Each year, hundreds of millions of monarch butterflies over several generations find their way to the high mountains of central Mexico. There, they overwinter by clinging to the branches of the sacred fir tree Abies religiosa in massive blankets of fluttering orange and black.
This migration miracle is threatened as global warming in the coming decades pushes temperatures too high for the forests of this revered tree to survive, even at the summits of the mountains where they now grow. But a recent experiment suggests this rare habitat might survive, if humans help the trees make a journey of their own.
“In this era of the Anthropocene, it may be possible to establish high-elevation A. religiosa stands that can successfully provide overwintering sites for migratory Monarch butterflies in the context of a warmer world,” researchers reported Oct. 18 in Frontiers in Forests and Global Change.
The experiment testing the ability to grow tree plantations for butterflies illustrates the desperate measures being considered to tackle two intertwined problems caused by climate change. One is the need for both plants and animals to move as their current habitat becomes inhospitable. The other is the plight of high-altitude organisms that are effectively trapped on shrinking islands, unable to escape warming mountaintops.
For the monarchs, the forests of A. religiosa currently max out around 3,500 meters on the summits of peaks inside the Monarch Butterfly Biosphere Reserve, a 56,000-hectare area protected from logging and other threats. But by the end of the century, nearly all of that area is predicted to be too warm and dry for the forests to endure. The monarchs rely on dense stands of the trees to protect them from rain and cold as they huddle together to survive through the winter.
Roughly 75 kilometers to the southeast, the Nevado de Toluca, an extinct volcano, offers a tantalizing alternative. It reaches more than 4,600 meters, higher than needed by the trees even at the end of this century.
But a forest doesn’t grow overnight. If conservationists are to create a higher-elevation forest home for the monarchs, it will take the better part of half a century. That creates a tricky balancing act. Is it possible to grow this tree at elevations that are higher than their comfort zone right now, in anticipation of temperatures decades into the future?
To find out, the researchers in Mexico gathered seeds from the existing forest. In 2021, they planted hundreds of seedlings at different elevations on the northeast flank of Nevado de Toluca. Then, they watched to see how well the trees grew. Even at the highest elevation site, at 4,000 meters, nearly half of the trees survived, although they barely grew at all and showed signs of frost damage. A little lower, at 3,800 meters, things looked more promising: two-thirds of the seedlings survived and they grew 54% less in height and 27% less in mass than the trees planted at 3,400 meters, roughly the same height as they came from in the monarch reserve.
The results were encouraging enough that it indicates the potential to successfully plant a future forest of A. religiosa at between 3,600 and 3,800 meters on this new mountain, the researchers reported.
“These planted stands could ultimately serve as overwintering sites for the monarch butterfly under warmer climates,” said Cuauhtémoc Sáenz-Romero, the lead author and a researcher at Mexico’s Universidad Michoacana de San Nicolás de Hidalgo.
The experimental sites are, however, a long way from the kind of dense forests that serve as a butterfly haven. Conservation groups and land managers would need to embrace this strategy and embark on a massive planting program. And, of course, the butterflies would need to be persuaded to fly just a little further and change their migratory destination.
There are signs this is already happening on its own.
“Monarch butterflies have over recent year established new and large colonies at colder places within the Nevado de Toluca, which suggests that they already are searching for new places to overwinter,” says Sáenz-Romero. “Once our seedlings are fully grown, they will hopefully discover our planting site, too.”
Sáenz-Romero, et. al. “Establishing monarch butterfly overwintering sites for future climates: Abies religiosa upper altitudinal limit expansion by assisted migration.” Frontiers in Forests and Global Change. Oct. 18, 2024.
Image: By dubassy/Envatobutt
