PS: there are only 3 terrains used in the above image. If you want to use this technique but memory is a problem, try saving space elsewhere, for instance render your tree (or the whole background part of your image) separately and merge them in photoshop. Re the atmospherics, volumetric world isn't likely to help as it will just lighten your picture. I'd be inclined to up the haze a bit and then play around with the sky settings (sky dome colour, sun colour, etc) until you get the effect you want.