The most common types of pruning are:
1. Crown Thinning
2. Crown Raising
3. Crown Reduction
Crown thinning, primarily for hardwoods, is the
selective removal of branches to increase light penetration and air movement
throughout the crown of a tree. The intent is to maintain or develop a tree's
structure and form. To avoid unnecessary stress and prevent excessive
production of epicormic sprouts, no more than one-quarter of the living crown
should be removed at a time. If it is necessary to remove more, it should be
done over successive years.
Crown raising is the practice of removing branches from
the bottom of the crown of a tree to provide clearance for pedestrians,
vehicles, buildings, lines of site, or to develop a clear stem for timber
production. Also, removing lower branches on white pines can prevent blister
rust. For street trees the minimum clearance is often specified by municipal
ordinance. After pruning, the ratio of the living crown to total tree height
should be at least two-thirds (e.g., a 12 m tree should have living branches on
at least the upper 8 m).
Crown reduction pruning is most often used when a tree
has grown too large for its permitted space. This method, sometimes called drop
crotch pruning, is preferred to topping because it results in a more natural
appearance, increases the time before pruning is needed again, and minimizes
stress. Crown reduction pruning, a method of last resort, often results in
large pruning wounds to stems that may lead to decay. This method should never
be used on a tree with a pyramidal growth form. A better long term solution is
to remove the tree and replace it with a tree that will not grow beyond the
available space.