About 600 volunteers started planting the first wave of trees, about 15,000 altogether this year, on 23 acres Saturday at the Flight 93 memorial in Shanksville, Pa.
The spot is dedicated to the 40 passengers and crew who fought back against terrorists who highjacked control of the plane on 9/11.
More than 150,000 trees are to be planted at the site.
“The 40 heroes of Flight 93 rose up against their captors and spared the nation further harm on September 11,” Gov. Tom Corbett said. “We are planting these trees not only in their memory, but in honor of the futures they assured for countless citizens by planting the first seeds of resistance to assure our nation’s freedom.”