Preserving Mexico’s spruce forests can help monarch butterflies

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

When scientists decided to plant hundreds of small oyamel firs (Abies religiosa) about 100 kilometers from their native habitat, they weren’t sure how many trees would survive. Today, most of the seedlings are flowering, researchers report on September 17 Forest boundaries and global change. Even at an altitude of 3,800 meters, above where the trees normally grow, almost 70 percent of the seedlings survived at least three years.

While moving an entire 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 was not involved in the research. “If we don’t help organisms move, you know, we’re just going to lose a lot of ecosystems.”

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

“I know this sounds crazy, but we need to move the forests to a higher altitude,” says Cuauhtémoc Sáenz-Romero, a forest geneticist at the Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Mexico. The Oyamel fir, which grows from about 2,400 meters to 3,500 meters, needs cool mountain air to survive. The high-altitude cold also works to slow the butterflies’ metabolism, allowing them to survive the long winter. As central Mexico gets warmer, new generations of oyamel fir are likely to creep higher up their native slopes. Soon they may run out of mountains to climb.

Sáenz-Romero wants to move the trees to higher mountains, but he knows they won’t get there on their own. “Unfortunately, the scene in lord of the ringswhere the trees are marching to 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 within the Monarch Butterfly Biosphere Reserve in Michoacán state and temporarily grew them in a tree nursery. Then, in partnership with the indigenous community in Calimaya, the team planted about 960 trees at four different elevations in the community forest on the Nevado de Toluca volcano.

Some saplings were planted at 3,400 meters – similar to the typical oyamel fir that lives inside the butterfly tank. But Sáenz-Romero wanted to determine how high an oyamel could get. Other trees were planted higher, in colder climates, at 3600, 3800 and 4000 meters above sea level.

If the spruce can take root at higher-than-normal elevations, the trees may thrive there in the future as temperatures warm, Sáenz-Romero hoped.

Three years after planting, the team found that the young spruces were smaller and shorter the higher they sat in Nevado de Toluca. However, many survived their first year, which may indicate long-term survival. On average, 80 percent of seedlings that were moved to locations 2.3 degrees Celsius colder than their regions of origin survived at least three years.

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

During the winter of 2023–2024, several large monarch colonies did not hibernate within the boundaries of the Monarch Butterfly Biosphere Reserve. They flew to other forests. “My guess is that monarchs are already looking for cooler places,” says Sáenz-Romero.