I do especially like the atmosphere and the clouds, which are fantastic, and very realistic!
I also like the foreground, however compared the photorealistic feel of the rest of the image, the trees have a noticably flatter look to them. Perhaps the trees would be the #1 reason why anybody could suspect this is CG.
I'm wondering, if a little reflections on the needles would help.
But anyway, one of the best images I've seen for a while now. Congratulations.
Cheers,
Frank