So I've been playing with this a bit to see if I can get something that works and got to this:
conifers.jpg
The tree outline was quickly inked in. The interior colour was painted in using a large fuzzy brush set to use colour from gradient and Gimp's built in greens gradient. The layer was set to multiply to get nice dark conifers.
The overall light and shade on the trees was done with a 50% grey overlay layer that I then painted onto with a large fuzzy brush with white for the top left edge of the forest and black for the bottom right edge, to give overall bulk and form to the forest.
I created a small brush to do the light and shade on the trees, by drawing a light line and a dark line that faded off towards the bottom on a new layer:
tree.png
I then cut this out from the image and because gimp can use the image on the clipboard as a brush, I had a ready made tree overlay brush. I set the brush spacing to 100 and created a new layer. I then painted over the forest with this until it looked like the trees were well spaced and then set the layer to overlay. I also did this with a much blurrier version of the above brush which did a nice job of giving more general form to it.
For those who are interested, here's the .xcf and the above brush as a .gbr file. Hope this gives foor for thought to the discussion. It can definitely be refined and I'll be interested to see what comes up.
Conifers.zip