Climate variability and the susceptibility of Engelmann spruce to spruce beetle outbreak in northwestern Colorado

Hall, Sarah Jane

Graduate Thesis Or Dissertation

Climate variability and the susceptibility of Engelmann spruce to spruce beetle outbreak in northwestern Colorado Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/tt44pm94w

Abstract

Future changes in climate are expected to affect disturbance regimes across the globe, which will have important consequences for ecological and social systems. In Colorado, spruce beetle (Dendroctonus rufipennis) outbreak is one of the most important disturbances. Future warming is expected to promote outbreak through direct effects on beetle population dynamics. However, little is known about if or how climate change will affect the host, Engelmann spruce (Picea engelmannii). The central goal of this research was to determine the susceptibility of Engelmann spruce to spruce beetle in the context of climate variability in northwestern Colorado. To gain insight into the effects of climate on spruce beetle outbreak, I examined a 350- year tree-ring record of outbreak. I found spruce beetle outbreaks were initiated by drought, which affects both beetle population dynamics and tree resistance to infestation. To understand if recent warming had affected the susceptibility of spruce, I compared historical (1940s) and current (2001-2011) tree- and stand-scale constraints on outbreak. I found that stand-level constraints appear to be less restrictive of the current outbreak, likely due to drought-driven declines in host resistance. To determine how climate and forest structure interact to influence spruce beetle outbreak, I examined landscape-level outbreak. Outbreak was constrained by forest structure, which reflects past disturbance. In particular, previous outbreak created a landscape template characterized by depleted susceptible hosts, which was less susceptible to subsequent outbreak. A better understanding of the landscape-scale patterns of outbreak requires better scientific monitoring of spruce beetles. This need motivated the development of remote sensing methods for spruce beetle outbreak. This dissertation has revealed important insight into how climate variability alters the susceptibility of spruce to spruce beetle outbreak. Notably, this dissertation highlights the importance of drought in decreasing host resistance to spruce beetle infestation. Future changes in climate will likely promote outbreak through direct effects on host resistance and spruce beetle population dynamics, however how disturbances interact to affect the forest structure and composition will likely be equally or more important in determining future outbreak.

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