Well, one thing you could do is model a tree, duplicate it several times, place them where you want, then hide each copy. Once you are done modeling the rest of your scene, simply unhide them and render it out. Once you unhide them, however, you should not move your camera around at all. Get your camera where you want the render to be taken, then unhide them. This should prevent your system from crashing. Another way you might be able to do it is to create a few trees, take a picture of the trees (either render it out, then save as jpeg or Alt+Print Screen and paste it onto microsoft paint), then place that image onto a plane. You can then place this behind your scene and follow this proceedure to illiminate the black background you will have.
Hopefully you understood all that.