Study reveals delayed tree mortality impacts forest recovery; surviving trees die years after 2020 Oregon wildfires, reducing forest regeneration potential by 8.5%: Portland State University

May 28, 2025 (Targeted News Service (Press Releases)) –

PORTLAND, Oregon , May 28 (TNSjou) -- Portland State University issued the following news release:

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Slowly dying trees impact forest recovery post-wildfires, PSU study of 2020 fires finds

By Cristina Rojas

Across the western U.S. , wildfires are becoming larger and more severe -- and even trees that initially survive are dying in subsequent years, making it harder for forests to regenerate, according to new research from Portland State University .

Building on previous research exploring fire refugia -- the green islands of live trees that remain after forest fires -- researchers in PSU's Global Environmental Change lab mapped annual changes in the extent of live tree cover up to three years after the unprecedented 2020 Labor Day fires in Oregon's western Cascades. The study, published in Landscape Ecology, is believed to be the first study of its kind to quantify changes in the spatial distribution and attributes of fire refugia as a result of delayed tree mortality.

The researchers say that if a fire event doesn't immediately kill a tree, there is potential for delayed fire effects to cause trees to die in subsequent years, including direct burn injuries as well as a combination of direct and indirect effects related to climate, insects, pathogens and heatwaves. Under warmer and drier conditions, both immediate and delayed fire effects are expected to proliferate into the future.

"These refugia act like lifeboats, protecting trees and seeds that can help the forest regrow," said Andres Holz , associate professor of geography. "This 'delayed tree death' is changing where these lifeboats are and how effective they can be."

Alec Dyer , who graduated in 2024 with a master's in geography and works as a geospatial data scientist at Leidos , led the study. The researchers found that the amount of living forest decreased by an additional 8.5% in the three years following the fires due to delayed tree death. Though prevalent across all forest types, older, mature conifer trees and those species that are naturally more sensitive to fire were the most likely to die later.

The team mapped where live trees were standing right after the five megafires and then checked those same areas again each year for up to three years following the fires. This allowed them to see how the areas of living trees changed as some fire-damaged trees eventually died.

The analysis revealed a trend of increasing isolation among fire refugia patches over time, potentially hindering seed dispersal. However, fire refugia patches large enough to encompass core areas of deep, unburned forest were resilient to delayed fire effects, providing critical habitats for species dependent on shaded and cool conditions for nesting and foraging. In response to delayed mortality, the area with few or no nearby seed sources for new trees to grow increased dramatically by 375% -- an area of nearly 19,000 acres.

"Understanding that different tree species have different abilities to survive fire and its aftermath can help forest managers develop better plans for managing forests after fires," Dyer said. "This is especially important as fire patterns continue to change, and we need new ways to help our conifer forests stay healthy and resilient."

Co-authors included Sebastian Busby , a former PhD student who now works for the Nature Conservancy ; Cody Evers , a research associate at PSU; and Matthew Reilly and Aaron Zuspan with the U.S. Forest Service .

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Original text here: https://www.pdx.edu/research/news/slowly-dying-trees-impact-forest-recovery-post-wildfires-psu-study-2020-fires-finds