The forest symbolizes freedom to True Son. It represents the nature-focused lifestyle of his adoptive Indian tribe.
True Son was kidnapped by the Delaware Indians when he was four years old. He has completely internalized the Delaware Indians’ values by the time the book starts when he is fifteen. When he is in the forest, he is free.
When True Son is forced to return to his biological home among the White Men, he finds it confining and depressing. The White Men wear heavy and uncomfortable clothes and ugly shoes. They live in annoyingly closed houses away from nature. He cannot understand this way of life.
Perhaps the Ruler of Heaven and Earth had imprisoned him to make him value the freedom when he got out. Never even along the Tuscarawas had he tasted such savor in the open trail, the sweet air, the green forest. (p. 83)
True Son only feels happy and free when he is in the forest. He feels at home, and comfortable there. Therefore the forest represents everything he has lost. It symbolizes freedom, which for him also means returning to his Indian life.