Mountain pine beetle (Dendroctonus ponderosae) outbreaks during the past decade have caused widespread mortality across western North American forests. These outbreaks have been notably intense in subalpine whitebark pine (Pinus albicaulis) communities – areas historically considered to be climatically marginal for beetle reproduction. Higher temperatures associated with climate change are thought to be important drivers of these outbreaks by lengthening the beetle development period, reducing the frequency cold-induced mortality events, and possibly diminishing host resistance to attack. The scale of recent high elevation outbreaks appears to be historically unprecedented, yet there is only limited understanding regarding the dynamics of past outbreaks in whitebark pine ecosystems. My thesis work focuses on the dynamics of a high elevation mountain pine beetle outbreak in the Pioneer Mountains of southwest Montana that occurred during the late 1920s and early 1930s. This research focuses both the landscape scale and individual tree scale, with the intention of providing context for current high elevation outbreaks. Specifically, I identify (1) the timing of the outbreak and assess the factors that caused the outbreak to end, and (2) differences in growth patterns between whitebark pines that were killed and those that survived the outbreak. The outbreak in the Pioneer Mountains, Montana, began in 1924, peaked in 1930, and abruptly ended in 1933. The termination of the outbreak was synchronous with the coldest recorded temperature for Dillon, MT, and high numbers of suitable hosts for attack persisted following 1933. Had this cold event not occurred, the beetles would have likely caused greater pine mortality and more heavily influenced successional patterns. During the outbreak, mountain pine beetles killed the most vigorous whitebark pines. Whitebark pines that survived the outbreak had consistently slow growth and their ring-widths responded more strongly to very wet and dry years. Whitebark pines killed earlier in the outbreak had slower growth than those killed later, suggesting a switch in host selection as the outbreak progressed. These results suggest that there may not be a linear relationship between host stress in whitebark pines and susceptibility for mountain pine beetle attack.
University of Minnesota M.A. thesis. December 2011. Major: Geography. Advisor: Kurt F. Kipfmueller. 1 computer file (PDF); ix, 113 pages.
Margoles, Daniel Speeter.
Mountain pine beetle-whitebark pine dynamics in a subalpine ecosystem of the pioneer mountains, Southwest Montana.
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