Hi there Metisan, welcome to the Guild! Hmm, did you by chance use my tutorial for the trees? If so, they look a bit flat for some reason. I'm wondering if you might accidentally have bump mapped the wrong layer or something. If you didn't use my tutorial, you may wish to look at it (it's in my link), I think it will help give your forests a bit more depth. As to the desert, well there are tons of different ways to do this. Probably the simplest way would be to add a new layer, use a fuzzy brush to paint a tan color onto the area you want the desert and then play around with layer modes and opacity settings till you get something you like. You could also find a texture of sand or dunes or something and, on a new layer, use your clone tool (again with a fuzzy brush) to paint in the texture, then once again, play with the layer mode and/or opacity till you are happy.

Cheers,
-Arsheesh