Flirting with Danger

Somewhere in a forest, in the middle of a city, a boy hangs 50 feet in the air. Here amid the oaks and the tulip poplars, high above Philadelphia’s tony Chestnut Hill, the only thing that stands between him and the ground is some flimsy netting, its holes as wide as his foot. This is the new elevated walkway at Morris Arboretum.

Out on a Limb is a Swiss Family Robinson–style adventure for adrenaline junkies. Set down in the heart of the University of Pennsylvania’s 92-acre tree preservation, it zigzags 450 feet over an incline, giving visitors a bird’s-eye view of the grounds, whether they’re romping around in the giant hammock or roosting in a supersize “oriole’s nest” that dangles 25 feet over the forest floor. The less courageous can relax in a Japanese-style tree house or gaze at a vast collection of trees, some older than the Declaration of Independence. “We wanted to make opportunities for visitors to experience trees in ways they never normally could,” says Aaron Goldblatt, of the Philadelphia firm Metcalfe Architecture & Design, “and to do so in a way that had the perception of risk.”

The trick was to seem dangerous without actually being dangerous. Working with CVM Engineers, the architects laid the wood walkway on beefy steel towers, each structurally independent so that a freak windstorm or a toppling tree (or both) wouldn’t wipe out the structure entirely. The oriole’s nest is suspended via an industrial-strength triangular apparatus similar to gin poles, which are used in mining. And the hammock is made of 5/8-inch-thick polyethylene rope layered over two supporting nets. “We tried for five minutes to cut it with a knife and couldn’t,” Alan Metcalfe says. Phew.

Out on a Limb doesn’t endanger the environment, either. Morris Arboretum exists primarily to educate the public about trees, and that mission is written all over the walkway. Everything from the locally harvested black-locust deck to the nest’s prefab steel framework, which minimizes on-site construction work, was selected to leave scarcely a trace. Chopping down mature trees was clearly out of the question, so the architects built around them. Some pierce straight through the promenade.

The project has proved wildly popular. Forty-one percent more people have frequented the gardens since it opened last summer, says Robert Gutowski, director of public programs. Maybe it’s the novelty of scrambling with the squirrels or the romance of strolling in the tree canopy. Whatever the reason, it prompted one kid to say something never before uttered about an arboretum: “I’ve waited my whole life for this.”