The Joy and Forgetfullness Blog has a useful post on modeling good looking tree bases.
Matakishi’s Tea House has a tutorial on making scouring pad pine trees. I ran across it while looking for instructions on building scouring pad hedgerows. No luck there yet.
There was a guy in our group who constructed an entire forest out of nails and sponges. It seemed like a good idea until one of the players leaned over the table and his hand came down on one of the trees.
Still, if you’re brave enough, here’s how to do it.