Landscapes of Oregon’s Western Cascades have historically burned more often than previously thought

Forests on the western slope of Oregon’s Cascade Range experienced much more frequent fires between 1500 and 1895 than previously thought, according to new research by University of California scientists. State of Oregon.

The results provide important insights into how landscapes might adapt to climate change and future fire regimes, the authors say.

James Johnston of the OSU College of Forestry led the study, which was published in Ecosphere.

“Wildfires are a fundamental process of the forest ecosystem,” he said. “With temperatures rising and burned areas increasing, we need to know as much as possible about the long-term variability of fires.”

Johnson and colleagues from Oregon State, the University of Oregon, and the U.S. Forest Service collected tree ring data at 16 sites in the southern portion of the Forest National Forest. Willamette, near Oakridge.

Trees form scars after cambial cells are killed by the heat of wildfires, he said. These scars are partially or completely covered by new tissue as the tree grows, and the tree rings tell the story of fire exposure.

Using chain saws, scientists collected samples from 311 dead trees: logs, short snags and stumps. Seventy-three percent of the samples were Douglas fir and 13% were ponderosa pine. The remainder consisted of sugar pine, noble fir, red fir, incense cedar, western redcedar, mountain hemlock, and western hemlock.

“We cross-referenced a total of 147,588 tree rings and identified 672 cambial injuries, 479 of which were fire scars,” Johnston said. “The scars allowed us to reconstruct 130 different years of fires that occurred at one or more of the 16 sites before a federal fire suppression policy took effect in the early 20th century.”

The main points to remember:

• Fires were historically much more common in western Oregon’s Cascade landscapes than previously thought.
• Indigenous peoples likely used fire to manage large areas to harvest resources and likely altered landscapes and fire regimes significantly.
• There are significant opportunities to restore fire-adapted systems in western Oregon today.

“Additionally, our study produced little evidence of the type of large wind-driven fires that in 2020 burned 50,000 to 75,000 hectares in watersheds immediately north and south of our study area” , Johnston said. “Only 39% of fire years were recorded at more than one site, only 11% were recorded at more than two sites, and only 3% were recorded at more than three sites – in a 37,000-acre study area , strongly suggesting that most fire years were recorded at more than one site.The fires were relatively small.

Across all 16 sites, the average fire return interval (the duration between fires) was as short as six years and as long as 165 years. In general, differences between these means were strongly associated with vapor pressure deficit or VPD, essentially drying. power of the atmosphere. The higher the VPD, the shorter the time between fires.

However, historical fires in successive stands of Douglas fir (stands that, if left alone, would eventually have Douglas fir as the dominant tree species) were much less strongly linked to dry air.

“We interpret the extraordinary pace of fires in these stands and the climate pattern associated with the fires there as indicative of indigenous fire management,” Johnston said. “We observed some of the most frequent fire return intervals ever documented in the Pacific Northwest, but the enormous volume of biomass accumulated by these rainforests over time is often attributed in part to the long intervals between fires. forest fires.”

The authors note that humans have occupied the southern portion of what is now the Willamette National Forest for at least 10,000 years. Various indigenous cultures, including the Molalla, Kalapuya, Tenino, Wasco, Klamath, Northern Paiute, and Cayuse, likely used the area for trading, hunting, and plant gathering.

“The removals happened very quickly, with most of the natives being taken to the Grand Ronde, Warm Springs and Klamath reservations,” said co-author David Lewis, a member of the Grand Ronde tribe and assistant professor of anthropology and in Native Studies at the College of OSU. Liberal arts. “The removal of tribes removed their practices of cultural stewardship, their use of annual cultural fires, from the land, radically changing the way forests were managed.”

By 1856, most of the remaining members of the Willamette Valley and western Oregon Cascades tribes had been forcibly relocated to reservations. Extensive clearcutting in the Willamette National Forest began in the late 1940s and continued for four decades.

“Now Forest Service managers want accurate information on forest vegetation and historical disturbance dynamics to manage lands in ways that promote resilience to climate change,” Johnston said.