On the crest of the eastern Andes, about an eight-hour drive on a dirt road from Cuzco, Peru, is an expansive vista of one of the most diverse forests on earth. Storm clouds boil up in the pink evening sky, and fog advances over the foothills. The rain and fog suffuse the mountains with the moisture that makes them so astonishingly full of life.
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Miles Silman, a biologist from Wake Forest University in North Carolina, brought me to this ridge to introduce me to the cloud forests of Peru. Clouds born of moisture rising from the Amazon River Basin sustain a great variety of trees, which in turn support ferns, mosses, bromeliads and orchids that struggle to lay down roots on any bare patch of bark. It’s these epiphytes (“epi” means “on top of,” and “phyte” means “plant”), plus the wet humus soil, the thick understory of plants and the immersion in clouds, that distinguish cloud forests from other types.
Silman and other scientists are attempting to catalog and understand the plant and animal life in Andean cloud forests before it’s too late. Oil companies, having found petroleum and natural gas in the surrounding lands, are cutting roads and pipelines that scientists say are damaging some plant populations. Also, local farmers and ranchers clear cloud forest to expand their operations and harvest firewood.
Most significant, the cloud forests here are threatened by climate change. In other parts of the world, warmer temperatures in the past century have pushed native species toward the geographic poles or altered their seasonal growth and migration. In North America, for example, the ranges of the blue-winged warbler and other songbirds have shifted north; barn swallows and other birds are migrating earlier in the spring than they once did; and plants are blooming sooner. But cloud forests may be particularly vulnerable to climate change.
Of 25 biodiversity hot spots worldwide that conservation groups say deserve special protection, the tropical Andes is the richest by far, says biologist Lee Hannah of Conservation International. The region has almost twice as many plant species and four times as many endemic plants—native species found nowhere else in the world—as the next place on the list, the forests between central Mexico and the Panama Canal.
Many of the Andean plants have “shoestring distributions.” That is, the area where they can root, grow and reproduce stretches over hundreds of miles horizontally—but only hundreds of feet vertically. Says Silman, “I could stand upslope and throw a rock across the elevational range of many different species.” These plants’ preferred altitudes—and therefore the altitudes of the birds and other animals that feed on them, pollinate their flowers and disperse their seeds—are determined largely by temperature. And as the Andes heat up through global warming, these plants may be evicted from their natural homes.
I head toward the cloud forest with six biologists and one field assistant. We plan to hike about 75 miles round trip, gaining and losing 9,000 feet in elevation over passes approaching 13,000 feet. The first day, we climb from the Yavero River west to the summit of an unnamed mountain in Peru’s Manú National Park, one of the largest rain forest reserves on earth. Our goal is Callanga, a small valley in the heart of Manú. The initial pace of the hike has me breathing deeply, and I wonder if I will be able to keep up.
The Andes comprise high parallel ridges that follow the Pacific Coast of South America. In the north, these ridgelines can be moist on both sides, but in Peru, the western slopes are bone-dry and the eastern slopes are saturated by the mist and clouds. On the drive from Cuzco to our trail head, steep terraced farms covered the sides of these tall mountains like a mosaic. In the canyons between ridgelines the forest is mostly pine and eucalyptus, both introduced species. Farmers going back even before the Inca have removed much of the natural vegetation. Only when you get to the ridgeline next to the Amazon Basin, where we do our hiking, do native species start to dominate.
We reach the Manú Park outpost, above the tree line, just before sundown. In the morning we fill up on oatmeal and start down the other side of the mountain. Trudging toward the trees below, Silman points out that they are even farther down the mountain than they should be. For more than 5,000 years people have gathered firewood from this highest layer of vegetation and cleared the land for farming and grazing. The Inca, whose civilization flourished here from 600 to 500 years ago, were masters of terraced farming. Burning or harvesting trees is now prohibited in this national park, but enforcement on these isolated slopes is difficult. “We should be walking in forest,” says Silman, as we follow the muddy paths surrounded by low bunch grasses.
The trail descends into forest—and clouds. In places I can barely see the path in front of me for the fog. Everything drips. At 6,000 feet of altitude, forests get up to 20 feet of moisture a year from rain. Water from clouds may add another 5 to 20 feet. The moss, ferns, bromeliads and orchids that cover the tree limbs strip moisture from the clouds and hold it, acting as a giant sponge. At the same time, trees extend roots directly from their branches into the epiphytes, to steal moisture and nutrients. The forest is a massive twisted tangle of roots, trees and epiphytes, what Silman calls “stuff on top of stuff.” All of the water-swapping interactions among plants slow the flow of moisture as it makes its way downhill into the headwaters of the Amazon.