Sure thing jtougas -

1) Select the Paintbrush tool. I use a brush that has holes in it, this time it was "Galaxy (AP)". (Not sure if that came with Gimp or if I picked it up somewhere but any sort of roundish holey brush should do.)
2) We are going to check the box 'color from gradient' so select two colors for the forest. I use a dark olive green and sort of a burnt umber color.
3) Paint in the forest on it's own layer (if you haven't already, create a layer and call it "trees" or something). Paint till you have the layer loaded with about the right consistency and coloring you want.
4) Select the forest area you have painted and Filters>Artistic>Gimpressionist:
4a) Within Gimpressionist (which I'm still figuring out) do the following tab selections:
- Presets>Dotify
- Paper>Default or marble
- Brush>Grad02.pgm
- Size> set Maximum to 10, you can play around with the radio buttons if you want but I'm not sure how much it matters.
5) Filters>Distort>Emboss: select Bumpmap and wiggle the sliders around till you get something that looks good.
6) Filters>Noise>HSV Noise - sliders till you get what looks good (This creates a speckled look which adds some texture to the trees)
7) I then paint shadows to create low spots and high spots. You can do this with the Dodge/Burn tool or better yet create a layer called Shadows filled with 808080 grey color and set to overlay. Then just burn & dodge to your hearts content.
I touch up by using the brush with holes to delete some areas so there are some thinned out spots in the forest and around the edges so it's not so abrupt.
9) I use the curves a lot to change the coloring at this point just depends how dark or light it came out but that's entirely up to your preference.

Anyway, hope I explained that well enough. Some of the steps may not be necessary (still getting Gimpressionist down) and sometimes I do the HSV thing and then a 1 pixel Guassian blur. Half the time I try something new in there just to see what happens.

Now I've got to go beat up my mountains more because while rifts are cool they aren't what I'm after. DOH!!