What I found works well for creating the forest texture in that respect is to not use the magic wand tool, but the "Colour Select" (in photoshop anyhow, not sure about gimp). That way instead of a hard selection edge, it goes into a smooth selection when removing the black. That's what happened here I think - there's a hard defined edge between the black (removed, now transparent) and the almost black.

Very small point, and doesn't detract from my enjoyment of the map!

it's a wondeful piece, tutorial basis or not (I've done my share of similar mapping style, nothing wrong with that!), and conveys a lot of information without being overcrowded.

Definitely worth posting, thanks! (and i love the parchment/legend/etc on the left, wonderful job, couldn't have done better myself!)