and the Susceptibility of Engelmann Spruce to Spruce Beetle (Dendroctonus Rufipennis) Outbreaks in Northwestern Colorado

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This doctoral dissertation research project will investigate the effects of warmer temperatures on the susceptibility of spruce trees to infestations by spruce beetles in subalpine environments of western North America. Changes in climate during the late 20th and early 21st centuries are widely viewed as the primary driver of a dramatic increase in bark beetle activity throughout western North America. Although the mountain pine beetle (MPB; Dendroctonus ponderosae) has caused most bark beetle related tree mortality across North America over the past two decades, concern has been growing that outbreaks of the spruce beetle (SB; Dendroctonus rufipennis) could be similarly extensive. The area affected by SB in Colorado and Wyoming nearly doubled from 460 km2 in 2009 to 840 km2 in 2010. The vast extent and substantial mortality of this and other bark beetle outbreaks over the last two decades are consistent with predictions drawn from studies of beetle population dynamics, which predict that a warmer climate stimulates beetle activity through decreased winter mortality and shifts from two-year to one-year life cycles. However, comparatively little is known about the potential constraints on SB activity posed by spatial variation in spruce tree susceptibility, as mediated by physical environmental factors interacting with forest dynamics in response to previous disturbances by fire, wind, and past outbreaks. Early research suggests that variation in tree susceptibility may have limited the extent and severity of previous SB outbreaks, but the feedback of tree susceptibility on outbreak dynamics appears to be greatly reduced under the warmer climate of the current outbreak (1998 - present). The central goal of this project is to determine the susceptibility of Engelmann spruce (Picea engelmannii) trees to SB outbreaks across the complex subalpine environment of northwestern Colorado in the context of climate variability. Using an integration of mapping via GIS as well as tree-ring analysis the research seeks to reconstruct historical and contemporary SB outbreaks. The research seeks answers to the following two research questions: What are the climate conditions related to historic spruce beetle outbreaks in northwestern Colorado? 2) What abiotic and biotic factors determine susceptibility to the current period of SB attack at the tree and stand-level, and how do these relationships compare with historic relationships (pre-1998) between susceptibility to SB and the same variables?The results of this research will provide insight into how climate variability alters the susceptibility of spruce trees to SB outbreak across the subalpine environment in North America. An increased understanding of the relationships between increased temperatures, beetle infestations and other forest dynamics will help inform management decisions and evaluate hazards to nearby communities related to high tree mortality. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.