Depending where your model will be used
the bottom may be sufficient...
You could make a decent Normal map / bump texture that would "fool" the eye into taking more detail from the shape then it currently provides..
but I would suggest as posted above.. that you add some more poly's in the base and "rough" it up a bit to give it a more natural look.. most roots look uneven and some even "knobby"
here is a quick pick of a tree i made for a DOD:S map i made.. it shows the actual polygons, but also the details baked in from the normal map
( not the bark etc.. just the general form of the tree )
and here is an example of some roots i fooled around with...
hope this helps ya out