If I may share some advice: look at the tree distribution, its not natural in two regards. For one, it's not dense enough. It needs to be much denser. Secondly, the distribution is too even. I suggest to use a powerfractal or cloud fractal shader as a distribution shader, and experiement with that a lot to figure out a natural distribution.
Other than that I must say that the scales and colors are chosen well, and the image could certainly develop from this good start.
