Thanks for the comments Korash. I see what you mean about the rivers. I can play around with that (I have them as a separate layer, so it's relatively easy to change).

For the trees, when I tried making them more separated it just looked weird. I either ended up with a packed forest or a desert with one lonely tree in it. I couldn't figure out how to find the middle ground between the two. Do you have any tips or links to good forest examples?