Douglas fir is the most abundant tree species in the Pacific Northwest. It’s Oregon’s official state tree. And it’s arguably the region’s most important timber species.
But like true firs, western redcedars and big leaf maples, Douglas firs have been experiencing a dieback in recent years due in part to drought.
Last year, researchers at the U.S. Forest Service recorded Douglas fir (Pseudotsuga menziesii) trees dying on approximately 463,900 acres (over 720 square miles) of land in Oregon and Washington.
This is the largest Douglas fir dieback recorded since 1989-1990.
Yet while the Douglas fir dieback occurred throughout the two states, most of this dieback hasn’t been severe. The exception is occurring in southwestern Oregon.
The region has seen “elevated levels of Douglas fir mortality” with as many as 30-50% of Douglas fir killed at some locations, according to the USFS.
A recent study from Oregon State University and the USFS helps explains why southwestern Oregon has seen so much Douglas fir mortality.
It also provides a glimpse into how future climate change could lead to a still larger regional dieback.
Published in March in the Journal of Forestry, the study suggests a combination of drought and insect attacks have been killing southwestern Oregon’s most valuable timber tree.
But the study does more than just assign likely causes of death. It also systemically links those causes together in time.
Trees under attack
Understanding how multiple causes interact with each other to weaken and kill a tree is essential to understanding the current dieback in southwestern Oregon, according to study coauthor David Shaw, professor and forest health specialist at Oregon State University’s College of Forestry.
“We know in some cases [of tree mortality] that an insect or disease can kill a tree whether the tree is healthy or not,” says Shaw. “But in most cases of tree mortality, it’s more complicated than one simple factor. You might find a beetle on a tree, but that might not be the whole reason. And that is what we found.”
The study determined that a wood-boring beetle called the flatheaded fir borer (Phaenops drummondi) was a likely culprit behind southwestern Oregon’s Douglas fir dieback.
From 2015 to 2020, more than 250,000 Douglas firs died in the region due to the beetle, more than the previous four decades combined, according to the study.
But, the study notes, the beetles could only kill trees that were already weakened by the region’s intense drought.
Since 2015, southwestern Oregon has regularly experienced long periods of above-average temperatures coupled with below-average precipitation levels.
These climatic conditions, says Shaw, have stressed Douglas firs, making the trees vulnerable to the fir borer.
And this is exactly what the study found. Tree death and beetle infestation were linked to multiple measurements of drought, including low precipitation levels and above normal temperatures.
But the study did more than link drought and beetles, it also linked a third factor to both: location.
Location, dieback, ‘decline disease spiral’
The largest number of dead Douglas firs were found by the researchers at sites that are between 400 to 1,000 meters (1,312–4,593 feet) in elevation.
As the study notes, these low-to-mid elevations areas contain “hot, dry sites” that are “climatically marginal for the species,” existing at the lower range of what Douglas firs can tolerate in southwestern Oregon.
This makes the trees’ location a “predisposing factor,” according to the study.
“If a tree is unlucky enough to grow there [at a hot, dry low-to-mid elevation site], this would predispose the tree to some impacts that other trees may not experience,” says Shaw. “When you combine that with the drought we are having—then these biotic agents can really pile on, pushing the trees over the edge.”
The study employed what’s called the Mannion “decline disease spiral.”
Developed by forestry scientist Paul Mannion, the decline spiral is a way to conceptualize tree death due to multiple factors occurring one after the other.
According to the study, southwestern Oregon’s Douglas fir dieback looks like a classic decline spiral, with location making the trees more vulnerable to drought—drought making the trees more vulnerable to beetle infestation and beetle infestation leading to death.
The reason Douglas fir is growing on these “climatically marginal” sites is a story in itself.
How Doug fir became dominant
The study focuses on the Oregon side of the Klamath Mountains Ecoregion, an 18,452-square-mile area that traces the path of the Klamath and Siskiyou mountains from California’s Sacramento Valley in the south to Oregon’s Willamette Valley in the north.
