I agree with the above and did a quick pass on the screenshot to show the direction I would take it. I think the main issue you have is color saturation, along with some value issues. For example, the sky is very dark in your image, while in reality, the sky is usually one of the brightest elements in an environment during a nice day. Even if you are going heavily stylized, looking to reality for value relationships will be useful to you when you want to convey a certain kind of feeling.
Color-wise, most colors are very saturated, and when they are, they create more of a stressful impression than the calmer (I assume) nature feel you're after. Doing some color picking, most of them are in the right half of the color gradient (see images), while I've kept them in the left in the modified screenshot.
I've also added some warm brown and less cold blue to the scene and your shadows. In the end, some of it is personal taste, but I think this would improve the general impression of your environment.
Modified:
Highly saturated space, use very sparingly or for specific effects:

Desaturated space, calmer:

Hope that helps, good luck!