Perhaps green scrubbing brush material (like for scrubbing dishes) as the woods? That's what I use for a lot of big woods in 2 and 3mm, on top of thin cork to give the appearance of space under the canopy. It'd work for the abstract scale of Rommel I think (I'm doing Rommel and Eisenhower in 3mm).
A friend who does big 6mm games uses it as the base of his woods and then adds clump foliage to make it more 3D, and then brown poster pins as the 'trunks' stuck upside down into the scrubber. An alternative I've scene is to use black foamcore as the base and then glue the clump foliage to that, and then use the pins again.