In 1981 , he used an appearance at the Kassel Documenta 7 exposition to dye the Rhine .
In 1981 , he joined German artist Joseph Beuys in planting 7,000 oaks .
In 1981 , he used an appearance at the Kassel Documenta 7 exposition to dye the Rhine , and joined German artist Joseph Beuys in planting 7,000 oaks .