I think the key to sucessful pixeling in this size is recognition of basic elements, along with the relevance given to them. You seem to be putting up too much detail and stray pixels everywhere that compete with the basic shapes, making the whole piece lack depth and look busy.
Simplified elements along with light and shadow help alot when defining shapes, and most of the time can be enough to make something look good. Some details can also be implied to the eye by altering the main shapes slightly, without having to waste more pixels.
Technique wise, something important is to avoid using lone pixels to define textures like this, as they alter them always in a negative way, and grab unwanted attention.
Here's my take of it. Light and shadow mostly on the trunk, different leafy layers for depth.
There were a bunch of tree threads here some weeks ago filled with excellent examples, btw. I recommend a quick search.