Factors Influencing the Spatial and Temporal Dynamics of Engelmann Spruce Mortality during a Spruce Beetle Outbreak on the Markagunt Plateau, Utah

Abstract

Host conditions are known to influence spruce beetle population levels, but whether they influence the spatial and temporal patterns of beetle-caused mortality during an outbreak is unknown. Using dendro- chronological techniques, we quantified the spatiotemporal dynamics of a modern (late 1980s through the early 2000s) spruce beetle outbreak in Engelmann spruce on the Markagunt Plateau, Utah. Erupting beetle populations occurred in multiple areas across the Plateau over the course of 8 years. Early in the outbreak, the timing of spruce mortality was strongly and predictably related to latitude, slope, aspect, stand structure, host species composition, and site productivity potential. Exogenous forcing from landscape-scale drivers such as winter minimum temperatures, summer maximum temperatures, and landscape-wide host suitability probably contrib- uted to spruce beetle population success and heightened spruce mortality. As the outbreak progressed, the timing of spruce mortality became less well correlated to stand and environmental variables. The outbreak subsided when suitable host material was depleted, including smaller spruce in remote stands at the fringe of the spruce-fir zone. Tests for spatial and temporal autocorrelation of beetle-cased spruce mortality did not provide strong evidence for the common perception that spruce beetle outbreaks originate in a specific location (epicenter) and then spread across the landscape; rather, support for positive spatial synchrony of multiple, building popula- tions during development of the outbreak was found. Based on these results, silvicultural intervention before the onset of an outbreak might mitigate Engelmann spruce losses; however, the same is almost certainly not true later in an outbreak of this scale and magnitude. FOR .S CI. 58(1):1-14.