Why not seperate the maps altogether. You have a height map, a color map, and and detail map already. Why not just add some new map types like:
Treemap : this would be used to plase trees, like you've done already.
Roadmap : this would define an area that block the spread function, because you don't want trees on your roads.
I'm sure there are others that can be thought up.
Treemap : this would be used to plase trees, like you've done already.
Roadmap : this would define an area that block the spread function, because you don't want trees on your roads.
I'm sure there are others that can be thought up.