Can redwoods and sequoias produce offspring in the Pacific Northwest?

If not, new efforts to migrate the species to the region might need continued human management

Everybody wants some: Could this scene at California’s Humboldt Redwoods State Park, which claims the largest expanse of ancient coast redwoods left on the planet, be replicated elsewhere? Photo: Visit Redwoods

By Nathan Gilles. March 21, 2024. In October of last year, The New York Times ran a story asking, “Can We Save the Redwoods by Helping Them Move?”

The story centered on Washington state resident and redwood enthusiast Philip Stielstra. A retired former Boeing employee, Stielstra is the president of PropagationNation, a nonprofit working on migrating coast redwoods and giant sequoias to the region, partly to help preserve the species for future generations.

This strategy, called “assisted migration,” follows the idea that people can, and some argue should, help species migrate to help prevent climate change-induced extinction.

The goal of assisted migration varies from using migrated trees for timber to preserving plant genes to creating self-sustaining populations of trees that have “naturalized” or established in their new environment.

While the Times story was largely sympathetic of Stielstra, PropagationNation and similar organizations, it nonetheless ended on what for me was something of an ecological cliffhanger: Once planted here, do the trees go on to reproduce, creating self-sustaining forests?

As author Moises Velasquez-Manoff writes at the end of the story, “The experts I asked knew of no instances in which the giant sequoia had naturally produced offspring anywhere beyond its native groves in the Sierra Nevada.”

Velasquez-Manoff further hints that coast redwoods might also be having trouble producing offspring outside their native range.

Philip Stielstra. Photo: PropagationNation

Velasquez-Manoff’s ending caught my attention because starting sometime in 2020 I began wondering the same thing.

My young son had been going through a tree phase at the time and had gained an uncanny ability to spot naturally growing “volunteer” trees orphaned in bad spots—under play structures, in gutters, next to buildings—that we just had to dig up and take home and love and add to his “tree school.”

While we found lots of oaks, maples and cedars—native and nonnative alike—and lots and lots of Douglas firs, we never found a single redwood or sequoia volunteer despite the fact that both trees are common where we live in the Portland metro area.

This got me curious.

I nevertheless filed the idea away as “interesting but maybe not useful.” That is until The New York Times story. Then I started making phone calls and emails. One was to Stielstra.

Stielstra told me on the phone that he had “not seen any evidence where there are young [redwood and sequoia] trees coming up that are definitely not planted,” adding that he had only heard of “rumors” and “speculation.”

The ability of a tree or other plant to produce viable offspring, I would learn, is called “recruitment” by scientists. It’s an important step in establishing self-sustaining populations of migrated species.

According to my own investigation, the answer to the question do redwoods and sequoias recruit in the Pacific Northwest? is probably no for giant sequoias and maybe but not often, for coast redwoods.

If it is true that the two tree species are not recruiting—or not recruiting well—outside their native ranges, this suggests, say experts I spoke with, that the trees might not naturalize in their new homes.

This further suggests that efforts to migrate the trees to the region might require ongoing human management to be successful, begging the question, is this what successful assisted migration should look like?

Migrants taking root—or not?

All this got me wondering what PropagationNation’s ultimate purpose of their ambitious goal of “bringing a million coast redwoods and giant sequoias to the Northwest” actually is.

For his part, Stielstra, while acknowledging that PropagationNation is performing assisted migration, says he doesn’t see naturalization as his group’s goal.

Appearing to contradict how the New York Times told his story, he further says he doesn’t see extinction prevention as his group’s goal either.

“I don’t think we see ourselves as the saviors of the redwoods [and sequoias] in the sense of saving them from extinction,” says Stielstra. “We just think that with our ecology and climate, they [redwoods and sequoias] can do really well here, so we’re just trying to help advance that cause.”

To support this claim, PropagationNation hired Russell Kramer, a forest ecologist at the environmental consulting company Resilient Forestry, to write a series of educational bullet points about migrating the trees to the region.

A statement attributed to Kramer on the group’s website states, the “current best available science” offers “a solid case for incorporating giant sequoias and coast redwoods into the mix of trees growing in the Pacific Northwest.”

Comfort zone? Potential coast redwood clones sprout from the base of mother tree in Vancouver, Wash. Photo: Nathan Gilles

One of Kramer’s arguments for planting redwoods and sequoias in the Pacific Northwest is that the region’s climate is currently suitable for both species.

But in an interview with me, Kramer said this statement was based on the observation that trees of both species—when grown in nurseries and planted in the ground—can survive in the region and is not based on the climate or environmental requirements of seedlings.

