Five years ago, the Labor Day Fires burned more than a million acres across Oregon and Washington.
Since then, researchers at Portland State University have been tracking the post-fire changes to the forests. What they’ve found reveals an evolving landscape, where unburned trees in the fire scars have succumbed to the delayed effects of the fires and will likely continue to die off.
During the 2020 fire event, dozens of fires were burning all at once, driven by unusually high winds. Five of the fires grew into “megafires,” each burning more than 100,000 acres on the west side of the Cascades.
But wildfires don’t burn uniformly across the forest. Wind, geography and forest management play a huge role in how severely the fires burn. And fires often leave behind large islands of living trees, seemingly untouched by the blazes.
These green patches in a sea of brown are important refuges for animals and plants and act as seed banks for the next generation of forests.
For three years, PSU researcher Andrés Holz and his team tracked the fate of these tree islands in the scars of Oregon’s five largest fires from Labor Day 2020: Beachie Creek, Holiday Farm, Archie Creek, Lionshead and Riverside. They found a large number of the green tree patches ended up disappearing. Mature conifer trees were the most likely to die during this time period.
Using high-resolution satellite imagery, the scientists found tree islands smaller than a football field tended to vanish by year three — leaving behind dead trees. And overall, the amount of living forest within the fire scars dropped by 8.5%.
Holz says a big reason for the delayed die-off of trees is that many were injured in the fire, but were able to hold on a few years before finally succumbing to their injuries.
But he thinks there’s another factor as well: increased exposure to the elements.
In a large forest, trees tend to protect each other. Wind can’t penetrate and dry out the soil. The unbroken canopy keeps the forest floor cool. But when a patch of forest is too small, there aren’t enough trees to achieve that critical mass of protection.
The consequences of losing these green islands could be substantial for forests west of the Cascades. The increased loss and fragmentation of forest habitat can hurt birds, rodents and other animals.
In addition, because so many patches of living trees disappeared in the three years following the fires, nearly 20,000 acres of burned forest lost access to those nearby seed banks. That could hamper long-term forest recovery.
The PSU researchers say their results provide information that could help forest managers target specific areas for post-fire tree planting. It also points to areas where salvage logging could be avoided to encourage resilient forests and wildlife habitat.
The research is published in the journal Landscape Ecology.
