Trees of Avalon — Elisabeth Waters

An uninvited guest can be a bother, especially if he's under a spell that prevents him from leaving. Eliona is a dryad, living peacefully in her tree in the Sacred Grove, until Nimue's spell binds the Merlin to her and makes him Eliona's permanent guest. Now Eliona must find a way to free him from her tree, or it--and she--will die.

This story was originally published in the anthology Return to Avalon, 1996.