Here are a couple fallen trees that I've been working on.
I started by gluing polystyrene tiles to old CDs and when dry, I trimmed away unwanted material and roughened up the surface a bit.
Basing material is then applied and left to dry.
All that is then required is to paint the basing material and add flock to taste.
I then went on to use the same method to make a larger, completely fallen tree.