What is ‘assisted migration’ and what are the risks?

As heat and drought continue to kill trees, humans are stepping in to help shift growing ranges. Not everyone agrees on what type of help is needed

“Trees of the Pacific Northwest” is a climate journalism collaboration between Columbia Insight, The Associated Press and Global Climate Desk. Today, the third of three stories co-published by the collaborating newsrooms examines issues affecting trees of the Pacific Northwest due to changing climate. The collaboration, developed over a six-month period, is part of The AP Global Climate Desk’s coverage of climate and will be distributed throughout the AP’s global network. Columbia Insight is proud to be part of this series and thanks our readers and donors for continuing to support local environmental journalism. —Editor

By Nathan Gilles. December 28, 2023. East of Seattle, on the banks of Snoqualmie River, row upon row of one- to three-foot tall tree seedlings sit snug in plastic cages intended to protect this experiment in adapting to climate change from the hungry mouths of deer.

The young trees are located at the Oxbow Farm and Conservation Center, in Carnation, Wash. Once covered in invasive species, the Oxbow site has been ecologically restored using Pacific Northwest native plants, including eight tree species.

But while all the trees at the site are native to the region, some started their lives as seeds hundreds of miles away.

Shiny metal tags mark the trees’ natal homes. One reads “Coos,” short for Coos Bay, Ore., over 400 miles to the south. Another tag indicates a seed source in Oregon’s Willamette Valley south of Portland.

Both trees are native big leaf maples, trees that have been dying in large numbers throughout western Washington due in part to abnormally warm summer temperatures.

This is the problem Matt Distler, Oxbow’s conservation program manager, hopes to solve: how to use assisted migration to keep Pacific Northwest native trees on the landscape as climate change kills them off.

Distler eyes his seedlings. They’re neatly laid out in a grid, labeled and wired to a station that monitors soil moisture and other factors.

Distler has the discerning eyes of someone accustomed to squinting in natural light. He wears a weatherworn leather hat, its brim folded on the sides, making him look somewhat like a cowboy from a Western.

“I think people are interested in this idea [assisted migration] because it is hopeful,” says Distler. “Meaning there are [climate] adaptation strategies that hold some hope in what seems like a grim reality.”

Because the current climate of Coos Bay and the Willamette Valley look similar to Oxbow’s predicted future climate, the maples grown from seed found in these southern locations should be better adapted to climate change.

In the coming years and decades, Distler hopes to test this idea, not only for big leaf maple, but also western redcedar, another native species that’s imperiled by warmer, drier summers.

‘Hopeful tactic’

Distler is a cautious practitioner of assisted migration rather than an evangelist.

He’s part of a growing number of practitioners concerned that many assisted migration efforts in the Pacific Northwest have lacked both scientific rigor and consent from local communities.

“Assisted migration is definitely a hopeful tactic,” says Distler.

But, he says, to be done right, it needs to be “carefully considered, well-monitored and collaborative.”

Matt Distler, conservation program manager Oxbow Farm and Conservation Center. Photo: Nathan Gilles

These rules and this public discussion, Distler and others claim, are needed because assisted migration is gaining in popularity and decisions about what gets planted today could have lasting ecological effects for decades to come.

In the Pacific Northwest, organizations from the U.S. Forest Service to the cities of Portland and Seattle and even a citizen-led effort are now practicing assisted migration as a way to help forests and prevent species extinction in the face of climate change.

But what’s emerged from these efforts is an ongoing debate about what type of assisted migration the region needs.

A clear divide has developed between organizations like Distler’s, which are advocating for a type of assisted migration that would help the region’s struggling native trees, and one that could instead see native trees replaced on the landscape by trees from the south, many of which are also threatened by climate change, including coast redwoods and giant sequoias.

The debate centers around two forms of assisted migration: assisted population migration and assisted species migration.

Forms of assisted migration

“There is a huge difference between assisted population migration and assisted species migration,” says Michael Case, forest ecologist at the Nature Conservancy.

Case currently runs an assisted population migration experiment at the Nature Conservancy’s Ellsworth Creek Preserve in western Washington. Distler’s project at Oxbow is also a form of assisted population migration.

Assisted population migration, also called “assisted gene flow,” involves moving a species’ seeds—and by extension its genes—within its current growing range.

Typically, this means moving seed sources from drier, warmer regions in the south to cooler, wetter ones in the north, like Distler is doing at Oxbow with big leaf maple.

A metal tag marks the origin of this big leaf maple at Oxbow Farm and Conservation Center in Carnation, Wash. An example of assisted population migration, this maple started its life as a seed collected in Coos Bay, Oregon, over 400 miles to the south. Photo: Nathan Gilles

Case’s project at Ellsworth Creek is a little different.

The project involves native Douglas fir and western hemlock.

