This is a very small image. Hard to see the real problem, but in my opinion, the textures of the trees are too dark.
When you are using texture maps, you can check which diffuse colour you have in the object's shader. If it's grey, turn it to white and see, if this helps. Otherwise just brighten up your image maps a bit in an image editing software.
Here's my favourite advice: don't use additional lightsources too brighten something up, unless, there is a real lightsource in the scene like lamps or fires. Play with the light that is natural and you'll get a natural looking image.
And to me the shadows are absolutely OK.