Topping is the act of cutting back the uppermost branches of a tree by removing the leaders growing up from the lateral branches. All too often, trees are topped in an attempt to control their height, or to significantly change their shape. It is a technique that is still common. This is unfortunate, as it is one of the most damaging techniques available, for two reasons.
First, the leaders carry the canopy, which is made up of the foliage that absorbs the most solar energy. Thus the removal of leaders is bad for the health and longevity of the tree.
Second, when a tree is topped, it is almost always with an upward facing cut. From this cut, the tree will quickly put out multiple vertical shoots which will grow rapidly, often at a rate of more than a metre a year, in an effort to replace its lost canopy. It quickly forms a crown that is denser than before, and within a few years the tree will probably match its original height. In short, the act of topping a tree is ultimately a self-defeating exercise.
Meanwhile, however, the upwards facing cuts will begin to rot. As the tree tries to heal, a lip will form around the cut, forming a bowl in which rain water will be trapped. Moisture, squirrels and insects then get to work to hollow out the area below the cut, quickly destabilizing the now towering new crown of branches. Insects, most notably carpenter ants, begin to hollow out the tree, often burrowing termite-like holes all the way between the crown and the roots. This effect is very apparent in the trees damaged in the ice storm of 1998, when many crowns were snapped off, and then ‘repaired’ as described above. Many of these damaged hardwoods have since become unstable, and recovery efforts continue to be common.
In summary, if you are concerned about the height of a tree, or you think it is a danger, or if you want more light through the tree, there are better techniques than topping. Topping will not only create an unstable tree, but will also make one that is even denser, and just as tall, as it was before it was topped. With the exception of some fruit trees, be sure to consider all other options before resorting to topping.