Predicting Tree Failure

Gary R. Johnson Associate Professor Urban and Community Forestry University of Minnesota
Ben Johnson Undergraduate Research Assistant Department of Forest Resources University of Minnesota

The key to preventing damage is to predict circumstances that could result in tree damage and take action to correct potential problems before storms strike. Look at the form of the tree, present decay and its extent, maintenance practices involving the tree, the presence of stem girdling roots, and existing site problems. There may be other predictable situations present, but these factors are the most common.

Tree Form

tree trunk

Image 1: Codominant leader with included bark = poor form and potential hazard. Photo by Gary R. Johnson

Trees may suffer from several natural form imperfections that lead to damage under storm conditions. Inclusion of bark at branch unions is a common weak point in trees. Branch unions should have a rough, protruding branch bark ridge where the branches meet. Without this proper union, branches or leaders have a tendency to separate during storm situations. Trees may also have codominant leaders. This occurs when a tree has two or more branches or stems that are trying to become the center point of the tree. Codominant leaders typically exhibit inclusion at their unions.

Decay

decay treeDecay is the natural degradation of tree stem, branch, and root tissue. Degraded tissue has very little strength and is the most common contributor to tree failure. Location of decay within a tree is critical in assessing the tree’s potential for failure. Decay located within a single branch is not always a critical situation since weak branches can easily be removed. But decay located within the main part of the trunk can be dangerous. Determining the extent of decay within the tree determines the tree’s decay and potential to fail. For every 3 inches of branch or stem diameter, solid wood should comprise at least 1 to 1.5 inches. Anything less than that often indicates a branch or stem that is more likely to fail during a storm.

 

Maintenance

Poor maintenance practices may encourage decay anywhere in the tree. Mechanical damage, including wounds from lawnmowers/grass trimmers, and poor pruning practices, often leads to decay. Piling mulch several inches against the stem may lead to stem girdling roots and other dysfunctional root systems. Incorrect irrigation can over saturate a soil and can encourage shallow root systems, stem girdling roots, and unstable trees. Improper use of staking equipment (wires around stems) often girdles and weakens stems.

Poor pruning practices that encourage decay or the formation of weak branches include: flush cutting (pruning too close to the lead branch or trunk), leaving long stubs, “topping trees,” or stripping bark when pruning. Trees should have between 2”-4” of coarse mulch, none of which is resting against the trunk of the tree. Staking equipment that is not properly installed or is left attached to the tree too long will damage stem tissue and encourage decay. Never attach staking wires or ropes to tree stems. Use wide, padding materials around the tree stem and connect these materials to the tree wires or ropes. There are several types of padding available at many garden centers or hardware stores, but strips of carpeting, old belts, or inner tubes also work well. No staking or guying materials should be left on a tree for more than 1 year without inspection and adjustment for increased stem growth.

Stem Girdling Roots

Stem girdling roots are roots at or below the soil surface that partially or completely encircle the trunk of the tree. Over time, they begin to stress the health of the tree, including the root system. The girdling roots eventually cause compression of the lower trunk, creating a weak point that is often the point of failure in high windstorms. Many stem girdling root problems can be prevented by root pruning pot bound trees before planting and planting all trees at the correct depth—the first branch roots just below the soil surface.

Site Problems

tree roots

Image 3: Blow over as a result of shallow root system. Photo by Gary R. Johnson

Common site problems that may contribute to tree failure are: poor soils, confined rooting areas, and inappropriate species for the location (such as large trees in small boulevard strips). Most of the problems are directly or indirectly related to lack of oxygen reaching the roots of the trees. When oxygen is lacking, root systems decline and tree stability declines. It is important to know your site situation, including soil types and rooting volume, before trees are selected and planted.

Summary

While tree failure can not be 100% predicted, there are ways to determine if tree failure is possible. Decay, poor maintenance practices, stem girdling roots, and site problems can be indicators of possible future damage. Take care to analyze these conditions and take preventative action if possible.

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