The severity of some diseases and pest infestations can be reduced by crown thinning, as it allows light and air moving freely through the canopy which, in turn, keeps the foliage drier, discouraging diseases.
Thinning is also used to reduce limb weight on mature tree, 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, which 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, resulting in too much weight being concentrated at the ends of the branches, which causes limbs to over elongate. It can also cause excessive epicormic growth – where branches inherently become weaker.
Hence, thinning evenly allows to transform a tree from being a huge, messy bush-like structure to being a monument of nature, where the beauty of the bark and branch structure can be much more visible and emphasized next to the foliage. It also allows more light in, around and under the tree, particularly valuable in small urban gardens, were light is scarce.