Inspired by a forest seen in the online magazine Warning Order, I had a go after speaking with the author.
It was quite a learning experience for me! :-)
The base is laser-cut mdf from Kraken Skulls, with some 1/8" dowel from the local DIY store glued down with No More Nails. I added in some various-sized gravel on the base while the glue was still wet. When dry, I primed it and spray painted it brown. I did a quick dry-brush of a lighter brown on the gravel and then used watered-down pva glue to add on some flock. The hardest part was next. The tree tops are clump foliage of three different colours mixed together. I tried putting it on with spray adhesive, which worked ok. Then I added in more with the pray in spots that needed it. Then I used watered down pva to soak the foliage.
I think next time I will make the tree-tops by soaking them in pva separately from the base, shaping it generally. It was a mess trying to do it on the base, and lots of the clumps fell off.