NEW YORK (AP).- After nearly nine years, life is returning to ground zero in a tangible way. Sixteen swamp white oaks arriving Saturday are the first of nearly 400 trees that will be planted at the World Trade Center site, where more than 2,700 people were killed when terrorists attacked the twin towers on Sept. 11, 2001. The trees will dot a cobblestone plaza surrounding two huge pools built on the footprints of the destroyed towers. “After all the tragedy, the idea of the first living component going back is emotionally significant to the rebuilding process,” said Tom Cox, CEO of Environmental Design, the Houston company that has cared for the trees and is taking them to the trade center site. Cultivated for four years at a nursery in Millstone, N.J., the 16 trees were to be loaded onto eight