Natsume's Book of Friends (2008)

S2E5 Tree of Promise

Natsume comes across a spirit who has been bound by his grandmother Reiko for half a century. She had written the spirit's name on a ripped page from the Book of Friends and secured it to a tree deep in the woods. Now, Natsume must find this lost page. What was Reiko's purpose in attaching it to the tree?