Forest Thinning Boosts Snowpack and Reduces Wildfire Risk in the Eastern Cascades

Mountain snow does far more than cover the landscape through winter: it serves as a natural reservoir. When spring and summer temperatures climb, the melting snow replenishes rivers and streams, providing water for forests, farms and communities.

In Washington State, the eastern Cascade Mountains form a crucial part of this water system. Climate change, however, is starting to alter it. Milder winters are reducing snowpack, while forests are becoming increasingly susceptible to wildfire.

Forest managers must therefore limit fire danger while safeguarding the snow that supplies the Yakima River Basin.

Researchers from the University of Washington and The Nature Conservancy examined whether forest management could address both challenges.

Their findings indicate that carefully thinning woodland can lower wildfire risk while allowing more snow to reach the forest floor.

Water held in mountain snow

The Cascade Mountains retain substantial water supplies for central Washington. Snow accumulates there over winter before gradually melting in spring. This prolonged thaw sustains rivers and streams for many months.

The Yakima River Basin is particularly dependent on this snow. Around 75 percent of the area’s water originates as mountain snow, on which farms, communities and wildlife all depend.

Snow is also important for forests and wider ecosystems. It maintains soil moisture, encourages vegetation growth and keeps streams flowing consistently, helping fish and other aquatic species survive.

Climate change is diminishing these snow reserves. Warmer winters bring increased rainfall and reduced snowfall.

Scientists estimate that snowpack in the Cascade Range may fall by roughly half over the next seventy years. Reduced snow would mean less water during the dry summer period.

Wildfire activity is increasing

Across the western United States, wildfires are growing both larger and more frequent. Higher temperatures and parched forests make fires more likely to ignite and more difficult to contain.

Researchers expect wildfire activity to continue increasing. Climate research indicates that the area affected by forest fires in this region could rise substantially in the coming decades.

Fire can also alter snow conditions. Dark ash and burnt debris commonly settle on snow after a wildfire. Because these dark materials absorb more sunlight than clean snow, they accelerate melting.

Meanwhile, reduced snow leaves forests drier and more readily ignited. Wildfire and declining snow cover can consequently intensify one another.

Forest thinning lowers wildfire risk

Forest managers already use a range of approaches to reduce the threat of wildfire, with forest thinning being a widely used option.

Workers remove selected trees to prevent forests becoming overly dense. Controlled burning is another approach, safely clearing dry vegetation.

Such treatments cut the volume of vegetation available to fuel severe fires. They can also help forests return to more natural conditions.

Scientists had not, however, fully established how these measures influence snowpack. As trees interact with snow in complex ways, the relationship required closer investigation.

Research in the eastern Cascades

The researchers examined woodland on Cle Elum Ridge in the eastern Cascades. Located at the headwaters of the Yakima River Basin, the site has an important role in storing snow.

The team thinned trees across approximately 61 hectares of forest. Some sections received minimal thinning, whereas more trees were removed in others. This produced several forest conditions for comparison.

The researchers then measured winter snow in 2023 and compared the results with snow levels recorded before thinning.

Several tools were used to assess the site. Time-lapse cameras monitored snow depth throughout the winter.

Scientists also used lidar, a laser-mapping method that produces detailed three-dimensional maps of forests and snow. Together, these methods showed how forest structure influenced snow accumulation.

Trees can obstruct or preserve snow

Trees influence snow in two key ways. Their branches may capture falling snow before it reaches the ground. Snow retained in branches often melts rapidly or becomes water vapour.

Conversely, trees can prolong snow cover by casting shade. This shade shields snow from direct sunshine and slows the rate of melting.

“Trees intercept snow and so can reduce snowpack, but trees also shade snow and so can retain snowpack,” said study senior author Jessica Lundquist, a professor of civil and environmental engineering at the University of Washington.

“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.”

Since trees can simultaneously intercept and shelter snow, forest structure has a strong effect on how much snow remains on the ground.

Forest thinning increased snowpack

The study found that thinning woodland increased the volume of snow retained on the ground.

On north-facing slopes, snowpack rose by about 30 percent. South-facing slopes saw an increase of approximately 16 percent.

More snow reached the forest floor because gaps in the canopy allowed snowfall to pass through the trees.

The researchers also estimated the water stored in this additional snow. On north-facing slopes, thinning added about 375 cubic metres of water per hectare, equivalent to 12.3 acre feet per 100 acres; on south-facing slopes, it added about 156 cubic metres per hectare, equivalent to 5.1 acre feet per 100 acres.

These gains can help maintain soil moisture, streams and regional ecosystems.

Small woodland gaps are most effective

The team established that tree spacing matters. Snow accumulated most effectively in openings roughly 4 to 16 metres wide.

These medium-sized gaps enabled snow to reach the ground while adjacent trees continued to provide shade. Dense woodland intercepted too much snow, whereas very large clearings could speed up melting.

The resulting balance of sunshine and shade created the most favourable conditions for snow storage.

A need for better communication

The study also found that forest managers and snow scientists frequently assess forests in different ways.

“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.”

Closer cooperation between these disciplines could support forest management plans that safeguard both woodland and water.

One answer to two challenges

The study’s results offer encouraging evidence. Forest thinning can both reduce wildfire risk and increase snow storage.

“At its core, this research shows that reducing wildfire risk and protecting water resources don’t have to be competing goals,” said Lumbrazo.

As climate change continues to reshape mountain environments, strategies that protect forests and water supplies will become increasingly important.

Thoughtful thinning may help sustain snowpack, support rivers and limit wildfire damage throughout the region.