Use a spline instead a surface, and turn on the Boundary->Size checkbox in the Areas rollout. Forest is optimized to work with splines, and include many specific options for them (as the boundary, falloff and others).
If you reuse the same Forest object with the surface assigned (although in ths case is not needed), remove it or turn off the "Surfaces Area" in the Areas rollout. If not, both areas (the surface and the spline) will be used simultaneously over the same region, creating overlapped objects.