The important thing is that neither kills the tree. Coppicing may involve just cutting of
some of the limbs, or with some species may be done close to ground. But pollarding
near where I now live, typically leaves no leaves or small branches, and top third of
tree is removed. I am astounded at how readily the trees again become attractive
and provide shade. The problem here is that lots are small, and trees cannot keep
enlarging their root systems. One of the first things I learned in botany (and one of
the few I remember) is that there has to be a balance between shoot and root.That's
why when one buys small trees, a good bit of the top has been removed, because
a lot of the tiny roots had to be sacrificed.