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Tree Risk Assessment

A Guide to Using ISA Best Management Practices
by

Urban Forestry

on 29 November 2017

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Transcript of Tree Risk Assessment

Our challenge: To keep the public safe while saving as many trees as possible.
This is the time to consider all possible targets.
Next we consider how the site influences the likelihood of failure...
Tree Risk Assessment
A Guide to Using ISA
Best Management Practices

Managing Risk and
Saving Trees

Balancing Tree Risk and Tree Benefits
Levels of Assessment
Performing a Level 2
Basic Assessment
Filling Out the Front Page
Target Assessment
Site Factors
Tree Health
&
Load Factors
Tree Defects
&
Conditions Affecting Likelihood of Failure
Assigning a Risk Rating
Time to fill out the back page!

Summarizing and Prioritizing
Inherent Risk
It is impossible to maintain trees completely free of risk.
But tree owners have a legal duty of care.
Risk vs. Hazard
Risk - "The combination of the likelihood of an event and the severity of the consequences."
Hazard - "A Likely source of harm."
So a tree is only hazardous after it has been assessed and found to pose a high to extreme risk.
Level 1
Limited Visual Assessment
ISA Tree Risk Assessment Manual, 2013, pg 6.
The best method for quickly inspecting large populations of trees to identify high risks.
Level 2
This is the level of risk assessment we will use most frequently.
Basic Risk Assessment
Level 3:
Advanced Assessment
Provides more detailed information about specific tree parts.
Usually a walk-by, drive-by, or fly-over inspection process

Used to identify trees needing a higher level of assessment or prompt action.
Level 1
Time to use the new form!
Level 3 Techniques Can Include:
Aerial inspection
Decay testing
Load testing
Root excavation
Let's get started!
Begin by designating a time frame for the inspection.
The "time frame" is the period of time in which you are estimating likelihood of failure.
This is typically 1 - 5 years.
Pedestrians on sidewalk
X
Vehicles on road
X
Power lines
X
Pedestrians on sidewalk
X
Vehicles on road
X
Power lines
X
Write down any targets and note their location.
Now consider 'Occupancy Rate'
Occupancy Rate
This is an estimate of the amount of time a target is actually in the target zone.
Rare
Occasional
Constant
Frequent
Bench
X
Assign an occupancy rate for each target...
Pedestrians on sidewalk
Vehicles on road
Power lines
Bench
X
X
X
X
3
3
4
4
Finally, consider if it is practical to move the target...
Pedestrians on sidewalk
Vehicles on road
Power lines
Bench
X
X
X
X
3
3
4
4
No No
No No
No No
Yes No
History of Failures
Pay attention to evidence of whole tree failures in the area.
This may indicate site conditions exists which could make tree failures more likely.
Look for site changes
Things like construction, trenching, grade change, or site clearance can affect health & stability.
Recent Tree Exposure
Trees which grew in the interior of a wooded area and are now in the open, may be more likely to fail when exposed to high winds.
Factors Affecting Soil Conditions:
Limited volume
Compaction
Saturation
Pavement over roots
Consider Common Weather Conditions:
Tree risk assessment takes into consideration typical weather conditions in the area.
Wind roses can be used to depict predominant wind directions.
Image from the Texas Department of Environmental Quality
Prevailing winds can affect tree growth patterns.
Changes to wind patterns can affect likelihood of failure
Precipitation...
It can happen and water is heavy!
Just a reminder that we are lucky to not have snow...
In this case the bench could be moved to eliminate one possible target.
Topography:
Trees growing on a slope or hillside might not be well rooted.
Aspect:
The compass direction of the slope.
North & East facing slopes tend to be colder and wetter, with rockier and thinner soils.

South & West facing slopes tend to be hotter and drier, with deeper soils.
A fundamental understanding of tree biology and mechanics is important when assessing tree risk.
Vigor:
Overall Tree Health
A tree's ability to compartmentalize wounds and resist the spread of decay is largely dependent on its overall health and vigor.
Foliage:
Look for problems like chlorosis or die back.
Tip or crown die back can indicate root problems.
Pests:
Look for signs of pests...
Some pest can directly affect tree health and others are positive signs of decay.

