I tried to add a simple surface layer and limit it's minimum altitude for a dusty snow addition, but for some reason it's doing some weird stuff in the fuzzy zone and in the masked out zone.
Minimum alt of -130 and 150 fuzzy zone with Use Y checked.
It's almost like it's trying to use position in texture and final position at once, since if you position in texture you get bottom of trees white everywhere.
Update: It looks like that becasue one of the young populations, with it's branches to the ground was still on position in texture. That's why. Oi.