In art it's usually easier to create a forest than it is a tree. For the tree covered hills in the background I first filled in the area with a dark forest green, and rubbed it in with my finger to fill in all the fiber of the paper. Next I used a brighter green to shape out the trees. I only used my finger to drag the edge of the brighter green into the dark, but kept the hard top edge so the trees had form, otherwise it's just a blurry green mass. A couple even lighter greens were used to add secondary highlights to the tips and a drab brownish-green was used for showing leafy branch clusters in the shadowed areas.
If I had thought of it, I would have taken pics of the steps I used and posted them as well.