Tree Risk Assessment

Most trees in an urban environment have potential for damage to nearby people and property. Most of the time trees are stable and their presence outwieghs the risk. Of course, it isn't easy to determine if that large tree hanging over your bedroom is a danger or not.

Don't buy a house without having the trees on the property evaluated for the risk they pose to the house and people living in it.

There are three levels of tree risk assessment. Level 1 is a quick first order look from afar, level 2 is an up close inspection of the tree, and level 3 inspections require advanced tools and canopy access.

Level 1

Level 1 tree risk assessments are done quickly with limited access or visibility. The results are likewise limited and some hazards may go unnoticed. Level 1 tree risk assessments are good for covering ground and quickly identifying high risk individuals. Taking a look at a tree from the sidewalk is an example of level 1 tree risk assessment.

Level 2:

Level 2 tree risk assessments are completed from the ground, but involve close up inspection of the visible roots and trunk, 360° around the tree. The canopy is inspected from the ground.

These assessments cover numerous factors including

  1. Targets of potential damage
  2. Site factors (tree history, soil conditions, wind, etc.)
  3. Overall tree health
  4. Species specific traits
  5. Pests and disease
  6. Load factors (canopy density, vines, wind, etc.)
  7. Structural defects (dead branches, bark inclusions, hollows, mushrooms, girdling roots, lean, canopy balance, cracks, taper, etc.)

After the above facotrs are compiled, an assessor makes a judgement about the likelihood of failure, impact, and residual consequences. These factors are used to calculate a risk rating ranging from "Low" to "Extreme".

As part of the Level 2 assessment, mitigation options are given with residual risk levels.

This tree was high risk because a major supporting root was cut to put in a walking path. This would have been identifiable with a walk around inspection.

This tree was high risk because a major supporting root was cut to put in a walking path. This would have been identifiable with a walk around inspection.

Beetle damage and root rot would have been identifiable with a tree risk assessment. This tree could have been taken down before falling on the property

Beetle damage and root rot would have been identifiable with a tree risk assessment. This tree could have been taken down before falling on the property

Level 3:

Not all defects are visible or observable from the ground. Many times hollow and branch attachments cannot be thouroughly inspected from underneath. The integrity of wood, root, and soil also requires a different persective than the naked eye.

Advanced assessment requires specialty tools and technical access, some of which are mildly invasive, others are not. Examples include:

This ash tree had a significant frature in the canopy that was not visible from the ground. From a climber’s perspective it is very easy to identify.

This ash tree had a significant frature in the canopy that was not visible from the ground. From a climber’s perspective it is very easy to identify.

Internal hollow only visible to the naked eye after a cross section is cut. This hollow could be observed through resistography or tomography.

Internal hollow only visible to the naked eye after a cross section is cut. This hollow could be observed through resistography or tomography.

  1. Climbing is a great way to get a clearer picture of a tree's canopy. Particularly the branch unions and hollows that can only be viewed from above. It is sometimes possible to inspect branch unions and hollows with drone videography, but drone access and effectivenss is limited.
  2. Air Spades are tools that blow away soil to expose roots without damaging them.
  3. Resistography measures the resistance wood gives to being drilled through by a small bit. Decayed sections and hollows inside the tree that aren't visible are revealed with resistographs sonic tomography, static integrated assessments, increment boring, and soil analysis.
  4. Sonic Tomography utilizes sound waves to map the condition of a tree's interior.
  5. Static Integrated Assessments, also known as pull tests, measures static compression and tension within the fibers of the wood at the base of a trunk in response to pulling the canopy with a rope.
  6. Increment Boring takes thin core samples of a trunk at various heights to assess decay and age.
  7. Soil Analysis gets to the root of the problem. A tree's health critically depends on the soil around it. If there are soil problems then there are tree problems.
 
 

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