PORTLAND, Ore. (AP) —
As native trees in the Pacific Northwest die off due to climate changes, the US Forest Service, Portland, Oregon and citizen groups around Puget Sound are turning to a deceptively simple climate adaptation strategy called “assisted migration.”
As the world’s climate warms, tree growing areas in the Northern Hemisphere are predicted to move further north and higher.
Of course, trees can’t get up and walk to their new climate homes. This is where assisted migration should help.
The idea is that humans can help trees keep up with climate change by moving them to more favorable ecosystems faster than the trees could migrate on their own.
Yet not everyone agrees on the kind of assisted migration the region needs – or whether it is always a good thing.
In the Pacific Northwest, a divide has emerged between groups advocating assisted migration that would help struggling native trees, and one that could instead replace native species in the landscape with trees from the south, including coast redwoods and giant sequoias .
“There’s a huge difference between assisted population migration and assisted species migration,” says Michael Case, a forest ecologist at the Virginia-based Nature Conservancy.
Case is currently conducting an assisted population migration experiment at the Conservancy’s Ellsworth Creek Preserve in western Washington.
Assisted population migration involves moving the seeds of a native species, and by extension its genes, within its current growing area.
Assisted species migration, on the other hand, involves moving a species far beyond its existing range, such as introducing redwoods and sequoias to Washington.
A third form of assisted migration, called ‘range expansion’, involves moving a species just outside its current range.
Case’s project involves testing whether varieties of native Douglas fir and western hemlock from drier parts of the Pacific Northwest can be used to help western Washington’s forests adapt to climate change. He says the Nature Conservancy focuses on population migration because it poses fewer ecological risks.
“Any time you plant something in an area where it is not locally grown, you increase the risk of failure,” Case said. “You increase the risk that potential ecosystem functions and processes are disrupted.”
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EDITOR’S NOTE: This story is part of a collaboration between The Associated Press and Columbia Insight examining the impact of climate on trees in the Pacific Northwest.
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Population migration is the only form of assisted migration currently used nationwide by the Forest Service, said Dr. David Lytle, the agency’s deputy chief for research and development.
“We are very, very cautious about not moving and establishing plant material over long distances outside and apart from a species’ historical range,” Lytle says.
The Forest Service is pursuing assisted population migration because it is likely to have little or no “negative impact” on ecosystems, he said.
Douglas Tallamy, a professor of entomology and wildlife ecology at the University of Delaware, said one potential negative consequence of species migration is the possibility that native caterpillars may not eat the leaves of migrated non-native tree species. Because caterpillars feed on birds and other animals, this can lead to disruptions in the food web.
This could happen if the city of Portland migrates oak species from places south, Tallamy noted. “Oaks are the most important wildlife-supporting plants we have in North America,” he said, “but if you take them out of reach, the things that are adapted to eat them can no longer access them.”
The City of Portland’s Urban Forestry program is currently experimenting with the assisted migration of eleven tree species, including three southern oak species: California black oak, canyon live oak, and interior live oak.
Asked via email about potential ecological disruptions, Jenn Cairo, manager of City Forester & Urban Forestry in Portland, said, “We use research from universities, state and federal sources, and local and regional field experience.”
Another advocate for species migration is the Puget Sound-based, citizen-led PropagationNation. The organization has planted trees in several Seattle-area parks and has an ambitious goal of “bringing one million coast redwoods and giant sequoias to the Northwest,” according to its website.
The PropagationNation website also recommends planting redwoods in areas where native western red cedar, western hemlock, Sitka spruce and bigleaf maple are already growing.
Western Red Cedar, Western Hemlock and Big Leaf Maple have all experienced dieback and declines in growth in recent years due to climate.
Philip Stielstra, founder and president of PropagationNation, and a retired Boeing employee, declined to comment for this story.
David Milarch, founder of the Michigan-based Archangel Ancient Tree Archive, which has supplied PropagationNation with redwoods and sequoias, says his trees are not intended to replace native Pacific Northwest species.
“All we are doing is expanding the range (of sequoias and sequoias) north in the hope that in 100 to 200 years they will still exist here and will not be included in the list of trees that are becoming extinct,” says Milarch.
Robert Slesak, a research forester at the Pacific Northwest Research Station, directs the Forest Service’s Experimental Network for Assisted Migration and Establishment Silviculture (ENAMES) project, which monitors population migration sites in Washington, Oregon and California.
Slesak called moving redwoods north a “risky proposition.” He said he has serious concerns about both assisted migration of species and assisted migration efforts that lack experimental rigor.
“Widespread assisted species migration without many experimental results to guide it is risky,” says Slesak. “Everyone knows we need to take action on climate, but there is a real risk of making things worse.”
Nevertheless, as the impacts of climate change increase, it is a risk increasingly borne by public and private groups around the Pacific Northwest.
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Nathan Gilles is a science writer and journalist based in Vancouver, Washington.
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Columbia Insight is an Oregon-based nonprofit news website covering environmental issues affecting the Pacific Northwest.
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The Associated Press’ climate and environmental reporting receives support from several private foundations. View more about AP’s climate initiative here. The AP is solely responsible for all content.