I actually had to move a bit closer to the screen to make sure it really was pixel art. That's amazing. I just recently felt all cool for making a good looking tree, and then I come across something like this...
Anyway, the large foreground tree at the left has some strange blocks of solid color near the top, maybe could use some more texturing. Also, in the base of that tree, the shadow between it and the rock wall has some strange clusters that might need to be cleaned up.