I began modeling the tree roots in Maya. I began with a polygon primitive cube and began subdividing the bottom faces, extruding them downwards, and continuing to subdivide and extrude each new face in a branching method, using scaling and twisting/rotating at times to organically shape them. Here is a pic of the beginning, with Smoothing turned on. (I cannot get smoothing to work once it is exported as and FBX, even with Smoothing Groups turned on.)
Once I felt I had enough branching roots, I imported it into Zbrush, and began shaping it more organically with the move tool.
I continued to shape the roots by pulling with the Move tool and applying some texture with Clay Buildup and Clay Tubes tools.