The tree from which Finny is pushed symbolizes a loss of innocence, and reflects the Biblical fall of man. As Gene returns to Devon in the first chapter, he seeks out the tree, almost as though he wants to face an old enemy. He describes as though it is a monster that tormented him as a child, but upon returning, he realizes it is nothing of which to be afraid. He calls it a shrunken old man, showing he has overcome his fear of it throughout his life. Finny's fall parallels the Biblical fall of Adam and Eve from the Garden of Eden. In that story, the tree of knowledge becomes a hiding place for evil. In the novel, the tree is the hiding place for Gene's evil. When Gene pushes Finny, not only does he lose his innocence by committing an act of violence, but he causes Finny to lose his by shattering his ideal world. That one act changes everything for the boys, and their relationship is never the same.
Trees are often symbols of life as they are planted on Arbor Day to symbolize new life and beginnings. For Gene, jumping on the limbs becomes an issue of jealousy and envy in his life to the point of competition. By jostling the limb, Gene sends Phineas crashing downward and the jealousy ends as Gene and Phineas become codependent. No longer does Finny cast a shadow over Gene's life; the tree "is weary from age."
While at Devon, Gene feels the shadow of Finny and realizes that the enemy has been himself. Blaming Finny for his problems, Gene has cast a shadow from the tree of competition, over himself. By knocking Finny from the tree, Gene has tried to liberate himself from the competition of Finny. By returning to Devon, Gene has essayed to lift the shadow of guilt from himself.