Another old-fashioned trick but I do never get rid of an old horse if he can still walk his mile ;-) so beginners may be interested in learning this. IN old 3D times, practically all of us used to build complex scenes by use of placeholders. I made myself some then: spheres on cylinders (in various shapes) for trees... the same for people, in different proportions of course! For all geometrical objects such as cars, combinations of boxes. If you build a few basic ones and save them, you can reuse them in your work. I assure you that if you take a little care in building them (like: make a round sphere for planetree crown, a stretched sphere for cypresses, etc.) it is not that bad. Oh and one very important tip which I forgot before! Start your scenes with all your terrains at 256x256. Turn them into higher resolution only when you are ready for render, and don't do that if you can avoid it: many people often blame heavy polycounts on vegetation elements, but I grant you that a couple of hi resolution terrains can weight as much as a dozen trees in your scene.