Teitelbaum, a former Colorado Division of Wildlife outreach educator and part-time professor of environmental science with DeVry University, believes that tree climbing is actually beneficial; he says it exercises the tree, making its tissue stronger with each trip. And because climbers pay such close attention, they are more likely to notice if a tree is sick or damaged. Those with enough training can safely trim dead or diseased branches, or notify others, such as park officials, who ought to know.

But Jody Rice, an education outreach coordinator who works with the Georgia Department of Natural Resources to develop authorized tree-climbing programs, says that climbers need to exercise caution about things like trimming dead branches. "If you take dead tree limbs, you might make the tree healthier, but not make the ecosystem healthier," says Rice. Woodpeckers, for example, will only build homes in dead trunks and branches. Removing a dead tree branch potentially destroys important habitat.

Determining the impact of a sport still in its infancy is difficult, if not downright impossible. "At this point, I imagine tree climbers are so dispersed, there's not yet a problem," says Matt Dietz, a conservation ecologist with The Wilderness Society. But he predicts that if tree climbing continues to grow in popularity, researchers and land managers will have to deal with its effects on the environment.

Dietz is particularly concerned about birds, especially during the breeding season. Unless nests are conspicuous, like the top-heavy platforms constructed by ospreys and eagles, climbers are unlikely to realize there are nesting birds in a tree until it's too late. A parent bird upset by even a single disturbance to a nest could flee to another tree, leaving its young to starve or become dinner themselves, says Dietz. Simply avoiding trees during breeding season is not a realistic option, because birds in the United States tend to breed during spring and summer - the very times most outdoor recreationists prefer to be outside.

To climb a tree, a person must first get to it, which brings up a host of other concerns. "The trees are one thing, but I envision lots of people wanting to climb the same tree," says Myke Bybee, a public-lands advocate for the Sierra Club. That leads to the formation of informal trails, which destroy the surrounding grasses, ferns and other plants and pack down soil around the tree's roots, limiting its ability to absorb water and nutrients, he says.

Still, Bybee is not opposed to tree climbing per se. "Outdoor recreation in any form from hunting to hiking in general is a great thing," he says. "But we would want to make sure people take measures not to damage the trees and environment."

Even Jenkins admits that climbers can get a little tree-happy. "You can over-climb trees," he says. "Humans tend to kill or destroy what they love through the years."

This is why Jenkins and Teitelbaum are trying to establish standards for environmentally conscious climbing. They recommend starting and finishing at the same place, and using mulch to cushion the ground around trees. Both Tree Climbing International and Tree Climbing Colorado have published guidelines for safe and environmentally conscious climbing. And climbers are developing more tree-friendly gear, such as rubber-soled shoes rather than spiked ones, and protective cambium-savers - hose-like tubes that slide over ropes to protect the trees from rope burn.

Despite these improvements, Sillett advises staying out of old-growth forests altogether. "Our impacts are foreign to that ecosystem," he says, "and there will be impacts."

Just a few decades ago, rock climbing was more of an underground cult than a full-fledged sport, and mountain biking was a handful of hippies converting clunky street bikes into rough-and-ready trail riders. For the most part, the early climbers and bikers saw their sports as a way to connect with nature. Sierra Club director David Brower, for example, was also a rock climber who used his experiences to draw attention to places in need of protection.

But what started as a spattering of independent enthusiasts quickly exploded. The International Mountain Bicycling Association, which now has 32,000 members, says that Americans ride single-track nearly 600 million times a year. Each year an average of 10,000 climbers visit Happy Boulders, a bouldering site just north of Bishop, Calif. The vast majority of recreationists may be conscientious, but there are always those who drill permanent climbing routes into cliffs, smear desert varnish with climbing chalk, batter the backcountry trails with their knobby tires and simply refuse to pick up after themselves. As a result, rock jocks and bikers often find themselves at odds with land managers and environmentalists.

Tree climbing shares the adrenaline rush, expensive gear, and conquest-of-nature mystique of rock climbing and mountain biking. And trees are poised to become the next frontier for the nation's weekend warriors. Public officials who want to avoid previous mistakes are determined to rein in tree climbing almost before the sport gets off the ground.

Denver flat-out refuses to permit tree climbing in city parks, citing liability concerns and the need to protect the trees. With people repeatedly climbing city trees, "we'd have to be replacing trees all the time," says Jill McGranahan, Denver Parks and Recreation spokeswoman.

Teitelbaum blames a bureaucratic "cover my ass" mentality, and Jenkins scoffs at the notion that tree climbing is dangerous. "Up to this point, we have had no injuries in this sport, none, nothing at all," he says. "You can't say that for rock climbers. You can't say that for cavers. You can't say that for hang gliders." (There have been documented climbing accidents - even fatalities - among professional arborists, however. And not long after Jenkins made this statement, a recreational climber fell and broke his back, recounting the accident on Jenkins' school's Web site.)