Awed before a yellow poplar, Walt Whitman sang of “the qualities, almost emotional, palpably artistic, heroic, of a tree.” Fondation Cartier’s “Trees”—a hybrid exhibition of art, philosophy, and biology—features site-specific sculptures (including one by the late, tree-like sage, Agnès Varda), work by indigenous tribes, and artistic meditations on the value of trees. The latest research shows that trees possess “plant intelligence,” enabling their symbiosis with other species. Listen to the lessons in the trees. —C.J.F.