A short clarification for those, who would like to help: the idea is to use a single height map for all the objects in the scene (you can see it, when change the heightFactor value in GUI, that instanced fir-trees and boxes move in accordance to the height of the “ground”).
PS I’ve got no idea of what’s going on iOS safari, as I don’t have Apple devices.
PPS The grid of island’s geometry was made specifically for that scene (see islandGeom() function)