Thinning is the selctive removal of small live branches to reduce crown density. Because the majority of small branches are at the outside edge of the crown, thinning is focused in that area. Proper thinning retains crown shape and size and should provide an even distribution of foliage throughout the crown.
Thinning increases sunlight penetration and air movement through the crown. Increased ligh and air stimulate and maintain interior foliage, which can encourage taper on scaffold branches. Thinning a limb should be considered if cabling will be performed. Thinning also can remove suckers from the base of the treeand some watersprouts on the interior. Excessive removal of watersprouts often produces more watersprouts, so it is not recommended. Vigorous production of watersprouts on interior limbs often is a sign of overthinning, topping, or lion tailing.
Excessive branch removal on the lower two-thirds of a branch or stem (lion tailing) can have adverse effects on the tree and therefore is not an aceeptable pruning practice. Lion tailing conentrates foliage at the ends of the branches, reduced branch taper, increased load on branch unions, and weakened branch structure. Lion tailing also changes the dynamics of the limb and often results in excessive branch breakage.
If the entire crown will not be thinned, the areas to be thinned must be specified. The size range and percentage of foliage to be removed also must be specified- usually in the 10 to 15 percent range- but should not exceed 25 percent of the foliage, especially in mature trees. Most thinning removes branches 1/2 inch to 2.5 inches in diameter. If larger branches are removed, large gaps may be created in the crown, or watersprouts can result.

Taken from ISA Best Management Practices