23220 Edmonds Risk Report 6.18.2015.pdfTree
Solutiontsnic
Consulting Arborists
Project No. TS - 4862
Arborist Report
TO: Kautz Route LLC c/o Steve Price
SITE: 23220 Edmonds Way Edmonds, WA 98026
RE: 23220 Edmonds Way Risk Report
DATE: June 18, 2015
PREPARED BY: Chris Madison, ISA Certified Arborist #PN-7671A
ISA Qualified Tree Risk Assessor
REVIEWED BY: Scott D. Baker, Registered Consulting Arborist 414
Board Certified Master Arborist PN0670B
ISA Qualified Tree Risk Assessor
Summary
The trees in question were damaged due to grading cut on a development site. A valid construction
permit was in place.
One tree has damage that makes it likely to be unstable in strong winds. This tree should be removed. A
second tree should be monitored or removed depending on the risk tolerance of the owner.
Assignment & Scope of Report
This report outlines the site inspection by Chris Madison of Tree Solutions Inc, on June 11, 2015. We
were asked to visit the job site and provide a Level 2 risk assessment for trees located on 23220
Edmonds Way in Edmonds, Washington. We were asked to provide a formal report including findings
and management recommendations. Steve Price, owner of Kautz Route LLC, requested these services
for informational purposes.
The tree size, species, health and structural condition and related notes and recommendations for each
tree can be found in Figure 1: Tree Inventory. Risk assessments for each tree can be found in Figure 2:
Risk Assessment Matrix. A site map with tree locations can be found in Figure 3: Site Map. Photographs,
Glossary and References follow the site map. Limits of assignment can be found in Appendix A.
Methods can be found in Appendix B. Additional assumptions and limiting conditions can be found in
Appendix C. Details of the risk assessment process can be found in Appendix D.
This risk assessment included in this report determines the presence of risk over a limited period of
time. The likelihood of whole tree or part failure is based on what is visible during the time of the
assessment and what would likely occur under normal weather conditions, over a one year time period.
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Observations
The Site and History
The site is a residential lot located in Edmonds. The site is currently being cleared and graded for
development by Kautz Route LLC. The site is going to be developed into a multi -family neighborhood
community.
We were contacted by our client soon after a grading cut to the western property line caused damage to
the roots of adjacent trees (see Photo 1). Our client requested that we assess the extent of the damage
for further planning purposes.
From my understanding all work was conducted under a 'Land Development Permit' approved by the
City of Edmonds.
All trees were located within six feet of the grade cut that occurred. The grade cut begins at the
northwest corner, and extends all the way down to the southwest corner. The grade cut varies in height.
The depth of the cut is smallest at the northern end (roughly less than a foot in depth) and deepest at
the south end (roughly four feet in depth). The grade cut adjacent to the trees varies in height from
roughly two feet to three feet.
Civil plans I reviewed call for a 3.9 foot tall retaining wall to be installed along the property line, just
inside where the grading cut took place. The wall is to be backfilled on the west side, with additional fill
to be added on the eastern side, raising the grade to roughly the original height.
Steve Price has been working closely with the neighbor ever since this incident had occurred. I met with
the neighbor, Scott, onsite during my inspection.
The Trees
I was asked to assess the root damage and associated risks of the five trees that were affected along the
property boundary (see Photo 2).
I excavated the soils adjacent to each tree to look for damaged roots. I noted damaged roots only near
Trees 1 and 2. 1 found no damaged roots adjacent to Trees 3 or 4.
I noted that the soils are sandy and well drained. This was also noted by the demolition contractor when
he was clearing the site.
Using a laser range finder, I measured the base of these trees as 109 feet away from the eastern edge of
the neighboring home. Only Trees 1 and 4 would be tall enough to strike the home were root failure to
occur. Tree 3 is 98 feet tall, only eleven feet shy of the distance to the home. All trees could strike
targets in the backyard area to the east of this home.
Tree 1 is a large Douglas fir (Pseudotsuga menziesii) which I measured at 43 inches in diameter at
standard height (DSH) and 159 feet tall. This tree was in good health and structure, and showed good
vigor. I observed four large structural roots which were damaged due to the grade cut. I measured the
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diameters of the damaged roots at 5, 10, 6, and 7 inches (see Photo 3). 1 excavated the base of the tree;
given the structure of the trunk flare, I believe the large roots which were severed were structural roots.
