SOLVED. YA (I think?) fantasy novel in which person emerges from a tree. [s]

[Adelaide]

I read this book within the past two years. It's a fantasy novel. All I remember is a family having a picnic under an old tree, in which there has always been the form of a person. I think the father tells some kind of legend about how he got in there. One night they fall asleep under the tree and one of the kids wakes up to see an actual person emerging from the tree. I'm not sure if the person runs away into the forest or speaks to the family and gets adopted or something.

That's all I remember. Any ideas? I think it's a fairly recent book.

[Justanotherbiblophile]

There's a scene like that in The Gate Thief.

[Adelaide]

I think that's it... Thank you!