By weekend's end, roughly 150,000 tree and shrub seedlings will have been planted at the Flight 93 National Memorial over the past decade/NPS

It has taken thousands of volunteers willing to get down on their hands and knees, and more than 150,000 seedlings just inches tall, but slowly a forest is regrowing around Flight 93 National Memorial in Pennsylvania.

The forest of trees -- red and white pine, red spruce, American chestnut, red oak, black locust, and many other varieties -- was called for when the memorial was designed to honor those who died on September 11, 2001, when they fought hijackers who had commandered a commercial airliner en route from Newark, New Jersey, to San Francisco. The hijackers had planned to crash the jetliner into the U.S. Capitol, but instead forced it down in a rural corner of the Commonwealth as a group of passengers tried to regain control of the jet.

This weekend, during the tenth annual Plant a Tree at Flight 93 event, roughly 500 volunteers are expected to plant the final 14,000 seedlings across 20 acres of the memorial on ground that once was gouged open by a surface coal mine. When all is done, more than 4,200 volunteers will have planted 41 varieties of native trees and shrubs. Past studies on the project have indicated a survival rate of better than 70 percent.

"The deep mining was still actually active on the day the plane crashed here, but the surface mining had been in early stages of reclamation," said Adam Shaffer, the memorial's chief of interpretation and education, during a phone call Tuesday. "It was around 1995 that they really started the reclamation process where the plane crashed."

To guide the reforestation effort, experts were called in to both specify the varieties of native trees that would normally grow on the site and blend in with the surrounding forest that rims much of the memorial, and also to prepare the ground so the trees would take hold, he explained.

"Talk about going from absolute desolate land to creating something here. ... These trees [from the first few years of plantings] are taller than me, and I'm 6-foot. We have some trees that are seven, eight feet tall now as a result of using this process," said Shaffer. "It's called the forestry reclamation approach. And we partnered with the Office of Surface Mining and their scientists.

"Basically, what we do to prepare the ground is to go in with a three-foot ripping shank on the back of a D-seven dozer on these legacy mine sites, and we rip it one direction and then go on contour with the landscape," the chief continued. "And at those intersections is where they put small plugs, or these small seedlings and that allows the roots to expand and do their best."

Flight 93 National Memorial, 2018 planting/NPS

Shaffer explained that volunteers are paired up, with one wielding a bar used for creating a pocket in the soil, and the other one pulling seedlings out of a bucket and planting them in the hole.

"It's all randomized," he said. "They pull out a seedling, they make sure that the roots are all going the right direction, put it in the ground, and then the person comes on the backside and they close that pocket up nice and tight so no air can get to the roots. And that's how we've successfully managed to do this project. Dating back to 2012, we're just around 200 acres that have been planted on the site."

The resulting forest, along with hiding the scars of coal mining, creates wildlife habitat and restores a pastoral setting to the landscape.

This coming Friday and Saturday the Friends of Flight 93 National Memorial, the National Park Foundation, and the National Park Service will return to the memorial with thousands of seedlings and hundreds of volunteers to finish the task.

“Plant a Tree at Flight 93 has been a tremendous event and represents one of the most successful and popular volunteer activities at the Memorial. This year will be no different. As we successfully close this chapter, we look forward to future opportunities to partner with our communities, the National Park Foundation and the National Park Service,” Fred Lukachinsky, president of the Friends of Flight 93, said in a release announcing the event.

This graphic depicts how many trees a year have been planted at Flight 93 National Memorial/NPS

“Although this year will culminate with the conclusion of Plant a Tree at Flight 93, the original design intent by Architect Paul Murdoch has been achieved. The Flight 93 National Memorial partnership plans to continue the tradition of bringing volunteers together and working on various elements across the 2,200-acre memorial landscape,” said Flight 93 National Memorial Superintendent Stephen M. Clark.