This was a short novel that was probably written in the 80s or 90s.
A young girl, maybe teen, lives in a small village area, and has magic powers. She can see some magical creatures that most people can't, such as some sprite or dryad being.
There's a man that's actually a demon, and I think he tries to get close to the girl by ingratiating himself with her caretaker or others in the village.
In the forest, there's this big tree, and something is wrong with it. I think there may have been demonic runes making the tree sick. If the tree died, then it meant something worse would happen. I think it meant the demon, the main antagonist of the novel, would overtake her, or the forest would die, or some demonic agenda would be set in motion. I can't remember which, or if I'm mixing up aspects of the book.
The demon knew about the girl and her magic, which was apparently a rare attribute. He was trying to gain control of her. His plans were thwarted when she healed the sick tree in the forest, but I don't think he was actually defeated/vanquished.
The end of the novel left it open for a sequel, but I don't know if one was every written.
I used to own the book, but can't find it, so it was probably lost in a move. I think it may have had a reddish cover, and it was possible written by a female author.