Why should you NOT top a tree??
STARVATION: Good pruning practices rarely remove more than one-fourth of the crown, which in turn does not seriously interfere with the ability of a tree's leafy crown to manufacture food.
Topping removes so much of the crown that it upsets an older tree's well-developed crown-to-root ration and temporarily cuts off its food making ability.
(Courtesy of the Arbor Day Foundation)