And there are no signs of the sport slowing down. In the last five years, Jenkins has watched his class load jump from one a month to nearly three times that many. Teitelbaum regularly leads classes attended by up to 10 people, as often as once a week during spring and summer. Schools have opened elsewhere, including Dancing With Trees LLC in Alto, Ga., and New Tribe's Tree Climbing Northwest in Grants Pass, Ore. Several Web sites and chat rooms are devoted to the fledgling sport.

Along with classes, New Tribe offers climbing gear, including harnesses ($47), ropes ($100-$500) and tree hammocks ($300), for sleeping in the treetops. Its Web site - one of many devoted to the sport - even gives instructions for building a gadget that couples a fishing reel with a slingshot to get your rope up into the tree.

New Tribe's vice president, Viola Brumbaugh, says she's seen the company's profits grow 20 to 30 percent a year over the last decade, with about 70 percent of its gear going to recreational tree climbers. Business is booming:, a general tree-gear supplier based out of North Carolina, fills about 15,000 orders for recreational tree climbing gear a year and receives 50 to 100 requests for recreational tree-climbing catalogues almost every week, according to Tobe Sherrill, the company's founder and CEO.

Brumbaugh and Jenkins attribute much of the sport's growth to recent press coverage. CNN, National Public Radio and National Geographic Adventure have all covered it. Last April, Richard Preston published The Wild Trees: A Story of Passion and Daring, a book documenting researchers' work in the canopy of California's redwoods. Most estimates place the number of recreational tree climbers at around 50,000 worldwide, but New Tribe co-founder Sophia Sparks says the sport is "mushrooming."

Hundreds of feet above the loamy ground in the Pacific Northwest is another kind of tree climber: the pinky-sized wandering salamander. This rare amphibian uses its long, slim legs and squared-off toes to climb ancient spruces and cedars. It spends most of its life in trees 300 feet high, never touching the ground.

The wandering salamander's world might as well be a different planet. Epiphytes - plants that grow free of the ground - latch onto trunk and branch in the muted light. Ferns, lichens and moss create new soil layers, providing water and regulating microclimates far above the ground.

The moss alone can take hundreds of years to grow. It traps sediment, which in turn traps moisture and then gradually releases it, altering the temperature and humidity.

The ferns also hold soil with their networks of fine, fibrous roots, creating mats among the branches that are sometimes three feet deep. These fern mats collectively act as reservoirs - a single tree can contain enough mats to hold up to 8,000 liters of water. They provide habitat for salamanders and other wildlife. Even the harpacticoid copepod, a common microscopic stream crustacean, can be found living in the treetops.

Lichens function as fertilizers in this airy environment. Cyanolichens convert nitrogen into a form plants can use to absorb nutrients. Stephen Sillett, a professor from Humboldt State University who studies forest canopy ecosystems, has seen entire new trees, such as hemlock and Douglas fir, sprouting from a host tree's fertile branches.

But this strange world is as fragile as it is vital, and even the most carefully placed hand or boot can take its toll, knocking the ecosystem out of balance.

"Even if you're an expert, you can accidentally bump off big clumps of moss," says Sillett, who winces at the damage he inflicts in spite of 20 years' experience climbing for research. He often sees the old scars left by what must have seemed, at the time, like an innocuous tap of the boot. "We're cruising through the trees with big rubber boots," he says. "We're not part of its natural equation."

But some climbers believe that keeping people out of the trees is harmful in other ways. "The othering of human beings from nature is very destructive," says Teitelbaum. Climbing trees and encouraging others to do so builds an appreciation for the forest and the organisms that rely on it, he says. He acknowledges that deliberately climbing in vulnerable trees, such as those containing epiphytes, is akin to "almost intentional destruction." But overall, he says, "I think there's more benefit than harm."