Tree 2 is a small Western redcedar which I measured at 13 inches DSH. This tree had some minor roots
cut; all of the damages roots were smaller than one inch in diameter.
Tree 3 is a medium sized Douglas fir. This tree has a poor taper and a small canopy, likely due to being
suppressed by the two larger trees (Tree 1 and 2) to the north and south. The grading cut occurred three
feet from the base of this tree.
Tree 4 is a large Douglas fir. This tree was in good health and form. The grading cut occurred three feet
away from the base of this tree.
Tree 5 is a small Cherry laurel (Prunus laurocerasus). I did not observe any major root damage to this
shrub.
All trees had invasive ivy (Hedera sp.) at their base. I observed ivy climbing some of the trees I was asked
to assess, as well as other trees in the nearby area.
Fiqure 1. Tree Inventory
Tree
Common Name
Botanical
DSH Drip General
Notes
No.
Name
line Health
Good vigor. Epicormic growth on
Pseudotsuga
trunk. Minor trunk kink at 78 feet.
1
Douglas f i r
43
27 E
Good
menziesii
Some over extended branches, mostly
on the southern side.
Western
13
8E
Suppressed by nearby trees.
2
Thuja plicata
Good
redcedar
Pseudotsuga
17.1
11E
Suppressed tree. Poor taper.
3
Douglas fir
Good
menziesii
Pseudotsuga
29.4
17E
Good health and structure. Grade cut 3
4
Douglas fir
Good
menziesii
feet away from base.
Prunus
2.5 A
3E
Good health, fair form. No major root
5
Cherry laurel
Good
laurocerasus
damage done to plant.
*Diameter at Standard Height (inches)
**Drip line was measuredfrom the outermost portion of the trunk to the outermost limits of the canopy
(fe e t)
/I Diameter taken at nursery caliper height- six inches above ground.
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Figure 2. Risk Assessment Matrix
Tree Common
No. Name
1 Douglas fir
Western red
2 cedar
3 Douglas fir
4 Douglas fir
Likelihood of
Part Likely to Pa rt Impacting Failure/impact Level of
Fail Failure Target Consequence
Roots Probable High Likely Severe
Roots Improbable Ve ry Low Unlikely Negligible
Roots Possible Low
Root Possible High
5 � Cherry laurel � Roots Improbable Very Low
The iSA Tree Risk Assessment method is explained in Appendix D.
Risk Rating
High
Low
Unlikely Significant Low
Somewhat
Severe Moderate
likely
Unlikely Negligible Low
Discussion
The Risk Assessment table above shows the likelihood of failure of the trees that were affected by the
grading cut. Scott, the neighbor to the west, requested that Steve Price have us assess the impact of
construction on these trees.
The grading cut to the east of these trees damaged some roots, and destabilized the soils. This risk
assessment only covers the new risk caused by this grading activity.
The two trees that have the highest risk are Trees 1 (high) and 4 (moderate). Trees 2, 3, and 5 all have
Low risk ratings.
From my observations on site, I believe that the major structural roots of Tree 1 were severed,
destabilizing the entire root plate to the east. Were strong winds to cause this tree to sway I believe it's
likely that the tree could fail at the base and fall towards to the home to the west. The upper portion of
the trunk (a part about 16 inches in diameter) would likely strike the home and cause severe damage to
the roof and targets within if they were present.
I recommend removal or snagging of this tree to reduce the risk. Work priority is high, and should be
done before the fall/winter seasons- when strong winds become more frequent.
Tree 4 is tall enough to strike the home to the west. I did not observe any damaged roots of this tree.
However, I believe the likelihood of failure to be possible due to the fact that soils were disturbed three
feet away from the base of this tree- which likely affected the stability of this tree. I believe the large
size of the upper tree part would cause severe damage to the home were it to fail. This tree poses a
'Moderate' risk. This tree could be retained and monitored- depending on the tree owner's risk
tolerance. If retention is desired, monitor the base of this tree after storm events for any cracking or
heaving of soils.
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The soils on the site, which are sandy and well drained, are conducive to tree stability.