“A tree can live somewhere, especially if it’s planted at maybe three years old,” says Kramer. “But it may not be able to reproduce there because the reproduction phase is the most vulnerable.”

The statement is telling.

Although redwoods and sequoias have been planted in the Pacific Northwest for decades and can easily be found growing and even thriving in yards, parks and arboretums in Washington and Oregon, these are trees that were grown under controlled nursery conditions and then planted.

What is far more difficult to find, say experts, are examples of the two species growing in the region without human aid.

In fact, many suspect that coast redwoods and giant sequoias are struggling to reproduce outside their native range because the environmental requirements for saplings and adult trees is different from what seeds need to germinate and seedlings need to grow.

But the exact reason is still something of a scientific mystery. Despite being two of the most iconic tree species in the world, there really isn’t a whole lot of information on how best to migrate them.

According to one study, coast redwoods might be struggling to reproduce north of their native range due to climate.

For giant sequoias, the reason is most likely about fire and fungi.

Fire and giant sequoias

Although the term “redwoods” is commonly used to describe both coast redwoods (Sequoia sempervirens) and giant sequoias (Sequoiadendron giganteum), the two species are quite different and so are the environments in which they naturally recruit.

Coast redwoods are native to the temperate foggy coasts of northern California and southern Oregon.

Giant sequoias are native to the western flank of California’s Sierra Nevada known for both snow and frequent fire.

Both redwoods and sequoias have thick bark that makes them resilient to fires. The recruitment of both species is thought to benefit from periodic, low-severity fires. But giant sequoia seeds, in particular, need low-severity fires to germinate successfully, says Nate Stephenson, scientist emeritus at the U.S. Geological Survey’s Western Ecological Research Center.

“I’ve pondered whether sequoias might be expected to naturally reproduce in the PNW and wondered if there are two main things limiting it: fire and/or pathogens,” Stephenson wrote me in an email.

Young duffer: A giant sequoia cone lands on infertile ground in a park in Vancouver, Wash. Sequoia seeds need bare mineral soil free of disease-causing fungi to germinate. Photo: Nathan Gilles

Based in Three Rivers, Calif., just outside Sequoia National Park, Stephenson has spent over 44 years studying giant sequoias.

In a phone interview, he said he didn’t know if sequoias were recruiting in the Pacific Northwest—or anywhere outside their native range—but said that if it’s not happening, a lack of fire could be a factor.

According to Stephenson, fire does several things that help sequoia seeds germinate.

First, fire heats up the cones on mature sequoia trees, forcing them to open and drop their seeds.

Fire also “punches a hole in the forest,” allowing seedlings access to light.

It also incinerates the forest floor litter, or “duff,” which sequoia seeds can’t germinate in. They require bare mineral soil.

And by burning off the forest duff, Stephenson says, fire might also “sterilize the upper layers of soil,” killing disease-causing fungi to which sequoia seedlings are highly susceptible.

“Adult sequoias are really good at fighting off fungal pathogens. The newly emerged seedlings aren’t as good at that,” says Stephenson, adding that fungi and lack of fire will likely limit the success of giant sequoia seedlings in the Northwest and other areas.

My investigation determined that Hoyt Arboretum in Portland, and the UBC Botanical Garden in Vancouver, B.C., which both contain adult sequoias, had no confirmed cases of sequoia seedling recruitment.

I was also unable to confirm a rumored case of sequoia recruitment in a planted grove in Washington.

This doesn’t surprise Stephenson. “If you plant a sequoia sapling in the right conditions in the Pacific Northwest, one that came from a nursery, it might do beautifully,” he says. “It’s just that [the] seedling stage might be a bottleneck that is preventing them from reproducing in the Pacific Northwest.”

Stephenson says this could change if the region starts to see more low-severity fires.

“If the fire regime [in the Pacific Northwest] starts to turn into something more like what you get in the southern Sierra Nevada’s of California, you might get more [sequoia] regeneration,” says Stephenson.

International sensation: Redwood seeds were first brought to England in the 1850s. These giant redwoods are found at Tredegar House in Wales. Photo: National Trust Images/Andrew Butler

While there are plenty of studies predicting that climate change will create ideal conditions for larger and more frequent wildfires in the Pacific Northwest, it is unknown if these fires will be suitable for future sequoia reproduction.

What is far more likely is recent high-severity fires in the tree’s native range are killing them.

Together, the 2021 Windy Fire and KNP Complex Fire are expected to lead to the death of as many as 3,637 large sequoias (trees over four feet in diameter), according to the National Park Service.

