Ah! So these are just the square edges of the population then. That makes sense. Well, what you can do is make a single, larger population, and then attach something like a Distribution Shader or Image Map Shader to your Density Shader in the Population node, and then you can control distribution of your populated objects using these other nodes. The Distribution Shader would allow you to control by slope and altitude for example, while an Image Map Shader would let you use an externally loaded image map to control distribution.
- Oshyan