This is drastic pruning, only appropriate with some species, carried out to make the crown smaller without spoiling the shape. It is not work which can be regularly repeated without harming the health of the tree. This type of work is the most difficult kind of pruning to do properly and must be carried out by cutting back to a side bud or branch to achieve a natural appearance without leaving stumps (which will die back and cause infection). This work may be justified in some circumstances to:
a. Prevent trees damaging buildings or power lines
b. To balance a misshapen tree after storm damage and reduce weight of unsafe branches
