‘Assisted migration’ has come to the Pacific Northwest, but experts don’t agree if it’s a good thing or a radical response to a warming world
In August, science writer Nathan Gilles reported on the dieback of western redcedars. But that icon of the Pacific Northwest isn’t the only tree species imperiled by climate change. In Part II of his investigation, Gilles reports on efforts underway to help save tree species from a warming climate by manipulating their growing ranges. The practice is both innovative and controversial. —Editor
By Nathan Gilles. September 15, 2022. U.S. Forest Service Climate Adaptation Specialist Andrew Bower can’t find his trees.
From the small town of Packwood, Wash., the site of Bower’s experiment in the future of forestry under an ever-warming climate is about a 45-minute drive, much of it along bumpy dirt roads that wind, dip and climb into the Gifford Pinchot National Forest.
“The challenge with this particular site is all the debris,” says Bower, a trained geneticist who bears a passing resemblance to actor Tim Blake Nelson (best known for his work in O Brother, Where Art Thou?).
In blue jeans, brown boots, forest green USFS T-shirt and yellow hard hat, Bower looks every bit the forester as he squints in an effort to find his research subjects: small spots of green, 6-to-8-inch-tall Douglas-fir seedlings grown from seeds that came from the Willamette National Forest and Siuslaw National Forest, both in Oregon and both nearly 200 miles further south and several degrees warmer than their current location.
In front of Bower is a stark reminder that the U.S. Forest Service still occasionally cuts down trees.
The 16-acre site is covered in the now supine remains of what was once a standing forest.
Logs and other woody debris—bleached bone-white by the sun—are surrounded by spots of green that Bower isn’t looking for: volunteer western hemlock saplings clinging to the base of a wide stump, knee-high vine maples, cottonwoods, bracken ferns, thistles and various grasses peak through holes in the debris where the gray, pebbly volcanic ash from the 1980 Mount St. Helens’ eruption still covers the ground.
Here and there stand full-grown Douglas-firs—about 80 feet in height and marked with orange spray paint, signaling that they should not be felled.
Also spared are trees the timber company that harvested the land didn’t want, including black cottonwoods as tall as the firs, and a much-shorter lone Yew.
And somewhere scattered among the living and dead organic matter are Bower’s baby Doug-firs, his climate migrants, his trees of the future.
This is an experiment in a radical new idea called “assisted migration” and a recognition by the Forest Service that climate change is here to stay.
What is assisted migration?
Assisted migration is the idea that because the climate is steadily warming, trees can—and, some argue, should—be moved from where they currently grow to where they are predicted to grow in the future.
This, the idea goes, will introduce trees better adapted to tolerate climate-induced warming and droughts.
While decades of research into assisted migration have provided proof of concept, these past experiments have tended to be small and far more controlled.
Unlike the neat, tightly packed rows of little trees grown in highly controlled conditions, Bower’s seedlings are spaced 17 feet part in exactly the sort of chaos foresters face when replanting after a disturbance, such as a harvest or a wildfire. (Bower has another experimental site even further north, in the Okanogan-Wenatchee National Forest where he’s planting ponderosa pines in a section of the forest that burned during the 2014 Carlton Complex fire.)
Testing in disorderly real-world conditions is the point.
Bower’s experiment is one of the first of many planned in the coming years by the Forest Service to see if the agency can put years of scientific research into assisted migration into operational practice.
After about five minutes of searching, Bower walks past the remains of a felled western redcedar—another species imperiled by climate change—and finds his first Doug-fir seedling.
Distance as time
“It’s a little yellow, but it’s still alive,” says Bower of the seedling.
This is to be expected. The small trees are just putting down their roots and are vulnerable.
Among the Oregon migrants are seedlings grown from seeds collected locally, both near this site and further downslope at a lower elevation.
In the years and decades to come, Bower expects to see clear differences between the locally sourced trees and the migrated trees.
“One scenario is that the local seed lots do better to begin with because they are adapted,” says Bower. “And maybe the seedlings from the warmest [climes] suffer from frost damage now, but over time as things warm up we start to see a shift where the productivity of the local seed lot decreases and those seedlings that are more adapted to warmer temperature do better.”
