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 as much as two to three times as wide as 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.

Heartwood and Sapwood

The internal part of a tree is called the heartwood. This is composed of dead xylem (water-moving vascular tissue) and is the main structural component keeping the tree rigid and moving water upwards. The heartwood is older than sapwood, which is generally the live tissue and cells that are dividing for growth. Xylem in both heartwood and sapwood provides strength, but the heartwood provides the majority of the strength of a tree as it is also less vulnerable to rot.

Some species of trees have weaker heartwood than others. This characteristic is seen in trees that are prone to breakage and typically short-lived, like boxelders and willows as compared to oaks and redwoods which live for many decades, even centuries.

The phloem is the vascular tissue that moves food made by the leaves to other parts of the tree. The phloem also provides some structural protection in the form of bark which is made of dead phloem that has been pushed out and crushed by the existing bark as new phloem forms. The bark keeps out pests and protects the internal vascular system from damage. Bark thickness varies from species to species, so the amount of protection is also variable. Trees with thinner bark are more likely to rip, which can expose or damage the phloem and xylem inside. Eventually, this can lead to rot if it is not properly protected from pests and diseases.

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
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.

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