New study sounds alarm, provides hope for western redcedars

First peer-reviewed research links cedar death to climate, details which trees are dying, which are surviving and how the species might be saved

High and dry: Dieback is a condition in which a tree or plant begins to die from the tip of its leaves or roots inward. Photo: Nathan Gilles

In August 2022, science writer Nathan Gilles reported on the plight of western redcedars and other tree species in the face of climate change. In this latest account from the field and labs where researchers are unraveling the causes behind the decline of the region’s most iconic trees, he brings us up to date on the state of Pacific Northwest forests. —Editor

By Nathan Gilles. February 23, 2023. Though no one realized it at the time, the beginning of the end for many Pacific Northwest western redcedar trees began about a decade ago.

Drought had come.

Early on, the cedars did what trees do: they adapted to the dry conditions. They conserved water and grew less.

This strategy had worked during past droughts. This time would be different. Some trees wouldn’t make it.

The drought was relentless. One drought year followed the next. Multiple summers during these drought years would be uncommonly long, warm and dry, especially on the coast.

All of this stressed the region’s western redcedars until it become too much.

For many, growth, already sluggish, stopped entirely and death came.

In 2017 and 2018, Oregon and Washington western redcedars would experience “anomalously high mortality rates,” according to a new study currently under review at the scientific journal Global Change Biology.

“It’s repeated droughts that is causing this tree mortality,” says study lead author Robbie Andrus, postdoctoral researcher at the Washington State University.

To arrive at this conclusion, Andrus and his fellow researchers collected tree ring core samples from both living and dead western redcedars in Oregon and Washington.

They then compared the tree ring data to recent climate data recorded for each location where the rings were collected.

Drought was strongly correlated with slower tree growth (thinner tree rings) and eventually tree death (partial tree rings), according to the study.

The lag of four to five years, says Andrus, suggests it took multiple years of drought-induced stress to eventually kill the trees.

This pattern was so consistent that fully 80% of coastal western redcedars sampled by the researchers died in 2017 and 2018, just four to five years after the trees started slowing their growth in response to drought, according to the study.

By examining climate data from 1975 to 2020, the researchers also found multiple drought years that were followed by wet years.

Tree rings during these periods showed a different pattern from the back-to-back drought years. As before, tree ring growth slowed (got thinner) during the drought years. However, normal growth resumed (got thicker) during the wet years that followed.

“If you have just one year [of drought], the trees can recover if the following year isn’t extremely warm or dry,” says Andrus. “If you have warm and dry conditions that reduces tree growth, and then warm and dry that follow warm and dry conditions, that leads to much slower recovery of tree growth verses having cooler and wetter conditions after warm and dry conditions.”

Diebacks upon diebacks

Andrus and his colleagues’ study is expected to be the first peer-reviewed research on what’s been called the western redcedar dieback, a term that includes dead, dying and struggling western redcedars.

The dieback is believed to be occurring throughout the tree’s current growing range, which runs from northern California to southeast Alaska.

The study’s conclusions echo those of an official government report published last year by the U.S. Forest Service, Oregon Department of Forestry and Washington Department of Natural Resources.

That report concluded that the western redcedar dieback was most likely caused by back-to-back drought years.

However, while “dieback” can refer to a large event affecting multiple trees, the term is also used to describe the behavior of individual trees.

Last stand: A nearly dead western redcedar near Vancouver, Washington. Photo: Nathan Gilles

To adapt to drought, a tree can slow its water loss and reduce its growth.

When this happens, a tree can die back from the tip of its leaves or roots inward to its trunk.

This shows up in western redcedars as dead branches, yellowing and browning leaves and “top kill,” when a tree’s top dies.

But dying back doesn’t work forever. If drought conditions occur for long enough, die-backs become die-offs.

“Not growing as much is a successful strategy to live, but it’s also an indicator that a tree is at risk of dying,” says study coauthor Henry Adams, assistant professor at the School of the Environment at Washington State University.

Both the current study and the government report failed to find a biotic agent that could be responsible for the dieback. This isn’t surprising.

“Tree of Life”

Western redcedars are naturally resistant to insects and fungi.

This resistance, and the tree’s evenly grained wood, has helped make the tree worth more per foot than the Pacific Northwest’s leading timber tree, Douglas-fir.

The tree also holds a special cultural significance.

Many Pacific Northwest peoples consider the tree a gift from the creator. The region’s original inhabitants call the tree, “Long Life Maker,” “Rich Woman Maker” and “Supernatural One,” among other names.

For millennia, the region’s indigenous peoples have used western redcedar to make homes, canoes, fishing gear, weapons, clothing, art and medicine.

The western redcedar is sometimes classified as a giant arborvitae. “Arbor vitae” means “tree of life” in Latin, a name also reportedly used by local indigenous cultures.

Yet despite its cultural and economic importance, not a lot is known about the western redcedar. The tree is largely “understudied,” according to sources interviewed for this story.

What is known is western redcedars are not “true cedars,” like those found in the epic Gilgamesh—which are in the genus Cedrus and members of the Pine family. Rather, the western redcedar (Thuja plicata) is a member of the Cypress family, the same family that includes coast redwoods, giant sequoias and junipers.

