Though no one realized it at the time, the beginning of the end for many Northwest Western red cedar 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鈥檛 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鈥檚 Western red cedars until it become too much.
For many, growth, already sluggish, stopped entirely and death came.
In 2017 and 2018, Oregon and Washington Western red cedars would experience 鈥渁nomalously high mortality rates,鈥� according to a currently under review at the scientific journal Global Change Biology.
鈥淚t鈥檚 repeated droughts that is causing this tree mortality,鈥� 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 red cedars 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 red cedars 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.
鈥淚f you have just one year [of drought], the trees can recover if the following year isn鈥檛 extremely warm or dry,鈥� says Andrus. 鈥淚f 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.鈥�
Diebacks
Andrus and his colleagues鈥� study is expected to be the first peer-reviewed research on what鈥檚 been called the Western red cedar dieback, a term that includes dead, dying and struggling Western red cedars.
The dieback is believed to be occurring throughout the tree鈥檚 current growing range, which runs from northern California to southeast Alaska.
The study鈥檚 conclusions echo those of an official government by the U.S. Forest Service, Oregon Department of Forestry and Washington Department of Natural Resources.
That report concluded that the was most likely caused by back-to-back drought years.
However, while 鈥渄ieback鈥� can refer to a large event affecting multiple trees, the term is also used to describe the behavior of individual trees.
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 red cedars as dead branches, yellowing and browning leaves and 鈥渢op kill,鈥� when a tree鈥檚 top dies.
But dying back doesn鈥檛 work forever. If drought conditions occur for long enough, diebacks become dieoffs.
鈥淣ot growing as much is a successful strategy to live, but it鈥檚 also an indicator that a tree is at risk of dying,鈥� 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鈥檛 surprising.
鈥楾ree of Life鈥�
Western red cedars are naturally resistant to insects and fungi.
This resistance, and the tree鈥檚 evenly grained wood, has helped make the tree worth more per foot than the Pacific Northwest鈥檚 leading timber tree, Douglas-fir.
The tree also holds a special cultural significance.
Many Northwest peoples consider the tree a gift from the creator. The region鈥檚 original inhabitants call the tree, 鈥淟ong Life Maker,鈥� 鈥淩ich Woman Maker鈥� and 鈥淪upernatural One,鈥� among other names.
For millennia, the region鈥檚 indigenous peoples have used Western red cedar to make homes, canoes, fishing gear, weapons, clothing, art and medicine.
The Western red cedar is sometimes classified as a giant arborvitae. 鈥淎rbor vitae鈥� means 鈥溾�� 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 red cedar. The tree is largely 鈥渦nderstudied,鈥� according to sources interviewed for this story.
What is known is Western red cedars are not 鈥渢rue cedars,鈥� like those found in the epic Gilgamesh 鈥� which are in the genus Cedrus and members of the Pine family. Rather, the Western red cedar (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鈥攊ncluding junipers and giant sequoias鈥攁re legendary for their drought tolerance, Western red cedars 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 鈥� drought 鈥� has caused the Western red cedar dieback, it does not conclude that human-caused, or anthropogenic, climate change is to blame.
This isn鈥檛 because the researchers tried but failed to prove a connection between Western red cedar mortality and anthropogenic climate change. They simply didn鈥檛 look for one. Doing so would have required a larger 鈥渁ttribution鈥� analysis.
鈥淧eople used to say you can鈥檛 tie just one individual event to climate change,鈥� says Adams. 鈥淲ell, you can. You just have to do your homework. And we haven鈥檛 done the homework on that, so to speak.鈥�
The study also doesn鈥檛 rule out climate change as the ultimate cause of the Western red cedar dieback either, and neither do its authors.
The study concludes that the recent Western red cedar dieback could be an 鈥渆arly warning鈥� that future droughts will lead to more Western red cedars deaths.
To nail that point home, the study鈥檚 working title is 鈥淐anary in the Forest?鈥�
鈥淚 think Western red cedar has been shown to be more drought-sensitive than a lot of its neighbors. That鈥檚 why I put the question mark on 鈥楥anary in the Forest,鈥欌�� says Andrus, 鈥渂ecause I really don鈥檛 have the answer to that question [the dieback鈥檚 connection to climate change]. But it does seem like a possibility.鈥�
Adams says it鈥檚 too soon to try to attribute the Western red cedar 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.
Climate change impact
鈥淲ithout being able to back it up, I don鈥檛 want say 鈥榠t is climate change that caused this,鈥欌�� says Adams. 鈥淏ut 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.鈥�
Adams says the drought years examined in the study can be considered as a glimpse into the region鈥檚 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 red cedar 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.
Temperatures during the drought years associated with the Western red cedar 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掳 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鈥檚 trees.
