You can replicate objects directly on a terrain object. You can use the shader or a map to control distribution of the objects on the terrain. If you use a shader mixer of black and white colors (or between) in the replicator modeler's menu, then objects will be placed on white but not on black. You can use slope to just place objects on flat surfaces (or just steep), use elevation to either fill or empty valleys, use a wires function to replicate stripes, etc. There is no need to convert the terrain to a vertex object first.