Saving Mexico’s fir forests could help monarch butterflies

An experiment to grow new forests in central Mexico offers hope that the crucial winter habitat for millions of migrating monarch butterflies could survive into the next century.

When scientists decided to plant hundreds of baby oyamel fir trees (Abies religiosa) about 100 kilometers from their native habitat, they weren’t sure how many trees would survive. Today, most of the saplings are flourishing, researchers report September 17 in Frontiers in Forests and Global Change. Even at an altitude of 3,800 meters, high above where the trees usually grow, almost 70 percent of the saplings survived at least three years.

While moving a whole forest may sound like a drastic measure, “desperate times call for desperate measures,” says Karen Oberhauser, a conservation biologist at the University of Wisconsin–Madison who wasn’t involved in the research. “If we don’t help organisms move around, you know, we’re just going to lose a lot of ecosystems.”

Each fall, after monarchs (Danaus plexippus) migrate from the milkweed-laden meadows of southern Canada to the mountains of central Mexico, they hibernate exclusively on the oyamel fir. Thousands might cluster on a single branch, causing it to droop under their collective weight. But the forests — and the butterflies who hibernate within — are at risk (SN: 4/4/11). Monarch butterfly populations continue to decline. And climate change projections predict that oyamel fir will vanish almost entirely by 2090.

“I know that this sounds crazy, but we need to move the forests to a higher elevation,” says Cuauhtémoc Sáenz-Romero, a forest geneticist at Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Mexico. Oyamel fir, which grow from around 2,400 meters to 3,500 meters, need cold mountain air to survive. The high-elevation chill also works to slow the metabolism of butterflies, allowing them to survive the long winter. As central Mexico gets warmer, new generations of oyamel fir will likely creep higher up their native slopes. They could soon run out of mountain to climb.

Sáenz-Romero wants to move the trees to taller mountains, but he’s aware that they won’t get there by themselves. “Unfortunately, the scene in The Lord of the Rings, where the trees are walking toward battle — it’s just fiction. It doesn’t happen.”

His team collected oyamel fir seeds from elevations between 3,100 meters and 3,500 meters inside the Monarch Butterfly Biosphere Reserve in Michoacán state and grew them temporarily in a tree nursery. Then, in partnership with the Indigenous community in Calimaya, the team planted around 960 trees at four different elevations in the community’s forest on the Nevado de Toluca volcano.

Some seedlings were planted at 3,400 meters — similar to typical oyamel fir that live inside the butterfly preserve. But Sáenz-Romero wanted to determine just how much altitude an oyamel could take. Other trees were planted higher, in colder climates, at 3,600, 3,800 and 4,000 meters above sea level.

If the fir could take root at higher-than-normal elevations, the trees might thrive there in the future as temperatures warm, Sáenz-Romero hoped.

Three years post planting, the team found that young fir trees were smaller and shorter the higher they sat on Nevado de Toluca. Still, many made it past their first year, which can indicate long-term survival. On average, 80 percent of the seedlings that got moved to locations 2.3 degrees Celsius colder than their home regions, survived at least three years.

Turning the experiment into reality likely would face many hurdles, including getting community and government support. And even if the trees can survive longer term, another question remains: Will the monarch butterflies find them?

During the winter of 2023–2024, some large colonies of monarchs didn’t hibernate inside the borders of the Monarch Butterfly Biosphere Reserve. They flew to other forests. “My guess is that monarchs are already searching for colder places,” Sáenz-Romero says.