I know that when I played my last druid I was a "plant" druid like the one you made. Unfortunately, pathfinder only focuses on wildshape as the "focus" ability for the druid. It is a great ability, with tons of versatility, but you have to become an animal. It doesn't seem to make a whole lot of sense for someone playing a briarborn to turn into a squirrel. Instead, work with your GM a little on this one, and change wildshape to plantshape. I had a druid who could turn into assassin vines, shambling mounds, treants and many other plant type creatures. Unfortunately, I don't have any specifics written down on that character and what creatures I could turn into and at what levels. It was handwritten and lost in a pile somewhere.
Thats not actually that bad of an idea, isn't their an actual plantshape spell for turning into different things? maybe I could ask them if I could use that