The main problems I see in this tree are the textures and perspective.
Now, assuming that the perspective is 3/4, the roots would never appear in such a way(they look almost as if they're viewed from the front), you can fix this by, firstly defining were they will be placed with a circle or whatnot, and then actually draw them along the circle.
The crown of the tree also suffers the same issue with the perspective, but that's mainly due to the way that the light source is displayed, try making the very top of the crown (with also the lower part of the crown that the light hits) be brighter than the rest.
The only issue I'm having with the textures so far (acknowledging that you barely did anything with the crown by now), is the tree trunk itself, you usually wouldn't encounter such tree trunks, the lines would usually go almost completely straight.
Here's an edit, just so that some of the stuff I said would make more sense.