鈥淲e are actually in a vulnerable place,鈥� says Adams. 鈥淲e just don鈥檛 realize it yet, because of the unique set up: it鈥檚 wet here, expect when it isn鈥檛. It鈥檚 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.鈥�
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 鈥渃limate tipping point鈥� 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鈥檛 think the deaths of juniper, pi帽on and ponderosa pines would continue. Fast forward 20 years and the region鈥檚 die-offs are not only not going away, they are expected to continue for decades as the climate continues changing.
鈥淭he hardest part about recognizing a tipping point is when you haven鈥檛 gone over it yet,鈥� says Adams. 鈥淭he best I can say is this could be what the beginning of what a tipping point looks like for this species (Western red cedar) and Northwest forests. And that鈥檚 what we鈥檙e arguing in the paper that this could be the beginning of something different.鈥�
That something different is the possibility that more diebacks could mean the Western red cedar could disappear from large sections of its current growing range.
This possibility was also hinted at in last year鈥檚 , which speculates, 鈥淲e could be seeing the range of (Western red cedar) shift or shrink due to changing conditions that reduce the suitability of some sites for this species.鈥�
Young, not old trees dying
Of course, some Western red cedars didn鈥檛 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.
Study coauthor Aaron Ramirez, assistant professor in Biology and Environmental Studies at Reed College, says he wasn鈥檛 surprised that younger trees are dying.
Ramirez says he鈥檚 observed the same phenomenon in separate research he conducted on Western red cedar dieback occurring in the Willamette Valley.
鈥淚n that (Willamette Valley) study we did see a similar result that dieback was concentrated in areas where it was mostly younger Western red cedar trees,鈥� says Ramirez.
Ramirez says one reason that could help explain the dieback鈥檚 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 鈥渄rier microsites.鈥�
He says he observed this pattern in the Willamette Valley where he found young Western red cedars growing alongside drought-hardy Oregon white oaks.
Ramirez says it鈥檚 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鈥檛 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鈥� research.
鈥淭his is the best analysis of Western red cedar dieback that I鈥檝e seen,鈥� says Peterson. 鈥淚t was a well-conceived study design. I think that鈥檚 the strength of this project.鈥�
Peterson says his own research into Western red cedar dieback also suggests older trees are living while younger trees are dying. But, he says, why this might be isn鈥檛 yet known.
鈥淭hey (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,鈥� says Peterson. 鈥淭hat鈥檚 always the big mystery. We don鈥檛 know anything about the genetics of what is going on out there, but it鈥檚 probably a pretty important issue.鈥�
Peterson, who has been a leading researcher of Northwest forests and their vulnerability to climate change, says he thinks it鈥檚 still too soon to tell if climate change is causing the current Western red cedar dieback, adding that climate attribution is 鈥渁 challenging issue.鈥�
The problem, says Peterson, is right now researchers only have data from one dieback, and this isn鈥檛 enough to find a trend.
鈥淚 think in this case what we really need is a longer time series,鈥� says Peterson. 鈥淚f we start to get additional episodes of dieback, then we have more evidence.鈥�
Still, Peterson says while it鈥檚 too soon to attribute the current Western red cedar dieback to climate change, diebacks are likely to continue as the region鈥檚 climate continues to change.
鈥淲e can鈥檛 say anything right now about (climate) attribution, but it鈥檚 (more diebacks) the kind of thing we鈥檒l likely see more of,鈥� says Peterson.
Tale of two distributions
Age wasn鈥檛 the only factor that made trees more vulnerable to death. Location also mattered, according to the study.
While most Western red cedars 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鈥� 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 red cedars an advantage.
鈥淚 was a little surprised by the results,鈥� 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鈥檚 official government report on the Western red cedar dieback. She says her role in the current study was limited.
Fischer expected the interior trees would 鈥渂link out before the coastal population鈥� 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.
鈥淚t makes sense when you think about it,鈥� says Fischer, 鈥渂ecause the interior population must be pretty adapted to drier conditions and perhaps they鈥檙e throwing roots down deeper in the ground.
鈥淲here over here on the west side, the trees are used to having water near the surface and they鈥檙e probably shallow rooted and as soon as that water table goes down, then everything that鈥檚 above water is not getting enough.鈥�
Fischer says although the two distributions of Western red cedars 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鈥� study also hints at this possibility, as well.
鈥淭here 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,鈥� says Andrus.
Andrus says doing this right will require an intensive research effort.
Currently, efforts are underway in British Columbia to breed interior Western red cedars with coastal Western red cedars. 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鈥檚 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鈥� exposure to extreme heat.
He says this should give people hope for the future of the species in the Northwest.
鈥淥ne thing I鈥檝e been hearing a lot of in the last couple of years is people giving up on Western red cedar, 鈥榦h, I鈥檓 not going to plant that because it鈥檚 going to die in climate change,鈥欌�� says Peterson. 鈥淲ell, 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.鈥�
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