On the slopes of a volcano in central Mexico, biologist Cuauhtémoc Sáenz-Romero and his team envision a climate refuge for oyamel fir trees and the monarch butterflies that depend on them. In 2021, the researchers trekked up the mountain to plant seedlings in biting cold temperatures—part of an effort to save the species from a climate-driven demise.
About 80 miles away, in the Monarch Butterfly Biosphere Reserve, millions of monarch butterflies (Danaus plexippus) find haven in the oyamel fir (Abies religiosa) forests after an arduous migration from Canada and the United States down to Mexico for the winter. Now, as rising temperatures, droughts, and disease threaten the forests of the monarch reserve, scientists are hoping to help these trees migrate.
“We’re doing something different,” says Sáenz-Romero, a researcher at the Universidad Michoacana de San Nicolás de Hidalgo in Michoacán, also home to the reserve. “If we don’t do this, the trees in the monarch reserve are going to die.”
By 2090, the reserve’s forest habitats, located around 11,000 feet (3,500 meters), are expected to deteriorate, as temperatures warm. Using a technique called assisted migration, researchers might be able to move these trees to the nearby volcano, Nevado de Toluca, Sáenz-Romero and his colleagues recently reported in Frontiers in Forests and Global Change.
(Follow the monarch butterfly on its nearly 3,000-mile journey.)
To give the seedlings on Nevado de Toluca the best chance of survival, the team planted the baby trees under “nurse plants,” existing shrubs that could provide protection. Here, Jose Humberto Garcia Miranda and Legna Perez Cruz use a similar technique at an oyamel fir tree nursery in Michoacan, Mexico.
Photograph by Jaime Rojo
Future forests
Forests move naturally; they shift in many directions and elevations as climate changes. In Mexico, trees are slowly migrating up the mountains, but they’re doing so at a slower pace than that of climate change. In the monarch reserve, for instance, the highest oyamel fir populations are already at the summits, leaving them with nowhere to go.
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Assisted migration helps the forest move more quickly than it would do so naturally. You collect seeds from a place with a specific climate, like the monarch reserve, and move them to an area that will have a similar climate in the future, explains Sáenz-Romero.
The experiment began in 2017, when Sáenz-Romero and his team gathered seeds from oyamel fir trees in the Monarch Butterfly Biosphere Reserve at various altitudes. For two years, the plants grew in a shade house and then for another year in a nursery at around 9,800 feet (3,000 meters) to help them adjust to the altitude. Then, in 2021, they worked with the local indigenous community of Calimaya to plant the seedlings under “nurse plants,” which protected them from harsh conditions, on the northeast slope of the Nevado de Toluca volcano. They focused on four different altitudes, ranging from roughly 11,000 to 13,000 feet (3,400 to 4,000 meters).
In 2023, six years after planting the seeds, they found that at two of those heights, 11,800 and 12,400 feet, nearly 70 percent of the seedlings had survived even at the higher elevation.
“These types of experiments are tremendously important,” says Sally Aitken, a professor in Forest and Conservation Sciences at the University of British Columbia, who wasn’t involved in the study. Aitken was also part of other field tests that involved moving whitebark pine, an endangered tree species in Canada, further north to protect it from warming temperatures and diseases like white pine blister rust.
However, assisted migration outside of a tree’s normal range comes with concerns, Aitken says. Oyamel firs don’t normally grow at this height on Nevado de Toluca’s slopes. Moving a species could have unintended ecological fallout for other species present in that ecosystem. There’s uncertainty around these field tests, she explains, but they’re crucial to better understand whether assisted migration could realistically save forest ecosystems.
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“We can’t implement these as solutions unless we know they’re solutions,” Aitken says.
In Mexico, droughts have made oyamel fir forests particularly vulnerable to threats like bark beetle infestations. Last summer, a mass mortality event occurred in Hidalgo, close to Michoacán, where many oyamel fir trees succumbed to drought and disease and had to be cut down.
To rehabilitate oyamel fir habitats in the Monarch Butterfly Biosphere Reserve, seedlings are grown in nurseries, like this one, and planted in reforestation areas. Efforts to restore reserve habitats through local planting programs are just as important as expanding the species’ range.
Photograph by Jaime Rojo
“It’s not that I’m negative or pessimistic,” says Sáenz-Romero, “I see the trees dying, and I know we need to take more active measures.”
For him, this meant taking on a more proactive approach for both the trees–and the animals that depend on them.
Monarchs on the move
Another big question is whether monarchs will find newly migrated forests. Research indicates that current wintering sites for monarchs in Mexico may become unsuitable for both the oyamel fir trees and the butterflies in the future, and some monarchs are already moving elsewhere. Last winter, monarch butterflies decreased by nearly 60 percent on Mexican wintering grounds. And surprisingly, conservationists found the largest colony several miles away from the reserve—roosting in the forests of San Antonio Albarranes, close to the Nevado de Toluca volcano.
(Why aren’t monarchs protected by the Endangered Species Act?)
The butterflies are looking for new, colder sites because the past sites are warmer, says Sáenz-Romero, who has done past research on the habitat’s historic and future climate. On Nevado de Toluca, the temperature at 11,100 feet is one degree Celsius colder than at the same elevation in the monarch reserve.
“If the monarch migration to this part of the world is to continue, both the trees and the monarchs will need to move,” says Karen Oberhauser, a biologist at University of Wisconsin-Madison who was not involved in the study. According to Oberhauser, who studies monarch butterfly ecology, assisted migration could be a possible solution; however, whether it will work remains to be seen.
“Unfortunately, climate change on the one earth we have is like a giant experiment with a sample size of one,” Oberhauser says, “not an ideal situation.”
For now, in Mexico, Sáenz-Romero hopes this new study convinces conservationists and government officials that assisted migration can establish new potential wintering sites for the monarch butterflies. Preserving the current monarch reserve is equally important as creating potential sites, he says.
These new trees are a field experiment, he adds, “we need to plant thousands more.”
