every year, at the beginning of November, one of the most impressive natural spectacles in the world takes place in Michoacán, Mexico. Hundreds of millions of migrating monarch butterflies settle in the forested massifs of the country’s Monarch Butterfly Biosphere Reserve, roughly 100 kilometers west of Mexico City. Having flown south for eight months, beginning their journey in the northern United States or southern Canada, they hibernate here for the winter before mating in the spring.

After flying for more than 4,000 kilometers, the butterflies land in the oyamel fir trees of the Ejido el Rosario region, where for weeks they congregate, protecting themselves from the wind and the cold nights. Without these trees, the butterflies would not be able to survive their exhausting journey.

The oyamel fir grows in a very small climatic space, one that is humid yet cold. “Its distribution is very limited to the highest mountains in central Mexico,” says Cuauhtémoc Sáenz Romero, a professor at the Universidad Michoacana de San Nicolás de Hidalgo. Sáenz Romero is the lead author of a recent study that anticipates that this forest will gradually deteriorate to the point of disappearance as a result of climate change, endangering the butterflies.

For the roosting monarchs, the oyamel canopy acts as a buffer to the local temperature and humidity, Sáenz Romero explains. “During the day, under the shade of the oyamel, the environment stays 5 degrees Celsius cooler than outside. It is a protection against high temperatures. At night it is the other way around, resulting in a 5 degree Celsius warmer environment.” The density of the canopy also protects against winter rain. “If the temperature drops below zero and the butterflies get their wings wet, they can freeze. That’s why these trees represent such a particular habitat,” says Sáenz Romero.

After awaking from hibernation and mating in central Mexico, the insects fly north to Texas in the United States, where they ...