Tree Risk Assessment

“Is my tree dangerous?” Is a question we hear often. The problem is that it’s difficult to trust tree service sales people because they have a natural bias toward selling tree work as they don’t get paid unless they sell something. To find out how we overcome this bias, check out our Unbiased Consulting Arborists page.

Disclaimer- This article, video, and it’s content is for informational purposes to show the typical process of tree risk assessment and SOME of the indicators of risk. The indicators discussed in this video/article do not cover all potential tree risks indicators. Tree’s can be dangerous and only a qualified assessor should perform tree risk assessments.

Below is a video of the typical tree risk assessment process and some of the things that we look for.

Three damage

Items we look for

Carpenter Ants

It is generally agreed that the presence of carpenter ants is considered a positive indicator of decay somewhere in the tree. Carpenter ants are scavengers, taking advantage of previously dead wood. In other words, carpenter ants do not “kill” a tree. Instead, they find a pocket of dead/dying/decaying wood within a tree and take advantage of its housing and protection that it affords them from the elements.
Also, if you see one, there are usually a LOT more that you don’t see. Carpenter ants don’t just “hang out” on the tree’s bark so usually, you only see one or two.

Gradation Changes

Grading and trenching can substantially affect a tree’s health. Trees have to “grow” toward the water. If we change where the water is flowing, or cut the tree’s roots, the tree is forced to use it’s resources to physically “grow” to the new water source which is tremendously taxing on the tree’s resources. If the tree is forced to allocate it’s growing energy to find water/nutrients, it will lack in resources to put toward producing leaves which can lead to an overall decline in the health of a tree. Given enough time, this will cause the death of the tree.

Mushrooms, conks, and fungal fruiting bodies

When decay is taking place, many times mushrooms, conks, or fungal fruiting bodies can be observed. These fungi can be positive indicators of decay. Their presence is weather dependent as many time they shrink up and disappear after a spell of no rain.

Inclusions and decay pockets

Whether from injury, fungal, bacterial, or improper growth trees develop decay pockets and inclusions within them. These represent weak spots within the tree as a whole where, many times, failure will happen first.
Our job is to try and identify these weak spots, asses how far into the tree they have gone, and thus try and determine how much weakening, if any, has occurred.

Dead Branch Tips

Dead branch tips usually indicates problems within the root system of the tree. When we see dead branch tips, we look on the ground for evidence of trenching, grading, installation of grass, tilling, or application of chemicals. It is important to realize that lots of dead branch tips could indicate a failing root system which could become hazardous.

Known Weak Tree Structures

Tree’s do their best to grow strong, however this does not happen all the time. Some of the structures we look for are:

  • Codominant Structures – A codominant structure is typically considered to be weaker than a wider branch/trunk attachment due to it’s acute attachment angle.
  • Heading cuts – Trees that have had heading cuts, or have been “topped” often times have decay pockets in their trunks where the advantageous sprouts have grown out.
  • Subordinate/thin trees – Upper story trees buried under the forest canopy focus so much of their energy to grow toward light, that they can become dangerously thin and unable to sustain their height when the weather picks up. A lot of the times these trees do not represent a significant risk if one’s life is defined as target, as the protection afforded by a house’s strength would be sufficient protection. However, if house is defined a the target, they offer a measurable amount of risk.
  • Overly Foliated Trees – Trees that have never been pruned, or have received too much fertilizer, can have too much “sail affect”. This is their ability to catch wind, snow, ice, and rain which can cause tree failure.
  • Leaning Trees – Trees lean for many reasons from growing toward the light, to roots giving way under pressure. Identifying the cause of the lean, its dynamic change over time, and the tree’s response growth to offset the lean are important factors in determining if the tree’s lean may cause failure.
  • Unbalanced Trees – Trees that have branches on one side can fail toward that side. A health and secure root system is mandatory to stabilize unbalanced trees.

Assessing The Risk

Once we look at the trees, we utilize the double matrix system developed by the ISA.
To do this we must first determine two things:

  • The timeframe for failure to occur, and the likelihood of the failed tree impacting the defined target.
  • Once we have the likelihood of failure and impact, we look at what are the consequences of that event to determine the risk?

Using this system, we are afforded a way to quantify a tree’s potential risk.
It’s important to note that “Low” risk never means “no” risk– Trees are heavy and they ALL come down in one way or another. If a tree is in the air, risk is there.

This is the first of two matrix's defined in ISA's best management practices in assessing a tree's risk.

Y Axis showing likelihood of tree failure within a specified timeframe. X Axis showing likelihood of failed tree impacting defined target

This is the second of two matrix's defined in ISA's best management practices in assessing a tree's risk. The consequences are largely defined by the tree manager's risk tolerances.

Y Axis showing likelihood of tree failure AND impact happening withing specified timeframe. X Axis showing the consequences of said event happening.

As stated above, there are just a few indicators that we look for in assessing tree risk. This certainly is not an all inclusive list. Likely, nothing can replace years of experience cutting and looking at trees and their risk so if you have questions, we recommend hiring a TRAQ Qualified Tree Risk Assessor.

If you are in the metro Atlanta area to include Roswell, Marietta, Sandy Springs, Woodstock, Alpharetta, John’s Creek, Milton, Buckhead, Smyrna, Decatur and surrounding areas, please feel free to give us a call at 770-Arborist (770-272-6747). We’d love to help!

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