Assessing regional variation in tamarack mortality from eastern larch beetle in the Great Lakes Region, USA

For over 125 years, tamarack (Larix laricina (Du Roi) K. Koch) across North America have fallen victim to the tree-killing activity of eastern larch beetle, Dendroctonus simplex LeConte (Coleoptera: Scolytidae) (Bright 2014). Localized infestations of this native bark beetle are not uncommon; instead, mortality events caused by eastern larch beetle have been reoccurring throughout the natural range of tamarack. While landscape-scale outbreaks of this insect are rare, the current outbreak of eastern larch beetles in the Great Lakes region of the United States (US), which has been ongoing since 2000, is unlike previous events in duration and in the absence of a preceding disturbance (e.g., defoliation and flooding). While infestations have been found in Michigan’s Upper Peninsula and north-central Wisconsin, the bulk of tamarack mortality has occurred in Minnesota, where approximately 86,500 ha have been affected. Based on forest inventory information from 5 ecological regions in the Great Lakes, a linear mixed effects model was used to identify regional variation in the relative importance of tree and site condition related to tamarack mortality due to eastern larch beetle over time. During the early outbreak period (2004 to 2008), diameter was an important predictor of mortality in the northern portion of tamarack’s US range, such that larger diameter trees experienced an increased probability of mortality. The positive relationship between diameter and mortality became regionally important as the outbreak progressed. The rate of tamarack mortality was highest in the oak savanna region of Minnesota, which represents the marginal distribution of tamarack. Within this region, the density of sympatric non-host gymnosperms was positively related to mortality. As the Great Lakes represents the southern portion of the range of tamarack and eastern larch beetle, changes in this region could have broader implications for the role of climate change on eastern larch beetle activity and the future extent of tamarack.