The basic rules for constructing a sukkah are:
The structure must be temporary, have at least two and a half walls, be big enough to contain a table, and have a roof made of shade-providing organic materials through which one can see the stars. (Source)
The Washington Square Park sukkah was built by Chabad House at New York University. Located slightly southeast of the Arch, it is made of variously sized wood boards and topped with branches of arborvitae (Thuja). Arborvitae is classified as a tree or a shrub and means "tree of life"; the Morton Arboretum describes it as long-lived with decay resistant wood.