Snow Avalanche Disturbance in Intermountain Spruce-fir Forests and Implications for the Spruce Bark Beetle (Coleoptera : Scolytidae)

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Spruce beetle outbreaks are the primary biotic disturbance affecting forests in the Intermountain spruce-fir zone. Major snow avalanches can produce large quantities of host material for spruce beetle colonization; however, few studies have examined the influence of avalanche disturbance on outbreak initiation. The purpose of my research was to investigate potential relationships between these two disturbance agents. In the first study, I used dendro-ecological analyses to date major avalanche years on the Wasatch Plateau in south-central Utah, and then determined what climate factors were associated with avalanche occurrence. The results indicated that mean January snowfall was significantly related to the probability of major avalanche years, although model correlations were extremely low. Potential dating errors, the lack of complete historical climate data, and the absence of snow pack information may have prohibited obtaining high model correlations. In the second study, Investigated how the seasonal availability of host material influenced spruce beetle brood production. I found that significantly fewer mean numbers of spruce beetles were produced in downed spruce felled during August 1997 than from trees felled in April of 1998. More brood adults also were present in, or emerged from the bottom surfaces of sample trees than either unexposed (North, East) or exposed (South, West) bole aspects. These results provided evidence that with greater potential for spruce beetle populations to build in host material produced in the host material produced in the spring, snow avalanche disturbance could contribute to the risk of outbreaks. I constructed snow avalanche and spruce beetle outbreak chronologies from historic documents in the third study, and used classification tree analysis to predict historic spruce beetle outbreaks form manor avalanche years and historic climate data. Major snow avalanche years were not identified in this analysis as important for the occurrence of spruce beetle outbreaks. Historic spruce beetle outbreaks, however, were significantly related to generally warm fall and winter temperatures and the drought. There results indicate that climate and other factors not considered in this analysis have a greater influence on spruce beetle epidemiology than the production of host material by snow avalanches.


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