The types of trees used in the Memorial are swamp oak white trees. Walker selected within the 500-mile radius of the World Trade Center site with the sturdiest trees with straight trunks, five inches in diameter and no more than 20 feet tall to symbolize strength, durability, and longevity. The trees are buried in 40,000 tons of soil were arranged in symmetrical rows and as the trees developed, and the branches were pruned to mature to create the effect of a gothic arch. The grove of trees brings green rebirth in the spring, provides cooling shade in the summer and shows seasonal colour in fall. The leaf colour ranges from yellow to a golden brown, sometimes pink in the fall and dark green in the summer. The trees in the present reach heights as tall as 60 feet in conditions similar to the plaza. The Memorial Glade, a small clearing in the grove, designates a space for gatherings and special ceremonies. With its grove of trees, the Memorial’s Plaza created a green roof for the structure of the 9/11 Memorial Museum, a train station and other facilities 70 feet below street level. Landscape architecture firm Peter Walker and Partners designed the plaza and a “suspended paving system”8 to support the swamp white oak trees growing on the plaza. The paving of the plaza lays a series of concrete tables that the plaza has of nutrients for rich soil for the planted trees. The system’s design allows for stable pavement on which people can walk, while providing a space for soil for healthy tree growth. Also in the pavement, snowmelt and spring rain are collected in drains, stored in tanks, shown in the image above, then pumped up for irrigation in summer and fall. The trees will never be identical, growth rate at different heights and changing leaves at different times, a physical reminder of living individuals. A Callery pear tree became known as the “Survivor Tree” after enduring the September 11, 2001, terror attacks at the World Trade Center. In October 2001, the tree was discovered damaged, with snapped roots, partly burned with broken branches. The tree was removed and placed in the care of the New York City Department of Parks and Recreation. After its recovery, the tree was returned to the Memorial in 2010, growing from eight feet to 30 feet tall, created “a visible demarcation between the tree’s past and present and today, the tree stands as a living reminder of resilience, survival, hope, and rebirth.”9 The waterfalls located inside the edge of the memorial pools in the field of trees were designed to the shape of the droplets falling into the fountain. The droplets were a “veil of tears beading the pools sides rather than the clinging walls of water.”10 The water droplets cascaded down to the dark grey granite panels that lined the interior of the pool. The sound of the waterfall provides the tranquility and rainbows created by the sun shining through the mist of the waterfalls.