My 'Jack the Ripper' Buckthorn destroyer

A while back I heard about a tool called a Weed Wrench.  There are other brands, but their use is to pull small trees and shrubs out by the roots.   So it seemed like a good project to build my own.   Some old parts from an old cultivator, and a corn planter and Voila!   We have lots of the invasive shrub Buckthorn so this may assist in the reduction of their numbers.  The above buckthorn was 1.5" diameter....  so what is the upper limit?   The jaws open over 3", but that seems a bit ambitious.