USGS scientist Erich Peitzsch holds the cross section of an avalanche-impacted tree. Courtesy USGS

Reading Between the Lines

USGS scientist gleans data from tree rings to date and predict large-magnitude avalanche events in and around Glacier National Park; findings show climate warming can contribute to future avalanche hazards

By Tristan Scott

With a newly minted doctorate in snow science, Erich Peitzsch has spent much of the past three years with his head in the books, poring over data to better understand how warming climate trends can affect future avalanche activities in the region surrounding Glacier National Park.

Even when he’s conducting field research in the remote backcountry of the Crown of the Continent, or scouring the steep contours of the Swan Range for evidence of avalanche activity and disturbances, Peitzsch, a scientist with the U.S. Geological Survey (USGS) Northern Rocky Mountain Science Center, is constantly reading — scanning and analyzing natural data sets to help describe the relationships between climate and avalanches and, subsequently, the repercussions to human safety, development and infrastructure.

For Peitzsch, however, the text he’s most keen to interpret is distinct from the scholarly articles of academic journals. That’s because he’s extracting the data directly from the heart of the ancient forests of Douglas fir and hemlock blanketing the Northern Rockies, literally reading the tree rings.

Indeed, the stories of past avalanches are preserved deep within the forests, inscribed on the cross sections of wood harvested from the dead and downed trees that strike haphazard poses in the slide paths, like bundles of super-sized pickup sticks. Smooth scars along the outside of the trees signal recent avalanche impact events that have chewed away bits of bark, while dark, irregular rings located inside the core of the wood (known as reaction wood) result after a tree has been struck. As the tree continues to grow, its attempts to heal and re-stabilize itself are marked by the atypical rings, an overcorrection that leaves an immutable impression.

Erich Peitzsch stands beside a “cookie” carved from an avalanche-impacted tree in the Lost Johnny Creek drainage in the Swan Range of Northwest Montana. Courtesy Daniel Stahle, USGS

To tap into these stories, Peitzsch has spent countless hours collecting “cookies,” though not the kind cached in a jar or on an internet browser. The cookies that Peitzsch collects by the hundreds must be harvested through labor-intensive means, by using a chainsaw to extract tree-trunk slices from toppled trees, then using a high-powered microscope to examine the rings for evidence of avalanche trauma.

Read enough tree rings, like Peitzsch has, and it’s possible to assemble a comprehensive chronology of avalanches in a particular region, filling in critical data gaps that have rendered written records incomplete, while the advent of data-gathering weather instruments is too recent to form a comprehensive historical record. By comparing the cookies from a particular avalanche path with related climate data, Peitzsch and his colleagues have gained a better sense of the climatic influences that are associated with large-magnitude avalanche events, as well as the frequency with which they occur.

“One of the shortcomings of studying climate is that we need an extensive record of at least 30 years, and while Glacier National Park has pretty robust records of avalanche activity going back to the early 2000s, and older records from ranger station logs and Department of Transportation archives and things like that, we don’t have a complete record,” Peitzsch said. “So in places where there isn’t a long and detailed record, we can use tree rings to develop an avalanche chronology.”

Over the past two years, relying on 673 cookies cut from 647 trees exhibiting 2,134 avalanche-related growth disturbances, Peitzsch and his team developed a long-term (1867-2019) regional avalanche chronology for the Rocky Mountains of Northwest Montana, which, when contrasted against the region’s 14% decline in snowpack between 1950 and 2017, reveals new information about avalanche frequency, and has produced groundbreaking findings that large-magnitude avalanche years in the region occur at intervals of approximately five years.

USGS scientist Daniel Stahle uses a high-powered microscope to examine the cross section of a tree for signs of avalanche impacts. Courtesy USGS

Moreover, the research detected an association of winters characterized by large snowpacks and “a potential increase in large-magnitude events driven by warming temperatures and spring precipitations,” according to a new study published in “Nature Scientific Reports,” which runs counter to the established correlations between heavy snowpack and major avalanche years — or, rather, adds another climate-driven dynamic. 

Although the study found that large-magnitude avalanche years in the region are primarily driven by storm frequency and a deep snowpack, and the probability of a large-magnitude avalanche cycle decreased between 1950 and 2017, that decrease “appears to be buffered by warming temperatures and spring precipitation which leads to regional large magnitude avalanche activity even in years with below average snowpack,” according to Peitzsch.

“So, this could mean an increasingly greater influence of wet snow avalanches or longer, drier mid-winter periods leading to weak layer development in a shallow snowpack,” Peitzsch said, leading to more frequent avalanche activity in warmer months, and at elevations where precipitation still falls as snow, such as the upper reaches of Glacier National Park.

Ultimately, the tree ring records collected as part of the study could do more than describe the avalanche history of the dozen study sites Peitzsch relied upon for his project (and his Ph.D. dissertation) — they may provide insight into future avalanche risk for infrastructure, transportation and recreational safety.

For example, around the same time that Peitzsch published the results of his study in “Nature Scientific Reports” on May 11, titled “Climate drivers of large magnitude snow avalanche years in the U.S. northern Rocky Mountains,” a party of bicyclists was trapped between two springtime avalanches on the popular Going-to-the-Sun Road, stranding them for hours until rangers were able to safely rescue them.

“As USGS scientists, we’re tasked with producing science that’s relevant, and in this case I think the relevance is two-fold,” Peitzsch said. “In terms of infrastructure planning on existing roads or railways in avalanche terrain, the study can help anticipate return periods of large-magnitude avalanches as a potential disturbance to those activities. And for avalanche forecasters and subsequently the public’s safety, establishing return periods of large-magnitude avalanches and the long-term climate drivers that cause them gives us a lot of good perspective and context for how to interact in the mountains.”

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