If pollarding is done repeatedly over the years, a somewhat expanded (or swollen) tree trunk will result, and multiple new side and top shoots will grow on it.
The main reason for this type of practice, rather than coppicing, was in wood-pastures and grazing areas where growth from the ground upwards was less practicable, due to the required area for grazing which would have been reduced by thickets of low tree growth. Pollarding above head height also protects valuable timber or poles from being damaged by browsing animals such as rabbits or deer.
An incidental effect of pollarding is the encouragement of underbrush growth due to increased levels of light reaching the woodland floor. This can increase species diversity. However, in woodland where pollarding was once common but has now ceased, the opposite effect occurs as the side and top shoots develop into trunk-sized branches. An example of this occurs in Epping Forest in London/Essex, UK, the majority of which was pollarded until the late 19th Century. Here, light levels on the woodland floor are extremely low due to the thick growth of the pollarded trees.
Good examples of trees which are regularly pollarded are willows in areas surrounding meadows.
A tree that has been pollarded is known as a pollard.
The term pollarding is also sometimes used in the practice of arboriculture for a particular form of tree management. This consists of the removal of all minor branches of a tree to leave just the trunk (to at least head height, or about 2 meters height) and a framework of major branches. The tree is then given some years to regrow, after which the process may be repeated.
See also: pruning fruit trees, Shredding