Well, it'd be a little messy, but you can use the 'flock' they are talking about without gluing it down. Just spread it over the ground and base of the trees. The kind I have came in a big plastic container with a screw on lid - so you can just lay it down, then sweep it up and store it between use.
A quick search gave me
http://www.cchobbies.com/scenery/wsgrass.htm - they have grass flock as well as lichen. I'm sure you've got a local hobby store that does train stuff you could look for what you want exactly though.