If the climate predictions the Forest Service is using prove accurate, by the middle decades of this century (2041-2070), the Willamette National Forest seedlings should be perfectly adapted to their current location. While by the end of this century (2071-2100), the Siuslaw National Forest seedlings will find their current location matches their evolved needs.
The lower elevation seedlings should be adapted in matter of years.
The local seedlings are being used as a control.
To understand how this works, imagine geographic distance as a proxy for climatic time.
Trees, and plants in general, have growing ranges, namely how far they can grow south to north and west to east, but also how high or low in elevation they can grow.
Ranges are determined largely by climate, namely what temperatures and levels of precipitation a tree evolved to grow in and tolerate.
Because global temperatures are predicted to rise steadily throughout this century, tree growing ranges in the northern hemisphere are tending to move north and upslope in elevation.
This means, the further south you go—or further downslope in some cases—the further into the climatic future you travel.
However, this isn’t a hard and fast rule. The seed collection site in the Siuslaw National Forest is actually less southerly (by about 20 miles) than the Willamette National Forest site. But it’s significantly closer to the coast, and so is warmer and further into the climate-changed future.
Climatic mismatch and extinction
As the climate warms, many plants, trees among them, are expected to become mismatched with their environments as the climate they experience fails to match the climate they evolved for. (Many animals are expected to experience a similar fate.)
In fact, researchers have confirmed that plant ranges have already moved due to climate change that’s already happened. For this reason, Bower’s local seedlings are already not perfectly adapted to the climate of today.
And those spared full-grown Doug-firs are adapted to the climate of 30 to 60 years ago. By middle and late decades of this century, the local trees are likely to be thoroughly out of synch with their environment.
Bower’s trees aren’t alone.
Earlier this year, researchers from the Forest Service and states of Oregon and Washington published the results of a multi-year study concluding that the Pacific Northwest’s iconic western redcedars are dying due to climate change.
The study concluded the species’ range could already be shifting north and upslope because the trees were in effect mismatched with their climate.
And redcedars aren’t alone.
Consider a recent informal survey done in conjunction with western Washington-based Forest Adaptation Network. The survey queried individuals doing ecological restoration in the Puget Sound area for local governments.
While small—just 22 persons responded—all respondents reported having observed “plant failures” due to a changing climate.
Further south, California’s beloved coast redwoods could lose the southern half of their range as early as 2030, according to scientists. California’s giant sequoias and Joshua trees are also threatened by climate change.
The list is likely to grow.
Keeping up with climate change
The problem is plants aren’t moving fast enough.
Plants migrate through reproduction, colonizing a landscape one generation at a time.
This is a slow process, but climate change isn’t. Plants, according to multiple studies, simply cannot migrate fast enough to keep up with the rate of change.
Trees—being long-lived perennials that often take years if not decades to get to productive age—are expected to be especially vulnerable to climate change.
Assisted migration essentially lends a helping human hand, moving trees fast and far enough to keep up with the rate of climate change.
However, assisted migration is not without risk.
Move trees too soon and they also risk environmental mismatch, or “maladaptation.”
Move trees from warm locations to cold ones and the cold can sometimes do them in.
Move trees from dry climates to wet ones and novel fungi will end their lives.
Critiques of assisted migration
Assisted migration, say its critics, is ethically fraught. If widely adopted, they say, it would entail a massive reordering of the natural world and could lead to unintended consequences.
What’s more, because one tree’s current home is another tree’s future home, saving one species could mean sacrificing another.
The City of Seattle is currently experimenting with assisted migration, including a small experiment to see if western redcedars can be replaced with nonnative incense cedars.
Seattle isn’t alone. Other groups are experimenting with assisted migration.
According to that same survey in western Washington, respondents reported having planted “near-native” species, including giant sequoia, coast redwood and incense cedar.
Coast redwoods’ range ends in southern Oregon. Incense cedars’ range extends into Oregon as far as the Willamette Valley, according to some estimates, but doesn’t naturally reach into Washington. (There’s some dispute about the natural range of incense cedars, and whether or not they occur naturally in the Willamette Valley.)
But survey respondents also held wildly different attitudes about using assisted migration.
