7000 Eichen (7000 Oaks) is Joseph Beuys's vast public artwork begun in 1982 and finished by his son in 1987 — one of the most ambitious works of land art ever attempted. Across Kassel, 7,000 oak trees were planted, each next to a basalt column shipped in from a quarry north of the city. As the trees grow taller and the rough stones stay the same size, the relationship between the living and the inert keeps shifting.
The original installation was a pile of 7,000 basalt columns stacked on the Friedrichsplatz. Each time a tree was planted, one stone was taken from the pile and placed next to it. The pile shrank as the city's tree cover grew. You can spot the pairings all over Kassel — once you know to look, the columns appear on street corners, in parks, beside playgrounds, and along the river.