Oak Architecture, Elm Art

The house is suspended 10 feet in the air by a compression ring that allows its sugar-maple tree to breathe and get water. Resembling a flying saucer, the platform is surrounded by sheets of stainless-steel mesh.

Other tree houses were designed to wrap around trees, stand next to trees or be attached to bands that skirt the trunks.

Peter Olin, a University of Minnesota horticulture professor and director of the arboretum, said the aim of the exhibit is to attract people, particularly families, to visit the arboretum.

"Everyone can relate to a tree house," he said. "You don't have to explain it. You know it from when you were a kid. Most people had a tree house, or their friend had a tree house."

Olin said the tree houses are far from the board-and-nails images that come to most people's minds. "They're interesting approaches to tree houses," he said. "Maybe 'tree places' is a more appropriate word. They're rather wild creations."

Like architect Jon Vandervelde's tree sculpture, titled "Growth." Using 4-inch-diameter carbon steel tubes, Vandervelde created a three-dimensional lattice that spirals up a tall red oak tree. Vandervelde welded the pieces, which resemble bits of a DNA strand, around the tree using rope, his rusted van and the help of a few passersby.

Some tree houses are interactive. "Tree Man" was already generating buzz among arboretum employees as a construction crew worked to bend long sandbar willows into the shape of a head and an arm hugging a sprawling oak tree.

Landscape architect Marjorie Pitz designed the tree to be fully accessible. Visitors are invited to walk through the side of the wickerwork man's head, stand inside the dome shape and take in the view through the woven willows before exiting through his wrapped arm. The arm even has a wheelchair-accessible entrance.

Pitz said the house represents the spirit of the tree, a concept found in many cultures, and that she hoped it would introduce architectural concepts to children. "From inside, it is like light through a cathedral gable -- but child-scale and made of natural material," she said.

Pitz said harvesting the limbs from native willows actually helped the trees because it promotes new root growth, which was part of the reason she selected them. Pitz, who was working with a construction crew from Witcher Construction Co. of Eden Prairie, Minn., said they cut about 500 big branches and about 100 smaller ones.

She said that the building process is trial and error, and that each step is an experiment. She was relieved to see the structure finally coming together.

And, as she worked on a way to support the fingers of the tree man, some of her target patrons -- schoolchildren -- passed by.

"Look at that big tree man. It's cool!" said one little boy. "I want to climb in it!"

On the Net: www.arboretum.umn.edu