Forests are such a piece of cake. I disdain the use of the GW trees. Not because they are ugly (I think they are really cool) but becasue of the cost. They are just too much for a decent sized forest.
What I did was get a bunch of twigs that look like mini trees. One or two branches are fine but they might break, so stick with `less is better'. Then I hotglued modeling lichen to them. I use the glue gun because it is a lot quicker and I am guaranteed results. I then either drill a small hole in the trunk and insert a small rod or just glue the sucker to a precut, corners rounded piece of fibreboard. Paint green and flock.
BAM. Instant forest.
 

Here is what the bottom looks like. I call it fibreboard but I have heard it called particle board, high density board, etc. Whatever it is, it gives you weight so the piece won't move too much when you plunk figs on it and it won't warp when painted.


 

I just felt like adding the cut down chunk of log and the stumps.