Feathers & Glass
The most interesting theory is that dryads are manifestations of their wards—usually individual trees at first, though their ‘hearts’ can grow larger as the Dryad claims more and more territory around it. The more powerful ones roam and protect entire great forests. The dryad’s heart, the tree that spawned it, is said to bleed when cut. Killing the heart will kill the dryad, and it is assumed that reverse is true as well, though it is harder to tell if this is direct causation or through willingness to ruin the tree on the murderer’s part. The more powerful Dryads who control larger areas do not seem to have hearts as easily findable as a single tree’s dryad. Researchers are unsure of the cause—mostly because of the effectiveness Dryads can weird keeping undesirables out of their realm. Small communities tend to find different ways to propitiate dryads to keep from harming a protected tree. More powerful ones can keep entire communities away, though may allow specific humans in if she needs help culling dead wood or some other problem. This is incredibly rare, though, as the Dryads are masterfully capable of balancing their small biospheres.
You’ve read in your oldest books that the more powerful ones will sometimes take the company of strange animal-human hybrids. The Dryads will not let them be studied so there is only speculation as to whether the companions are borne from the Dryad punishing a human, fucking a human, or if the creature is something else the forest can birth when the Dryad’s heart becomes powerful enough.