I basically followed this tutorial. For the edges I made short jittery strokes in little arcs. And on the bottom edges of the forests there are series of short vertical strokes. I treated the parts where rivers cut the forest as any other edge, but made the interior lines a bit darker.

photoshop specifics: I used a hard round brush size 3 or 4 with size varying with pen pressure.

Just keep playing around with it til it looks right. It took a few tries for me to get it how I wanted. If you look around you can seen that there's a bit of variation in how they were drawn. The forests under Greyhill have smoother edges and the shadows are a bit too uniform. The large forest north of Rivermeet has edges that are still too linear and they're drawn with smaller arcs than the ones on most of the other forests. Thinks started to come together in the region by Falinde and Talnisur.

I hope that helps.