But instead of taking seeds from the south, Case has selected seeds from drier regions to the east of his planting site.

The idea is that by selecting tree breeds from drier regions, these breeds will not only do better in a warmer, drier future, they’ll also increase the diversity of the local forest by adding to its gene pool.

By contrast, assisted species migration involves moving a species outside its current growing range, what moving redwoods and sequoias to Washington state entails.

There’s also a third form of assisted migration, called “range expansion,” which involves moving a species just beyond its current growing range.

All three forms of assisted migration are based on the now well-tested idea that trees and other plants have growing ranges that are largely determined by climate factors, including temperature and precipitation.

These factors limit how far north and south, west and east, and up and down in elevation a plant can grow.

However, as the climate continues to warm, plant-growing ranges in the Northern Hemisphere have long been predicted to move farther north and higher in elevation.

There’s even evidence that growing ranges are already shifting, with some researchers suggesting that current tree die-offs are the result of these climate-induced range shifts.

The problem, of course, is trees and other plants can’t get up and walk to their new climatic homes. Instead, plants migrate via seed, one generation at a time.

It’s been estimated that trees would need to migrate at about 10 times their natural rate to keep up with climate change.

This is where assisted migration comes in.

Assisted migration is the deceptively simple idea that humans can help trees and other plants—and even climatically displaced animals—keep up with climate change.

The problem, says Case, is plants and their ecosystems aren’t simple.

“Whenever you plant something in an area where it’s not locally found you increase the risk of failure [of the species],” says Case. “You also increase the risk of disturbing ecosystem functions and processes.”

Tree seedlings sit in plastic cages to protect this climate adaptation experiment at Oxbow Farm and Conservation Center from foraging deer. The project includes the assisted population migration of Pacific Northwest native Oregon white oak and big leaf maple. Photo: Nathan Gilles

Case says the Nature Conservancy is focusing on population migration but isn’t considering species migration because native trees are more likely to survive if moved within their growing ranges and are highly unlikely to disrupt local ecosystems.

This is also Distler’s position.

“When you are talking about moving populations of a native plant this is something to be done with care, this is not a dramatic act,” says Distler. “But move a species far north of its native range, and there are more potentials for risk.”

The Forest Service’s position

The U.S. Forest Service has taken a similar position and is practicing only population migration, according to Dr. David Lytle, deputy chief of research and development for the agency.

Lytle oversees research efforts for the agency nationally.

“The Forest Service certainly engages in the first [population migration] and we would carefully consider range expansion, but we are very, very cautious and do not engage in the long distance movement and establishment of plant material outside and disjunct from the historic range of a species,” says Lytle.

Lytle says his agency is concerned that species migration could create “negative consequences.”

One potential consequence is that an introduced tree species could become an invasive species in its new environment.

“You move a species outside its native range and you remove all the factors that it has evolved with and that control it—competition, diseases, insects,” says Lytle.

[perfectpullquote align=”full” bordertop=”false” cite=”” link=”” color=”” class=”” size=””]“It’s a fun game that horticulturists want to play … but it will probably cause more problems than not.” —Douglas Tallamy, University of Delaware[/perfectpullquote]

The USFS’s largest assisted migration effort in the Pacific Northwest is called the Experimental Network for Assisted Migration and Establishment Silviculture or ENAMES project, an effort spanning Forest Service operational and research offices in Washington, Oregon and California.

ENAMES currently has eight sites where migrated trees have been planted, with 29 more sites in the works.

ENAMES involves population migration for native Douglas fir and ponderosa pine.

While the USFS has experimented with assisted migration on a smaller scale for decades, ENAMES takes the next step. The project is an attempt to apply population migration on a large scale in part as a way to replant regional forests following wildfires and other climate disruptions.

Redwoods, sequoias, invasive species

Not everyone is critical of species migration, especially not for two popular California trees that are already common in Oregon and Washington cities and yards: coast redwoods and giant sequoias.

Probably the most well-known organization advocating for species migration is the Seattle-based, citizen-led PropagationNation.

Philip Stielstra, a retired Boeing employee and PropagationNation’s founder and president, declined to comment for this story.

Arguing that redwoods “belong here in the PNW” and are better at storing carbon than Pacific Northwest native trees, PropagationNation has planted coast redwoods and giant sequoias in several Seattle-area parks.

The group has also planted redwoods at an ecological restoration site in the Olympia area.

According to the PropagationNation website, many of these trees come from the Michigan greenhouses of the Archangel Ancient Tree Archive.

This long-distance movement of plant material violates a key recommendation of David Shaw, professor and forest health specialist at Oregon State University.

“Our recommendation is to grow your stuff locally,” says Shaw. “We’re seeing a lot of invasive species in nursery operations.”

