Mountaintop forests perpetually shrouded in mist are some of the most biologically diverse places on the planet—and, reports Jim Robbins from the Peruvian Andes, among the most endangered

Heights don't usually bother me, but the two-thousand-foot drop outside the van window makes my palms a bit sweaty. I bury my nose in a map as we slowly grind our way up one of those roads the Andes are famous for: a narrow switchback one-lane ribbon of ruts and rocks that slices across a precipitously steep mountain face. The road has claimed its share of travelers, and small shrines decorated with plastic flowers mark the spots where some poor soul took the long plunge. That was before the late nineties, my driver is quick to assure me, when the government got serious about road maintenance.

As we left Cuzco and drove up the west side of the South American cordillera, the landscape was bone-dry and nearly treeless, broken only by a patchwork of wheat fields and mounds of yellow corn drying in the sun. Cows pulled wooden plows through the red-brown soil. Then, after five hours, we burst over a mountain pass into a radically different world—the wet, eastern side of the Andes, where thick jungle is enveloped in a ground-hugging fog.

This is Peru's Kosñipata River Valley. Kosñipata is a Quechua Indian word that means "the place with smoke." It refers not to fires but rather to the clouds that smother the mountaintops, race through the valleys, drift like giant hot-air balloons across the landscape. Here, and in hundreds of mountainous forests around the globe, are where clouds live when they are on earth.

I am in the Kosñipata, one of the most biologically fecund places on the planet, to explore firsthand how this unique tropical-montane ecosystem works and to find out what impact climate change is having on cloud-shrouded preserves. Sixty percent of them are concentrated in Asia (mostly in Indonesia); a quarter are in southern Mexico, Central America, and the South American Andes; and the remaining fifteen percent are in Africa, including Rwanda's Volcanoes and Uganda's Bwindi national parks, home to the planet's last mountain gorillas.

Cloud forests are among the most imperiled ecosystems, and should they be diminished, the loss would be incalculable. Global warming is causing the mists that blanket these mountains to rise in elevation, leaving the forests open to the ravages of direct sunlight. In 2002, the United Nations and the World Conservation Union and other groups launched the Mountain Cloud Forest Initiative to raise the profile of these regions and to advocate for their protection. Critical because of the superabundance of species they harbor (there are as many types of trees in this one valley as in all of North America), cloud forests are also a reliable source of clean water for many developing nations. Those of Honduras's La Tigra National Park, for example, furnish forty percent of the water for the city of Tegucigalpa, and Dar es Salaam, in Tanzania, gets one hundred percent of its water from the cloud-shrouded peaks of the Uluguru Mountains.

While global warming is the chief threat to cloud forests, changes on the ground also pose hazards. The so-called death-by-a-thousand-cuts scenario is common in developing countries—the result of logging concessions, subsistence farms, new roads, oil pipelines. Cloud forests are giving way to coffee plantations in Colombia and cardamom farms in Sri Lanka. The mountain forests of the eastern escarpment of Madagascar have been reduced from 27,000 square miles in 1950 to fewer than 4,000 today because of clearing for subsistence agriculture. And in Vietnam's Tam Dao Mountains, logging has destroyed canopy cover for butterflies and other species.