Newsletter Date April 2013 Peter Jackson Ba Dip LA CMLI M.Arbor.ADesign Construction Management Services Limited Tree Preservation Order Applications

Borough of Cheshire East, Stoke on Trent, Stafford, Newcastle under Lyme Special points of interest:

∗ Fully Qualified Landscape DCMS offer a very unique and profes­Architect sional service in Staffordshire to

∗ Registered practice of the help home owners and developers to

Landscape Institute successfully achieve first time plan­ning permission for tree works.

∗ Tree Consultant Whether you are looking for planning

∗ TrustMark Approved advice, planning drawings, building regulations or a TPO tree work plan­

∗ ISA Certified Arborist ning application, DCMS are here to

∗ Professional Member of the help ease the planning process and Arboricultural Association to help you achieve first time plan­

∗ Planning consultant ning application success.

We have recently been granted con­sent to work on protected trees atthe Orange Tree in Stoke on Trent, StQuentin Nursing Home, Newcastle

Inside this issue: under Lyme and High Park Stafford. Applications have also been submit­

TPO applications ted to Stafford Borough Council to work on a mature beech tree in Barlaston, where a risk assessment

Planning application Newcastle under Lyme has been requested by Craig Watkins

who is the Tree Officer for Stafford

Planning advice in the Staffordshire borough of Newcastle under Lyme

Planning negotiations are ongoing Audley Road) I believe the first three "I don’t have any particular concern with an application for a single de­ reasons on the 2004 application with the design of dwelling (either as tached dwelling in Chesterton. The refusal have been addressed in the submitted or as proposed with the planning department have now com­ submission. However, I am still not amendment).” The proposed dwelling mented on the application saying convinced the fourth reason has however, can not be sited in an alter­“I've visited the site last week and been adequately addressed to over­ native location for other factors, and due to changes in planning policy and come our concerns (the form and therefore we have put this applica­the amendments to the vehicle ac­ character of the area and detriment tion before the Planning Inspectorate cess to the site being proposed (off to the street scene).” They also say in March 2013.

Design Construction Management Services Limited

My name is Peter Jackson. I am presently the Director of Design Construction Management Services; a

597 Etruria Road Development Consultancy based in Stoke­on­Trent where we specialise solely in planning applications, Basford Stoke on Trent landscape design, tree surveys, tree related planning applications, appeals and Public Inquiries. Prior to ST4 6HP

this I was Landscape Officer for Newcastle­under­Lyme Borough Council and RPS Landscape and Ecology Phone: 01782 713616 Mobile 07950 259905 Consultants in both Chester and Birmingham. E­mail: peterjacksondev@tiscali.co.uk

I have a Bachelor of Arts Degree (with Honours) in Landscape Design and a Diploma in Landscape Archi­

www.treesurveycheshire.co.uk tecture both from Manchester Metropolitan University (formerly Manchester Polytechnic). I became an www.BS5837treesurvey.com Associate of the Landscape Institute in 1994 and a registered practice of the Landscape Institute in 2012. I

have worked additionally as a consultant for 18 years.

I am also a Professional member of the Arboricultural Association, an Associate of the Institute of Char­

tered Foresters, an ISA Certified Arborist and a Trustmark Tree Consultant.

The BS 5837 arboricultural consultancy aims to provide a comprehensive, efficient and cost effective

service incorporating all aspects of arboriculture and planning. We provide a consultancy service on all

tree related issues involving a planning application for the private sector. We are committed to providing

specialist expertise in BS 5837 arboriculture to meet our clients' requirements and where appropriate we

will liaise with other professionals to provide structural engineering solutions to obtain planning permis­

sion.

We provide arboricultural advice to architects, planning consultants, developers and other professions

associated with maximising land within a prospective development. Our surveyors are all Professional

Members of the Arboricultural Association (M.Arbor.A) and therefore our reports are able to be given as

proof of evidence in any appeal or Public Inquiry.

We specialise in BS 5837 arboricultural consultancy, rather than conducting any tree surgery work. How­

ever we can provide schedules of work and recommend suitably competent and qualified tree surgeons

that will carry out any work to a high standard for us.

