probably make the shader a little bit brighter (the color component) is set to 0.5 or something like that I think.
As the lighting on the background is so nice, it also would make sense to render two passes - one as it is right now. Another one just with the trees and an extra light. nothing else, not atmosphere, no mountains. And then add this extra pass on Photoshop on top of the original render.
Of course later on it would be nice if lights could be restricted to certain objects.