If you look at clear skies, they almost always become darker higher up and lighter near the horizon. This could do without the extraneous trunk on the left. Also, since the sun is evidently casting light on the far side of the tree, and a little to the left, there would be a lack of light on most of the tree from the viewer's perspective getting lighter towards the left and top. Also, the palette is a bit unevenly ramped as there are a few colours which are very similar.