Tree distribution:
It is really quite easy to get this effect. I start by making the population and pasting the coordinates. Now I go to an overhead projection of the preview window. I adjust the population size so it will cover everything which needs covering. This makes a population which is in the millions on this type of render.
Now I add a distribution shader in the Blend By Shader input. Make Fractal Breakup selected. By using the measure tool, you can get the 'width' across the preview image. Divide this by 1/3 or 1/4. (It can be rounded up) This will give you the Scale of the Fractal Breakup. The Lead-In Scale is the full measurement. Set your altitude and slope constraints how you think they will work.
The last things to do are to reduce the coverage on the distribution shader and set the population density of the object. Preview B-box can show how the objects are placed in the landscape. If you are not happy, change the seed and /or the density of the population. If you leave the distribution shader open, it is easy to adjust these settings also.
This method can take a population down from millions of instances to a manageable couple of hundred thousand. I have not had much luck using this method for grass objects. Happy rendering!
