It might be useful to add an option to make the forest into a tiling texture (remove the edge constraint and allow the image to wrap around). You can also do this by starting with a larger canvas and rendering the individual symbols twice (one time on the left/top and one time on the right/bottom edge) to get proper wrap. The final output would be cropped down to the smaller area. While this would seem to defeat the basic idea and purpose of the program it might be a generally useful tool for generating fills.

The last iteration does have one visual issue for me: my primitive visual system wants closer items to always be in front of items farther away. It wants to see forest areas as stretching off into the distance as though they are being viewed from above and the jumble of overlaps makes my eyes water... I had this problem with CCx maps long ago but the SYMSORT command fixed it for me. Generating the symbol entities for the whole area, sorting them on bounding box (the appropriate metric is the hard part) and then rendering them might make the whole thing a little clearer.

This program reminds me of the ancient SymFill command I did for CC2 back in 2002 (the current drawing tools have similar functionality, I understand). http://www.ridgenet.net/~jslayton/SymFill/index.html has the toy from way back then. Some of the options on the UI dialog might be useful as suggestions for ways to get the forest variance up. There was a suggestion to use multiple symbols, but I never got around to it.