OK, I think I understood now. Great question.
First I would like to note that "Patches 2" map You are using is only for geometry distribution. White pixel means there is random(from the list) geometry item, black pixel - empty space.
I would highly recommend watching this tutorial
as it explains distribution maps in great depth.
That means You have to feather scale (not density as it is already chosen in distribution parameters) with some other means. And there many ways to achieve similar effect in ForestPack Pro.
One way could be to copy "Patches 2" bitmap and use it in Scale Map:
After playing with blur, contrast and brightness values You can have parametric solution to Your problem:
As an alternative You can use technique described in this post
when using custom bitmap Densities.
Hope this helps.