Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Nostalgic, Somber
From the very first sentence, we know that the narrator is recalling events from long ago. The phrase "[o]nce there was a tree" (6) immediately suggests a time gone by, and the second half of the sentence, "and she loved a little boy," hammers that feeling home. Little boys, after all, don't stay little forever, and we get the immediate sense that we're going to find out what happened as this one grew up.
This sense of nostalgia is palpable throughout the first several pages of the book as we hear how the tree and the boy used to spend their days. And then, with the illustration on page 30, we become aware of a somber tone that lasts throughout the book.
The boy, pictured leaning up against the tree (30), looks lonely and sad. It's a simple illustration, one in which his gaze is downcast, his mouth is straight, and his posture is resigned. We know that look: it's youthful isolation coupled with boredom, and it's clear our sweet story of the tree and her boy has taken a very serious turn.
Sure enough, from this point on, the tree is left alone more and more, and the boy only visits when he needs something from her. Despite the fact that the boy takes all of the tree's fruit, her limbs, and even her trunk, the story resists becoming macabre because, well, the tree is a tree. As horrifying as it is to see her "body" carried away to be turned into a boat, the cutting down of trees is familiar enough to us not to be gruesome. Instead, it's a somber tone that pervades the story, along with a nostalgia for the good times that once were.