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Abstract

North Pacific coastal temperate rainforests are important for carbon sequestration, biodiversity, ecosystem services, and provision of economic revenue through eco-tourism and resource extraction. Historically, disturbances in these forests occur at the scale of individual trees but may affect entire stands. Douglas-fir beetle (Drendoctonus pseudotsugae) is a major mortality agent of coastal Douglas-fir (Pseudotsugae menziesii var. menziesii), a predominant tree species within these temperate rainforests. Douglas-fir beetle is thought to exhibit pulse-driven population dynamics, whereby densities are determined by the availability of stressed, defensively impaired trees. By contrast, threshold-limited bark beetle species undergo density-dependent shifts in colonization behaviour and will preferentially colonize healthy trees when populations exceed a critical density. Climate change is likely to increase resource pulses through increased frequency, intensity, and severity of stress inducing events such as droughts and windstorms. I evaluated the pulse-driven dynamic of Douglas-fir beetle in coastal forests by testing the hypothesis that host-tree preference is not density dependent. Using a random forest modelling approach, I investigated forest conditions associated with endemic and epidemic Douglas-fir beetle presence within coastal Douglas-fir forests and compared the two models to quantify changes in host selection. I found the variables that affected low-density (endemic) and high-density (epidemic) beetle populations, and the form of their influence, were the same; however, the order of variable importance differed between the two populations rejecting the hypothesis that DFB does not exhibit density-dependent host selection behaviours. Endemic populations were associated with stands with high proportions of large Douglas-fir trees, dense stands relative to their age and slow growth rates. In contrast, epidemic populations following a catastrophic windstorm were driven by stand height and elevation. This nuanced yet distinct density-dependent shift in host selection behaviour challenges the current pulse-driven and threshold-limited dichotomy, suggesting host-tree preferences may exist on a spectrum for pulse-driven species. Forest management practices have altered species composition and successional patterns of coastal forests creating vulnerabilities during resource pulses to future outbreaks. Understanding population dynamics of Douglas-fir beetle equips land managers with the information to engage in proactive forest management practices to reduce risk and impacts from future outbreaks.