Walking the Tree by Kaaron Warren
An extraordinary island fantasy from the author of Slights. The island is the world, and the centre of the island is the Tree. When they are 18, each person must leave their home and walk the Tree. Years later, when they return -- if they return -- they will be changed. When Lillah embarks on her own journey, however, she never expected to deviate from the sacred path! and enter the Tree.