Thalia/Kate
Long story. But apparently, when you've just sacrificed yourself for your friends and you're laying there dying and you send a prayer up to your dad Zeus to help you (thinking maybe he might heal you a bit or maybe get rid of those hell hounds tearing you to shreds), his version of helping you is to turn you into a tree.
And apparently when you turn into a tree in your dreams, you turn into a tree in real life.