Thanks a lot for your comments.
I agree with Calico about the foreground trees, but I think it's not the translucency setting. They look much better when rendered in a 1.0 detail render setting. I tried it but it took ages to render, so I rendered them in a lower setting.
I have to say that I'm not very happy with the clouds either but "invert profile" is unchecked. The first try was without clouds, but it looked a bit empty.