Between the lines: Tree rings reveal avalanche history

Man in hardhat among trees
group examines tree rings
group walks through trees on a mountain
Man holds tree cross section
person in hardhat looks at tree cross section

Come late May in southeast Alaska, AK CASC scientists Eran Hood and Gabriel Wolken have something unexpected on their minds: snow. Specifically, avalanches. Standing among snags and downed trees on a steep slope near Eaglecrest Ski Area in Juneau, the evidence of disturbance is obvious to any bystander.

To Hood and Wolken, this landscape could be a vast source of information about the avalanche events that have occured here over the past five, ten, or even 100 years. They are scouting field sites for an upcoming historic avalanche reconstruction project that will use dendrochronology (the study of tree rings) as a method for dating large magnitude avalanche events.

At Eaglecrest, the story is written in the wood of the hardy mountain hemlock trees that remain after avalanche debris has melted away.

“When you’ve looked at thousands of tree rings like they have, you can read them like a book,” Hood says of Erich Peitzsch and Danny Stahle, two researchers from the U.S. Geological Survey (USGS) Northern Rocky Mountain Science Center that have flown in to collaborate on the project.

Peitzsch and Stahle are conducting similar research in Glacier National Park, which shares southeast Alaska’s avalanche-prone characteristics with steep topography and heavy precipitation.

The story of past avalanches is preserved in thick cross sections of wood harvested from downed and dead trees in the avalanche path. Smooth scars along the outside of the trees mark recent avalanche impact events that have scraped bark away, and dark, irregular rings in the core of the wood (known as reaction wood) result after a tree has been impacted by an avalanche. As the tree continues to grow, its attempts to heal and stabilize itself are marked by the atypical rings.

In the samples the team examines from the Eaglecrest site, an avalanche event from about 40 years ago is evident in nearly all of the cross sections. More easily apparent are signs of a recent event from 2012, which impacted a popular run right on the margin of the Eaglecrest ski area. Near the center of one disk, a dark mark near the interior of a cross section could indicate an event as far back as the 1800s. By comparing records from multiple trees located in a particular path, and comparing these dates with climate data, the team can get a better sense of the climatic influences that are associated with large magnitude avalanche events.

Ultimately, the tree ring records collected in this project could do more than describe the avalanche history of the four main study sites for the project; they may provide insight into future avalanche risk for infrastructure in the Juneau region. This information is of particular interest given that several neighborhoods in Juneau lie directly in avalanche zones, a danger that has sometimes hit too close to home. At one of the project study sites, the Behrends Avenue path, evidence of recent avalanche activity is evident just upslope from a downtown Juneau neighborhood. In March of 2017, residents looked on as an avalanche rumbled down the cliffs towards their homes. The debris stopped short of any damage, but left a strong impression on the community. Avalanches in Juneau have also cut power sources for weeks at a time, caused structural damage, and taken lives.

“The mine has an avalanche forecaster, the electric company has an avalanche forecaster, the ski area has an avalanche forecaster, the city has an avalanche forecaster; There’s clearly a lot of interest in the information this project will provide,” said Hood.

In partnership with the USGS Northern Rocky Mountain Science Center, this historic avalanche data will be critical for assessing the impacts of changes in climate on future avalanche hazards in the Juneau area. The AK CASC and USGS researchers, as well as UAS undergraduate student Mckenzie Wilson, will officially begin working on the project in Summer 2019, with four expected study sites around Juneau: near Eaglecrest Ski Area, Behrends Avenue, the Kensington mine, and the Snettisham Fjord, which houses Juneau’s main hydroelectric power plant.