1. Strengthening our Community Tree Canopy Through Education
Module #10: Hazard Tree Evaluation
Laura Sanagorski, Environmental Horticulture Extension Faculty

2. Strengthening Our Community Tree Canopy
2012 Urban Forestry Series
This series is a part of a project titled “Strengthening Our Community Tree Canopy Through Education”.
This project is made possible by  a grant through the 2011 National Urban and Community Forestry Grant Program and the Florida Department of Agriculture and Consumer Services, and the Florida Forest Service.

3. Hazard Tree Evaluation

4. Hazard Tree Assessment: The process of evaluating the likelihood that part or all of a tree will fail (fall) and cause damage and/or injury.

5. Arborists perform tree risk assessments to:
Enhance public safety
Protect workers on the job site
Promote tree longevity by predicting and preventing tree structural failure

6. Risk Assessment Basics
Failure potential: The likelihood that an entire tree, or part of a tree, will break and fall within a given period of time.
Targets: People or property that could be damaged, injured or killed if the tree fails.

7. Tree stability may be affected by:
Construction, grade changes and trenching
Removal of adjacent trees that previously served as a wind buffer
Replacement of sidewalks and root loss
Failure of nearby trees
Change in wind dynamics
Storms cause most tree failures
Either during the storm,
Or after the storm as a direct result of damage that occurred because of the storm.

8. Tree Inspection A Visual Tree Assessment (VTA) involves a thorough, systematic inspection, walking completely around the tree.
Photo: UF George Fitzpatrick
Visual Tree Assessment
Is there dieback, gaps or discoloration in the tree’s crown?
Is the tree leaning?
Are individual branches extending beyond the rest of the crown?
Does the tree have good tree taper?
Are there abnormalities in the trunk, root collar or root zone?
Photo: UF George
Fitzpatrick

9. Trunk Taper
Diameter should decrease as you measure higher up the trunk

10. Co-dominant leaders
Included bark; little or no
connective wood.
Photos: UF
George Fitzpatrick

11. Photo: UF George Fitzpatrick

12. Photo: Bill DuMond

13. Common Defects Associated with Tree Failure
Codominant stems
Included bark
Cracks and splits
Multiple attachments
Cross branching
Girdling roots
Wounds
Decay
Cavities
Fungal fruiting bodies
Bleeding
Loose or cracked bark
Nesting holes
Deadwood
Borers, termites ants
Leaning tree
Overpruned palms
Previous failure

14. Tree Decay Potential Indicators of Decay Cracks Seams Bulges Wounds
Basal rot can lead to tree failure
at the base.
Fungal fruiting bodies, mushroom like structures, can indicate significant decay inside the tree.
Potential Indicators of Decay
Cracks
Seams
Bulges
Wounds
Photos: UF George Fitzpatrick

15. Useful tools for tree inspections
Air excavation devices
Binoculars
Listening devices
Increment borers
Electric drills
Resistograph

16. Risk Assessment and Management
Requires knowledge of both tree biology and structure-function relations in trees
All observations, measurements and recommendations should be documented
Numeric formulas are available (i.e., Matheny and Clark).

17. Two-page form recommended by the ISA for Tree Hazard Evaluation

19. Hazard Rating FAILURE POTENTIAL (1-4) + SIZE OF DEFECTIVE PART (1-4)
TARGET RATING (0-4)
___________________________________________________________________________

20. Hazard Rating – Failure Potential
1 = low 2 = medium 3 = high 4 = severe

21. Hazard Rating – Size of Defective Part
1 = less than 6 inches 2 = 6 to 18 inches 3 = 18 to 30 inches 4 = greater than 30 inches

22. Hazard Rating – Target Rating
0 = No target
1 = occasional use
2 = intermittent use
3 = frequent use
4 = constant use

23. Hazard Tree Case Studies

24. Case Study 1
The tree in question is a Bael Fruit(Aegle marmelos), that is located in a botanical garden adjacent to a walkway at the main entrance, where a steady flow of pedestrian traffic occurs. The tree possesses numerous codominant trunks with bark inclusion and also has visual signs of decay and cracks at the areas of bark inclusion. Additionally, pruning cuts were made in the past that have not compartmentalized.
Beneath one of the codominant trunks is a bench that could be considered a potential target. Furthermore, a hanging basket with a staghorn fern is situated on this particular codominant trunk. Additionally, visitors to the garden should also be considered targets.
This tree could potentially experience branch failure from any one of the codominant trunks, especially the one with the excess weight of the fern. Upon inspection, it appeared that this particular 10”-12” branch was most likely to fail As such, the tree’s hazard rating would be:
Failure potential: 3
Size of defective part: 2
Target rating: 3
Hazard rating:

25. Case Study 1 Failure potential: 3 Size of defective part: 2
Target rating:
Hazard rating:

26. Case Study 2
This large Live Oak (Quercus virginiana), is approximately years old and is 51.5” DBH (diameter at breast height). It is located adjacent to a walkway that leads from a parking lot to the main entrance of a building and receives a fair amount of regular pedestrian traffic. The tree has numerous codominant trunks that are roughly the same diameter and originate low on the tree. There is also bark inclusion present and cracks are forming where the bark is trapped between the trunks.
The narrow branch angles on the tree appear to be retaining moisture and could possibly be harboring decay. A number of branches have excessive end weight and there is evidence that a number of pruning cuts and wounds have failed to seal over.
Given these defects, this tree has potential of failure from one of the large codominant trunks. Upon inspection, it was determined that a branch with a diameter of approximately 22”-24” posed the greatest threat for failure, though, while highly possible, it is not necessarily imminent. As such, the tree’s hazard rating would be:
Failure potential: 3
Size of defective part: 3
Target rating: 3*
Hazard rating:
* - Picnic tables formally underneath the oak were removed to reduce
the target rating, bringing it from a “4” to a “3”.

27. Case Study 2 Failure potential: 3 Size of defective part: 3
Target rating:
Hazard rating:

28. Mitigation Options

29. Mitigation: The process of reducing risk potential. May include:
Removing the target
Fencing off the area
Removal of dead and broken branches
Installation of bracing or cabling
Lightning protection systems

30. Liability and Negligence
Arborists have a duty of care and are held to higher standards because of their higher level of knowledge and experience.
Negligence is the failure to exercise due care.
Causation in fact means that the damage, injury or death can be traced back to a person’s action (or inaction).
Act of God is an occurrence due to natural causes that could not have been prevented by ordinary skill and foresight.
All landscape trees pose a risk and some trees pose more risk than others.
The property owner must decide what level of risk is acceptable.
The arborist can assess risk and make recommendations.

31. Strengthening our Community Tree Canopy Through Education
Module #10: Hazard Tree Evaluation
Laura Sanagorski, Environmental Horticulture Extension Faculty