Thanks!! The ivy objects are made with the ivy generator in 3ds max, rotated by 180° vertically and the pivot set to the appropriate point. The population square is more or less on the ground and not above the cliff, so I guess that's the reason, why the objects are on the underside.
It's crucial to connect the very last shader to the terrain input of the pop, so that the populator can read the displacement correctly.
By the way, here is another cliff test.