At the base of the tree, I can just make out Genevieve, getting ready to come up. Very early this morning, she set our ascent rope with a hundred-foot shot over a high branch from her crossbow, and now she fixes her ascenders to the anchored line and snaps them to her harness with a locking carabiner. Most of the techniques and gear we use have been employed by cavers and rock climbers for years, but the tree climbers' argot is mostly their own: A "flying traverse" is a Tarzan-like pendulum swing from treetop to treetop. "Bark bite" describes a tree-inflicted abrasion. "Tree surfing" is euphoria-induced skylarking on a windy day.

Unlike most other sports, tree climbing takes place in a living organism (or "being," depending on whom you're talking to). Maybe that's why there's a kind of built-in pantheism to it. Trees have long provided places for us humans to hang any anthropomorphic whimsy that crosses our fuzzy little minds. To children, trees are sprites and fairy-tale monsters. To adults they are shade, board feet, gods or goddesses. To tree surgeons they are fickle, uncooperative, occasionally violent clients. For related reasons, the naming of trees seems an important part of tree climbing; this tulip poplar I'm in has been christened Ariel. A pair of TCI training oaks are named Nimrod and Diana. Julia "Butterfly" Hill, the tree-sitting activist, called the 200-foot redwood she lived in Luna. And in Missoula, Montana, where I live, there's a 60-foot Siberian elm I've come to know as Butch.

TCI founder Jenkins reveals his flower-power lineage when he speaks of wanting "to transform mankind's relationship with trees." And member Robert Fulghum, author of the spiritual primer All I Really Need to Know I Learned in Kindergarten, offers this koan-like assessment: "Tree climbing is a place to be rather than a thing to do."

"The intimate experience you get from climbing a tree opens the mind and heart to what a tree really is," says Sophia Sparks, co-owner of New Tribe. Of course, Sparks sells gear without which, for many, this degree of intimacy would be impossible; her company's Ness climbing saddle costs $70, and its Treeboat Hammock (for arboreal spooning or leisure) goes for $112. There are also Treeboat Blankets, Treeboat Pillows, and an insulating Treeboat Cozy hammock liner. This is all good-looking equipment, and quite reasonably priced. But it's hard not to think, Is this another deal where, to get back to nature, you gotta have the gear?

Unlike my own ascent of Ariel, Genevieve's is not inchworm-like at all. Her movements are fluid and economical; she reaches Ariel's first branches in half the time it took me. Watching a person come straight toward you up a rope produces a weird binocular effect, like she's tunneling at you through thin air. Genevieve arrives unwinded and ties in above me.

"Hey," she says, regarding us on our perches. "Lookit all these handsome dudes."

It's been a calm day so far, but now the wind kicks up. It roars through the branches like a great wave, loosening showers of seeds. There are shouts and whoops from all over the tree, and the climbers above me take pendulum swings, kicking out from the tree and rotating on their lines as the big tree sways. "Wow!" someone exclaims. "Beautiful!" The earnest TCIers call this "deep fun."

On the other hand, it's impossible to be in a tall tree in a high wind and not feel, at first, a little bubble of panic. Then, like an infant, you give yourself up to forces stronger and larger than you—the rush of the wind, the creaking, the rocking. The movement becomes comforting and familiar. Even looking down becomes comforting and familiar. In a big tree, it all feels right.

It occurs to me that Ariel was a sapling about the time the Pilgrims arrived. In 400 years, the lower plates of bark have simply sloughed off, much like rock from an old vertical face. This is a "wild" tree, and up until ten years ago it was a pretty safe bet that no human had ever set foot in its branches.