So I've bee making a map for a game I'm running soon using Ascension's atlas style tutorial, here's were I'm at :

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Now.. I want to add a layer of forest that are better defined than just leaving it to the general colour variations in this tutorial.
I've been messing about with drawing a layer with dark green blobs for my forest then adding some noise and lighting, but though I've got some ok'ish results I'm sure there must be a better way!

...any ideas guys?