Some reported having no concern. Others reported being concerned about “unanticipated consequences to native habitats” and the possibility that employing assisted migration could mean putting “even more stress on our native struggling to adjust to climate change.”
The experiment is recognition by the Forest Service that climate change is here to stay.
In April of this year, the Portland-based Bonneville Environmental Foundation (BEF) released the results of a similar survey of individuals doing ecological restoration for local nonprofit, federal, state, municipal and tribal organizations in the Pacific Northwest.
While also small, just 51 individuals responded, the survey further confirmed that assisted migration is already happening regionally, including planting native plants that were grown from seeds outside their range and planting (or planning to plant) nonnative species, including incense cedar, giant sequoia and coast redwood.
The BEF survey also displayed a range of differing opinions about assisted migration, including debate about what constitutes a native species. Incense cedar, for example, were listed as both native and nonnative by respondents.
Types of assisted migration
Assisted migration is an umbrella term with varying degrees of controversy and varying degrees of risk of maladaptation.
Assisted migration encompasses everything from moving a species wholesale to a new location, a risky and controversial practice called “species migration” (what moving both species of redwoods and incense cedars into Washington would entail); to “assisted range expansion,” or the moving of trees just outside their current growing range (what moving incense cedars and coast redwoods further into Oregon would entail).
There’s also “assisted population migration” (or “assisted gene flow”), which involves moving a species’ seeds—and by extension their genes—within its current range.
Assisted population migration is both less controversial and poses far less of a risk of maladaptation. It’s also the point of Bower and the Forest Service’s current research efforts and the kind of assisted migration they’re advocating for.
Paradigm shift for foresters
Bower’s two sites are part of the Forest Service’s Experimental Network for Assisted Migration and Silviculture, or ENAMS project, an ambitious effort spanning Forest Service operational and research offices in Washington, Oregon and California.
The Washington Department of Natural Resources and Oregon Department of Forestry, which are also running experiments in assisted migration, have joined in the effort, as has the Bureau of Land Management. The Forest Service also plans to work with private landowners.
As of this writing, ENAMS has planted trees on three sites, including Bower’s two and an additional site in the Umpqua National Forest in Oregon.
Two more Oregon sites, both in the Willamette National Forest, are scheduled to be planted this fall.
But this is just the beginning. An additional 15-20 sites are expected to be planted by 2024. The overwhelming majority of sites are on lands affected by wildfires.
“This is intended to be very big and all encompassing,” says Robert Slesak, research forester at the Pacific Northwest Research Station and project lead. “The science is important but one of the most valuable things that could come out of it hopefully is that we are forming a foundation to get a shared purpose to work together collaboratively on this issue because it’s huge.”
It would be hard to overestimate the impact the ENAMS trials could have on forestry in the Pacific Northwest.
To date, regional foresters have been cautious about embracing assisted migration, believing that “local is best” when it comes to selecting seeds.
“We’re starting to move on this [assisted population migration], to match seed sources with the climate to which they are adapted to the climate to which they will be experiencing,” says Brad St. Clair, a retired (though still very active) former USFS research geneticist with the Pacific Northwest Research Station.
St. Clair has been a leading advocate for assisted migration in the Pacific Northwest. He’s also run some of the longest and most involved studies in assisted migration in the region.
This has involved, to put it bluntly, torturing trees, moving trees from one climate to another and seeing what happens.
What St. Clair and his colleagues discovered is that trees tend to tolerate climates that are either 2 degrees Celsius (3.6 degrees Fahrenheit) warmer or 2° C cooler than the climate they evolved for.
“The climate has changed by 1 degree Celsius [roughly 1.8 degrees Fahrenheit] so far and we are looking at 2-3° C [3.6-5.4° F] by another couple decades,” says St. Clair. “It’s going to look bad. So, we need to start moving now or we aren’t going to have any chance of keeping up with it.”
Questioning “local is best”
To address this rapid change, St. Clair, Slesak and Bower are recommending the Forest Service should reassess its “local is best” practice and embrace assisted migration.
Specifically, they’re hoping to change guidelines around “seed zones.”
Existing within a tree’s larger range, seed zones are the Forest Service’s way of describing what are, in effect, climate varieties, trees of the same species that form genetically unique populations based on adaptations to climate.
