Quote - ......
If you notice in my material, I have all 4 materials in a "pile". .......

OK - I've reconstructed my scene but I am not sure I did it correctly because I had a heck of a time adding "pure layers."  Each time I clicked the Add Layer button, a tree with two layers was added.  I am not sure exactly how I finally ended up with something resembling your structure.

Anyhu, a few straggler trees are still appearing on the higher (in your case) green elevation: