nice idea -- would look for a higher res background so you can get a clearer chop

The trees looks terribly cut and paste, since the dark in between the trees does not match the clear sunny sky around them, and the shape is too smooth and does not look like real trees, which are very irregular in profile...

I agree with MossyB. If you cant get the trees right, you better leave them out of this picture.

I agree with JStaes. I have to say, unfortunately that took away from the image. But just work a bit more on that for future images, and try to find trees that will better match the image you are working with, lighting-wise. Also a trick that I use when I can't cut the edges of trees out good enough, I make a brush using the type of leaves that are on the trees, and I paint around the edges and play with the colors and blending modes to make it look a little more realistic.