The following came to mind:

1) A shallow level of soil (dry or slightly damp)

2) A mix of coarse sand into greenish-brown paint. Paint an area of the base to make an opening in the trees. It should make an uneven, gritty, dirt-filled area. More detail can be added using tiny wood chips or cut-up twigs, sprinkled around. (a similar method to this can also be used to make particular desert scenes)

3) Undergrowth material from hobby / craft stores. The stuff used with train set models is really good for open areas between trees.

4) A combination of some or all of the above.

There's bound to be more.

Also have a look at the undergrowth in Footloose's dio pics. Very good stuff there.