Using one scene rendered as an alpha plane and then using it as a portion of your next scene is a great way to save on polygon count to be sure. If you group your trees and put them in a separate layer you can turn that layer off by clicking the small eye looking icon on the right. X-frog trees also can be made in a lower polygon count I think as I don't have that program I can not really test this out. I do know in V4Pro you can increase and decrease polygons in any given plant or tree in the plant editor and that really is helpful. Good luck on your effort no doubt you are learning a lot over all this and BTW so am I. Regards.