The branches that remains after an aggressive pruning are more vulnerable to failure, as they do not have more branches to act as a buffering shield. Outside limbs can also divert some wind from the center of the tree. Furthermore, trees need leaves to produce food and branches to store it. By have less leaves, the tree produce less food and leads to stress, which could attract pests and diseases. It also contributes to a general decline caused by "starvation". By removing branches, trees divert energy to closing off the wounds instead of growing new wood to stabilise itself.
Tree are known to be "self-stabilising", which means that they could move and add wood in reaction to wind and other forces, in order to resist against said force. After a major portion of a tree crown is removed, the tree would not move when force was applied, and thus could not figure out where to strengthen or put out anchoring roots. This method of tree pruning creates a scene of hustle and bustle, but contributes little to the stability of the trees.
However, other tree pruning methods such as selective crown-reduction, end-weight reduction, crown-lifting and dead-wood removal may be necessary to prevent branch failure. A guaranteed way by which to prevent tree failure is to remove the tree, as it may not be of a suitable size or species for its environment.
The information and ideas in this article were inspired by Greg Lukens, Certified Arborist, Lukens Tree Preservation.
