Another reason for pruning is to rejuvenate a stagnant plant that doesn’t respond to any other treatment. This “restorative” pruning is normally limited to shrubs and small trees. Severely pruning the plant places it in a “grow or die” situation, and is used only where you have little to lose. Many plants respond to this drastic treatment with vigorous new growth which may be used to redevelop the plant. Always ask a landscape professional before performing "restorative" pruning... (starnursey.com)