Trees in the Andes are moving uphill to escape warming

In the Andes the alpine landscape follows a fairly predictable pattern: tropical vegetation down below and far thinner vegetation at higher altitudes. Yet as the climate continues to warm, the tree line is creeping uphill.

Yet the trees are not migrating upslope fast enough to keep up with the pace of climate change, says an international team of tropical biologists from the University of Miami.

The scientists, who conducted a comprehensive study in the region, examined a database of nearly 200 small Andean forest plot inventories spread across more than 33.5° latitude (from 26.8° S to 7.1° N) from 360m to 3,360m above sea level throughout the Andean and Amazon forests of Colombia, Ecuador, Peru, and northern Argentina. The surveyed forest plots contained more than 2,000 tree species in all.

The researchers, who published a study in the journal Nature, found that tropical and subtropical trees across the Andes Mountains in South America are responding to warming temperatures by migrating to higher elevations with cooler temperatures in a phenomenon known as “thermophilization,” which means that heat-loving species can take the place of others that are less able to cope with high temperatures.

“Everything is moving up the mountain so the species near the tops of the mountains are running out of places to go and may soon face the risk of mountain-top extinction,” says Kenneth J. Feeley, the Smathers Chair of Tropical Tree Biology at the University of Miami who worked on the study.

Yet whereas tropical trees thrive in hot and humid areas, they are seeking optimal conditions, which they increasingly find further uphill in a trend that has been observed throughout the Andes. However, these tropical trees are far less adopted to life at cooler alpine areas than plant species in temperate forests, which are far more used to intense seasonal fluctuations in temperatures. This is posing a barrier to their uphill migration.

“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,” explains lead author Belén Fadrique, a PhD student in tropical ecology. “These ecotone barriers make it hard for plants to relocate their populations – and if they can’t relocate, they will go extinct.”

And that’s news for forest in the Andes. “Andean forests must be added to the growing list of ecosystems and species that lack the ability to quickly and cohesively respond to climate change and thus face high risk of extinction, biodiversity loss, and functional collapse,” Feeley said.

“Tropical forests are one of the most important players in the world’s global carbon cycle,” he added. “They slow down climate change by taking a lot of carbon out of the atmosphere and putting it into their growth. So the faster climate change happens, the faster we will lose our tropical forests, which in turn means that climate change will happen even faster.”