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Oral Presentation

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Student Competition

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Student 10-Minute Presentation Competition

Plant-Insect Ecosystems

Student Competition

Student

Grad 10-min Competition: P-IE, Climate Change

500 years of hemlock defoliator outbreaks in the Tongass National Forest

Monday, November 10, 2025

8:00 AM - 8:12 AM Pacific

Location: Oregon Convention Center, Portland Ballroom 254, OCC

Presenting Author(s)

MT

Marlee Theil

University of Washington
Seattle, Washington

Co-Author(s)

EG

Elizabeth Graham

USDA Forest Service, Forest Health Protection
Juneau, Alaska
MH

Michael Howe

Assistant Professor
Utah State University
Madison, Wisconsin
KN

Kellen Nelson

USDA Forest Service, Pacific Northwest Research Station
Juneau, Alaska
PT

Patrick Tobin

University of Washington
Bothell, Washington

Pacific coastal temperate rainforests comprise the largest and most intact temperate rainforest on Earth. In recent years, widespread outbreaks of two native insect defoliators, hemlock sawfly (Neodiprion tsugae) and western blackheaded budworm (Acleris gloverana), have affected over 4,300 km2 of hemlock and spruce forests across the Tongass National Forest of southeast Alaska. It is unclear whether the extent of these outbreaks is unique given the limited historical record of their outbreak activity. To enhance understanding of historical outbreak activity, I developed a dendrochronological dataset spanning the past 500 years to reconstruct the temporal dynamics of outbreaks and to help contextualize the most recent outbreaks. My analysis revealed a long-term decline in average outbreak duration since the early 1400s, coupled with an increase in outbreak frequency since the year 2000. This could be the first indication that the frequency of the most recent outbreaks is unique. These findings provide land managers with empirical support to make decisions regarding the protection of forest resources and preservation of forest health.