Hi, although Falloff options are available both for Surface areas as Splines, Forest uses an unique area for all surfaces.
Therefore it's not possible to apply the technique described in the tutorial directly with a surface.
Anyway, there is a simple process to extract a spline from a surface:
1) Convert the surface object to an Editable Poly.
2) Set "Border" selection.
3) Select All->Edit Borders->Create Shape from Selection.
Now you can use the generated shape, to create new spline areas in Forest.
In fact, internally Forest uses all areas as splines, and applies this process to get a spline from the Surface area.
Just remember to disable Areas->Surface Area->On, if not items will be duplicated, because they are created both from the Spline and Surface areas.