A pest we are lucky NOT to have:

Mountain Pine Beetles
Species Failure Profile:
Some species are prone to certain types of failures, like branch drop in pecans.
Load:
The result of various forces acting on an object.
The Two Most Important Loads for Trees are:
Gravity
Wind
Force is magnified by the length of the 'lever arm'
Lion's tailing trees increases the length of the lever arm leading to a higher likelihood of failure.
L
L
L = Length of Lever Arm
The lever arm length is usually about the center of the tree's crown, so when you raise the crown you lengthen the lever arm.
Relative Crown Size
The size of the crown compared to the size of the trunk.
A taller tree that has the same trunk diameter and crown size of a shorter tree, may be more likely to fail due to its longer lever arm.
L
L
L = Lever Arm Length
LCR
Live Crown Ratio:
The ratio of the height of the live crown to the height of the whole tree
70'
52'
LCR = 52'/70' x 100% = ~74%
35'
LCR = 35'/35' x 100% = 100%
60'
10'
LCR = 10'/60' x 100% = ~17%
If LCR is less than ~ 33% there may be a higher likelihood of failure.
Unbalanced Crown
and
Over Extended Branches
Most of the crown is on one side of the tree.
Branches which grow beyond the rest of the canopy.
Cracks
Image: Tree Risk Assessment Manual, International Society of Arboriculture, 2013, pg 90, Figure 5.31
The four main stresses which tend to cause cracks.
Rot & Decay
Some of the main forms of decay fungi include:
White Rot
Brown Rot
Soft Rot
Mostly break down lignin and leave cellulose

Keeps some tensile strength but loses compressibility.

Wood will appear spongy and and white.

Most commonly seen in hardwoods.
Lignin is Brown
and provides stiffness and strength when wood is under compression.
Cellulose is white and provides flexibility and strength when wood is under tension.
Mostly break down cellulose and leave lignin

Keeps some flexibility but loses tensile but loses compressibility.

Wood will be brittle, crumbly and brown.

Most commonly seen in conifers.
Typically break down cellulose first, and then lignin.

Create pockets of decayed wood that are softer than surrounding tissues.

Usually less significant than white and brown rots.

Most commonly seen in hardwoods.
Response Growth
New wood formation in response to damage or load.
Reaction Wood
Wood formed in response to gravity.
Compression Wood (CW)
Typically formed in conifers
Formed on underside of branches
High in lignin
Tension Wood (TW)
Typically formed in hardwoods
Formed on upperside of branches
High in cellulose
Compression Wood
Tension Wood
Woundwood
Wood formed in response to damage
Image: Trees, Associates & Shigo, CD
Image: Trees, Associates & Shigo, CD
Woundwood is denser and more resistant to decay than normal wood.
Look for response growth as an indicator of weakness or decay.
Load on Defect
Minor
Moderate
Significant
Likelihood of Failure
An estimation of how likely a tree or tree part is to actually fail
Improbable
Possible
Probable
Imminent
Lean
The angle of the trunk measured from vertical.
There is an app for that!
Risk Categorization
Branches
Overextended branches
12"
25'
1
None
X
X
Likelihood of Failure & Impact
From Matrix 1
12"
25'
2
None
X
X
X
X
X
X
Risk Rating of Part
From Matrix 2
Low
Moderate
Notes, Explanations, Descriptions
Draw a diagram and/or make notes as needed.
Barton Springs Rd.
Sidewalk
Tree has been pruned for powerline clearance and is very unbalanced. Branches are overextended and directly over a very busy street and sidewalk.
Mitigation Options
List any mitigation options which could lower risk.

List preferred option first.
Prune to reduce branches over power lines, road and sidewalk
Move park bench
Low
Low
Use the matrix to help calculate the residual risk after mitigation options.
Tree Based Mitigation Options

Prune
Remove
Cable/Brace/Prop
Improve site
Move Target
Restrict Access
Vs.
Target Based Mitigation Options
Overall risk rating is the highest risk determined for the tree.
Overall residual risk is the risk remaining after the highest risk tree part is mitigated.
Work Priority
Limitations and Recommendations
Assign a priority rating based on urgency of work, with one the most urgent. Schedule work accordingly.
In the case of an extreme risk rating it is a good idea to restrict access until the risk can be mitigated.
X
X
X
X
X
1 year
X
If monitoring is needed, recommend an inspection interval.

If an advanced assessment is needed, mark form as preliminary, and make recommendations.

Note any limitations of the assessment.
Ganoderma...
Is a white rot which decays the roots of trees.
Laetiporus
"Chicken of the Woods"
Is a brown rot which affect hardwood trees
Or to restrict access to the area.
Consequences of Failure & Impact
When rating consequences of failure injuries to people are always considered severe.
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