Given my observations on site, I believe that failure of Trees 2 and 3 and impact to the neighboring
home is unlikely. The backyard area is occasionally used, but I believe the likelihood of failure and
impact to a target in the backyard to be unlikely. These trees would be most likely to fail during a wind
storm, and the space would likely be vacant during that time.
Tree 5 is a cherry laurel, which is a species that can grow into a large shrub/small tree. This species is
considered to be an invasive species in our region. I observed little to no damage to the root system of
this shrub. This shrub has no major targets, and no parts large enough to cause any damage.
In my opinion the removal of any of these trees would not cause an increase of wind exposure to the
remaining trees onsite. The canopies of these trees are distinct, and they are well adapted to wind
loads. The trees also have a shared rooting area which is also a benefit to tree stability. There is also a
heavily wooded area located to the south of the neighboring property, which acts as a wind buffer for
the property and trees located within.
The installment of the retaining wall and fill soil to the east of these trees should help improve the
stability of the remaining trees. Care should be taken to insure that no additional roots will be damaged
during the installation of the retaining wall. I advise the footing to be placed at current grade so that no
further excavation is needed. The backfill should be tamped down so the soil is lightly compacted.
I recommend the removal of any ivy climbing these trees, and removal of as much ivy on the ground as
possible. Ivy can climb trees, adding significant weight which could increase the likelihood of failure. Ivy
can also cause other tree health problems including the girdling of vascular tissues.
All of the trees affected by the grading cut are located on the neighboring lot to the west. Due to the
location of these trees Scott, the neighbor, is the tree risk manager. All decisions to manage/mitigate
the risk of these trees are ultimately up to him.
Recommendations
• Remove or snag Tree 1. If a snag is left it should be 15 feet or less in height. A new fir tree
planted next to the snag will help it blend in to the landscape.
• Do not grub or grind the stumps of trees that are removed. Stump grinding would add to the
disturbance, and possibly further destabilize the adjacent trees. Leave stumps to rot in place.
• Either: retain and monitor Tree 4 or remove to eliminate risk.
• Retain and monitor Trees 2 and 3.
• Place footings for the retaining wall at the current grade. Tamp the backfill so that it becomes
lightly compacted.
• Remove all invasive species at the base of the trees- specifically remove any ivy and vines from
the base of all trees.
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Figure 3. Site Sketch (not to scale)
�6
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Photographs
Photo I- Four trees and one shrub located on the western border of the developed property. Photo taken looking
West.
Photo 2- Damage caused by grading cut. Photo taken looking Northwest.
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Photo 3- Example of roots that were damaged due to grading cut. Roots were large, and likely main structural
roots of the tree. Pencil in photo indicates scale. Left: Root D- measuring seven inches in diameter. Right: Root 13-
ten inches in diameter.
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Glossary
advanced assessment: an assessment performed to provide detailed information about specific tree
parts, defects, targets, or site conditions. Specialized equipment, data collection and analysis,
and/or expertise are usually required (ISA 2013)
ANSI A300: American National Standards Institute (ANSI) standards for tree care
basic assessment: detailed visual inspection of a tree and surrounding site that may include the use of
simple tools. It requires that a tree risk assessor walk completely around the tree trunk looking at
the site, aboveground roots, trunk, and branches (ISA 2013)
bending moment: a turning, bending or twisting force exerted by a lever, defined as the force (acting
perpendicular to the lever) multiplied by the length of the lever (see moment) (ISA 2013)
cabling: installation of hardware in a tree to help support weak branches or crotches (Lilly 2001)
chlorotic: foliage with whitish or yellowish discoloration caused by lack of chlorophyll
codominant stems: stems or branches of nearly equal diameter, often weakly attached (Matheny etal.
1998)
cracks: defects in trees that, if severe, may pose a risk of tree or branch failure (Lilly 2001)
crown: the aboveground portions of a tree (Lilly 2001)
crown cleaning: selective pruning to remove one or more of the following parts: dead, diseased, and/or
broken branches (ANSI A300)
DBH or DSH: diameter at breast or standard height; the diameter of the trunk measured 54 inches (4.5
feet) above grade (Matheny et a/. 1998)
deciduous: tree or other plant that loses its leaves sometime during the year and stays leafless
generally during the cold season (Lilly 2001)
drive -by (assessment): limited visual inspection from only one side of the tree, performed from a slow -
moving vehicle; also may be called a windshield assessment (ISA 2013)
epicormic: arising from latent or adventitious buds (Lilly 2001)
evergreen: tree or plant that keeps its needles or leaves year round; this means for more than one
growing season (Lilly 2001)
force: any action or influence causing an object to accelerate/decelerate. Calculated as mass multiplied
by acceleration. Is a vector quantity (ISA 2013)
increment: the amount of new wood fiber added to a tree in a given period, normally one year.
