This process is when trees are pruned back nearly to the stem, enabling them to produce a dense mass of branches.
It is sometimes done for aesthetic purposes and/or to keep a beloved tree from outgrowing its bounds, necessitating removal.
Pollarding begins on young trees, but can be started later in the trees life. The process is repeated throughout the life of the tree (Often Annually).
Pollarding is only suitable to certain types of trees.
Pollarding is also appropriate where the crown of a tree is too large for its location or in some instances where extensive defects and weaknesses might make the tree unstable. Severe pruning of this type can cause vigorous re-growth on the tree.