When the World Trade Center towers fell on 9/11, debris rained down near Manhattan's historic St. Paul's Chapel. But an old sycamore tree took the brunt of the blast, and the 1766 chapel where George Washington once worshipped didn't even lose a window.

Sculptor Steve Tobin paid tribute to the tree and the tragedy in a monumental 20-foot bronze casting of the sycamore's stump and roots. The 2004 work, known as the Trinity Root, is the only art memorial on display near the site of the disaster.

The project was a turning point for Tobin. After years of working in bronze, he felt he could not make another piece to compare to the Trinity Root. He switched to working with steel, and the style of his root sculptures shifted from naturalism to modernist. The result, a series of works called Steelroots, is on display outdoors at the Minnesota Landscape Arboretum in Chanhassen starting Saturday and running through January.

While the arboretum has hosted special exhibitions for nine years, this is the first with a single artist whose work falls into the fine art tradition, said Sandy Tanck, the arboretum's manager of interpretation.

"It's a magical fit," she said. "This artist works out of reverence for nature, which resonates with us. It brings parts of plants that people often ignore out of the ground so we can consider them. And the way the art works in the landscape is fascinating to me."

Sixteen of Tobin's sculptures are on display within about a half mile of the arboretum's Oswald Visitor Center. Made from reclaimed steel pipe that show the scars of use, they range from a playful white piece with curly projections that rattle and bounce up and down when touched to a rust-colored, 40-foot giant that seems to grip the earth even as a lone root reaches for the sky.

"My training is in math and physics, but I've always been interested in nature and the earth," Tobin said by phone from his home in Pennsylvania. He has worked in clay, glass and bronze and is well known for sculptures of African termite mounds, the forest floor and sweeping bronze "walls" of bones. But Tobin said roots are a subject that could keep him busy for much of the rest of his career.

"The power of the unseen is what fascinates me," he said. "We look at trees and we focus on what's visually apparent. But what's important is what's below ground.

"That works for everything -- you look at people, it's the part where the content lies that is invisible. It's the same with books. We see only the cover."

Steelroots was displayed last year at the Morton Arboretum in suburban Chicago. Tobin said arboretums seem a natural place to display the works.

"People who come to the arboretum are predisposed to the message in the work," he said. "People who come to an art museum might see it just as art. My priority is nature.

"People come [to the arboretum] to worship trees and light and flowers. These pieces are intended not to dominate those elements, but to enhance them with these geometric lines."

Near the arboretum's visitor center is "Romeo and Juliet," naturalistic giant castings of the roots of a pair of trees that grew on two sides of a stream. The roots were "going down for support and reaching toward each other," Tobin said. "It seemed very romantic."

The only other named work in the arboretum exhibit is "Tango," another romantic piece that depicts two leaning figures in a dance-like move. Many of Tobin's Steelroots sculptures look like they might be dancing. But he rarely names them.

"I will sometimes name a piece so people know what the artist was thinking," he said. He prefers to have viewers put their own spin on his pieces. "If something is untitled, they have to bring their own life experiences to interpret it," he said.

So viewers who don't know the name of the angled black sculpture may well picture the massive, swooping black pipes as an elephant or dinosaur. At this time of year, the pipes in the sculptures echo the bare branches of the huge oaks that surround them. Walk under the works and the perspective changes, framing the sky or a distant tree or barn. That will change when trees leaf out.

"We wanted [the sculptures] there for all seasons," Tobin said. "I think viewers have to visit and revisit. In the snow, the dark pieces are very stark. The white pieces stand out in summer and spring, but in the winter we see the shadows more than the actual pieces."

While a couple of the sculptures move when someone pushes on them, people provide the kinetic element for the pieces, Tobin said. "As you move, they change shape," he said.

One sculpture -- a white three-legged swirl of pipes that spans a walking path -- casts the shadow of a peace sign. People who know Japanese will see characters for "mankind" and "great" in the same work.

Tanck said she hopes people walk through and under the pieces and sprawl on the ground to look up through the sculptures at the sky and the trees.

"They invite you to interact," Tanck said. "And they engender cloud-watching."

Mary Jane Smetanka • 612-673-7380