(Dunster 1996)
ISA: International Society of Arboriculture
included bark: bark that becomes embedded in a crotch between branch and trunk or between
codominant stems and causes a weak structure (Lilly 2001)
landscape function: the environmental, aesthetic, or architectural functions that a plant can have (Lilly
2001)
lateral: secondary or subordinate branch (Lilly 2001)
level(s) of assessment: categorization of the breadth and depth of analysis used in an assessment (ISA
2013)
lever arm: the distance between the applied force (or center of force) and the point where the object
will bend or rotate (ISA 2013)
limited visual assessment: a visual assessment from a specified perspective such as foot, vehicle, or
aerial (airborne) patrol of an individual tree or a population of trees near specified targets to identify
specified conditions or obvious defects (ISA 2013)
mitigation: process of reducing damages or risk (Lilly 2001)
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moment: a turning, bending, or twisting force exerted by a lever, defined as the force (acting
perpendicular to the lever) multiplied by the length of the lever (ISA 2013)
monitoring: keeping a close watch; performing regular checks or inspections (Lilly 2001)
owner/manager: the person or entity responsible for tree management or the controlling authority
that regulates tree management (ISA 2013)
pathogen: causal agent of disease (Lilly 2001)
phototropic growth: growth toward light source or stimulant ( Harris et OL1999)
Resistograph drill: a drilling instrument used to determine the density of wood by measuring the
amount of resistance presented to the drilling needle as it is driven into the wood. The drilling
resistance profiles show clearly where compression wood, annual rings, rot in various stages and
other defects have been encountered by the drilling needle
retain and monitor: the recommendation to keep a tree and conduct follow-up assessments after a
stated inspection interval (ISA 2013)
significant size: a tree measuring 6" DSH or greater
snag: a tree left partially standing for the primary purpose of providing habitat for wildlife
soil structure: the arrangement of soil particles (Lilly 2001)
sounding: process of striking a tree with a mallet or other appropriate tool and listening for tones that
indicate dead bark, a thin layer of wood outside a cavity, or cracks in wood (ISA 2013)
structural defects: flaws, decay, or other faults in the trunk, branches, or root collar of a tree,
whichmay lead to failure (Lilly 2001)
tomography: a technique for obtaining 2-D cross sections or 3-D pictures of the interior of an object by
passing sound waves through the object and measuring the travel times of the acoustic signals as
the object absorbs or scatters them on ray paths between source and receiver.
Visual Tree Assessment (VTA): method of evaluating structural defects and stability in trees by noting
the pattern of growth. Developed by Claus Mattheck (Harris, et al 1999)
walk -by (assessment): a limited visual inspection, usually from one side of the tree, performed as the tree risk
assessor walks by the tree(s) (ISA 2013)
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References
ANSI A300 (Part 1) — 2008 American National Standards Institute. American National Standard for Tree
Care Operations: Tree, Shrub, and Other Woody Plant Maintenance: Standard Practices (Pruning).
New York: Tree Care Industry Association, 2008.
Dunster & Associates Environmental Consultants Ltd. Assessing Trees in Urban Areas and the Urban -
Rural Interface, US Release 1.0. Silverton: Pacific Northwest Chapter ISA, 2006
Dunster, Julian A., E. Thomas Smiley, Nelda Matheny, and Sharon Lilly. Tree Risk Assessment Manual.
Champaign, Illinois: International Society of Arboriculture, 2013
E. Smiley, N. Matheny, S. Lilly. Best Management Practices: TREE RISK ASSESSMENT, ISA 2011.
Lilly, Sharon. Arborists' Certification Study Guide. Champaign, IL: The International Society of
Arboriculture, 2001.
