Natural Notes: Tree Risk Management

This post consists of my notes from a lecture on tree risk management for urban trees as part of a training to become a certified arborist by the International Society of Arborculture.  The speaker was Lindsay Ivanyi, a natural resources specialist with the Forest Preserves of Cook County and her lecture took place on January 28, 2015 at the Salt Creek Resource Management facility in Willow Springs, Illinois.  

Failure potential
Fungi growing at the base of a tree, or basal rot, is a sure sign of decay on that tree and could pose a risk!  This kind of damage to a tree cannot be repaired because the tree will compartmentalize off the damaged area, compromising its structural integrity.  

Recognizing Risk
Codomiant stems (see figure 1) are a result of improper pruning in the early stage of growth of the tree.  

It's important to understand that trees are supported in a manner  that is more like a wine glass, with a wider "plate" of roots at the bottom (figure 2).  Codominant growth leads to the problem of included bark (figure 3).  Less included bark is good because it means there is a stronger union between the tree branch and the trunk whereas too much included bark is bad because there is weaker branch union (see figure 4.

Figure 1:  Codominance in tree growth

Figure 2:  Trees are supported more like a wine glass on a plate.  

Figure 3:  The structural problems that arise from too much included bark

Figure 4:  The less included bark the stronger the branch union

Tree Risk Assessment Overview
Some of the components of a tree risk assessment include:

• The history of the tree's growth
• Information about the site and weather conditions
• The species of tree
• The tree's growth pattern
• Branch attachments

And more.  

When examining the history of the tree and its site, the property's history and the tree's growth history are both important factors.  The top three "tree targets" are (1) people, (2) property, and (3) vehicles. 

Never fill or seal off a wound on a tree.  It will only lock in fungus or other pathogens.  Let the tree compartmentalize on its own, or remove the tree if letting it heal itself will be too risky.  

 When damage, injury, or death occurs because of a defective tree, the law usually holds the landowner responsible.  

What is a hazard tree?  It is a tree that has a structural defect that may cause the tree or a portion of that tree to fall on someone or something of value (a target).  

Site

Edge trees are trees that are on the edge of natural areas bordering roads, parking areas, etc.  

Native trees that are near new land development may look healthy but good still be damaged.  In wet sites, trees have a shallow root system that could make them vulnerable to wind.  Vulnerability to wind is also a problem in shallow soils.  

Tall trees are susceptible to lightning damage, and species of trees include ash, beech, black locusts, elm, horsechestnut, hemlock, maple, oak, pine, poplar, spruce, sycamore, and tulip poplar.

It is also good to consider the tree's previous branch failures when doing an assessment.

It is BAD to perform internodal pruning (topping) of trees adjacent to utility lines (see figure 5).  This results in weak, small branches that exhibit stunted growth.

Figure 5:  Internodal pruning (topping)

Topping and crowning of trees is also bad because it can create dead branches and limbs that can act as a "widow maker" if they fall down (see figure 6).  Trees will naturally prune themselves, as well in response to snow, ice, rubbing limbs, and other odd growth patterns.

An unbalanced crown is one in which the live crown ratio is less than 60%.

Figure 6:  "Widow-maker"

 It should be noted that a tree can be healthy but can still be structurally unsound!  

During a risk assessment, check the trunk for included bark and codominant stems (figure 7).  Also look for wounds and cracks (see figure 8) - vertical cracks are bad!  

Figure 7:  Check for codominant stems and included bark.  

Figure 8: Vertical cracks are bad!

Decay deteriorates the tree from inside.  The outer ring bears most of the load (weight) if the tree.  This is not a problem if the tree is not leaning towards a target.

Seventy-five percent of tree failures are root related.  The key tree roots are the flares and buttresses.

Soil failures occur when the soil does not have the strength to support tree roots.

Root failure occurs when the force of wind or gravity exceeds the strength of the roots.  Healthy trees can come down when the wind exceeds 60 mph.

Root collar disorders can occur when soil and mulch covers the root collar which makes the tree susceptible to disease and infestation (figure 9).

Figure 9:  Root collar disorders can occur when there is too much mulch.