I've done many scenes set in forests and they can be a pain to do. Why you didn't ask, well here is the answer. A good 3d model of a tree will either have lots of leaves modelled of a lot of transparency maps and both have downsides. For those unaware a transparency map will make parts of a 3d face opaque of even transparent depending on whether it's black or white… or where it is in between. Essentially they look like an inverted silhouette. So to get a nice looking tree I might use a picture or a branch with leaves and put it on a rectangular facet or plane. I'll then apply a transparency map to make only the branch and leaves visible. These are easier on computer processing power than more complex models but don't look at good most of the time. Another downside is render time when ray tracing. Multiple trans maps can greatly increase the time it takes to render.