Trees are so difficult! I drew heaps of them before I drew one I was actually fully happy with!
I think you could work on showing where your light source is coming from, so then you can tell easier where darker parts of the tree, and darker shades go.
Don't forget a little on the trunk as well :)
Here is a couple of pages that might help :)
http://fav.me/db5nz7a
http://fav.me/d9cr2dk
http://fav.me/da43513
Hope you find these use-full!

Got a as it that's a nice looking tree! Good work!

I have to say this doesn't look bad but a good simple pixel art to be honest. All you need is the highlight shading on the trees and a shade on the wood as long as you position the light source. Keep practicing and stay strong. :)