The title points to a ring of timber posts, and the poem pictures one tree turned over: roots in the air, spreading like branches into a "dark sky of the dead." The image is strange and a little unsettling. Something that should reach up toward light has been made to reach down, or up into darkness, as if the tree now belongs to the underworld.
The second stanza shifts to "who," meaning the dead. They "feed & return," so death here is part of a cycle rather than an ending. They stand in circles, like the posts themselves, and every spring they sprout again with "leaves of malachite," a green stone-like color that makes the new growth feel both alive and ancient.
The speaker doesn't state strong feelings outright. Instead the tone is quiet, watchful and reverent. The mood is eerie but oddly comforting, suggesting that the dead are not gone but keep nourishing the living world.