Silence.

Absolute silence is all Ken Christensen can remember hearing as he walked near the old hemlock grove with others visiting the United Flight 93 crash site in rural southwestern Pennsylvania.

The future 2200-acre park is in an area commonly known as the Laurel Highlands and lies on a plateau between the Allegheny Mountains on the east and the Laurel Mountains on the west.

In the present temporary memorial, there are lots of trinkets, coins, rocks and hats - little items left by visitors to honor the memories of lives that were ended much too soon.

"It's a sobering experience," says Christensen, a biologist with Stow, Ohio's Davey Resource Group. "It reminds you that these people - these mothers and fathers and sons and daughters - they were just going about their normal daily lives. And then what they did - they're about the closest you can get to real heroes."

That's why the National Park Foundation is leading a $30-million campaign to fund the construction of an actual memorial in full partnership with family members, federally-appointed advisory commissioners, local residents and the National Park Service.  Construction is underway and the memorial will be dedicated in 2011.

Christensen volunteered to visit the site in April because Davey Tree is donating its services to the cause.

The memorial plan will preserve many of the existing features and introduce newly constructed and natural elements. As envisioned in the General Management Plan, visitors enter the park via Route 30, and will first encounter the Tower of Voices, which will be set among rings of pine trees and house 40 wind chimes. The sound is meant to mimic many voices on phones as well as rushing wind - the last distant memories of the people who gave their lives on Sept. 11, 2001 to save so many.

Then after crossing through former mining areas, a bowl-shaped center will become the Field of Honor. Surrounding the field will be a tree-lined walkway on a raised earth form with 40 memorial groves comprised of 40 trees - one for each of the 40 heroes. A visitor center will sit where the plane initially crossed into the circle, near the Sacred Ground, the final resting place of the passengers and crew. Here, visitors will be able to approach the edge of the crash site.  A grove of hemlock trees will be visible in the background, as witnesses to the historic events that took place here on that day.

The trees chosen to surround the Field of Honor will exhibit vibrant fall color - mainly reds and yellows.  Christensen and others at Davey are helping recommend trees that best fit the landscape architect's plan and the site conditions. The site suffers from years of mining, which scarred the landscape. Acid mine drainage continues to challenge the area.

So far, Christensen believes that trees that seem to be doing best are those that adapt well in wetland situations, including red maples and service berries. The sugar maple on the other hand, prefers a more high and dry location.

"Ideally," Christensen says, "we like to suggest four or five different genera to ensure a healthy forest environment."

The problem with just one species is that one insect or disease that impacts one tree could mean not only devastation for that tree, but also other similar trees in the area. An example is the emerald ash borer affecting ash tree-lined streets.

"This way, if something goes wrong with one species, the whole area isn't potentially affected," Christensen explains.

In addition to building a permanent, publicly-accessible memorial at the crash site, the renovated area is meant to educate and inspire visitors about the heroism and human courage exhibited by the 40 passengers and crewmembers from United Flight 93.

"It should be a really wonderful project when completed," Christensen says. "This will be a permanent living monument to not only what happened, but to real life heroes."