Why we never top a tree
Proper Cuts at the Branch Collar and Why Topping Harms Trees
Each cut is made just outside the "branch collar", the swollen ring where a branch meets the trunk, with no stub left and no flush cut into the trunk, and no more than 25 percent of a tree's live, leaf-bearing limbs comes off in one year.
"Topping", cutting the top of a tree back to stubs, weakens its structure and opens it to disease. Professionals do not top trees, and it is not a recognized pruning practice; when a tree has outgrown its spot, crown reduction is the cut that brings it down. Stripping the inner branches and leaving leaves only at the tips, "lion-tailing", is the over-thinning mistake on the other side.
Those are the pruning standards the trade works to, set out through the Tree Care Industry Association.