Strengthening our Community Tree Canopy Through Education

Module #10: Hazard Tree EvaluationLaura Sanagorski, Environmental Horticulture Extension Faculty

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.

Hazard Tree Evaluation

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.

4

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

5

Risk Assessment Basics

6

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.

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.

7

Tree InspectionA Visual Tree Assessment (VTA) involves a thorough, systematic inspection, walking completely around the tree.

8

• 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

Photo: UF George Fitzpatrick

Visual Tree Assessment

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

12Photo: Bill DuMond

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

13

Tree Decay

14

Fungal fruiting bodies, mushroom like structures, can indicate significant decay inside the tree.

Basal rot can lead to tree failure at the base.

Potential Indicators of Decay• Cracks• Seams• Bulges• Wounds

Photos: UF George Fitzpatrick

Useful tools for tree inspections

Air excavation devices Binoculars Listening devices Increment borers Electric drills Resistograph

15

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).

16

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

17

18

Hazard Rating

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

Hazard Rating – Failure Potential

1 = low2 = medium3 = high4 = severe

Hazard Rating – Size of Defective Part

1 = less than 6 inches2 = 6 to 18 inches 3 = 18 to 30 inches4 = greater than 30 inches

Hazard Rating – Target Rating

0 = No target1 = occasional use2 = intermittent use3 = frequent use4 = constant use

Hazard Tree Case Studies

23

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: 3Size of defective part: 2Target rating: 3

 Hazard rating: 8

Case Study 1

24

Failure potential: 3Size of defective part:

2Target rating: 3 Hazard rating:

8

Case Study 1

25

This large Live Oak (Quercus virginiana), is approximately 38-40 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: 3Size of defective part: 3Target rating: 3*

 Hazard rating: 9

* - Picnic tables formally underneath the oak were removed to reduce

the target rating, bringing it from a “4” to a “3”.

Case Study 2

26

Failure potential: 3Size of defective part:

3Target rating: 3 Hazard rating:

9

Case Study 2

27

Mitigation Options

28

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

29

Liability and NegligenceArborists have a duty of care and are held to higher standards because of their higher level of knowledge and experience.

30

• 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.

Strengthening our Community Tree Canopy Through Education

Module #10: Hazard Tree EvaluationLaura Sanagorski, Environmental Horticulture Extension Faculty