Very neat. Looks like an LTTP tree on realism crack. (that's a good thing)
The thing jumps out at me most about this is that it's quite large, yet the detailing you're doing is very monotonous and repetitive. I suggest some little detail gimmicks here and there to break it up. It has to do with a certain design principle - you ought to give the eye little things to jump around on, rather than allow the tendency to pass over repeated detail to cause your viewer's eyes to not spend more time taking it all in, so much work can go to waste. To illustrate - think of a big field of black and white checkerboard squares - the very second you see it, your eye realizes what it is and that there's nothing but a repeated pattern. Therefore, the eye does not look at every individual square does it? No. The eye passes it over, on account of a lack of 'visual interest'. - An extreme example but it proves the point. Well, I think you have a lesser example of that happening here, so far.
The thick vertical trunk looks like strictly upright textured cylinders butted up to one another, arrayed in a circle. Why not take that bark texture and swirl it around a little into more interesting patterns. It's verticalness is a little plastic.
Also, this is huge. From what I've seen: almost everyone that embarks on a huge pixel project like this does NOT finish it. Just a reality check. Do you really want to proceed? The ROI may not be there for you.
Curious to see how you'll style the foliage at the top. Always my favorite part of a pixelled tree. But I do love gnarled roots. So fun to draw, too.
What's the context? Is this a crop of a larger image? Is the tree to be the focal point of the final outcome?
