Evaluating regeneration and stand dynamics in tamarack stands impacted by eastern larch beetle in northern Minnesota, USA

Abstract

Tamarack (Larix laricina DuRoi K. Koch) forests in Minnesota are currently threatened by a landscape-level outbreak of eastern larch beetle (ELB) (Dendroctonus simplex LeConte), a native bark beetle. Since 2001, ELB has impacted 816,833 acres of tamarack forest over 20 consecutive years. This has resulted in many management questions related to stand development, especially in terms of regeneration. To explore how ELB has influenced stand structure and composition, 33 sites across a gradient of ELB damage severity and outbreak timing in north-central Minnesota were sampled. Older mortality sites had the highest overstory tree density, driven largely by a high volume of standing dead tamarack. On average, regeneration exceeded a commonly used standard of 600 trees per acre in older high mortality stands only and was below the suggested stocking level in recent high mortality and low mortality stands. Tree ring analysis of 150 tamarack tree cookies revealed understory tamarack saplings aged 9 to 82 years old, suggesting tamarack may have more robust shade tolerance than previously believed. Tamarack established consistently throughout a period of 64 years, suggesting advance regeneration is a part of tamarack stand dynamics. Finally, we identified temporal differences in establishment periods in stands with varying levels of ELB impacts. The combined results of this work highlight complex and understudied dynamics related to tamarack regeneration. Regeneration may take 10 to 15 years to establish after an ELB outbreak. However, it is not only new germinants present in the seedling, sapling, and small tree layer but also advance regeneration.