How Pruning Affects Tree Strength

Written by August Hoppe, President Hoppe Tree Service, Certified Arborist WI-0477A, and John Sepulveda, Certified Arborist WI-1454A

Trees have evolved unique characteristics to be able stand upright, grow tall and withstand heavy wind events. In this article, we’ll talk about this truly fascinating ability, and ways pruning can affect tree strength.

The Many Roles of Roots

The internal vascular system and roots provide the necessary strength and nutrient and water transport that allow trees to grow upward and stay strong enough to resist breakage and toppling. Underground roots are a main source of water intake and nutrient uptake from the soil. They also provide a base that anchors the tree to the ground to prevent tipping over in windy conditions. The roots may spread greater than five times the height of the canopy. Trees have smaller, nutrient-absorbing roots and larger, structural support roots that both serve important functions. The root collar is the area where tree roots merge into the main stem of the tree.

Why Do Some Branches Fail?

Trees are one of the few plants that exhibit branches. Branches form off of stems and are the structural support for leaves. They carry the water from the soil to the leaves that is essential for photosynthesis, the process by which trees convert energy from the sun into food. Branches also store food the tree will use during the dormant season.

Included bark at a junction point in the tree. Had the limb on the right been pruned years ago to slow its growth or removed to create one central trunk, the tree would not have the trouble it now faces with this serious crack.

The strength of branches is based on their attachment to the parent stem. Generally, trees that grow with narrowly-angled attachment points to the stem are more likely to fail and spilt. This is due to a phenomenon called included bark. Since the attachment to the tree is so narrow, the main stem and the branch start growing together and the bark from both stems get included with solid wood growing around it. Branch areas with included bark are weaker than solid bark, and become a stress point for trees. Branch attachments with widely-angled attachments are stronger due to the fact that there is more space between the stem and branch and hence more room for them both to grow and develop solid wood without growing into each other.

Maximize Tree Strength with Pruning

Areas where two limbs of roughly equal size come together is another potential area for included bark. This is called codominant stems. Proper pruning when a tree is young can eliminate or reduce the potential of included bark and codominant stems. Pruning to create a strong central leader in the tree is usually preferred and helps the tree maximize its strength.

All of these factors together help to keep trees strong and help them to survive storms and other stress events. It is important to set urban trees up for a long life by training them early with structural pruning and consistent care. It is much easier to do a training pruning on a young tree in order to develop a strong set of limbs than to try years later to adjust the structure of a mature tree.

Hoppe Tree Service provides expert pruning on both young and mature trees. Contact us to visit with a certified arborist and receive a quote for services.