CORVALLIS, Ore. – A new analysis shows that the Pacific Northwest's mature and old-growth forests are most at risk of severe wildfire in areas that historically burned frequently at lower severity.
The study by scientists at Oregon State University and USDA Forest Service Research & Development is important because those forests are culturally, economically and ecologically significant, supporting biodiversity while storing vast amounts of carbon, and they are under increasing threat of stand-replacing wildfire.
Researchers used state-of-the-art modeling techniques to gain knowledge that can help guide risk mitigation efforts and return forests to healthier fire regimes.
Led by Bruno Aparicio and Meg Krawchuk of the OSU College of Forestry, the research highlights the impact of fire exclusion by showing that 75% of the forest areas with the biggest risk of severe wildfire are places that used to see widespread low- and mixed-severity fires.
The exclusion of fire from Northwest landscapes began with the disruption of Indigenous fire stewardship, the researchers say. Indigenous peoples were forcibly removed from their lands in the 1850s, and putting out wildfires became federal policy following the Great Fire of 1910, which destroyed several towns and burned an area roughly the size of Connecticut across forests in Idaho, Montana, Washington and British Columbia.
Prior to fire exclusion, frequent low- and mixed-severity fires had played a key role in maintaining the forest ecosystems of the western United States. Since then, older forests and trees have seen major structural shifts as shade-tolerant and fire-intolerant species have moved in.
"Now, as wildfire activity intensifies under climate change, understanding where and why mature and old-growth forests are most vulnerable is essential," said Aparicio, a postdoctoral researcher in Krawchuk's group within the college's Department of Forest Ecosystems and Society. "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."
Fire refugia are pockets of landscape less prone to crown fire than adjacent areas and may hold the key for scientists, conservationists and land managers seeking to preserve vulnerable species in a changing climate.
Informally described as the "lifeboats" or "slow lanes" of biodiversity, refugia have spawned the new field of refugia science , which has learned that some locations have inherent characteristics – such as terrain, vegetation, proximity to bodies of water, and slope-face direction – that can buffer them from stand-replacing fire with a degree of predictability.
In the context of this study, which looked at federal forests in the 24 million acres of western Oregon, Washington and California covered by the Northwest Forest Plan, estimates indicate fire refugia could reduce the overall risk of severe fire by up to 20%. Roughly 7 million acres of the study area are mature or old-growth forest, 1.8 million acres of which is in fire refugia area.
The analysis suggests the study area's subalpine mature and old-growth forests – those at the timberline – are both relatively rare and have high vulnerability, and that the Klamath Mountains are a hotspot for both wildfire exposure and potential carbon emissions.
"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.
Through the 20th century, the authors note, mature and old-growth forests on national forests saw major declines because of logging. In recent years, wildfire has replaced logging as the largest threat, and since 2000, federal lands have seen a net loss of 2.6 million acres of mature forest and 700,000 acres of old-growth forest. Those acres were designated for multiple uses including recreation, timber production and wildlife habitat, providing important ecosystem services.
"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."
Developing land management strategies to address the loss of mature and old-growth forests to stand-replacing wildfire is crucial for maintaining these vital socio-ecological resources, the researchers say. That starts with quantifying the spatial footprint of fire exposure by combining geographic data with fire ecology analytics.
"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."
Also collaborating on the study, published in Natural Hazards , were Michelle Day, Raymond Davis and Rachel Houtman of the Forest Service, which provided funding for the research.
