You need to develop the modelling of the tree more. Forget texturing until you have a pleasing shape and volume.
When you have developed that kind of organic shape, opportunities to highlight texture are literally at every turn.
quick non-pixel-art edit:

This fixes up some of the modelling problems (each level being too distinct; at each level, the surface being flat rather than somewhat variable in depth..) and adds some texture.
Hope that gives you some ideas.