One way to have a whole forest for the price of a few poligons is to use a plane and apply a tree texture to it, with the appropriate transparency map. You could take a 3D tree, do a number of renders of it from various angles, each time saving the render and the alpha channel (save as TIF and you'll get both. You can then easily separate the 2 in a paint program). You then create a simple plane and load the textures to get a flat tree that you can put anywhere. You can do this with just about anything (houses, cars, furniture,...) and place these in the background at a very low cost.