There can be no such thing as forest benders in the way you're speaking of them. Benders use elements, and obviously a "forest" isn't an element. Like Hama, I'm certain water benders can learn to some extent to control the water in the nearby flora and fauna and move them as they would water.
This is similar to how Azula can use lightening, and Toph as an earth bender can manipulate metal. There are obviously no "metal" benders or "lightening" benders as a separate group--rather these are just extensions of one of the original four bending arts.