Hi,
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
Kind regards,