Branches die off for a number of reasons including light deficiency, pest and disease damage, and root structure damage. The main reason deadwooding is performed is safety.
The deadwooding process speeds up the tree's natural abscission process. It also reduces unwanted weight and wind resistance and can help overall balance.
Crown and canopy thinning increases light and reduces wind resistance by selective removal of branches throughout the canopy of the tree. This is a common practice which improves the tree's strength against adverse weather conditions as the wind can pass through the tree resulting in less "load" being placed on the tree. The shape is vital for the survival of the tree and lopping off the wrong sections of a tree if it has surpassed its height limit can actually be extremely damaging. This (topping) can hinder its growth or cause an overbalance.
Crown lifting involves the removal of the lower branches to a given height. The height is normally achieved by the removal of whole branches.The branches are normally not lifted to more than one third of the tree's total height.
This pruning technique is usually used in the urban environment as it is for public safety and aesthetics rather than tree form and timber value.
When a branch is removed from the trunk, it creates a large wound. This wound is susceptible to disease and decay, and could lead to reduced trunk stability. Therefore, much time and consideration must be taken when choosing the height the crown is to be lifted to.
Reducing the height and or spread of a tree by selectively cutting back to smaller branches and in fruit trees for increasing of light interception and enhancing fruit quality.
Pollarding, a pruning system involving the removal of the upper branches of a tree, promotes a dense head of foliage and branches.If done on larger or mature trees, will promote decay and possibly death of the tree. It is normally started once a tree or shrub reaches a certain height, and annual pollarding will restrict the plant to that height.