Technically speaking, you should not be putting any kind of lens flare effects into a matte painting, or anything that’s supposed to be moving, such as moving water. Matte paintings are meant to be used as virtual backdrops, and they should not contain anything that is meant to be moving, because they’ll appear to be “frozen” in the actual final result (birds frozen in the sky, moving water frozen in place, falling leaves frozen in mid-air, etc). As for lens flares, they are mechanical in nature, caused by the way lenses are designed, and that shouldn’t be part of your matte painting (especially if the shot is meant to have camera movement–your lens flare will be frozen into the background–again, very strange). The images you see on matte artist’s websites that contain moving elements, are usually screengrabs from the actual movie, when those moving elements have been composited into the shot. The original mattes themselves would not contain any moving elements.
I agree with Timothy that your image appears a bit flat. The turning of forms in your individual small shapes such as the trees aren’t prominent enough.
Your form/cast shadows should match the color cast of the sky dome more, since they receive the bounced ambient light mainly from the sky. Also, I think the shadows are a bit too dark, as if they aren’t getting any ambient light from the sky at all.