Selective forest thinning in the eastern Cascades supports both snowpack and wildfire resilience

(Press-News.org) As climate change nudges weather in the eastern Cascades in extreme and volatile directions, forest managers in the region have a lot to juggle. Hotter, drier summers are contributing to bigger and more frequent wildfires. Meanwhile, warmer winters may cause the Cascades to lose 50% of its annual snowpack over the next 70 years. Mountain snow supplies the Yakima River Basin with 75% of its water supply, making it a crucial reservoir for both nature and agriculture across a broad swath of central Washington. Less winter snow also leads to drier and more fire-prone forests in the summer.

To encourage fire resilience, forest managers use tried-and-true tools like controlled burning and the selective felling of trees to thin out the forest. Both methods remove fuel and help return forests to historical conditions — but less is known about their impact on snowpack.

To address this knowledge gap, a team of researchers at the University of Washington and The Nature Conservancy (TNC) embarked on an ambitious, multiyear study of snowpack along Cle Elum Ridge, an area of the eastern Cascades in the headwaters of the Yakima River Basin. The group experimentally thinned the forest to varying degrees in a roughly 150-acre area. Then, they measured the amount and duration of snowpack during the winter of 2023 and compared it to a previous winter before the forest treatment. 

The results were encouraging: Forest thinning efforts increased snowpack by 30% on north-facing slopes and by 16% on south-facing slopes. Thinning aided snowpack the most where it created a patchwork of gaps in the forest rather than a more even density; gaps of 4-16 meters in diameter seemed to be the “sweet spot” for snow. 

The research points toward more refined forest management practices that can optimize for both wildfire resilience and snowpack.

The results were published March 3 in Frontiers in Forest and Global Change.

“At its core, this research shows that reducing wildfire risk and protecting water resources don’t have to be competing goals,” said lead author Cassie Lumbrazo, a postdoctoral researcher at the University of Alaska who completed this work as a UW doctoral student of civil and environmental engineering. “That’s genuinely good news for a place facing both growing wildfire threats and increasing water vulnerability. So much of the climate conversation focuses on loss, which makes findings like this especially meaningful.”

Predicting snowpack in forested areas, especially those at higher altitudes, hinges on understanding how much snow reaches the ground and how much lands in the forest canopy. Snow on the ground is more likely to stick around through the season, whereas snow in the trees may either melt or sublimate back into water vapor. In either case, it wouldn’t add to the reservoir of water that melts in the spring and summer.  

“Trees intercept snow and so can reduce snowpack, but trees also shade snow and so can retain snowpack,” said senior author Jessica Lundquist, a UW professor of civil and environmental engineering. “The dominant effect depends on winter temperatures, and the Cascade crest near Cle Elum is right on the border where the effect flips from trees decreasing snow to trees saving snow.” 

Previous work by Lundquist and her team found that natural gaps in the forests of the eastern Cascades accumulated more snow. This, combined with other research, gave the team reason to hope for a positive connection between forest thinning and snowpack, though it wasn’t a sure thing. Other studies have found that open areas elsewhere in the Western U.S. saw reduced snowpack.

Thus, it was time for a direct — and complex — study of managed forests.

Researchers picked Cle Elum Ridge for the work, where TNC’s forest managers were planning thinning treatments to improve forest health and wildfire resiliency. The orientation of the ridge allowed them to compare north- and south-facing slopes — southern slopes in the region see more sunshine and less snow retention on average. From October 2021 to September 2022, the researchers worked with TNC’s forest managers and local contract loggers to remove trees on both slopes in a gradient, from no thinning to extensive. The team also set up time-lapse cameras at several strategic points to measure snow depth over time.

Then, they waited for snow to fall.

By March 2023, the area was close to its peak snowpack, and the team returned with staff and equipment from the UW Natural Hazards Reconnaissance Facility (RAPID). The RAPID crew flew a specialized drone that generated a detailed 3D map of the study area using a laser-mapping technology called lidar. 

By comparing the new 3D map and timelapse imagery to lidar data captured before the forest treatment, the team was finally ready to calculate two things: the change to the forest structure, and its effect on the snowpack.

Across the whole study area, the team found that thinning helped the forest recover 12.3 acre-feet (or about four million gallons) of water in the form of snow per 100 acres on north-facing slopes, and 5.1 acre-feet (or about 1.5 million gallons) per 100 acres on south-facing slopes. 

As expected, areas where the thinning opened gaps in the canopy were most effective at restoring snow storage that had been previously lost to environmental degradation and climate change. Gaps of 4-16 meters in diameter seemed to retain the most snow, though there were few gaps larger than 16 meters to evaluate.

One surprising result: The way forest managers thin forests doesn’t reliably create gaps. Forest managers map out their reductions using the density of trunks in an area, not canopies, as their primary measurement.

“Imagine a group of 100 people all holding umbrellas in the rain,” said co-author Susan Dickerson-Lange, director of the UW Climate Impacts Group. “They’re standing close enough together that their umbrellas overlap, so none of the rain hits the ground. If you remove 10 of the umbrellas randomly, you’d still have plenty of coverage overall. But, if you remove 10 umbrellas that are right next to one another, you create a gap in the umbrella ‘canopy,’ and you get a 10% increase in the amount of rain that hits the ground.”

That realization adds a nuance to the findings. It’s likely that forest thinning can benefit both wildfire and snowpack resilience at the same time, but only if managers keep canopy gaps in mind. 

“One thing we all learned was that snow people and tree people speak different languages,” Lumbrazo said. “Different experts look at totally different variables to help them decide whether or not to cut down a single tree. So an important goal is to get everyone speaking the same language. And I think this paper is one step towards better communication.”

Overall, the results suggest practical changes to forest management practices in the eastern Cascades. For example, managers might consider more tree-thinning on north-facing slopes, since snowpack gains may be greater there. With further research, these learnings may also extend to other regions in the Pacific Northwest. 

The work could also aid collaboration between forest managers and hydrologists at a time when the region needs all the water it can get.

“As we lose snowpack, everything becomes really squeezed,” said co-author Emily Howe, a senior aquatic ecologist at TNC who earned her doctorate in aquatic and fishery sciences at the UW. “We are currently in our third consecutive year of water restrictions in the Yakima River Basin, and are staring down one of the lowest snow years on record. However, our research shows that the treatments currently used for restoring fire resilient forests are compatible with the forest structure needed for supporting water security. And in a world where climate change is reducing water supplies and increasing wildfire severity, we are pleased to report that the same forest treatments can support both goals.”

Co-authors include Steven Pestana, a former UW graduate student of civil and environmental engineering; John Cramblitt, a former UW undergraduate student of atmospheric and climate science; Karen Dedinsky, a data processing specialist at the UW RAPID facility; and Kyle Smith, director of Forest Conservation and Management at The Nature Conservancy.

This research was funded by The Washington Department of Natural Resources, The Nature Conservancy and the National Science Foundation. 
For more information, contact Lundquist at jdlund@uw.edu, Dickerson-Lange at dickers@uw.edu or Howe at emily.howe@tnc.org.

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