But the region’s seed zones haven’t changed in decades. And so, like the Doug-firs left standing on Bower’s site, they represent the climate of the past, not the present or the future.
“The common consensus is that western redcedars aren’t going to make it. So people are already replacing the species.”
What St. Clair, Slesak and Bower are recommending isn’t so far-fetched. Very similar rule changes about where seeds can be collected and planted have already gone into effect in Canada.
In 2019, the British Columbia Ministry of Forests changed its rules around its “seedlot” selection system—B.C.’s version of seed zones—to incorporate climate change and assisted migration. On April 1 of this year, the Ministry of Forest’s chief forester made using the new rules obligatory.
Greg O’Neill, climate change adaptation scientist for the British Columbia Ministry of Forests, outspoken advocate for assisted migration and project lead for the Ministry’s Assisted Migration Adaptation Trial (AMAT) says he’s pleased to see the rules change.
“Essentially what we are doing is putting trees back where they came from,” says O’Neill. “We are restoring them [trees] to their climate homes.”
“Driving ecological simplification”
Assisted migration has critics.
“Quite frankly, it [assisted migration] seems to be a recipe for driving ecological simplification,” says Mark Schwartz, plant ecologist and professor emeritus at the University of California Davis.
In 2012, Schwartz was the lead author on a paper in the journal BioScience outlining potential problems with assisted migration.
Calling themselves the Managed Relocation Working Group (“managed relocation” is another term for assisted migration), Schwartz and 31 coauthors detailed the ethical, ecological and even political problems that could arise from assisted migration.
Problems include: a lack of clear policies and laws around assisted migration; the possibility that the practice could lead to “severe negative ecological consequences”; and the possibility that assisted migration poses a “moral hazard,” because people might think it’s a solution to climate change, which could discourage efforts to lower emissions and slow global warming.
The paper also points to how assisted migration tends to “privilege a species-centered approach to conservation,” one that tends to focus on single dominant species, like trees, and just a handful of dominant trees at that.
“We shouldn’t be comfortable with saying that we need to change the dominant tree species, and everything will be okay,” says Schwartz.
As an example, he says, consider what would happen if people started moving coast redwoods in large numbers out of their current range in northern California and southern Oregon further north into northern Oregon and southern Washington. Doing this, says Schwartz, would only move the trees not the whole ecosystem.
The bay trees that live in the forest understory, the fungi that live in the soil, the salamanders that swim in the water that collects on the forest floor and all the other organisms and complex interactions that make up the forest would be gone, an ecology simplified.
“What kind of redwood forest would that be if it succeeds?” says Schwartz. “It might be a very impoverished redwood forest because we haven’t brought in the hundreds of things that fill in around the redwoods.”
The critique is a valid one.
The Forest Service ENAMS trials involve just two dominant tree species: Douglas-fir and ponderosa pines. Though Slesak says he hopes to expand the number of species in the future.
O’Neill’s AMAT project includes 15 species. However, missing are many of the understory trees, including vine and big leaf maples.
What’s more, the attention afforded the 15 species isn’t distributed equally. Seeds for Douglas-firs come from 11 seedlots, whereas only 2 seedlots are used for western redcedar.
Assisted migration for timber production
In general, says B.C. forester O’Neill, research dollars tend to go to dominant tree species, which often are dominant timber species as well.
While O’Neill and the Forest Service’s research has clear applications beyond timber production, timber production could certainly benefit from adopting assisted migration and for a very obvious reason: trees, at least in theory, could be harvested before they become maladapted due to climate change.
Trees could keep up with the rate of change by being cut down.
On the other hand, trying to use assisted migration to restore an ecosystem in which trees will not only not be cut down but will be expected to live for decades if not centuries, and hence will need to endure the long years of climatic mismatch, is another story altogether.
“If you’re not planning on harvesting then the assisted migration is more complicated, because you will expect trees to last longer than 60 or 70 or 80 years,” says O’Neill.
Assisted migration in Seattle
During last year’s heat dome, Ronda Strauch, climate change research and adaptation advisor for Seattle City Light (Seattle’s municipal electricity provider) was worried.
Her trees at the City’s restoration site at Stossel Creek near the Marckworth State Forest east of Seattle were still seedlings. And the heat dome wouldn’t do them any favors.
