Regional temperatures associated with tamarack mortality due to eastern larch beetle in Minnesota, USA

Background/Question/Methods

Pure stands of tamarack (Larix laricina) help to define the forests of Minnesota’s ‘North Woods’. Although tamarack continues to be a dominant component of northern Minnesota forests, a history of major anthropogenic and biotic disturbances, such as harvesting associated with European settlement and outbreaks of eastern larch beetle (ELB, Dendroctonus simplex LeConte), have altered the extent of tamarack on the landscape. Native to North America, ELB has been documented in Minnesota since 1938. While outbreaks are typically sporadic and short-lived, the most recent outbreak of ELB has been ongoing since 2000. Defoliation is frequently a disposing factor to trees undergoing attack by ELB, but defoliation activity has not been apparent prior to the current outbreak. Hence, we studied whether climatic factors such as recurring periods of drought and changing temperatures/seasonality were associated with outbreaking populations of ELB in Minnesota. Aerial survey data, as well as ground observations from the Forest Inventory Analysis program of the USDA Forest Service were used to analyze trends in tamarack mortality and abundance/distribution in Minnesota between 1977 and 2012.  Spatial autoregressive models were employed to examine ELB mortality as a function of annual change in temperature between 2001 and 2012.

Results/Conclusions

Tamarack mortality quadrupled from 124.6 thousand m³ in 1990 to 560.6 thousand m³ in 2012. County-level analysis indicated that Lake of the Woods County, which experienced the highest mortality and severe drought, saw an increase in MRATIO from 0.16 to 11.31 between 2004 and 2012. Preliminary results show that within the current outbreak there is an increase in the rate of mortality following successive years of drought, with highest mortality occurring in areas with the most severe, prolonged drought. Moreover, spatiotemporal autologistic models at 10 km grid scales indicated that tamarack mortality increased with later frost-free periods in a previous year and later annual dates reaching 10 degrees C in a current year. These patterns likely reflect beetles emerging synchronously and attacking tamarack with frozen root systems. Continued analysis of temperature as it relates to spatial patterns of tamarack mortality and ELB activity will help to further elucidate historic patterns of ELB outbreaks and inform future management practices.