Paul Warchol

Many architects found their calling while building childhood treehouses, but Metcalfe Architecture & Design landed the dream project of designing a soaring canopy in the trees. “Out on a Limb” is a new permanent exhibit at the Morris Arboretum of the University of Pennsylvania in Philadelphia.

The Arboretum sought to attract a younger audience, but getting kids excited about trees is no easy feat. However, focus groups revealed that interjecting elements of risk and danger pique children’s interest, and a canopy walk proved the ideal solution. Principal Alan Metcalfe and partner Aaron Goldblatt worked with Arboretum staff to select the site, a steep ravine that would allow an ADA-accessible 450-foot walkway to project above the terrain.

Visitors enter via a wood hoop tunnel into the Tree Pavilion, a safe haven for acrophobics. Inspired by Chinese garden design, a moongate transitions from solid ground and frames a 250-year-old chestnut oak, the ostensible linchpin of the structure. Next, visitors choose between a slightly swaying suspension bridge to the “Bird’s Nest” or a metal grated walkway to the “Squirrel Scramble.”

The Bird’s Nest hangs by a chain from two chopstick-like columns and is woven from sustainably harvested branches. The enclosed space contains “eggs” that double as seats, allowing children to feel safe while taking in a bird’s-eye view of the forest. The Squirrel Scramble features rope netting that surrounds two trees. Though it’s completely safe, the experience of lying suspended 50 feet above ground is thrilling.

Paul Warchol

Trees guide the form of the canopy walk, and zigzagging switchbacks, also borrowed from Chinese garden design, draw visitors to different corners of the forest. At the end of the platform, breathtaking views shift with the seasons; trees form a solid wall in the summer, but winter brings clear perspectives of the valley and creek beyond.

Although local woods, including black locust and ironwood, form portions of the canopy walk, the architects selected recycled galvanized steel for the structure to avoid competing with the trees. Fabricated off-site, it was boomed into place by cranes to avoid disrupting the landscape. An arborist helped determine locations for a system of “micropiles,” 6-inch-diameter pipes driven as much as 100 feet into the earth, without disrupting roots. Unlike traditional treehouses, no structural elements actually touch any trees. On the ground, protective wood slat casings were installed around the trees to protect them during construction and extensive layers of mulch with stabilizing fabric insulated many of the roots from foot traffic.

Metcalfe Architecture & Design’s canopy walk successfully answered a call: Since the exhibit opened in 2009, the number of visitors to the Arboretum has increased 66 percent.