Nice forest. I'd apply some texture pattern to the canopy if I were you, but it can look fine without one. If you want/don't mind perfectly straight tree trunks, here is a trick that could save you a lot of time and work:

1. Create a small image of vertical stripes (the trunks) that extend from the top of the image to the bottom of it, so that it can work as a tiling pattern. Save it as a GIMP pattern file in the folder where your GIMP is set to look for patterns.

2. In the map image, make a new layer called "trunks" or w/e. Apply a pattern-fill to this layer with the pattern you created in step 1 (you'll need to close and restart GIMP first). Create a black mask for this layer.

3. Make a selection of the forest canopy on what ever layer you have it, switch to edit the mask of the trunks layer, and fill the selection with white.

4. Float the selection and move it some distance down (depending on how long you wish the trunks to be).

Now you should have a neat 'outline' of trunks visible below the canopy, in similar manner as a drop-shadow would be. Of corse, you'll need to keep the canopy layer above the trunks layer for this to work as intended.