Anyway I worked mostly on the bits of pine needles and twigs in the hollow of the rock. I'm using negative drawing to accomplish the desired results. When finished, it creates a nice feeling of depth and layers. It can be a rather slow process but it is definitely worth it.
Basically to do this, you draw in your main pieces of twigs, needles, leaves, etc. leaving them white and then fill in the abstract background areas as dark as you can. Then with a light touch and a 2HB I go lightly over the mid-ground pieces and then add a vein or slight shadow to the foreground pieces. You can even go in with Blu-Tack and create leaves or twigs that you didn't actually draw in.
It can be a time consuming process and you do have to pay a little bit of attention to where your twig ends and begins but if you make a mistake it really won't matter that much. I'm sure there are bits and pieces in there that don't align.