ZBrush - Floating Tree Progress

Being new to ZBrush I have created a number of exercises designed to help me discover the tools and navigate the UI. 
This project, my floating tree, is going to be an attempt at improving my 2D rendering. I plan to model this as best as I can, bring into KeyShot and the Photoshop to create a final illustration. 
I want to understand how light affects textures and how depth and composition are affected when working in this way. 
So here's my progress thus far on the floating tree.