Matheny, Nelda and James R. Clark. Trees and Development: A Technical Guide to Preservation of Trees
During Land Development. Champaign, IL: International Society of Arboriculture, 1998.
Mattheck, Claus and Helge Breloer, The Body Language of Tree�.: A Handbook for Failure Analysis.
London: HIVISO, 1994.
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Appendix A - Limits of Assignment
Unless stated otherwise: 1) information contained in this report covers only those trees that were
examined and reflects the condition of those trees at the time of inspection; and 2) the inspection is
limited to visual examination of the subject trees without dissection, excavation, probing, climbing, or
coring unless explicitly specified. There is no warranty or guarantee, expressed or implied, that
problems or deficiencies of the subject trees may not arise in the future.
Tree Solutions did not review any reports or perform any tests related to the soil located on the subject
property unless outlined in the scope of services. Tree Solutions staff are not and do not claim to be
soils experts. An independent inventory and evaluation of the site's soil should be obtained by a
qualified professional if an additional understanding of the site's characteristics is needed to make an
informed decision.
A Hazard Tree is defined as a tree that has been assessed and determined to have characteristics that
make it an unacceptable risk for continued retention. A hazard tree, or a hazardous component, exist
when the sum of the risk factors equals or exceeds a predetermined threshold of risk. The
predetermined threshold for risk and the actions required to reduce the risk below that threshold is
established by the risk manager.
As a Qualified Tree Risk Assessor, my job is to provide the risk manager, in most cases the property
owner, with technical information required to make informed decisions. The risk manager must make
the decision about how to implement the actions required to reduce risk to acceptable levels.
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Appendix 13- Methods
I evaluated tree health and structure utilizing visual tree assessment (VTA) methods. The basis behind
VTA is the identification of symptoms, which the tree produces in reaction to a weak spot or area of
mechanical stress. A tree reacts to mechanical and physiological stresses by growing more vigorously to
re -enforce weak areas, while depriving less stressed parts (Mattheck & Breloer 1994). An understanding
of the uniform stress allows me to make informed judgments about the condition of a tree.
Using the International Society of Arboriculture (ISA) Tree Risk Assessment Qualification method, I
assigned a risk rating to the tree. I performed a Level 1, 2, or 3 risk assessment of all trees as outlined in
the Best Management Practices companion publication to the American National Standards Institute
(ANSI) A300 Part 9: Tree Shrub and Other Woody Plant Management — Standards and Practices, Tree
Risk Assessment. This approach provides assessors a structured process, based on good science and
arboriculture, to assign recommended thresholds for action for the purpose of informing risk managers.
Additional information regarding the method can be found in Appendix F.
I measured the diameter of each tree at 54 inches above grade, diameter at standard height (DSH). If a
tree has multiple stems, I measured each stem individually at standard height and determined a single -
stem equivalent diameter by using the method outlined in the Guide for Plant Appraisal, 9th Edition,
published by the Council of Tree and Landscape Appraisers.
I used a steel soil probe to test soil depths.
I used binoculars to inspect the upper parts of the trees.
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Appendix C- Assumptions & Limiting Conditions
1. Consultant assumes that any legal description provided to Consultant is correct and that title to
property is good and marketable. Consultant assumes no responsibility for legal matters.
Consultant assumes all property appraised or evaluated is free and clear, and is under responsible
ownership and competent management.
2. Consultant assumes that the property and its use do not violate applicable codes, ordinances,
statutes or regulations.
3. Although Consultant has taken care to obtain all information from reliable sources and to verify the
data insofar as possible, Consultant does not guarantee and is not responsible for the accuracy of
information provided by others.
4. Client may not require Consultant to testify or attend court by reason of any report unless mutually
satisfactory contractual arrangements are made, including payment of an additional fee for such
Services as described in the Consulting Arborist Agreement.
5. Unless otherwise required by law, possession of this report does not imply right of publication or
use for any purpose by any person other than the person to whom it is addressed, without the prior
express written consent of the Consultant.
6. Unless otherwise required by law, no part of this report shall be conveyed by any person, including
the Client, the public through advertising, public relations, news, sales or other media without the
Consultant's prior express written consent.
7. This report and any values expressed herein represent the opinion of the Consultant, and the
Consultant's fee is in no way contingent upon the reporting of a specific value, a stipulated result,
the occurrence of a subsequent event or upon any finding to be reported.
