What is the importance of Greenwood in the short-story collection, Through the Dark?

Greenwood is the imaginary world that Sam, Lucas, and Mia created when they played in the treehouse at the Orefo’s home in Bedford. Although Greenwood itself is not real, it is incredibly significant in both "Sparks Rise" and "Beyond the Night. In "Sparks Rise," memories of Greenwood are used to bring Sam and Lucas back together. In "Beyond the Night," Sam, Mia, and Ruby are able to reach the last piece of Lucas’ mind through a specific story that he held onto when the rest of his memories were repressed at the facility. They visit the treehouse itself at the end of "Beyond the Night" in search of the written stories that Lucas left for Sam when his family moved away.