Estimates of dead and dying trees are even higher for 2020’s Castle Fire, which could lead to the death of as many as 10,600 large sequoias.

Asked if he thought more destructive fires might cause sequoias to go extinct, Stephenson said the tree’s popularity makes extinction unlikely.

“I’m not too worried about them going extinct. What does concern people is that we could lose the big old ones [in the Sierras],” says Stephenson.

Although the sequoia’s native range is limited to just 70 California groves covering a measly 44 square miles, the tree has been planted on nearly every continent on Earth, including the United Kingdom, according to a recent study published by Royal Society Open Science.

A BBC story covering the study ends by noting that “while the trees are doing well in the UK, there’s little chance of them taking over our native forests any time soon—they’re not reproducing here as they need very specific conditions to take seed [germinate].”

Coast redwoods—seedlings and clones

While giant sequoias reproduce via seeds, the situation with coast redwoods is a bit more complicated.

Coast redwoods can reproduce both sexually via seed and asexually by producing clones of themselves that grow as green shoots, or “suckers,” at their bases and roots.

As Velasquez-Manoff hints in his New York Times story, this self-cloning ability makes it difficult to determine if a redwood sapling found under a sexually mature “mother” tree is a clone or has grown from seed.

In fact, it’s simple to witness redwood’s self-cloning powers for yourself. Green shoots can easily be seen sprouting from the base of redwoods wherever they grow, including in cities. Seedlings, however, are rare.

During my reporting, I heard a handful of rumors about the trees naturally producing seedlings outside their native range but was able to confirm just two possible cases.

Both are at Portland’s Hoyt Arboretum. According to arboretum curator Martin Nicholson, the young trees “appear to have grown from seed and are not root suckers.”

The UBC Botanical Garden has no confirmed cases of redwood recruitment.

Seattle Parks and Recreation staff confirmed that they had seen no signs of redwood recruitment in the city’s parks and wild spaces, though they and other experts interviewed for this story say tree recruitment in cities is rare in general.

Whether self-cloning could play a role in redwoods naturalizing themselves in the Pacific Northwest is something of an open question. But Greg Ettl, associate professor at the University of Washington’s School of Environmental and Forest Sciences, thinks it might.

“In general, I think naturalization of [coast] redwoods [in the Pacific Northwest] should be possible, but much of this would come from clonal spread,” Ettl wrote in an email.

He went on to say, “It is likely that reproduction from seed is rare [in the Pacific Northwest] due to limited seed production, viability, small size and (maybe more importantly) lack of suitable sites.”

Ettl is currently testing the suitability of growing coast redwoods in the Pacific Northwest, including the species’ drought and frost tolerance. He’s one of the first researchers to do this.

Coast redwood germination—climate and fog

Another reason we don’t see more redwood seedlings in the Pacific Northwest might have to do with climate, according to a study by Richard Winder, a retired research scientist at the Pacific Forestry Centre in Victoria, B.C.

Published internally by the government agency Natural Resources Canada, Winder’s study aimed to determine if redwood seeds could germinate on Vancouver Island, and how current germination rates on the island might compare to those within the tree’s native range in California and southern Oregon.

Moving on up: Coast redwood thriving in Vancouver, Wash. Photo: Nathan Gilles

To figure this out, Winder and colleagues germinated redwood seeds in greenhouses heated to mimic the springtime temperatures of the tree’s native range.

They also germinated seeds in the cooler springtime temperatures currently found on Vancouver Island.

The number of germinated seeds under Vancouver Island temperatures was “tiny” compared to seedlings grown under warmer conditions, leading Winder to conclude that natural seedling recruitment isn’t currently optimal there to “sustain ongoing populations.”

“You would want it to be warmer [on Vancouver Island] than it currently is in order to foster recruitment,” Winder told me over Zoom.

In other words, the further north you move the tree outside its native range, the less likely recruitment will become because temperatures will simply be too cold.

Winder’s findings do suggest that recruitment on Vancouver Island will likely increase as temperatures rise under future climate change.

But there’s a big caveat to that: fog.

Redwood recruitment is believed to benefit from the fog that blows in from the Pacific Ocean. This fog keeps the seedlings from drying out while their young roots burrow down in the forest duff in search of water in the soil.

Winder’s study assumes Vancouver Island’s current fog patterns won’t change in the future. And that, he admits, might not happen.

That’s because the fog that rolls in off the ocean is the result of ocean currents, currents that could alter—and with them the fog they produce—as climate change continues.

Whether the future has enough fog when redwood seedlings need it, says Winder, is uncertain.

In the meantime, I plan to keep looking under mother trees for those rare recruits.