ColorEdge doesn't work as density map in Forest because it is computed at render time, but scattering process is done in a previous stage. I'm not used GradientEdge... may be it works in a different way.
Anyway, it should not be necessary. In the latest releases, Forest supports area falloff directly from the surface, without requiring a spline as before. Just turn on the "Surfaces Area" and adjust the falloff parameters.