It's a Chokecherry Tree

"After I left you, those boys came in there and took my milk. That's what they came in there for. Held me down and took it. I told Mrs. Garner on em. She had that lump and couldn't speak but her eyes rolled out tears. Them boys found out I told on em. Schoolteacher made one open up my back, and when it was closed it made a tree. It grows there still." pg 19-20

Sethe is beaten by Schoolteacher's two nephews. They leave a scar on her back in the shape of a tree which reminds her in the present day about all the bad memories and terrible things that happened while being a slave at Sweet Home. This tree on her back represents the pain of her past which is constantly following her. The emotional and physical scars caused by Schoolteacher continually plague Sethe, who, before Beloved returned was engaged in a constant battle with keeping those memories at bay, for the good of herself, and the good of Denver.