As Karyn made the wish, she began to lie down upon the ground, beginning to feel as if she were falling apart. Then she was absorbed into the soil and into the roots below. Because her wish was for her to become nutrients for the tree, only her body disintegrated, leaving her clothes and (more importantly) the wishing stone sitting on the lawn.
And, because of her wish, Karyn and Jon wouldn't be able to change back to normal for another month. Meaning that anyone could come by and take the stone. And Jon and Karyn wouldn't be able to stop them. Of course, Jon was a tree now and trees didn't have minds, and neither did nutrients. Both of them would be no more than just inanimate objects for the next four weeks. They wouldn't even know if the stone had been taken or taken by whom.
As the night ended and the sun came up ...