Does it need to be procedural? If you want it realistic, go get a picture of some tree bark and edit it in Gimp or photoshop to make it seamless. Use this as your color component.
Run a threshhold over it so the valleys are dark and the tops are white, blur slightly, and you have you normal component.
Anything else and what you will be ultimatly doing is drawing tree bark by hand…
You could fake something with the proceedural textures, but I doubt you’d get it that realistic.
Try a Wood-BandNoise (hard noise) as color (brownish grey, and dk-brown) and normal (1.2-1.5) scaled about 4-5 in the X and .5 in the Y Tube mapped. Low spec (0.15) High hardness (100).
Layer on that some Clouds with the brightness down and contrast up with a medium grey color, and a norm about 1.0. Tube mapping. Scaled about x=3.5, y=4.
Should give you a start, but for this an image texture is the way to go.