In fiction, trees typically serve as inanimate props and backdrops. Human characters climb trees (and fall out of them), use them as targets, build treehouses in them, carve hearts and initials on their bark and use them as hiding places or lookouts.

A few authors, however, elevate trees to the status of sentient, (usually) noble characters in their own right. Tolkien's Treebeard is one classic example. Lightleaf in the King of the Trees series is another.

If you can find other examples, feel free to include them in the list.