What is Tree Topping?
Tree topping is the drastic removal, or cutting back, of large branches in mature trees, leaving large, open wounds which subject the tree to disease and decay. Topping causes immediate injury to the tree and ultimately results in early failure or death of the tree.

Other names for this malpractice include stubbing, heading, heading-back, stubbing-off, tipping, hatracking, topping off, dehorning, lopping, or roundover.

In short, topping by any name is the worst thing to do to the health of a tree.

more here: