I played around with how to make the lowest polygon tree that still looks good and attached are some pictures of what I figured out. Basically, individual leaves and a round trunk aren't needed. Most of the 'quality' and smoothness comes from the texture. Make a whole branch a one piece bowl-shaped polygon. That way it looks full (3D) from any angle on the ground. Also, with a single bowl-per-branch, it's much easier to set the UV and swap out branch textures to make new trees.
You could put low-polygon trees here and there along the wall-edge with some kind of flat tree-wall texture between the 3D trees (like the attached bmp). It would work especially well if you used a river to keep the player at a distance from the tree wall.
You could put low-polygon trees here and there along the wall-edge with some kind of flat tree-wall texture between the 3D trees (like the attached bmp). It would work especially well if you used a river to keep the player at a distance from the tree wall.