Eastern Oregon's Malheur National Forest boasts some of the state's oldest trees, including pine and larch that live more than 500 years. But many of those ancient trees are dying at an alarming rate, a new analysis shows.
Between 2012 and 2023, a quarter of trees more than 300 years old in randomly located sites in roadless areas died, the study found. A triple whammy of drought, bug infestations and competition with younger trees is likely driving the decline.
"It's sad to see so many old trees dying," said lead researcher James Johnston , an assistant research professor in the University of Oregon's Institute for Resilient Organizations, Communities, and Environments.
"In the moistest and most productive Douglas-fir-dominated forests in Western Oregon, setting forests aside as protected reserves has proven to be a successful strategy for protecting old trees," he said. "But this research shows that we need active management to remove younger trees in order to protect old trees in dry forests of Eastern and Southern Oregon."
Johnston's team published its findings July 8 in the journal Forest Ecology and Management.
The study period stretches back to when Johnston, a fire ecologist, was working on his doctoral dissertation. His research back then involved taking core samples from old-growth trees in unlogged, roadless sections of the Malheur National Forest where trees were left undisturbed.
Old-growth trees, such as the older pines, firs and junipers of Malheur, store away carbon and provide critical habitat. Their extensive root systems also act as filters as water moves through the soil, which helps maintain water quality and supply.
A decade after his initial work, Johnston came back to the area to see what trees were still alive. He and colleagues located 1,617 trees from the previous project, the ages of which spanned a few decades to more than 600 years old. A third of trees beween 150 and 300 years old and a quarter of trees greater than 300 years old had died within the last 10 years.
"We're talking about trees that easily live to be 500 years old, and a quarter of them died in just 10 years. That's really alarming," Johnston said.
The team then looked for causes of mortality. One trend that emerged was that trees whose growth had slowed down in the last five years or so were more susceptible to death. Older trees in areas that hadn't burned in a wildfire in the last 130 years also were at greater risk.
That's because wildfire helps thin out the forest, removing younger trees that compete for water and nutrients. Competition is one of the major driving forces behind the Malheur's losses, Johnston said.
In protected areas of the forest — such as the unlogged, roadless areas he sampled — it's generally illegal to cut down trees. That means there's an overgrowth of younger trees, compared to unprotected areas where land managers thin the forest.
Competition for resources overlaps with another issue: drought. Although Eastern Oregon's forests are considered dry forests, meaning they've adapted to seasonal dry spells, they still need water to survive. And the area is significantly drier than it once was.
To top it all off, several species of defoliating insects, such as western spruce budworm and Douglas-fir tussock moth, chowed down on the trees during the study period.
Trends toward increased fire and insect invasions are likely to continue, Johnston said. When the team ran statistical models based on those trends, they found that younger trees can't keep up with the death rates of their elders.
The models predict less than a quarter of old-growth trees will remain in the next 50 to 60 years. In part, that's simply due to the time it takes a tree to grow.
"You can't replace a 300-year-old tree in 10 years or 20 years," Johnston said.
Despite the disturbing news for the Malheur's old-growth trees, there's still a way forward. Reducing competition between trees is the most promising way to preserve the old-growth, Johnston said. In areas where trees can be thinned regularly, Johnston expects a more positive outlook. But data in those forests are limited, because old-growth trees far outlive the lifespan of a single researcher.
"We need significantly more research about the fate of old trees and dry forests," Johnston said. "These are systems for which we don't have nearly enough data."
— By Jude Coleman