8. All photographs included in this report were taken by Tree Solutions Inc. during the documented site
visit, unless otherwise noted.
9. Sketches, drawings and photographs in this report, being intended as visual aids, are not necessarily
to scale and should not be construed as engineering or architectural reports or surveys. The
reproduction of any information generated by architects, engineers or other consultants and any
sketches, drawings or photographs is for the express purpose of coordination and ease of reference
only. Inclusion of such information on any drawings or other documents does not constitute a
representation by Consultant as to the sufficiency or accuracy of the information.
10. Unless otherwise agreed, (1) information contained in this report covers only the items examined
and reflects the condition of the those items at the time of inspection; and (2) the inspection is
limited to visual examination of accessible items without dissection, excavation, probing, climbing,
or coring. Consultant makes no warranty or guarantee, express or implied, that the problems or
deficiencies of the plans or property in question may not arise in the future.
11. Loss or alteration of any part of this Agreement invalidates the entire report.
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Appendix D - Qualified Tree Risk Assessment
The International Society of Arboriculture has developed a standardized and systematic process for
assessing tree risk. This approach evaluates the likelihood of whole tree or partfailure and any
associated consequences, based on what is visible during the time of the site visit and what would likely
occur under normal weather conditions, over a limited time period.
LEVELS OF RISK ASSESSMENT
Level 1- Survey
Level 1 shall be a limited visual assessment of an individual tree or a population of trees to identify specified
conditions or defects. Conditions to be identified should include obvious defects. Level 1 assessment shall be from
a limited, specified perspective, such as drive -by, walk -by or aerial patrol. Level 1 survey assessment methodology
shall be specified. Periodic assessments, monitoring, and follow-up recommendations should be made based on
the outcome of the assessment and the objectives.
Level 2 — Basic
Level 2 assessments shall include a 360-clegree, ground -based visual inspection of the tree crown, trunk,
above -ground roots, and site conditions around the tree. Use of hand tools, trowels, binoculars, or probes,
shall not be precluded from a Level 2 assessment. A mallet or other tool should be used to sound the trunk,
root collar and above ground buttress roots in order to detect large hollows and loose bark. Level 2 shall
provide a detailed visual inspection of a tree(s) to detect the conditions specified and tree defects in relation to
surrounding targets.
A basic assessment should include the identification of conditions indicating the presence of
structural defects including, but not limited to:
• Dead, diseased, broken branches, stems, and roots;
• Weakly attached branches and co -dominant stems;
• Mechanical damage and cracks into the wood;
• Abnormal growth such as swelling, ribs, flat areas, or seams;
• Indications of decay and cankers;
• Root plate lifting, abnormal trunk flare, lack of trunk flare, soil cracks, grade change, restricted
or undermined roots;
• Unusual tree architecture including lean, low live crown ratio, poor taper, and crown asymmetry
Level 2 inspections should be conducted annually; more frequently if species, tree size, tree condition or
other factors indicate a need for a more frequent interval. Scheduling inspections shall be the responsibility
of the tree owner. Monitoring and follow-up recommendations should be made based on the outcome of
the assessment and the objectives.
Level 3 —Advanced
Level 3 assessments shall include all Level 2 requirements. Level 3 shall include advanced method(s) to provide
more detailed information on tree structural strength, the extent of specific structural defects, conditions, or other
factors in relation to a target. Level 3 assessment shall include, but is not limited to, one or more of the following
tree assessment techniques: Aerial assessment of branch or stem defects; Microresistance Drilling; Evaluation of
target risk; Increment boring; Probing; Pull testing; Radiation assessment (eg. radar, x-ray, gamma ray); Sonic
assessment; Sounding; and, Sub -surface root and/or soil assessment.
