For the sixth year, volunteers plant trees in a field at the Flight 93 National Memorial as part of a re-forestation initiative. (Jeff Swensen/For the Washington Post)

A few dozen people spread out on a bumpy, bulldozed hillside, piercing the ground, twisting up the dirt, digging holes and planting delicate seedlings.

Someday this rocky former mining site will be covered with tens of thousands of baby trees rising out of the ground. But volunteer efforts to plant along this Southwestern Pennsylvania expanse aren’t meant to cover up what happened here. Instead, the trees are meant to help people remember.

On Sept. 11, 2001, United Airlines Flight 93 crashed into this field, a hijacked San Francisco-bound jet ultimately felled after passengers and crew members revolted, preventing terrorists from reaching their intended target in Washington, D.C. Officials believe the terrorists wanted to crash into the U.S. Capitol Building — but ultimately hit the ground in Pennsylvania at more than 500 miles per hour — rather than fully lose control of the hijacked plane.

The “Plant a Tree at Flight 93” project — which has provided thousands of volunteer-planted baby trees in designated spots at the 2,200-acre site since 2011 — aims to honor the victims of Flight 93 with something beautiful and beneficial, officials said. This year, some 500 volunteers came out on Friday and Saturday to plant more than 11,000 new seedlings among 17 acres. By 2020, leaders hope to have 150,000 trees.

“It will help heal this scarred land where the crash site is, and where the mine used to be,” said Henry Scully, executive director of the Friends of Flight 93 National Memorial. “It is all part of the healing process.”

A US Forest Service ranger explains the planting of an American Chestnut seedling on the 6th consecutive year where volunteers planted trees in a field at the Flight 93 National Memorial as part of a re-forestation initiative. (Jeff Swensen/For the Washington Post)

The trees — including 15 species, such as black cherry, black locust, American chestnut, red oak and white pine — will help make the memorial site “a place of healing, respect and tranquility,” Scully said. The Pittsburgh native lost several friends in the 9/11 terrorist attacks on the World Trade Center in New York.

Flight 93 was one commercial airplane out of four that were hijacked in the United States on Sept. 11, 2001, with al-Qaeda terrorists taking control of the plane after it left Newark on its coast-to-coast flight. Because its takeoff was delayed, it was still airborne after airplanes hit the World Trade Center and the Pentagon. Passengers and crew, learning of the attacks and a national grounding order for all aircraft, worked to thwart the hijackers’ plans to use Flight 93 in a similar attack.

All 44 people aboard were killed — 33 passengers, seven crew and four hijackers — but no one on the ground was injured.

The memorial, part of the National Park Service, surrounds the area where the flight hit in a rural part of Pennsylvania, which by flight time is about 20 minutes from Washington. The tree-planting effort is intended to reforest the area and provide a windbreak for the memorial.

Volunteers broke off into teams of 20 Friday, with each member pairing up with a partner, one to do the digging, another to do the planting. The seedlings have about a 75 percent survival rate, with the rest succumbing to damage from winter weather or deer, Scully said. The baby trees stand about 1 to 3 feet tall, but after several years of growth, they will serve as a shield against the strong winds that blow across the memorial site.

The planting event attracted many people in conservation lines of work, like biologist Natalie Shearer, 39, of Pittsburgh. She squatted down with a seedling, while her co-worker and planting partner — Jesse Killosky, 29, of Finleyville, Pa., — stomped on a dibble bar to carve out the dirt. Both women work for AECOM, a Pittsburgh-based engineering firm that does environmental consulting.

“It’s really a good experience,” Shearer said. “It’s such a sacred site.”