The study confirms for the first time that, like many other plant and animal species around the world, tree species from across the Andean and Amazon forests of Colombia, Ecuador, Peru and northern Argentina have been moving to higher, cooler elevations. But unlike the world’s more temperate or boreal forests, which are far more accustomed to dramatic seasonal shifts in temperature, tropical trees are running into environmental roadblocks at higher elevations that are thwarting their upward migration and threatening their survival.

“In the Andes, the ecosystems can change very fast and very dramatically, for example, from sunny and dry premontane forests to sopping-wet cloud forests. These changes, called ecotones, appear to be blocking species migrations,” said lead author Belén Fadrique, a University of Miami Ph.D. candidate who designed and carried out the study with Feeley, her advisor and UM’s Smathers Chair of Tropical Tree Biology. “These ecotone barriers make it hard for plants to relocate their populations—and if they can’t relocate, they will go extinct.”

For the Nature study, the researchers set out to answer a scientific call to include more tropical plants in the studies that investigate and predict the effects of climate change—the very call that Feeley and his fellow tropical biologists have been issuing for years because, as he notes, “the tropics include most of the world’s species and we know next to nothing about what those species are doing or how they are responding to climate change.”

Aiming to fill that void, the UM researchers joined forces with 18 other researchers from around the world to create a new database that tracks the livelihoods of thousands of highland trees on 186 plots of land across what is known as the Tropical Andes Biodiversity Hotspot.

Sitting at elevations from 300 to over 3,000 meters above sea level, the plots, most of which are about the size of an American football field and have been inventoried multiple times over the past couple decades, are spread across Columbia, Ecuador, Peru, and Argentina. Collectively, these forest plots contain an astonishing diversity—a total of 120 different plant families, 528 genera, and more than 2,000 tree species. That is more than double the species of trees than are found in all of the United States and Canada combined.

Using newly developed modeling techniques that combined the data from all the plots into a single comprehensive analysis, the researchers confirmed that, as temperatures in the high Andes rise due to global warming, heat-loving tropical trees are seeking more optimal conditions by migrating to higher and cooler altitudes. Consequently, the abundance of these heat-loving, or thermophilic, species is increasing on these plots through time while, in contrast, the abundance of cold-loving species is declining.

The researchers also were surprised to learn that, while this phenomenon of thermophilization is widespread across the Andes, the rate of the changes in forest composition are not uniform across elevations. They suggest “ecotonal roadblocks” may be, in part, responsible. In other words, as Andean species get pushed upslope by rising temperatures, they may quickly find themselves out of their comfort zones as other aspects of their habitat, such as rainfall and cloud cover, become intolerable.