Well...
A troubled boy must rescue a Princess -- whom only he can see - who is trapped in a tree by a sorcerer's spell, who will die when the tree is cut down to make way for a mall.
Klunky. And 39 words. Here's the madness in my method:
Stories of the magical and sci-fi genres often face an up hill struggle to persuade the viewer to suspend belief, to buy into alternate rules of reality that define the story world. It's a challenge to distill those alternate rules of reality into a logline, but occasionally necessary to do so.
'whom only he can see'
I'm explicitly stating an alternate rule that is only implied in the original logline. I mean, if everyone else could see the Princess, the kid wouldn't have a dramatic problem, would he?
'by a sorcerer's spell'
That emendation is to provide an answer to the question: who is the antagonist, the villain? I infer that the Princess's predicament is created by an agent with the power to trap her in the tree. The mayoress may be corrupt, but does she have that power? Or is the 'corrupt mayoress' merely a pawn, a dupe, of the agent with the magical chopes to trap the Princess?
'who will die when the tree is scheduled to be cut down'
Again, I make explicit what seems to be implied. This is to answer the question: what is at stake? Well, obviously it's not the tree's life -- it's the Princess's life. (Which I infer is linked to that of the tree.)
Oh yeah: suggested title "Tree of Life". (Recognizing that Terrence Malick's 2011 film was titled "The Tree of Life".)
fwiw