David Shaw. Photo: OSU

Shaw is concerned that assisted migration, if broadly adopted, will become a “pathway for invasive species” to enter the Pacific Northwest.

The problem, he says, isn’t that the migrated trees themselves will become invasive, but instead that the trees will carry invasive species on them. Something he says that’s happened elsewhere in the nursery industry.

To avoid this, Shaw says, while seeds have to be collected from afar, those seeds should always be grown locally.

Shaw, who is working on a paper on best practices for assisted migration, recently presented his recommendations at the annual convention of the Society of American Foresters.

Shaw says he’d recommend that PropagationNation not plant any of its trees grown in Michigan in wild spaces intended for ecological restoration.

“Growing something in the Midwest and transporting it to the wildlands of the Northwest is probably not the greatest idea,” says Shaw.

David Milarch, founder of the Archangel Ancient Tree Archive, says he understands concerns that growing trees non-locally could unintentionally introduce invasive species. But he says his greenhouse works closely with inspectors from the Michigan Department of Agriculture and Rural Development to ensure that his trees aren’t unintentionally transporting invasive species.

“We’re not in the business of sending invasive species anywhere,” says Milarch. “All we are doing is extending the range [of redwoods and sequoias] north in the hopes that they will still be here in 100 to 200 years and not join the list of trees that are going extinct.”

Many of Archangel’s redwoods are clones grown from cuttings taken from living giants in California, shipped to Michigan to be grown, and then shipped back to the West Coast to be planted.

Milarch says because clones are genetic copies of their parent trees, this preserves the parent trees’ genes for future generations.

The Archangel Ancient Tree Archive and Milarch’s interest in cloning started when he purportedly began talking with angels following a near-death experience brought on by alcohol withdrawal.

Since this experience, Milarch has become a leading proponent of assisted migration as a way to save the world’s trees from climate change-induced extinctions.

Milarch’s story is told in the book The Man Who Planted Trees.

Milarch says he has been unable to successfully grow giant sequoia as clones and has started growing the trees from seed.

Redwoods and sequoias are unlikely to become invasive in the Pacific Northwest, according to Milarch, because the two species don’t reproduce easily from seed and the seeds that do germinate are unlikely to do so far from their parent tree.

But both Archangel’s and PropagationNation’s efforts raise the possibility that Pacific Northwest native plants could be displaced by redwoods.

PropagationNation’s website recommends planting redwoods in areas where native trees, including climate-imperiled western redcedar and bigleaf maple, already grow.

Milarch says Archangel’s redwoods and sequoias aren’t intended to replace Pacific Northwest native species, adding that his organization’s trees are only there to “augment native forests.”

“By no means do we have enough trees or the will or the design to plant even a 40-acre forest,” says Milarch.

Milarch also says his redwoods and sequoias are unlikely to replace native plants in local ecosystems because most of them are being planted inside urban areas, which are already largely unnatural spaces filled with nonnative species.

Population migration in Seattle

Michael Yadrick, plant ecologist at the City of Seattle Parks and Recreation, who is also a member of the Forest Action Network, says his department is currently practicing population migration for six native species, including Douglas fir and western redcedar.

Yadrick says his department isn’t considering planting redwoods or sequoias as part of its assisted migration efforts.

“I think there is a concern with incorporating species that in the near history weren’t found here although they could have been here in deep time,” says Yadrick.

Forest Ecologist Rolf Gersonde oversees assisted migration efforts for Seattle Public Utilities. He’s also a member of the Forest Action Network.

Gersonde also says his department isn’t considering assisted migration for redwoods or sequoias, in part because it’s unclear how well-adapted the trees are to the region.

According to Gersonde, a better candidate for long-distance migration is incense cedar.

City of Bellevue Forest Management Program Supervisor Rick Bailey stands among dozens of juvenile giant sequoias in Bellevue, Wash. Photo: AP Photo/Manuel Valdes, File

Incense cedar’s range is believed to extend through California and Oregon, ending near the Washington border south of Portland. (Though the popular guidebook Trees and Shrubs of the Pacific Northwest claims the species has a disjunct population in Washington state north of the Portland metro area.)

Incense cedar has been planted extensively along Interstate 5 in both the Portland and Seattle areas.

Gersonde helped design Seattle’s Stossel Creek assisted migration project, which includes a small experimental plot of incense cedar.

The rest of the project involves population migration.

Incense cedar is often discussed as “surrogate” or replacement for western redcedar, though Gersonde says this wasn’t the intention behind including the tree at Stossel Creek.

“We are using [incense cedar] because it overlaps with [the range of] western redcedar in southern Oregon. But incense cedar is much better adapted to warmer drier conditions,” says Gersonde “We want to increase the genetic diversity of our local [forest] communities. Not replace it, but add to it.”