Yet while other members of the Cypress family—including junipers and giant sequoias—are legendary for their drought tolerance, western redcedars are not.

The trees have been shown to be highly sensitive to drought. And this makes them especially vulnerable to climate change, which is predicted to lead to more droughts in the Pacific Northwest.

Canary in the forest?

While the current study demonstrates that a recent climate event—drought—has caused the western redcedar dieback, it does not conclude that human-caused, or anthropogenic, climate change is to blame.

This isn’t because the researchers tried but failed to prove a connection between western redcedar mortality and anthropogenic climate change. They simply didn’t look for one. Doing so would have required a larger “attribution” analysis.

“People used to say you can’t tie just one individual event to climate change,” says Adams. “Well, you can. You just have to do your homework. And we haven’t done the homework on that, so to speak.”

[perfectpullquote align=”full” bordertop=”false” cite=”” link=”” color=”” class=”” size=””]“The western redcedar could disappear from large sections of its current growing range.”[/perfectpullquote]

The study also doesn’t rule out climate change as the ultimate cause of the western redcedar dieback either, and neither do its authors.

The study concludes that the recent western redcedar dieback could be an “early warning” that future droughts will lead to more western redcedars deaths.

To nail that point home, the study’s working title is “Canary in the Forest?”

“I think western redcedar has been shown to be more drought-sensitive than a lot of its neighbors. That’s why I put the question mark on ‘Canary in the Forest,’” says Andrus, “because I really don’t have the answer to that question [the dieback’s connection to climate change]. But it does seem like a possibility.”

Adams says it’s too soon to try to attribute the western redcedar dieback to climate change.

However, he says, the current dieback could provide us a glimpse into what climate change is likely to do to the species in the decades ahead.

How climate change will alter forests

“Without being able to back it up, I don’t want say ‘it is climate change that caused this,’” says Adams. “But I will say this is what climate change is going to look like. This is what it is supposed to look like in the Northwest.”

Adams says the drought years examined in the study can be considered as a glimpse into the region’s future.

Consider precipitation.

The Pacific Northwest is known for its rain. But that precipitation is seasonal.

Precipitation tends to fall from October to June. While the dry period normally runs from July to September.

But, as the study notes, the back-to-back drought years associated with the western redcedar dieback included years during which warm and dry conditions started as early as May and June. This lengthened the time the trees were growing without water from the sky by two months.

Climate predictions look similar.

Current and predicted distributions of western redcedar under a high-warming climate change scenario. Red indicates the highest degree of viability. Yellow indicates less viability. Maps by Nicholas Crookston and colleagues

Under future climate change, Pacific Northwest summers are expected to receive less precipitation than they have historically even while the region’s annual precipitation could remain constant or even increase.

Temperatures during the drought years associated with the western redcedar dieback could also be seen as an analog for the future. They include 2015. Still the warmest year on record for Oregon and Washington, temperatures during 2015 were 3.9° F warmer than average for both states. This is roughly similar to future temperatures predicted for the region by the middle of this century, according to past research.

All this has Adams worried about the future of the region’s trees.

“We are actually in a vulnerable place,” says Adams. “We just don’t realize it yet, because of the unique set up: it’s wet here, expect when it isn’t. It’s that period of time when we turn off the faucet from sky, that drizzle, as that gets longer and hotter, then we have a lot of potential, I think, for fast changes in our Northwest forests.”

Tipping point

Adams, who spent the early part of his career in the American Southwest, says he thinks the Pacific Northwest could be at the beginning of a “climate tipping point” that will lead to more tree deaths like the drought-induced die-offs that hit the Southwest starting in the early 2000s.

At the time, he says, plenty of researchers didn’t think the deaths of juniper, piñon and ponderosa pines would continue. Fast forward 20 years and the region’s die-offs are not only not going away, they are expected to continue for decades as the climate continues changing.

“The hardest part about recognizing a tipping point is when you haven’t gone over it yet,” says Adams. “The best I can say is this could be what the beginning of what a tipping point looks like for this species [western redcedar] and Northwest forests. And that’s what we’re arguing in the paper that this could be the beginning of something different.”

That something different is the possibility that more diebacks could mean the western redcedar could disappear from large sections of its current growing range.

This possibility was also hinted at in last year’s government report, which speculates, “We could be seeing the range of [western redcedar] shift or shrink due to changing conditions that reduce the suitability of some sites for this species.”

Young not old trees are dying

Of course, some western redcedars didn’t die in the recent drought.

And here, too, the study has a very interesting finding: young trees died, old trees survived.

Of the trees sampled by the researchers only trees less than 150 years of age died. Trees older than 150 years of age lived.

Golden oldie: Healthy western redcedar. Photo: Nathan Gilles

Study coauthor Aaron Ramirez, assistant professor in Biology and Environmental Studies at Reed College, says he wasn’t surprised that younger trees are dying.

Ramirez says he’s observed the same phenomenon in separate research he conducted on western redcedar dieback occurring in the Willamette Valley.

