This lovely exhibit by Tadashi Kawamata was recently installed in Madison Square Park, which I view as "my park" since I walk through it twice a day to and from work. It was marvelous to smell the pine boards and watch with wonderment while these works of art were being built high up in the trees. According to the New York Sun, Mr. Kawamata creates "site-specific sculptural installations." Please google him and look at his other works which vary amazingly.
PS - The squirrels in the park seem to love the tree houses as much as I do.