Editor's note: The following release was prepared by the staff at Flight 93 National Memorial in Pennsylvania.

On April 17-18 the National Park Service, the National Park Foundation, and Friends of Flight 93 National Memorial are continuing a major reforestation effort that will eventually result in 150,000 new trees at Flight 93 National Memorial, the nation’s permanent memorial to the 40 passengers and crew of United Flight 93, and all of those lost on September 11, 2001.

Every year during National Park Week, America’s national parks host public events and coordinate volunteer opportunities to celebrate the special occasion. For a fourth year, Flight 93 National Memorial will honor National Park Week with Plant a Tree at Flight 93. This event is one of the many activities the National Park Service will offer as part of the 2016 Centennial celebration. The goal of the National Park Service Centennial is to connect with and create the next generation of park visitors, supporters, and advocates.

The tree-planting events will take place from 9 a.m. to 2 p.m. on April 17-18. Businesses and organizations have already committed to providing more than 150 volunteers each day, but 50 additional individuals are still needed each day to complete the work. All interested volunteers must pre-register online (first-come, first-serve) and be at least 14 years of age. Flight 93 National Memorial will still be open to the public during regular visiting hours (9 a.m. to 5 p.m., with last entry at 4:30 p.m.) on the days of the tree-planting effort.

New this year, special teams of volunteers will begin to remove invasive species such as autumn olive which threaten the native plants at the memorial, including the tree seedlings that are being planted. Teams will remove these plants near the current tree planting site as well as other key areas at the memorial under the direction of invasive species specialists.

To date, over 1,500 volunteers have assisted with planting over 52,000 tree seedlings at Flight 93 National Memorial. This year, a team of more than 400 volunteers under the guidance of tree-planting experts will have the special opportunity to continue this effort and plant an additional 22,000 seedlings at the memorial. According to Keith Newlin, Western Pennsylvania National Parks Deputy Superintendent, “The 32-acres planted by volunteers  this year will begin the development of the Tower of Voices site. We are working with the architect to implement the planned memorial landscape.”

Many sponsors have joined with the National Park Foundation to make this effort possible including Arbor Day Foundation, Enterprise Rent-A-Car, Roy A. Hunt Foundation and The UPS Foundation.

Additional National Park Service partners include the American Chestnut Foundation; Appalachian Regional Commission; Appalachian Regional Reforestation Initiative; Department of Agriculture, U.S. Forest Service; Department of Interior, Office of Surface Mining, Reclamation, and Enforcement and U.S. Fish and Wildlife Service; Green Forests Work, Home Depot; REI; Indiana University of Pennsylvania; Penn State University Altoona; Penn State University DuBois; Pennsylvania Department of Conservation and Natural Resources, Bureau of Forestry; Pennsylvania Department of Environmental Protection, Bureau of Abandoned Mine Reclamation; Pennsylvania Game Commission; Starbucks, Somerset.

As a kick-off to National Park Week, visitors can enjoy a Fee Free Weekend on April 18 and 19 at all national parks nationwide. For a full listing of all National Park Week events in Pennsylvania and across the country, go to www.nationalparkweek.org.