“In that [Willamette Valley] study we did see a similar result that dieback was concentrated in areas where it was mostly younger western redcedar trees,” says Ramirez.

Ramirez says one reason that could help explain the dieback’s prevalence in young trees is the possibility that older trees have already taken all the best habitat and many younger trees are now growing on marginal “drier microsites.”

He says he observed this pattern in the Willamette Valley where he found young western redcedars growing alongside drought-hardy Oregon white oaks.

Ramirez says it’s also possible older trees might be more resilient to droughts because they can access water deeper in the soil profile that younger trees simply can’t reach.

Professor of Forest Biology at the University of Washington Dave Peterson, who was not directly involved in the study, says he was impressed with Andrus and colleagues’ research.

“This is the best analysis of western redcedar dieback that I’ve seen,” says Peterson. “It was a well-conceived study design. I think that’s the strength of this project.”

Peterson says his own research into western redcedar dieback also suggests older trees are living while younger trees are dying. But, he says, why this might be isn’t yet known.

“They [older trees] are the winners in the grand competitions of life. And so, they have probably a better location to grow and maybe they have some genetic advantages as well,” says Peterson. “That’s always the big mystery. We don’t know anything about the genetics of what is going on out there, but it’s probably a pretty important issue.”

Peterson, who has been a leading researcher of Pacific Northwest forests and their vulnerability to climate change, says he thinks it’s still too soon to tell if climate change is causing the current western redcedar dieback, adding that climate attribution is “a challenging issue.”

The problem, says Peterson, is right now researchers only have data from one dieback, and this isn’t enough to find a trend.

“I think in this case what we really need is a longer time series,” says Peterson. “If we start to get additional episodes of dieback, then we have more evidence.”

Still, Peterson says while it’s too soon to attribute the current western redcedar dieback to climate change, diebacks are likely to continue as the region’s climate continues to change.

“We can’t say anything right now about [climate] attribution, but it’s [more diebacks] the kind of thing we’ll likely see more of,” says Peterson.

Tale of two distributions

Age wasn’t the only factor that made trees more vulnerable to death. Location also mattered, according to the study.

While most western redcedars in Oregon and Washington can be found on the wet west side of the Cascade Range rain shadow, the trees can also be found in drier eastern Washington, and even Idaho and Montana, where the Rocky Mountains take a sharp turn westward on their journey into Canada.

The climate experienced by this interior population is significantly drier than the climate west of the Cascades. Its summers are also warmer and its winters are colder as well.

Yet according to Andrus and colleagues’ analysis, these interior trees did far better during the drought than trees west of the Cascades.

According to study coauthor Melissa Fischer, these drier conditions might have even given interior western redcedars an advantage.

“I was a little surprised by the results,” says Fischer, forest entomologist at the U.S. Forest Service for the western regions of Oregon and Washington.

Fischer, who formerly worked for the Washington Department of Natural Resources, was a lead coauthor and researcher on last year’s official government report on the western redcedar dieback. She says her role in the current study was limited.

Fischer expected the interior trees would “blink out before the coastal population” because summers in the east are so much warmer and drier than conditions on the west.

The opposite ended up being true.

Interior trees died in fewer numbers than did trees on the coast, which in the study included everything west of the Cascades, including the Willamette Valley and Puget Sound areas.

“It makes sense when you think about it,” says Fischer, “because the interior population must be pretty adapted to drier conditions and perhaps they’re throwing roots down deeper in the ground.

“Where over here on the west side, the trees are used to having water near the surface and they’re probably shallow rooted and as soon as that water table goes down, then everything that’s above water is not getting enough.”

Well fed: Western redcedar doing what it should. Photo: Nathan Gilles

Fischer says although the two distributions of western redcedars are very different, they are nonetheless the same species, and this means drought tolerant interior trees could be bred with drought intolerant coastal trees. This, she says, could help the species adapt to climate change.

The very last sentence of Andrus and colleagues’ study also hints at this possibility, as well.

“There is a possibility that we could bring in a seed stock or somehow breed some of the genes [from the interior population] into the westside population or plant seed stock from the interior to allow cedar to persist in an area [west of the Cascades] where they are dying off,” says Andrus.

Andrus says doing this right will require an intensive research effort.

Currently, efforts are underway in British Columbia to breed interior western redcedars with coastal western redcedars. This project, however, has just begun and is expected to take years to produce results. 

No similar efforts in the United States are currently underway, according to sources interviewed for this story.

But, Peterson says, the study points at another possibility as well: that there’s still suitable habitat for the trees where they already are. After all, not every tree died.

These sites could include wet areas in valleys and northern-facing slopes and other places that provide plenty of water and limit the trees’ exposure to extreme heat.

He says this should give people hope for the future of the species in the Pacific Northwest.

“One thing I’ve been hearing a lot of in the last couple of years is people giving up on western redcedar, ‘oh, I’m not going to plant that because it’s going to die in climate change,’” says Peterson. “Well, you just have to be careful about where you plant it. We will have to be even more thoughtful in the future with tree planting in order to ensure that trees are going to be in resilient places on the landscape.”