This is the first tutorial I wrote for the Cartographers Guild some years back. It is a walk through on how to create photorealistic forests like the ones shown above using GIMP. In addition to the tutorial (link below) you will also need to download the Forests Patterns zip folder. I could not figure how to upload the file directly to this post, but you will find a link to it at the original tutorial thread here. To install unzip and save the patterns to the patterns folder of GIMP. Then open or refresh GIMP and they should show up in the patterns dialogue.
Photorealistic Forests Tutorial
ars phantasia 
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