Themes of The Overstory include the importance of nature, conviction, and the interconnected nature of things.

The importance of nature is one of the main themes of the book. This is clear from Richard Powers's repeated use of the types of wood around the characters, their desire to save the virgin forest, and what they're willing to sacrifice to do so. However, the trees aren't only passive placeholders in the story; they have their own agency and do things like save the characters and bring them together to help save themselves.

Conviction is another important theme in the story. It's best modeled in what happens to Olivia. She and the group of people fighting to save the forest are out blowing up pieces of construction equipment. She's injured and on the brink of death when one of her friends suggests calling the police to get emergency medical help. She tells them not to; she knows she's going to die. All she wants is for them to finish. Her conviction that saving the forest is important is worth more to her than her own life.

Finally, the book takes a look at the interconnected nature of the world and how its shaped by time. Without the forests and the trees, humans—including the humans in the story—would not have the lives they do. The life and health of one are connected to the life and health of the others. The separate lives of the many characters that Powers describes in the first section of the story also mirror this because they come together—in many cases—to work toward a common goal with each other and with the trees themselves.