Tree- and stand-level factors associated with tamarack mortality due to eastern larch beetle (Dendroctonus simplex LeConte) in Minnesota

Minnesota has experienced outbreaks of the native eastern larch beetle (ELB), Dendroctonus simplex LeConte (Coleoptera: Curculionidae, Scolytinae), since 1938. Historically, these outbreaks have been intermittent, often separated by decades, and short-lived. In contrast to historic patterns, the current outbreak of ELB has continued from 2000 to the present. Tamarack (Larix laricina) extends across 1.1 million acres of northern Minnesota, where it is a dominant component of bogs and peatlands. Data collected by the USDA Forest Service, Forest Inventory and Analysis program show that tamarack mortality in Minnesota has more than tripled over the past decade, increasing from 5.8 million ft³ in 2003 to 19.5 million ft³ in 2013. This increase in tamarack mortality is largely attributed to the activity of ELB. Factors that predispose tamarack to attack by ELB are not thoroughly understood, however, physiological stress, most often defoliation, is generally a precursor to infestation. As a marked contrast, predisposing factors are not apparent within the current infestation. In this study, forest inventory data collected between 2004 and 2013 were used to examine differences between tamarack trees that were killed by ELB and those that survived or were not attacked. Probability of tamarack mortality from ELB based on a logistic regression model was significantly (p<0.01) negatively correlated to diameter, crown ratio, stand age and stand basal area, and positively correlated to height.