There are differences?
The only differences I ever saw was the woody root was below the ground, and the woody stem was above the ground. The second that the woody root erupted through the surface of the ground (and from what I saw just about all of the roots did this eventually, or so it seemed) it immediately turned into a stem, and a new poplar tree sprang to life. Or it did until I ran over it with my dad's lawnmower.
There were weekends mowing that lawn that the -only- thing that had grown during the week was the poplar suckers.
If your such an expert, name 5 things that poplar wood is used for OTHER than burning or making paper.
Ted