I recently finished this; a 6-inch across wood (suitable for the "woodland ambush" special rule) with removable trees to accommodate units.



(I included an undercoated high elf mage for scale)
I bought three plastic trees (better quality and much cheaper then the GW ones), then cut out from dense 5mm-thick foam-board 3 discs each just over an inch wide and one more six inches wide, then I rounded the top edges with a stanley knife and glued the trees to the smaller discs.
I coated the discs (top-side only) with PVA and dipped them into a bowl of modeling sand (the six-inch disc had to have the sand sprinkled on it). When the glue dried, I then applied a clear matte varnish to the sand, which removed the dust and made it darker, and prevents its from falling off.
After that had dried, I applied patches of PVA to the larger disc in spaces where the trees wouldn't sit and applied static grass.
So, what do you think?