Titled 7000 Eichen - Stadtverwaldung statt Stadtverwaltung (English: 7000 Oaks - City Forestation Instead of City Administration), was an idea in "land art" created by German Fluxus artist Joseph Beuys (1921 - 1986).
The work was inspired by Guy Debord and the Situationist International's Theses on Traffic (read that here), which was first published in Internationale Situationniste, issue #3, in 1959.
The 7000 Oaks project in New York City was funded by The Dia Art Foundation, and was first installed in 1988, with only five trees (Oak, Bradford Pear, Gingko, Sycamore, and Linden). In 1996, eighteen new trees were added (including the aforementioned, as well as Pin Oak, Elm Honey Locust, and Red Oak).
If you find yourself in the area to check these out, also take a walk down High Line Park, which is a 1.45-mile-long (2.33 km) garden park, created using a section of New York Central Railroad's West Side Line.
Originally built in 1831, most of it was demolished in 1960. Originally, it ran from 23rd Street to 129th Street, but was extended south to Spring Street in 1851.
The section that remains stretches from 12th Street to 33rd Street, but the park opened in phases, with the first being in 2009 (Gansevoort St to 20th St), then phase two in 2011 (20th to 30th St), and phase three in 2014 (finishing just before 33rd St). The section pictured first (the 30th Street Spur) will open this year.