The Douglas fir dieback has also been reported across the border in California. The study does not include numbers on the extent of the dieback there.
The Klamath Mountains Ecoregion is one of the most botanically diverse areas on the West Coast, containing 29 different conifer species alone.
Amid this diversity, the most abundant tree in the region is Douglas fir.
However, in those areas where the dieback has been greatest, the presence of Douglas fir in large numbers is most likely a recent phenomenon, according to the study.
Using data from a 1936 survey to reconstruct the region’s forests of the past, the study concludes that Douglas firs only recently moved into the Klamath Mountains’ low-to-mid elevation sites in large numbers.
In total, 50% of the areas containing Douglas fir dieback and 47% of trees killed were found on lands where drought-tolerant Ponderosa pine and oaks once dominated, according to the study.
In fact, Douglas fir dieback was strongly correlated with the presence of one drought-tolerant species in particular: Oregon white oak (Quercus garryana).
The explanation has to do with the oak’s ability to tolerate especially dry sites with poor soils.
Oregon white oak is also a fire-tolerant species. For thousands of years, Native American peoples have used fire to cultivate the oak—and the benefits it brings, including acorns and habitat for game animals—by burning the saplings of competing species, especially Douglas fir.
“There is pretty good evidence that fires were frequent, and with suppression and exclusion, there has been a huge change in the landscape. That’s a story that’s pretty well accepted,” says study lead author, Max Bennett, extension forester and associate professor at Oregon State University.
The study notes that a combination of fire suppression and the exclusion of the traditional use of fire by Native Americans probably allowed Douglas firs to colonize large sections of the Klamath Mountains’ low-to-mid elevation regions, regions that provide marginal habitat for the species, especially in drought years.
Though, says Bennett, it’s not a simple story of Douglas fir merely moving onto land where it shouldn’t grow.
“I think some people hear this and they think ‘oh Douglas fir is an invader’ or something. It’s a little bit more complicated than that,” says Bennett.
Bennett says Douglas fir probably did exist historically at the low-to-mid elevation sites where dieback is now occurring. But, he says, the historical record suggests they did not exist in as large of numbers or in as dense of stands as they do now.
Complicating the story is the fact that not all Douglas firs at lower elevations have died.
“And that is intriguing,” says Bennett. “Why are these individual trees still hanging in there? We have all sorts of hypotheses, but I don’t think we really know.”
Climate change and Douglas fir
Bennett says it would be wrong to dismiss the current dieback as nature merely correcting itself.
As a counter-example, he points to the recent drought that led to the dieback, which includes some of the warmest years on record for Oregon.
“We’ve had drought, we’ve had precipitation deficits in southwest Oregon in the past several years,” says Bennett. “But if you look at the historic record, it’s really nothing extraordinary. There was a long-term drought in the late ’80s. There was a pretty sharp drought in the early 2000s. And you can go back before these. What is really notable [about the recent drought] is the increase in temperatures.
Bennett says the recent drought looks less like the droughts of the past and more like the “hot droughts” predicted under future climate change.
For this reason, he says, the Douglas fir dieback happening at the lower elevations could be seen as a glimpse into what the future might hold for the species at higher elevations.
This is more than idle speculation. While the study did not try to link the current Douglas fir dieback to human-caused climate change, it does include future climate change predictions.
According to these predictions, by the 2050s the same warm and dry conditions that contributed to the current Douglas fir dieback will become far more common and widespread in southwestern Oregon.
If this happens, lands that are climatically marginal for Douglas fir are likely to move up in elevation, encroaching on areas that are now ideal for the trees to grow.
“I guess we could all debate if [the recent drought] is climate change or not,” says Shaw. “But it’s consistent with predictions for climate change. Looking into the future we’re afraid that Doug fir will continue to die at these lower elevations and then [the dieback] will start creeping up into the higher areas. That’s what we’re concerned about. That this will continue to intensify.”
