Researchers are moving a *forest* uphill to save monarch butterflies.

Nature reports on a peculiar, passionate project led by Mexican scientists who are trying to save an endangered species by transplanting hundreds of fir trees to higher, cooler elevations, about 400 meters uphill:

Forest geneticist Cuauhtémoc Sáenz-Romero at the Michoacan University of Saint Nicholas of Hidalgo (UMSNH) in Morelia, Mexico, has been relocating oyamel firs (Abies religiosa) in the Monarch Butterfly Biosphere Reserve, about 100 kilometres northwest of Mexico City, for the past 3 years. A study reporting the results of the experiment is currently under review at a scientific journal.

For nearly two decades, the idea of ‘assisted migration’ — moving species to new areas to rescue them from rising temperatures — has stirred controversy among ecologists. Opponents worry that species introduced into other regions could spread so much that they threaten organisms already living there.

Over the past 20 years, the number of monarch butterflies (Danaus plexippus) in North America has dropped by more than 80%, according to the Center for Biological Diversity, a non-profit group in Tucson, Arizona.

Rising temperatures and habitat destruction at the butterflies’ breeding sites in the United States and Canada are the major drivers of monarch declines, says Karen Oberhauser, a conservation biologist at the University of Wisconsin–Madison.

Extreme climate events threaten the Eastern monarch butterfly’s habitats at their wintering sites in Mexico, Oberhauser says. In 2016, for example, a severe storm damaged thousands of fir trees in the mountains of central Mexico. The loss of habitat, followed by freezing temperatures, killed 31–38% of the monarchs.

And Sáenz-Romero has estimated that rising temperatures will shrink the habitat suited to oyamel fir trees in Mexico nearly 70% between 2025 and 2035.