Population migration in Portland

Both Gersonde and Yadrick’s migrated trees are being planted in Seattle’s more wild forested spaces, including in sections of the city’s watershed.

But according to Douglas Tallamy, professor of entomology and wildlife ecology at the University of Delaware and leading advocate for planting native species, U.S. cities in general need to plant more native species even in their more developed urban spaces.

“Plants are not just decorations; they have ecological function,” says Tallamy.

Tallamy is the author of Nature’s Best Hope. The book argues that native plants support more wildlife than non-native plants.

But because our cities and backyards now mostly contain non-native species, these spaces have become, in effect, ecological dead zones incapable of supporting local wildlife.

While many have written off cities’ urban spaces as ecological lost causes, Tallamy has not.

He argues that there’s simply not enough land left to set aside as nature preserves. If we want to avoid extinction and ecological collapse, Tallamy says, we should transform our backyards and urban spaces by planting native species.

Unsurprisingly, Tallamy is, in his words, “not a fan of assisted migration” and is especially critical of species migration.

“It’s a fun game that horticulturists want to play, but ecologically it’s not necessary and will probably cause more problems than not,” says Tallamy.

The problem, says Tallamy, is simple: moving plants outside of their growing ranges removes those trees from the communities in which they evolved.

This, he says, will be the likely outcome of the City of Portland’s current species migration and range expansion efforts.

Woodpecker nesting holes in a dead western redcedar at Magness Memorial Tree Farm in Sherwood, Ore. Redcedar and other tree species in the Pacific Northwest have been dying because of climate-induced drought, researchers say. Photo: AP Photo/Amanda Loman

Portland Parks and Recreation’s Urban Forestry program is experimenting with moving 11 tree species, according to department spokesperson Mark Ross.

Several of these species have growing ranges that extend through California and end in southern Oregon, including coast redwood, California black oak and canyon live oak.

Also on Portland’s list are Interior live oak and MacNab cypress or Shasta cypress. The ranges of both species are currently confined to California.

Tallamy says the three migrated oak species are unlikely to support the same number of local caterpillar species—and by extension the birds and other animals that feed on them—as Oregon white oak, the Pacific Northwest’s reigning native oak species.

Research from Tallamy’s lab at the University of Delaware suggests that the number of caterpillars supported by any given oak species declines the farther outside its native range the tree has been moved.

“Oaks are the most important plant for supporting wildlife that we have in North America, but when you move them out of range, the things that are adapted to eating them no longer have access to them,” says Tallamy.

Tallamy says the City of Portland’s migration of oaks is misguided for another more obvious reason: Oregon white oak will not only support native caterpillars; the tree is also widely recognized by scientists for its extreme drought and heat tolerance and ability to adapt to multiple climate conditions.

In fact, Oregon white oak not only grows on both sides of the Cascades’ rain shadow, but its growing range extends from British Columbia through most of California, reaching as far south as Los Angeles County.

“Oregon white oak goes almost all the way to southern California in its natural range,” says Tallamy. “It is extremely adaptable. There is no reason why we need to expand the number of oaks. Why not expand the planting of Oregon [white] oaks? It is already adapted to do what you want it to do.”

Representatives from the City of Portland Urban Forestry program declined to an interview request for this story.

Asked in an email about potential ecological disruptions, City Forester Jenn Cairo responded: “We use research from universities, state and federal sources, and local and regional field practitioner experience.”  

Cairo provided the same email response for four other questions submitted via email.

Need for public discussion

The City of Seattle’s Gersonde says groups practicing assisted migration need to not only apply scientific methodology to their efforts, they also need to be open about what they’re doing.

“We do have to have the discourse and the outreach to our local communities,” says Gersonde. “That’s just as important as our planting projects.”

Distler agrees, saying communication needs to start before migrated trees are even planted.

Oxbow’s assisted migration project sits on the traditional lands of the Snoqualmie Indian Tribe.

Distler says he consulted with the Snoqualmie before starting the project. The Snoqualmie are now helping fund the effort.

A representative from the Snoqualmie couldn’t be reached for comment.

EPA weather monitoring equipment in Willamette National Forest in Oregon. Photo: AP Photo/Amanda Loman

Distler says one of the messages that needs to be conveyed to the public is that assisted migration in all its forms isn’t a technological fix or solution to climate change.

Assisted migration, like tree planting generally, doesn’t lower the amount of greenhouse gases being emitted. What assisted migration is, says Distler, is a way to adapt to climate change that is already happening.

“I think there is a moral hazard in focusing too much on assisted migration because it can take energy and attention away from cutting emissions,” says Distler.

Distler worries that the growing popularity of assisted migration could draw attention away from other climate adaptation and ecological restoration efforts.

“Even within climate adaptation, there are things like this,” he says gesturing to his rows of native trees, “that even without assisted migration are probably more important.”

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