“I was so panicked when the heat dome hit,” says Strauch. Thinking her recently planted trees would “get baked,” she drove to as close to the site as she could, then rode her bike on a dirt road uphill the rest of the way.
“I was like, ‘Oh my God are you still alive?’”
What she found surprised her. Though perhaps it shouldn’t have.
“They did amazing, I was shocked,” she says. The western white pine and shore pine were “going nuts.” But the western redcedar were “hanging on.”
The Stossel Creek site is an experiment in using assisted migration for ecological restoration.
A land that was partially logged, the 154-acre Stossel Creek site was purchased by Seattle City Light in 2015 as part of its efforts to restore salmon and steelhead habitat in the area. About a third of the site is being actively restored, though only a small part using assisted migration.
Using methods similar to Bower’s, the Stossel Creek site includes seedlings representing six local trees species but migrated from sites further south, an example of assisted population migration. Trees include Douglas-fir and western redcedars.
The site also includes an example of species migration. Incense cedar seedlings from California were planted at the site.
Incense cedar is being investigated as a “surrogate”—meaning ecological replacement—for the climate-imperiled western redcedar. Incense cedars were selected when the project was unable to locate western redcedar seedlings from California.
While the site’s plot of incense cedars is small, the site’s operators, Strauch among them, are in effect experimenting with replacing a local species threated by climate change with a non-local species.
This has some concerned.
“The common consensus is that western redcedars aren’t going to make it. And so, yeah, people are already replacing the species,” says Brenda Clifton, senior restoration botanist for the Skagit River System Cooperative, which oversees natural resource management for Sauk-Suiattle Indian Tribe and the Swinomish Indian Tribal Community. Clifton isn’t a member of either group and does not claim to speak for them.
Without naming any one project, Clifton says she worries that because the western redcedar is experiencing a major climate-induced dieback that people are rushing to replace the species when they should be working harder to protect it.
“For me replacing the western redcedar is problematic,” says Clifton. “It’s an iconic species and certainly has its niche and I would worry that if you go too gung-ho and try to fill that niche with a species from somewhere else you might advance the decline of the western redcedar.”
Strauch says this is oversimplifying the situation.
“I don’t think incense cedar is going to replace western redcedars,” says Strauch. “But that’s part of the experiment, to see how incense cedars are going to do right next to western redcedars … If for some reason people don’t want that species [incense cedar] on the site, we can always go in and cut them down”
A different type of assisted migration
Clifton, who plants trees as part of her salmon restoration work in Skagit Basin, says she’s been reluctant to embrace assisted migration. (She’s tried growing seedlings from seeds south of the Skagit Basin, but they didn’t survive.)
She says she’s been determined to find local seed sources because she hopes to find local climate adaptations.
Here she’s defining “local” broadly, though not as broadly as proponents of assisted migration.
She collects tree seeds from all over the Skagit Basin, including on both sides of the Cascade rain shadow.
“I’m just trying to get seed from as many different [local] sources as possible, which is its own type of assisted migration, but not the one that people think of,” she says.
Dr. David Shaw, professor and forest health specialist at the Oregon State University in the College of Forestry and another critic of assisted migration, also thinks more attention should be paid to local genetic variability.
“I believe our native forests represent biodiversity hotspots,” says Shaw. “My big contention is, since we don’t know where climate change will take us, ultimately, that we should look for local seed sources and drought tolerance—because we may have it here, we just don’t know because we haven’t looked.”
Echoing Schwartz, Shaw says the assumptions underlying assisted migration tend to simplify or overlook the complex interactions that occur in healthy undisturbed forests. He thinks assisted migration, even the least controversial and risky assisted population migration, is selling local genetic diversity short.
Trees that have survived climatic disturbances like droughts should be sought out and their seeds collected, he says.
“In some evolutionary theory,” says Shaw, “disturbance actually increases the resilience of the forest because it kills the trees that aren’t adapted to the disturbance and leaves the ones that are. As disturbance comes in and causes a lot of mortality, the question is what do the survivors represent?”
Nathan Gilles is a science writer based in Vancouver, Washington.
RELATED: The secret power of old growth