Chops on larger trunks force taper into the tree. Typically with a trunk chop, the grower chooses one of the new buds that pop up near the chop site (this is typical tree response to wounding--hormones that used to travel up the chopped portion have nowhere to go once that larger portion is gone. Those hormones trigger new buds to replace the missing portion. This is a key ingredient in bonsai--hard pruning induces tighter, closer branching through new bud production).
Matching up the original trunk to the new apex branching takes some time. The bigger the original trunk, the longer the replacement branching takes to fill in--the fastest way to do it requires the tree be planted back in the ground or in a largish training container (ANY container will slow growth to some extent, as top growth depends on the amount of roots supplying it).
The top third of this oak was regrown from a sprout after the bigger trunk was severed. It took about 20 years to get to where it is now. I started it with a pinkie finger sized shoot that sprouted at the top and went from there. It took so long because all of that time, the tree has been in a container. I could have halved that time if I'd planted it back in a grow bed (which I don't have room for in my backyard).
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