Random points, in no particular order:
Your light source appears to be top-right, but the foliage of the trees (the nearer ones especially) doesn't reflect the "right" part very well. Put more shadow on the bottom-left.
What's that spotted grey stripe just below the horizon supposed to be? It looks like the side of a rock cut or gravel pit, sort of.
Shadowing on the cave-mouths of the mound things in the background is completely wrong for a top-right light source. The one on the right especially should also cast some shadow.
The trunk of the largest tree on the far right doesn't work well for two reasons. First, what I assume was meant to be bark texture looks more like the lines where someone glued the tree back together—if you want to try again, go for something more vertical. Secondly, while there are a few kinds of trees where the branches stick out at right angles from the trunk, generic deciduous hardwoods like the ones you seem to be trying to depict aren't among them. You want to have the branches either spread out from a crotch at a level near where the leaves begin, or have them angle up rather than curve. I'd suggest searching for "winter tree photo" or similar to get an idea of what branch patterns in real trees look like.
I'm not sure those irregular stripes in the sky are a good idea. I'd even them out and maybe even do some dithering or blending with single-pixel horizontal lines in alternating colours.
I'll ignore the river for the present since you say you haven't worked on it yet.
Hope that helps.
