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Master of Science (MS)
Bunn, Andrew Godard
Medler, Michael J.
The western spruce budworm is recognized as the most ecologically- and economically-damaging defoliator in western North America. Like other defoliating insects, the western spruce budworm consumes the needles of host tree species like the Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), causing limb and tree mortality, regeneration delays, and reduction in tree growth rates. Synchronous western spruce budworm outbreaks can occur over much of a host species' range, and we need a better understanding of the mechanisms driving the species’ population dynamics in order to predict climate change effects, mitigate ecological and resource management impacts, and understand ecosystem dynamics. To contribute to our understanding of this species' outbreak dynamics, we used dendrochronological methods to reconstruct multicentury outbreak records for four sites in the Okanogan Highlands of central Washington State. By comparing these reconstructed records to historical and reconstructed regional drought data, we were able to test drought history as a potential driving factor of western spruce budworm population dynamics, as well as see how human impact may be affecting these population dynamics. Outbreak synchrony was found to increase after the late 19th century, possibly due to anthropogenic factors. This change is more readily apparent as the intensity of outbreaks increases. Drought records show that outbreaks tend to occur as the drought severity decreases, and moisture availability increases. As the variability of climate conditions is projected to increase, trending towards warm and dry conditions, the intensity and frequency of high-intensity western spruce budworm outbreaks could likely increase as well.
Western Washington University
Okanogan Highlands (Wash.)
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Ellis, Todd M., "Climatic Drivers of Western Spruce Budworm Outbreaks in the Okanogan Highlands" (2016). WWU Masters Thesis Collection. 528.