It looks ok, the colours seem to be a bit of an eyesore, maybe dim them down a bit. Also, should the tree have no shadow? do leaves in real life gradient from darkest at the bottom do brightest at the top? The flat base of the tree makes it look like the tree is lying on the ground, add a bit of a curve to it. I know the way I've put all this across may seem harsh, but as you said, constructive criticism. Oh and with any advice, you don't actually have to listen to any of it.