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LIKELIHOOD OF FAILURE
improbable: the tree or branch is not likely to fail during normal weather conditions and may not fail in many
severe weather conditions within the specified time frame
Possible: failure could occur, but it is unlikely during normal weather conditions within the specified time frame
Probable: failure may be expected under normal weather conditions within the specified time frame
Imminent: failure has started or is most likely to occur in the near future, even if there is no significant wind or
increased load. This is a rare occurrence for a risk assessor to encounter, and it may require immediate action to
protect people from harm
LIKELIHOOD OF IMPACTING A TARGET
Very Low: the chance of the failed tree or branch impacting the specified target is remote. This is the case in a
rarely used site fully exposed to the assessed tree or an occasionally used site that is partially protected by trees or
structures. Examples included a rarely used trail or trail head in a rural area, or an occasionally used area that has
some protection against being struck by the tree failure due to the presence of other trees between the tree being
assessed and the targets
Low: it is not likely that the failed tree or branch will impact the target. This is the case in an occasionally used
area that is fully exposed to the assessed tree, a frequently used area that is partially exposed to the assessed tree,
or a constant target that is well protected from the assessed tree. Examples include a little -used service road next
to the assessed tree or a frequently used public street that has a street tree between the street and the assessed
tree
Medium: the failed tree or branch may not impact the target, with nearly equal likelihood. This is the case in a
frequently used area that is fully exposed on one side to the assessed tree or a constantly occupied area that is
partially protected from the assessed tree. Examples include a suburban street next to the assessed street tree or
a house that is partially protected from the assessed tree by an intermediate tree
High: the failed tree or branch will most likely impact the target. This is the case when a fixed target is fully
exposed to the assessed tree or near a high -use road or walkway with an adjacent street tree
Likelihood of
Failure (Tree)
Likelihood of Impacting Target (Person or Property)
Very Low Low Medium High
Imminent
Unlikely
Somewhat
likely
Probable
Unlikely
Unlikely
Somewhat
likely
Possible
Unlikely
Unlikely
Unlikely Somewhat
likely
Improbable
Unlikely
Unlikely
Unlikely Unlikely
Figure 1: Risk rating matrix showing the level of risk as the combination of likelihood of a
tree failing and impacting a specified target.
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CONSEQUENCES OF FAILURE
Negligible: consequences are those that involve low -value property damage or disruption that can be replaced or
repaired, and do not involve personal injury.
Minor: consequences are those that involve low -to -moderate property damage or small disruptions to traffic or a
communication utility.
Significant: consequences are those that involve property damage of moderate -to -high value, considerable
disruption, or personal injury.
Severe: consequences are those that could involve serious personal injury or death, damage to high -value
property, or disruption of important activities.
Likelihood of
Failure and
Impact
Negligible
Consequences (to target)
Minor Significant Severe
Very likely
Low
Moderate
Likely
Low
Moderate
Somewhat
Low
Low Moderate Moderate
likely
Unlikely
Low
Low Low Low
Figure 2: Risk rating matrix showing the level of risk as the combination of the likelihood of
a tree failing and impacting a specified target, and the severity of the associated
consequences.
2940 Westlake Ave. N (Suite #200) - Seattle,WA98109 - Phone 206.528.4670 - Fax206.547.5873
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June 18, 2015
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RISK RATING CATEGORIES, TIMING FOR MITIGATION
In the tree risk assessment matrix, four terms are used to define levels of risk; low, moderate, high, and extreme.
These risk ratings are used to communicate the level of risk and to assist in making recommendations to the owner
or risk manager for mitigation and inspection frequency. The priority for action depends upon the risk rating and
risk tolerance of the owner or manager.
Extreme —The extreme -risk category applies in situations in which failure is imminent and there is a high likelihood
of impacting the target, and the consequences of the failure are "severe." The tree risk assessor should
recommend that mitigation measures be taken as soon as possible. In some cases this may mean immediate
restriction of access to the target zone area to avoid injury to people.
High —High -risk situations are those for which consequences are "significant" and likelihood is "very likely" or
"likely," or consequences are "severe" and likelihood is "likely." This combination of likelihood and consequences
indicates that the tree risk assessor should recommend mitigation measures be taken. The decision for mitigation
and timing of treatment depends upon the risk tolerance of the tree owner or manager. In populations of trees,
the priority of high -risk trees is second only to extreme -risk trees.
Moderate— Moderate -risk situations are those for which consequences are "minor" and likelihood is "very likely"
or "likely"; or likelihood is "somewhat likely" and consequences are "significant" or "severe." The tree risk assessor
may recommend mitigation and/or retaining and monitoring. The decision for mitigation and timing of treatment
depends upon the risk tolerance of the tree owner or manager. In populations of trees, moderate -risk trees
represent a lower priority than high- or extreme -risk trees.
