How do i get the randomness?
I'm afraid in this case it is necessary to create a separate Forest object by level. Currently Forest uses the same random seed for each surface, so the distributions are identical. (I think we would change this).
But there is an alternative way to get it:
1) Scatter them using an unique UV Surface.
2) Switch to Custom Edit (Tree Editor).
3) Change to the sub-object edit mode and select all items.
4) Click Tree Editor->Randomize