What is the author's tone in Z for Zachariah by Robert C. O'Brien?

The tone and mood are complicated and fluctuate throughout the story. The tone oscillates between happy, uncertain, and terrified. Ann is at many points quite happy, particularly when Mr. Loomis begins to feel better, when she farms, when she visits the church, and when she picks flowers. These activities take her mind off of the difficult reality she lives in.