Structural Pruning is considered the best way to thin trees.
Crown Thinning is the removal of select branches to reduce density evenly throughout the crown without modifying crown size or shape. Entire tree crowns are commonly thinned because tree owners and arborists believe this is what should be done to trees, like the appearance of a thinned crown, believe that thinning somehow improves tree health, or think it is a traditional pruning practice.
Thinning typically does not change the overall size and shape of a tree. Thinning the entire crown without addressing the structure accomplishes little merit in terms of tree health, structure, and longevity and often waste precious resources for both the tree and owner.
Crown Reduction is pruning to reduce the height or spread of a tree. The objective of reducing crown dimension includes providing clearance for building, streetlights, or for view; reducing the risk of tree or branch failure; reducing the amount of shade beneath a tree; mitigating broken branches when restoring following storms; preserving mature trees; or when the customer simple wants the tree to be smaller.
Unfortunately, many people prune when it is not necessary, or they misapply or misunderstand thinning, and subscribe to the practice of only removing branches from the interior of the crown. This is often referred to as lions-tailing, overlifting, overthinning, stripping out the interior, or cleaning out. Little or nothing is removed from the ends of the limbs, and this is a mistake. Lions-tailing causes structural weakness. Maybe it is done because it is easy to generate large piles of brush on the ground. It sometimes seems like people that perform this are charging the unaware customer by the pound because of the enormous amount of brush on the ground.