Officials say it will take decades to replace and revitalize trees at the national memorial in western Pennsylvania to the crew and passengers who died there when a hijacked airplane crashed on Sept. 11, 2001.
About 700 dead or unhealthy trees were removed nearly a year ago and work has begun to add topsoil and plan for what is expected to be a lengthy effort to rehabilitate the trees in 40 memorial groves and along a central walkway.
