There's a technique in photoshop where you could apply the texture of the terrain under the shadow to the shadow so as the shadow wouldn't look painted on. Its a little involved but pretty convincing. It uses "apply image". Basically you duplicate your image, blur it (2 pixels or so) subtract it from the original image (offset +127) and that should give you a texture or relief image. You then take the blurry image - make a new layer paint your shadows on that new layer and then blur them (about 2 pixels). Merge down and apply the blurry image to the relief image (offset -127) add. viola - textured shadows.
oh and foremost- the cast shadows of the planes don't match the cast shadows of the trees. For it to work, its going to have to look like the light source is the same.
as nobaddy said , you don't need the shadows. given the angle light and its diffusion, you wouldn't see the shadows in the picture except maybe at the very bottom.
Oh and yet another edit- GE is on the board now - he cand give you tons of advice. He's an extemely talented illustrator.