For the last eight years, Forever Young has built universally accessible tree houses around the country, including at Paul Newman's Hole in the Wall Gang summer camp in Ashford, Conn., for children with cancer.

Forever Young recently created Lookout Loft, one of three tree houses in Longwood Gardens' "Nature's Castles" exhibit, and is a consultant for Morris Arboretum's canopy walk, which opens next year.

What motivates someone like Allen to make a life - and a living - out of tree houses that everyone can experience?

The former Penn Mutual life insurance agent says it's a desire to "give everyone, regardless of age, ability or agility, the chance to see the world differently, to play together and to enjoy the peace and freedom tree houses offer."

The world is no longer "a men's club that excludes people who are different. We're more about community," Allen says.

Whether community means family, block, city or planet, trees are treasures at every juncture.

They keep our air supply fresh by absorbing carbon dioxide and producing oxygen. They offer shade and shelter, lower heating and cooling costs, stem erosion, blot out noise and glare, reduce stormwater runoff, and minister to wildlife.

And for humans, being tree-borne - even for an hour at an arboretum near Philadelphia - can be thrilling. Fun, too.

You're rocketing through the sky. You're hiding from soldiers. You're daydreaming. You're snoozing. And, at least at Longwood Gardens and Morris and Tyler Arboretums, you're safe - unlike Allen, who describes his childhood tree house back in Charlestown, N.H., as "pretty hazardous."

This he says with a child's loud laugh.

- Virginia A. Smith