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Stanford Report, April 21, 1999

Global warming endangers tropical mountaintop cloud forests, study suggests


Global warming may endanger the survival of tropical montane cloud forests, according to a new study published in Nature April 15.

The forests, often located on mountaintops or ridgelines, rely on moisture from clouds to survive. A computer simulation study by Stanford biology doctoral student Christopher J. Still, Stanford biology Professor Stephen H. Schneider and Prudence N. Foster, an astrophysicist from the University of Tokyo, shows that increased temperature caused by doubling the level of carbon dioxide in the atmosphere may raise the altitude of clouds so much that they recede from mountaintops, in the process threatening complex ecosystems that contain many endemic species.

"This is an example of one location, or sets of locations, where seemingly small changes on the global scale can be very dramatic locally," says Schneider. "This could involve thousands of species that took eons for the co-evolution of climate and light to occur, that will just get wiped out."

Frequent cloud contact enables the forests to work as important regional watersheds. The sponge-like properties of epiphytes such as mosses and ferns that grow on the trees help provide flood and erosion control during the rainy season and water storage during the dry season.

"They’re really amazing forests," says Still. "When you see the tree limbs they’re just draped in epiphytes, moss and thick layers of humus and cacti. They’re almost glued to the trunks."

The forests are also home to a range of endemic plants and animals. For example, more than a third of endemic vertebrates in Peru rely on cloud forests as their primary habitat. In Monteverde, Costa Rica, the cloud forest is also part of the migratory pattern for exotic birds such as the male Resplendent Quetzal, a bird with a three-foot-long tail revered in Mayan culture.

There are only a few hundred montane cloud forests worldwide, but they are threatened by the same man-made pressures such as wood harvesting, grazing and resource logging that affect the better-known tropical rain forests.

Instead of focusing on deforestation, the Nature paper, "Simulating the effects of climate change on tropical montane cloud forests," looks at how increasing temperatures may affect the height of cloud formation. The study compared simulated vertical profiles of temperature and humidity for a doubled CO2 scenario and a present-day scenario. Doubled CO2 conditions are likely to occur by the end of the 21st century, according to scientists including Still and Schneider.

Schneider says that the argument gets complicated because increasing humidity creates clouds. "When you heat up the world you get more humidity, so why don’t the clouds go lower?" he asks. "The reason is the atmosphere gets even warmer and that evaporates the water and prevents [the vapor from] reaching condensation. So, you’ve got two factors fighting each other. One is the extra water vapor, which should make the clouds lower; the other is the extra temperature, which drives the clouds higher. In our model, the temperature wins."

Still says the research indicates that the height of cloud formation may rise during the winter season, the dry season when the forests typically rely most on the moisture from cloud contact. "Some places, such as Monteverde [in Costa Rica], essentially sit on a ridge line and, if the clouds on average form a couple hundred meters higher than they do now, then they’re not sitting on the forest," Still says. "In those situations, some localized species may be driven out."

The study looks at four montane cloud forests on four continents, located at varying altitudes and distances from oceans. In Central America, Monteverde offers a broad range of climate, vegetation and zoological research. In South America, the lower altitude forest at Serrania de Macuira sits on a peninsula on the northern shore of Colombia. The third site is Mount Kinabalu on the island of Borneo. Mount Virunga in Africa is a land-locked, high-altitude forest, close to Lake Victoria. It is home to the endangered mountain gorilla.

Some of these cloud forests, such as Monteverde, already are protected but Schneider says that more action is needed to save them.

"I would like people to pay attention to cloud forests, not just from the conventional concern of encroaching human land use and habitat destruction but from the synergism, the interaction of climate change and direct human disturbances like deforestation," he says. A combination of the pressure from expanding populations encroaching on territory, plus climate change, may try to push endemic species upward to higher elevations. "But how are they going to go up if they have nowhere to go?" Schneider asks, referring to the plants and animals that already live on mountaintops. "Once they’re gone, they’re gone." SR