Reducing the crown of a tree can ensure a suitable size and shape for it's location is maintained. Phased reductions are recommended over several years if the goal is to reduce the size of the tree extensively.
Crown lifting involves the removal of lower branches within the crown of the tree to maintain headroom below the tree for people or vehicles.
Crown thinning is carried out when the sail area of the tree must be reduced to allow more wind to pass through. It can also allow for more light below the tree.
What it says on the tin. Removal of deadwood ensures a tree is safe from shedding dead branches onto people, vehicles or buildings below.
Some species of tree can have all of their upper branches removed once established at a suitable height, which promotes the growth of a dense head of foliage and branches. It is recommended this is completed periodically dependent on species and location.
Some species of tree can be cut down to near ground level and will regenerate. This can be completed if removal is not required but the tree is too large for its location or if the wood is to be harvested.