Crown thinning is a pruning technique primarily used on hardwood trees. Crown thinning is the selective removal of stems and branches to increase light penetration and air movement throughout the crown of a tree. The intent is to improve a tree's structure and form while making life uncomfortable for tree pests.

On occasions the severity of some diseases and pest infestations can be reduced by crown thinning because more light and air
can move through a thinned canopy, this keeps the foliage drier, which discourages diseases.

Thinning is used to reduce limb weight on mature trees in order to compensate for structural defects such as cracks, hollows,
and cavities. This involves the selective removal of secondary branches within the crown to open up the tree. Crossing and competing branches are mostly removed. This also reduces the risk of infection through abrasions in the bark where it has been rubbed off. Thinning also reduces the risk of wind throw (branches snapping off due to heavy winds).

unfortunately, a lot of tree surgeons misunderstand the reasons for thinning by only removing branches from the
interior of the canopy. This is often referred to as lion tailing. Little or nothing is removed from the ends of the limbs, The consequence of this is that too much weight is concentrated at the ends of the branches which causes limbs to over elongate. It can cause excessive epicormic growth, if the tree is constantly pruned this way the branches will inherently become weaker and as a consequence could possibly cause wind throw.