Pruning large trees: thinning out vs topping
How a tree is pruned will greatly affect the growth form, vigor, and stability of the tree. Two common types of pruning are thinning out and topping. As explained below, the effects of the two kinds of pruning are very different.
Thinning out is also known as selective cutting or drop-crotching. It involves complete removal of a branch back to the main stem, or to another lateral branch, or to the point of origin. With thinning out, the overall general shape of the tree is kept. Pruning wounds are closer to the stem and heal more rapidly. In addition, stimulation of new growth is distributed over many growing points.
Topping is a more severe type of pruning and consists of cutting the top of a tree in a "flat-top" or "snowball-cone" shape. With topping, effects will be far more negative. Numerous new shoots will develop rapidly, producing many fast-growing, succulent sprouts. The tree will appear bushy, and the new shoots will generally form more structurally weak junctures with the main branch of the limb. Branches will tend to angle up very closely to the tree trunk, producing weak crotches.
Topping trees vastly reduces the number of leaves they have, thereby limiting the trees' ability to produce food energy through photosynthesis. It can result in their early death. In addition, topping produces large pruning cuts that are slower to heal and more vulnerable to decay.
An ideal ornamental tree shape is a straight tapered trunk with scaffold branches coming from the trunk. The most structurally sound branches are those that come out at wide angles from the trunk.
By keeping in mind a general structural framework for trees that encourages strong growth, the trees will be better able to withstand adverse weather and environmental conditions.