Interactions Between Fire and Spruce Beetles in a Subalpine Rocky Mountain Forest Landscape

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Interactions between natural disturbances are widely recognized as important determinants of vegetation patterns in forested landscapes but have only rarely been investigated quantitatively. In a subalpine forest landscape in northwestern Colorado, we quantified spatial associations of fire and spruce beetle (Dendroctonus rufipennis) outbreaks over more than a century and developed a multivariate logistic model of probability of occurrence of spruce beetle outbreaks. The study area, an 2800 km2 landscape in White River National Forest, was affected by severe widespread fires around 1879 and by a spruce beetle outbreak in the 1940s, which affected most of the forests that were dominated by Picea engelmannii (Engelmann spruce) and Abies lasiocarpa (subalpine fir). The boundaries of the fires ca. 1879 and of the 1940s spruce beetle outbreaks were digitized based on an 1898 vegetation map and modern aerial photograph interpretation, and overlaid in a Geographic Information System. The areas disturbed by the ca. 1879 fires and the 1940s beetle outbreak were also overlaid with 303 fires recorded after 1950 as well as with topographic and forest structural variables. Forests that had burned in 1879 were less affected by the 1940s outbreak than older stands. On the other hand, areas affected by the 1940s spruce beetle outbreak showed no higher susceptibility to subsequent fires. A multivariate logistic model indicated that, although fire history had the greatest effect on stand susceptibility to spruce beetle outbreak, dominance of neighboring stands by spruce as well as elevation were also important predictors of outbreaks. Predictive modeling of spatial interactions between fire and spruce beetle disturbances needs to consider the high degree of variability in the nature of these interactions related to contingencies such as time since last major disturbance, topographic position, and weather during windows of potential interaction.