Low —The low -risk category applies when consequences are "negligible" and likelihood is "unlikely"; or
consequences are "minor" and likelihood is "somewhat likely." Some trees with this level of risk may benefit from
mitigation or maintenance measures, but immediate action is not usually required. Tree risk assessors may
recommend retaining and monitoring these trees, as well as mitigation that does not include removal of the
tree.
Source: E. Smiley, N. Matheny, S. Lilly. Best Management Practices: TREE RISKASSESSMENT. ISA 2011.
OPTIONS FOR MITIGATION
Remove the risk altogether, if possible, by cutting off one or more branches, removing dead wood, or possibly
removing the entire tree. Extreme risk situations should be closed off until the risk is abated.
Modify the risk of failure probability. In some cases it may be possible to reduce the probability of failure by
adding mechanical support in the form of cables braces or props.
Modify the risk rating by moving the target. Risk ratings can sometimes be lowered by moving the target so that
there is a much lower probability of the defective part striking anything. Moving the target should generally be
seen as an interim measure.
Retain and monitor. This approach is used where some defects have been noted but they are not yet serious and
the present risk level is only moderate.
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DEFINITIONS (RISK)
acceptable risk: the degree or amount of risk that the owner, manager, or controlling authority is willing to accept
(ISA 2013)
acceptable threshold: the highest level of risk that does not exceed the owner/manager's tolerance (ISA 2013)
consequences: outcome of an event (ISA 2013)
consequences of failure: personal injury, property damage, or disruption of activities due to the failure of a tree
or tree part (ISA 2013)
likelihood: the chance of an event occurring. In the context of tree failures, the term may be used to specify: (1)
the chance of a tree failure occurring; (2) the chance of impacting a specified target; and (3) the combination
of the likelihood of a tree failing and the likelihood of impacting a specified target (ISA 2013)
likelihood of failure: the chance of a tree failure occurring within the specified time frame (ISA 2013)
likelihood of failure and impact: the chance of a tree failure occurring and impacting a target within the specified
time frame (ISA 2013)
likelihood of impact: the chance of a tree failure impacting a target during the specified time frame ISA 2013)
likely (likelihood of failure and impact): defined by its placement in the likelihood matrix (see Matrix 1 on page 2
of the Tree Risk Assessment form); imminent likelihood of failure and medium likelihood of impact, or
probable likelihood of failure and high likelihood of impact (ISA 2013)
limited visual assessment: a visual assessment from a specified perspective such as foot, vehicle, or aerial
(airborne) patrol of an individual tree or a population of trees near specified targets to identify specified
conditions or obvious defects (ISA 2013)
mitigation: process of reducing damages or risk (Lilly 2001)
mitigation options: alternatives for reducing risk (ISA 2013)
mitigation priority: established hierarchy for mitigation of risks based on risk ratings, budget, resources, and
policies (ISA 2013)
residual risk: risk remaining after mitigation (ISA 2013)
risk perception: the subjective perceived level of risk from a situation or object, often differing from the actual
level of risk (ISA 2013)
risk rating: the level of risk combining the likelihood of a tree failing and impacting a specified target, and severity
of the associated consequences (ISA 2013)
risk tolerance: degree of risk that is acceptable to the owner, manager, or controlling authority (ISA 2013)
target: person, object, or structure that could be injured or damaged in the event of tree or branch failure (Lilly
2001)
target -based actions: risk mitigation actions aimed at reducing the likelihood of impact in the event of tree failure
(ISA 2013)
target management: acting to control the exposure of targets to risk (ISA 2013)
target value: the monetary worth of something; the importance or preciousness of something (ISA 2013)
target zone: the area where a tree or branch is likely to land if it were to fail (ISA 2013)
tree risk assessment: a systematic process used to identify, analyze, and evaluate tree risk (ISA 2013)
tree risk evaluation: the process of comparing the assessed risk against given risk criteria to determine the
significance of the risk (ISA 2013)
tree risk management: the application of policies, procedures, and practices used to identify, evaluate, mitigate,
monitor, and communicate tree risk (ISA 2013)
unacceptable risk: a degree of risk that exceeds the tolerance of the owner, manager, or controlling authority (ISA
2013)
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