The trees and the foreground areas are underexposed, at least to me they are. You have run into the problem that a lot of landscape photographers run into - a foreground/middle ground that is much darker than the lighter/brighter sky. There a few ways to "fix" this, you can dodge the foreground/middleground areas in the hopes of lightening them up a bit, you can shoot a 3 stop HDR image (not sure if 3 stops will do it though) or you can use a graduated ND filter. If you shoot in raw, then you can "edit the image twice (two copies) - once to bring up the trees and grass and a second time to bring out the sky, which is what this image does, and then combine the two images in Photoshop. MY 0.02¢ FWIW.
WesternGuy