SAUKVILLE, Wis. — When Steff Merten asked who wanted to go first, of course it was the guy wearing a Spider-Man T-shirt who raised his hand.

Merten had shown members of Riveredge Nature Center’s adult tree climbing club how to tie knots and buckle into their harnesses and gone over the basics of climbing. As sunlight knifed through the tree canopy on a warm, humid day, it was finally time to head skyward.

Jeff Wojcik, a 32-year-old Spider-Man fan from Port Washington, volunteered to be first. Inchworming his way up, stopping to tie safety knots every five feet, Wojcik was soon six stories up, swaying in the same breeze undulating the branches and leaves around him.

“It’s nice up here,” Wojcik said.

Wojcik was soon joined by the other four members of the Saukville nature center’s first adult climbing club — a 69-year-old father and his 34-year-old daughter, and a married couple in their 40s — who climbed ropes tied to nearby red oaks.

Riveredge’s adult climbing club is not an outlier — quite a few adults are climbing trees. In some respects, it’s becoming a stampede into the trees as many folks around the United States and world are rediscovering the joys of scampering up oaks, evergreens, elms and maples. At a time when adult coloring books are exploding in popularity, tree climbing is becoming a trendy way to de-stress while also getting an unusual workout and reconnecting with childhood.

“They want to go back to being a kid,” said Merten, Riveredge Nature Center adventure program manager whose climbing helmet sports the phrase: “I speak for the trees.”

The Natural Resources Foundation of Wisconsin schedules tree climbs as part of its popular annual schedule of field trips throughout the state. After the first year’s tree climb field trip at Riveredge filled up quickly, organizers boosted the number of field trips to three last year. This year, all three field trips — open to 18 people in each session — were quickly snapped up, said Michelle Milford, the foundation’s outreach coordinator.

People signing up for the tree climb field trips say they want to try something different and add adventure to their outdoor activities, Milford said.

“It’s like yoga in the trees,” Milford said. “It gives you an entirely different perspective on where you are in the forest. It lets you be a bird or a squirrel for a day.”

This is what’s known at technical tree climbing, meaning climbing with ropes, as opposed to free climbing, or scrambling up a tree without equipment.

Harv Teitelbaum opened Tree Climbing Colorado in the early 2000s and is the founding president of the Global Organization of Tree Climbers, a nonprofit that spreads the practice of safe tree climbing. He trained a team at Riveredge that included Merten and has trained tree climbing facilitators at other nature centers and parks and rec departments across the U.S.

“A few years ago we sent out a blanket email to nature centers around the country letting them know how great tree climbing is, here’s what you can do with it. We got a tremendous response from Wisconsin,” said Teitelbaum.

When people join climbing clubs like the one at Riveredge, which has offered a youth club for a few years, they use the club’s gear: helmets, rope, lines, weights, carabiners and saddles. Most of the equipment is arborist gear and the saddle is a low-slung harness worn around the waist that feels like a chair when climbers sit back and dangle from a tree limb.

Teitelbaum has climbed trees for decades, an experience that constantly changes since every tree is different and the same tree can change from season to season, from tall redwoods that provide a straight up-and-down experience to spreading live oaks, which offer a lot of horizontal space to roam.

Based in Colorado, Teitelbaum said he’s learned things about trees he would never know if he hadn’t climbed them.

“Often times, ponderosa needles form an upturned basket and after it snows, the snow will partially melt during the daytime and refreeze, so there’s a little clump of ice in the basket of needles,” Teitelbaum said. “That clump of ice will melt and the tree gets to drink on that for a while.”

There are books for tree climbers or folks thinking about climbing trees, ranging from the technical “Tree Climber’s Companion” published in 2000 by Jeff Jepson to Jack Cooke’s “The Tree Climber’s Guide: Adventures in the Urban Canopy,” which came out in April and extols the virtues of climbing trees in London.

In an email interview, Cooke said he wrote his book to help people rediscover their childhood, remind them of simple pleasures and help them step back from their routines.

“When adults climb trees, they reconnect to the natural world in the most primal manner. Hanging from a branch, you take your life in your own hands. The world ceases to be composed of the manmade, replaced instead by natural rhythms like the movement of insects in the bark or the call of birds nesting high above you,” Cooke said.

This is the third year Riveredge has offered tree climbing and the first for an adult club; members pay $120 for five sessions throughout the summer. If club members can’t make one of the sessions, or if they join after the season begins, they can come during another tree climb on Friday evenings or Sunday afternoons for youths or prorate their fees, plus they must join the Riveredge Nature Center. The adult tree climbing club will meet from 3:30 to 6 p.m. on July 17, Aug. 21, Sept. 18 and Oct. 15.

Merten pointed out to club members that the first 20 feet of climbing are the hardest, especially the first time as climbers get the hang of it, struggle to tie knots and pull themselves up using the correct line. But as they go higher, it becomes easier and the rewards — spectacular views — are great.

Tom Rogers, 69, an Eagle Scout who is interested in ropes and climbing, thought it would be fun to join Riveredge’s new club with his 34-year-old daughter Megan Burgard, both of West Bend.

The first thing Burgard did when she climbed up to around 20 feet off the ground was a bat hang, hanging upside down with her arms stretched out.

“You feel weightless, like you’re floating in the trees,” Burgard said. “I was probably 10 or 12 the last time I climbed a tree. You don’t scurry up like I did when I was a kid. It’s a workout. You’re doing something healthy and you don’t realize it.”

Susannah Bird, 48, and her husband Jon, 47, of Saukville and natives of England, are Riveredge members who often hike the trails but wanted to see a different view of the nature center.

“I didn’t know what to expect,” Jon Bird said. “I was talking to people at work about what I was doing this weekend and when I told them I was climbing a tree, they thought I was mad.”