There are several ways how to control Distribution areas using UV Surface Mode
. In Distribution Map
roll-out you can assign any Map
to define distribution areas. Please check attached screenshot showing effect of checker map over your surface. So, you can for example unwrap your surface mapping and prepare a texture that'll suit your needs.
Another possible way is to use Vertex Color
map and paint over your surface using Vertex Paint
modifier. In this case you should add some vertexes to your model. Attaching an example showing it with combination of Forest Pack Pro
I did some time ago testing it.
Hope this helps you