Thanks for your reply.
Animating the slope and height range was not an option in this case, since that results in trees suddenly popping up in other unwanted areas.
I did use UV-mode to prevent the scattered objects from sliding around when scatter geometry is moving: that worked well.
The limit by Material IDs however, are sadly not an option when in UV-mode. that's why I had to delete some of the unneeded geometry.
So the question still remains: in future releases: can an option be implemented to lock these settings to a certain frame?