Wildfire Threat to Pacific Northwest Old-Growth Forests

Wildfire is an increasing danger to some of the Pacific Northwest’s oldest forests, landscapes so long established that they have become part of the region’s identity.

Old-growth forests lower air temperatures, retain water in the ground and provide habitat for plants and animals unable to survive elsewhere. They also lock away vast quantities of carbon in both their timber and the soils below.

A new analysis delivers a stark warning: many mature and old-growth forests in the Pacific Northwest now face their greatest likelihood of severe wildfire in the very areas that historically experienced more frequent, but less destructive, burning.

When frequent fire disappears

Across much of the western United States, fire was once a routine feature of the landscape. These were not necessarily fires that destroyed everything; rather, they were frequent low- and mixed-severity burns that removed smaller trees and undergrowth.

By doing so, these fires kept forests more open and made them better able to endure severe wildfires.

When smaller, lower-intensity fires are absent for extended periods, forest conditions begin to alter. Trees become more densely packed, while dead wood accumulates across the forest floor.

Shade-tolerant, fire-intolerant species establish themselves in locations where they previously struggled to persist. As time passes, the likelihood of high-severity wildfire increases.

The forests people rely on

Scientists from Oregon State University (OSU) and USDA Forest Service Research & Development carried out the study. Bruno Aparicio and Meg Krawchuk, from the OSU College of Forestry, led the research.

The findings matter because mature and old-growth forests are not simply “pretty old trees.” They serve as reservoirs of biodiversity and carbon stores, while also holding cultural and economic importance.

In addition, old-growth forests are facing a growing threat from stand-replacing fires, which kill most or all of the trees within a forest.

Using advanced modelling, the researchers identified the locations in old forests most likely to experience severe wildfire.

A key result was that 75 percent of forest areas facing the greatest risk of severe wildfire lie in places that historically had widespread low- and mixed-severity fires.

How past fire policies shaped risk

Fire did not vanish from Northwest landscapes by chance. The researchers identify the disruption of Indigenous fire stewardship as a pivotal change.

Indigenous peoples were forcibly removed from their lands during the 1850s, interrupting fire stewardship practices that had existed for generations. Wildfire suppression then became federal policy in the early 20th century.

This change followed the Great Fire of 1910, which destroyed several towns and burned an area approximately the size of Connecticut through forests in Idaho, Montana, Washington and British Columbia.

The transition mattered because regular low- to mixed-severity fires had been a vital element of western forest ecology. Once these fires were excluded, the structure of older forests and trees began to change as new species established themselves and fuels accumulated.

“Now, as wildfire activity intensifies under climate change, understanding where and why mature and old-growth forests are most vulnerable is essential,” said Aparicio.

“This work provides a spatial framework to help land managers prioritize where protection may be sufficient, where active restoration is needed, and how fire refugia can be integrated into conservation and wildfire risk planning.”

The role of fire refugia

Forests do not burn uniformly. Even in extreme wildfires, certain areas commonly escape the most severe damage.

Scientists call these locations fire refugia: areas with a lower chance of crown fire than the surrounding landscape.

Fire refugia allow vulnerable species to persist and can provide starting points for recovery after a fire. They have also contributed to an emerging research field sometimes known as refugia science.

The principle is straightforward: certain places possess inherent characteristics that make them more likely to withstand intense fire, including terrain, vegetation, closeness to water bodies and the direction a slope faces.

For the study, researchers examined federal forests across the 9.7 million hectares of western Oregon, Washington and California covered by the Northwest Forest Plan.

About 2.8 million hectares of this land consists of mature or old-growth forest, with 730,000 hectares of these older forests located within fire refugia. Estimates from the analysis suggest that fire refugia could lower the overall risk of severe fire by up to 20 percent.

Where wildfire risk is highest

The analysis indicates that mature and old-growth subalpine forests at the timberline are comparatively scarce and particularly vulnerable.

It also identifies the Klamath Mountains as a hotspot for wildfire exposure and possible carbon emissions.

Older forests contain carbon built up over decades or centuries. When a stand-replacing wildfire passes through, it can release much of this stored carbon, while the forest’s capacity to absorb carbon is reduced until regrowth is well established.

“Protecting mature and old-growth forests isn’t just about preserving the past – it’s a key strategy for climate mitigation, ecosystem resilience and long-term forest stewardship,” Krawchuk said.

From logging losses to wildfire

The authors note that logging caused a steep decline in older forests on national forests throughout the 20th century. More recently, however, wildfire has become the largest threat.

Since 2000, federal lands have recorded a net loss of 1.05 million hectares of mature forest and 283,000 hectares of old-growth forest.

These areas were assigned to multiple uses, including recreation, timber production and wildlife habitat, and they deliver ecosystem services that people often overlook.

“Current trends show a continuous increase in area burned and wildfire severity in the western U.S. over the last decade,” Aparicio said.

“The increase is driven by a growth in the number of human-caused fires; warmer and drier fire seasons and worse droughts, all exacerbated by climate change; and the presence of landscape fuels created through management and fire-exclusion policies.”

Using maps to guide forest care

The researchers maintain that reducing losses of mature and old-growth forests to stand-replacing wildfire begins with understanding where exposure is greatest and what causes it.

This requires combining geographical data with fire ecology analysis to produce a clear map of risk across an enormous landscape.

“Our work can help prioritize stewardship actions,” Krawchuk said. “Overall, fire exclusion has left historically fire-resistant forests disproportionately exposed to severe fire now.”

“Our analysis can underpin the stewardship of mature and old-growth forests to promote their resistance and resilience.”

Put simply, the study highlights a difficult but practical truth: old forests cannot be protected through a one-size-fits-all approach.

Some locations may remain resilient with basic protection, whereas others could require active restoration to reduce fuels and restore healthier fire patterns.

In a warming climate, places that naturally avoid the most damaging flames may ultimately prove more important than anyone anticipated.

The complete study was published in the journal Natural Hazards.