As far as I am aware, Dryad's currently have no impact on the environment,
My thoughts would be, to have them as the wander around, reset the stunted trees that are growing naturally now since the patch that tree's now spawn in the wild when certain conditions are met. In addition to the above, adding in a hearth magic ritual that can summon a dryad for a limited time, (15-40 sec) , this would be useful for tree farms.
The effect could be either resetting the tree to 10% restarting its growth just as if newly planted, or unblocking the stunt and continuing from where it stopped growing, or even as a random RNG percentage all options retaining the percentage chance of "re-stunting".
This may have already been suggested if so please forgive my lack of forum search skills.