Tree topping is a term used to describe the cutting back of large branches that make up the canopy of a tree. This method of pruning is not a good thing. To me it disfigures the tree, making it look like a giant shrub instead of a stately tree. Also, topping will cause eventual health problems and a shorter life.
Most homeowners mean well when they have trees topped. The most common reason is the fear that their tree has grown so tall that a strong wind may blow it over on the house or other structures. The root system of a healthy tree usually has roots extending out twice as far as the longest branches, so a healthy tree is not likely to blow over except under extreme winds.
Some believe that topping is actually good for the tree, stimulating new growth and a thick canopy. Trees do vigorously grow back long upright sprouts in a desperate attempt to grow back their food producing leaf canopy that was lost through topping. Sprouts growing from large stubs are usually weakly attached and more prone to damage by wind or ice accumulation. They are also less pleasing in appearance as a natural looking canopy.
Other reasons for topping include overhead utility wires, view obstruction, branch encroachment on buildings, etc. These problems result from the wrong tree being planted in a site with limited space, and has outgrown it. In some cases corrective pruning of one or two major branches might solve the problem without resorting to topping. Total tree removal may be the only ultimate option.
Topping damages trees in other ways as well. Removing a large percentage of the tree canopy upsets the crown to root ratio and seriously reduces the tree’s ability to produce enough food. This stresses the tree, making it less healthy and prone to insect and disease problems. Removal of the tree canopy suddenly exposes the bark to direct sunlight, often causing scalding or even death to those exposed tissues.
Large branch stubs from topping seldom heal completely and usually provide an entry point for wood rotting fungi. Decay that begins at these wounds often works its way into the main trunk and can structurally weaken the tree.
All of these stresses reduces the life expectancy of a severely topped tree to about half that of a properly pruned tree, so think hard about having it done. There are pruning methods that are much less harmful, so study up on it or talk to a professional arborist.