I literally just learned what banding was about an hour ago...  lol 
And I actually watched a deviantart tutorial on making trees with detail, but I didn't work with nearly enough pixels as they did and the detail suffered . I was going for shading with all the different shades, but I guess I'll do that instead lol. Thanks for the feedback 

You've got banding, colors that don't read and hardly any leaf design. The canopy of a tree is set apart into layers. Yours doesn't really have layers. The layers also shouldn't be outlined, they need to be integrated into one another. There's plenty of good pixel tree tutorials. I recommend this one: http://kiwinuptuo.deviantart.com/art/Pixel-Art-Tutorial-Tree-196679114