A close up of my Black Tree Design woodsmen and leaf litter. These two are from their "Good People of the Land" sets I and II.
The trees have a peg that fits into the root base. The plastic base is glued to the board, but the trees just rest in the hole. This allows them to be removed if need be (although it does leave a stump in the way).
For more tree-building articles see Iron Mitten and Cursed Treasures.