I feel as if it's uneasible to plant trees on anything but either a) a conveniently flat patch of forest that you include in your base layout
or b)a fugly unflat land of forest that you less conveniently have to include but can't terraform.
Seeing as most people are OCD about flattening, and how forcing people to center their town placement around something as derpy as existing forest tiles that don't give obsessive people aneurysms, could we at the very least have trees spread wald much faster than is presently the case? At the very least one tree should, by its completion, have spawned terrain far enough to allow for 2 other trees to grow fast. As it stands our fresh trees haven't spread anything at all yet, not even a measly tile under themselves.