Warming is chasing cloud forests steadily uphill
Deforestation higher up on mountains will leave these Mesoamerican forests nowhere to retreat
Mist swallows the treetops in a cloud forest in the state of Oaxaca in southern Mexico. Bromeliads, mosses and other epiphytes cover the tree trunks, greatly increasing the surface area of the canopy, thereby allowing it to collect vast amounts of water from tiny fog droplets.
Santiago Ramírez-Barahona
Cloud forests are strange and ghostly places — akin to coral reefs hidden high on tropical mountains. Stunted trees loom in the mist, gnarled trunks and branches crusted in moss, lichens, orchids, ferns, bromeliads and even climbing cactus vines. Arboreal frogs and salamanders spawn in fog-fed bromeliad pools, and spider monkeys pause to sip drinks.
But these enigmatic forests are being squeezed by warming and deforestation.
Hundreds of tree and plant species that make up Mesoamerican cloud forests are being chased uphill by rising temperatures, at an average rate of 1.8 to 2.7 meters per year, researchers report in the March 7 Science. From 1979 to 2010, these forests retreated 84 meters uphill. At the same time, cattle grazing and deforestation higher on the mountains is pushing the forests downward 6.3 meters per year — squishing these ecosystems into ever narrower bands of territory.
The results are “alarming,” says Emily Hollenbeck, a tropical ecologist with the Costa Rica–based Monteverde Conservation League who was not part of the study. “These species are running out of space, and they’re running out of time.”
While cloud forests cover just 0.4 percent of Earth’s land, they harbor 15 percent of its bird, mammal, amphibian and tree fern species. Wander into one on a foggy day, and you may find that you’ve walked into a miniature rain shower: Tiny fog droplets condense onto the leaves above and drip to the ground, providing plants with extra water to supplement the rain they receive.
What’s more, “they produce water for a lot of communities,” says Santiago Ramírez-Barahona, an evolutionary biologist at the National Autonomous University of Mexico in Mexico City.
Ramírez-Barahona was long curious about how global warming would affect these forests. During his childhood in the 1980s and 1990s, he often visited his father’s coffee plantation in Coatepec, Mexico. He saw banks of cool fog descend each day on the Sierra Madre Oriental mountains, a major cloud forest site 15 kilometers west. When he returned in 2015, he was shocked to see that the clouds didn’t come down as far as they used to.
Rising temperatures and falling humidity were causing the base of the daily cloud layer to rise by a meter or two per year.
Starting in 2019, Ramírez-Barahona and colleagues tried to find out whether the forests were also migrating uphill, chasing these clouds. They analyzed old records from dozens of biologists who had visited cloud forests from Mexico to Cost Rica and collected plant samples for museums. They mapped the elevations where species were collected and spent five years writing and refining computer code to analyze the data, encompassing 362,000 plant samples and 1,021 species.
At least 380 of those species are retreating uphill from the lower reaches of their ranges, the team found. This included everything from sweetgum trees to tree ferns, climbing cacti and parasitic mistletoe. But encroaching pasturelands at the upper ends of their ranges prevented these plants from migrating into higher, cooler, wetter zones, says Ramírez-Barahona. “They are just hitting a ceiling, because there’s no forest remaining up there.”
The loss of these forests could reduce the water supply for farmers who live lower down, says Sybil Gotsch, a forest ecophysiologist who studies cloud forests at the University of Kentucky in Lexington. This kind of forest functions like “a sponge” with extra soil, moss and plants piled high in the trees — and thick layers of leaves and twigs on the ground. “The cloud forest is a giant three-dimensional structure that slows down and stores water,” she says, releasing it into streams even during the dry season, when rain is not falling — but fog is still present.
A 2017 study found that when Mexican cloud forests were cleared for pastures, the stream flow during dry seasons decreased by up to 50 percent, because the pastures weren’t absorbing fog or storing as much water.
Kenneth Feeley, a tropical forest ecologist at the University of Miami in Florida, who helped with statistical analyses on the new study, predicts that shrinkage of cloud forests will threaten many of the plants inhabiting them. “These species tend to have very small ranges, which means that they have very small population sizes,” he says. “They are inherently at risk for extinction.”
