STEP NINE: IN THE JUNGLE, THE MIGHTY JUNGLE
Forest time--one of my favorites! First I will--yep, you guessed it--create a new layer called (surprise surprise!) "Forests".
Basically I'm following the same sequence I followed with both the water & the land. In this way, my entire map will flow. So, choosing a light green hue that coordinates with the rest of my map (a really tough job for me!), I use a sharp-edged brush to color in my forests.
Next, on a new layer called "Forest Colors" I randomly mark up the forests with a couple darker colors, using a soft brush & a very low fill (about 15%). Then I merge the Forest Color layer down into my Forests layer, and apply the Filter>Artistic>Sponge about 3 or four times. The first image below is my end result.
THE TREE PATTERN
Now, although like the mountains I could draw on the forest details by hand, I will instead save time by creating a pattern for them. It'll still require some hand-drawing, but not nearly as much.
So first I create a new Photoshop document (300px square & 300 px/inch) & make a new layer above the white background. Then, using a soft 5px brush, I draw squiggly tree outlines in the center, never touching the edges. (see image below) As you can see, I also add a few thinner "leaf squiggles" inside the trees. DO NOT TOUCH THE EDGES, though.
When the center is filled with trees, I apply Filter>Other>Offset. The settings don't matter much, as long as "Wrap Around" is clicked. As you see, the image shifts, leaving me new areas to fill with trees. I keep Offsetting until I have nowhere else to fill.
Finally, I need to define this as a pattern, so I go to Edit>Define Pattern, give this a name (TreeFill, or something), and it's ready. I close this little document (you can save it if you want to) & go back to my Thracioth map.
APPLYING THE TREE FILL
Back in my map I Magic Wand tool select the forests by clicking anywhere in the green. Then I create a new layer above it called "Forest Details."
With that new layer slected, and with the ants marching around my forests, I click the "new fill or adjustment layer" button at the bottom of my layers screen (it looks like a half black-half white circle), and I choose Pattern. The screen will pop up that allows me to select the forest pattern I just created, plus to adjust the scale (mine is at 30%). Click OK & voila! I get chills every time!
I now Select>Deselect my forests. I also decided to set the blending mode of that Forest Details level to "color burn", because I like the orange-ish edges, but this is not required.
Finally I need to play with the edges, since my fill is imperfect there. I could stroke these, but it looks too smooth for my tastes, so I will go in by hand & draw the edges with my squiggle lines, then hand touch-up with green. Below is my final version.
EDIT, June 11, 2007: The above system for tree fill works great, but had I the chance to redo, I would instead create an entire fill layer / layer mask for my forest pattern, then "paint" them in. While essentially the same result, it does offer more flexibility. (For a wonderful video tutorial on this system, watch Butch Curry's video tutorial "Fantasy Cartography with Adobe Photoshop 3")