The Giving Tree is the story of a consensual, non-monogamous BDSM relationship between a man--the Dominant, and a tree--the submissive.
The tree loves the boy, and for his part he enjoys touching her limbs, tasting her fruit, even carving his name into her skin. This makes the tree happy. Later, the boy uses her limbs to build a house for his wife, and the tree gladly allows this, too, and is very happy. She probably always knew that the boy had other subs in his life, but the exquisite pain made her feel things again she hadn't felt in years.
Eventually, the boy makes a boat from the tree's trunk, which she's not too keen on, actually, because really, everyone's got limits. Then the boy returns again as an old man and wants a place to rest, and the tree allows him to sit on her stump and is happy again, because it's really hard to find a good Dom when you live outside a major city.
Always establish a safeword.
Silverstein, Shel. The Giving Tree. New York: Harper & Row, 1964.
The Giving Tree (Slipcased Mini Edition)