Sometimes you may look at a tree and think it's getting too tall, but the time to start addressing this issue is when the tree is young because removing main branches to reduce height in a mature tree is called topping, an unacceptable practice in the professional tree world.
Crown reduction is used to reduce the size of a tree and the size of cuts should be as small as possible to reduce the chances of decay. The tree trimmer will consider the number and location of remaining branches and the possibility of sun scald on newly exposed branches. Healthy and vigorous branches will be kept to maintain structural integrity and form of the tree.
Topping temporarily reduces the tree size by cutting live branches and leaders to stubs without long term tree health or structural integrity.
The following are the reasons why a tree should not be topped an its effect on the tree:
Starvation
Topping removes a large portion of the crown, reducing the trees ability to manufacture energy resources to recover from the pruning cuts. It also removes the carbohydrates stored in branches.
Insects and Diseases and Decay
Large stubs of a topped tree have a difficult time forming wound wood. The location of these cuts and their large size prevent the tree defense system from being able to do it's job. These wounds are vulnerable to insect invasion.
Weak Branch Attachments
A new branch that sprouts after a larger branch is shortened is more weakly attached than a branch that develops normally.
Rapid New Growth
The objective of topping is size management, usually reduction in height, but this is only temporary. The resulting sprouts are far more numerous than normal new growth and the rapid rate can return the tree to its original height in a short time.
Tree Death
Some trees are more tolerant to topping than others, surviving for a few years. Many trees do not have the ability or resources to recover from this extreme pruning, leading to their decline and eventual death.
Ugliness
A topped tree is disfigured. Even with its regrowth, it never regains the grace and character of its normal form.
Crown Restoration
If a tree has been topped and has responded by growing lots of sprouts a crown restoration can improve it's structure and appearance. It involves the selective removal of some sprouts and stubs. Well spaced sprouts on damages branches are chosen to become permanent branches to reform the crown. This is not a quick process and can take many pruning cycles to accomplish.
*All information has been collected from the International Society of Arboriculture.