Here's something I'll throw out there: what if this "imprisonment" has been done before? Perhaps this is a sort of chrysalis, and eventually, after an appropriate span of time, this bark-skinned elf girl with a tangled of leaves in her hair will find her weather-hardened skin cracking open...and out steps a newly-born dryad.
How did she survive without food for so long? Perhaps she was literally "rooted" to the spot: her outer skin took on the appearance of bark, and roots grew down into the ground from the parts of her legs touching the forest floor. She would have literally become a plant, or at least a hollow plant, inside which was held an elf spirit undergoing metamorphosis into a true nature spirit - a dryad.
There are some interesting implications, if this is how dryads are "born" in your game world. The whole time she was in her crysalis, she might have been communing with nature, and this might have a calming effect, completely purging all of the horrors of her previously rough life as a young elf. This might also easily explain why she was not devoured by insects or foxes during her crysalis stage: she was in empathic contact with any natural being within a certain radius - no matter how tiny or nonintelligent - and "persuaded" them not to devour her. There could still be squirrels' nests in her hair or whatever; as a nature being, she wouldn't mind having her body put to such nondestructive use.
Any dryads in the area (or other woodsy guardian types - unicorns, nymphs, sprites, etc.) would have been looking after this crysalis form as well.
If the PCs are on hand when she "hatches," they could very well be the first people she meets up with in her new life as a dryad. It could make for a very interesting encounter, and give them a future point of contact with the "faerie world," should you ever want to involve them in any such adventures.
Just an idea.
Johnathan