wildflowerwhimsy-blog:

In Greek mythology, the dryads are female spirits of nature (nymphs), who preside over the groves and forests. Each one is born with a certain tree over which she watches. The lives of the dryads are connected with that of the trees; should the tree perish, then she dies with it.

If this is caused by a mortal, the gods will punish him for that deed. The dryads themselves will also punish any thoughtless mortal who would somehow injure the trees.