For your style of tree it can help to think of a sphere being lit by a single light. Imagine how that light would build highlights and shadows and ultimately work that into your tree piece. Right now there are dark shadows all around the edges, save for the foliage clump at the top. This is creating that pillow-shaded impression another was talking about...
There is also this very abrupt shift in hue in your darkest shade which isn't really present elsewhere. I understand what look you were going for, but drop the saturation way down. It's not really working to your best interests.
I made some quick changes to demonstrate a direction you could go in, but please don't rely on it as a 100% accurate lighting model.
Edit: Here is also a somewhat unrelated lighting setup that you could use to help you think of your tree's light distribution in terms of spherical..
ganbatte kudasai!!