We offer a service in the following counties:­

Chester & Cheshire West, Cheshire East, Cheshire, Newcastle under Lyme, Stafford, Lichfield & Cannock

Chase, Staffordshire, Stoke on Trent, South Derbyshire, North West Leicestershire & Melton, Leicester­

shire, Telford and Wrekin, Shropshire, Rugby & Warwick, Warwickshire, Walsall, Sandwell, Birmingham,

Bromsgrove & Solihull, West Midlands, Worcestershire and other areas by special arrangement.

Town and Country Planning Act 1990

(as amended)

TPO Application to fell trees covered

by a TPO risk assessment

Proposal : Reduce Beech T1

Site : Barlaston Lea, Station Road,

Barlaston, Stoke on Trent. ST12 9DA

Document date : May 2013

Client : B Hygienic Ltd Unit 3, Newstead

Industrial Estate, Trentham, Stoke-on-

Trent. ST4 8HX

Contact details for access:

07769 715500

Richard@bhygienic.co.uk

1. Introduction

Section 198 of the Town and Country Planning Act 1990 empowers a Local Planning

Authority to protect trees with amenity value by making Tree Preservation Orders. These

Orders bring under the control of the Authority the felling, topping, lopping, uprooting and

wilful damage of specified trees and woodlands. Orders may be made in respect of both

individual trees and woodlands, but cannot be applied to bush shrubs or to hedges as such.

The DETR booklet Tree Preservation Orders, a Guide to the Law and Good Practice

provides advice on the making of Tree Preservation Orders. In general, orders should be

made to protect selected trees and woodland if their removal would have a significant impact

on the environment and its enjoyment by the public. Trees may be worthy of preservation for

their intrinsic beauty, for their contribution to the landscape, or because they screen an

eyesore or future development. The value of trees may be enhanced by their scarcity. Other

facts, such as their importance as a wildlife habitat may also be taken into account although

on their own these may not be sufficient to warrant an Order.

The Beech tree (T1) that is subject to protection from the Tree Preservation Order is located

in the front garden of Barlaston Lea, and adjacent to the rear annex of Lea House. The

dwelling at this site is a detached house with significant landscaped gardens to the front and

to the rear. The surrounding area is in the majority residential premises with a similar

character, of well maintained plots with car parking and modest gardens to the front and

larger amenity spaces with trees and vegetation to the rear much of which could be

considered mature.

2. Prudent Landowner

The National Tree Safety Group was formed in 2007 to look to discuss how best to codify

the best, generally accepted and balanced approach to managing risks from trees. It stated

that we are most aware of unlikely and unusual events and those things which are most

important to us are often the least appreciated and taken for granted. Terrorist attacks,

severe earthquakes or tsunami are part of our news but not part of most of our lives. Few of

us ever experience the tragic consequences of a fatality or a serious injury as a

consequence of tree failure. However, while, happily, such tree-related events are rare,

when they do occur they are newsworthy because of their very infrequency. The infrequency

of tree failure events is in contrast to the ubiquity of trees, which define our landscape from

the centre of our capital cities to the most remote of places. The majority of us see trees

every day; they populate our countryside and towns, our parks and gardens to such an

extent that we can easily forget their importance to our quality of life. Naturally and rightly,

fatal and serious accidents are investigated and can result in litigation. In such cases, there

is a need for the health and safety authorities and the courts to understand both the value of

trees and the context of the management of trees and good practice against which any

individual case can be compared. This also provides a benchmark for managers to work

with. However, because of the importance of trees in our landscape and society, the

infrequency of tree failure events and the wide range of environments in which trees occur,

great care needs to be taken not to create another level of burdensome regulation.

However, under both the civil law and criminal law, an owner of land on which a tree stands

has responsibilities for the health and safety of those on or near the land and has potential

liabilities arising from the falling of a tree or branch. The civil law gives rise to duties and

potential liabilities to pay damages in the event of a breach of those duties. The criminal law

gives rise to the risk of prosecution in the event of an infringement of the criminal law. The

owner of the land on which a tree stands, together with any party who has control over the

tree’s management, owes a duty of care at common law to all people who might be injured

by the tree. The duty of care is to take reasonable care to avoid acts or omissions that cause

a reasonably foreseeable risk of injury to persons or property. if a person is injured by a

falling/fallen tree or branch, potential causes of action arise against the tree owner in

negligence for a breach of the duty of care, in the tort of nuisance and, where the injured

person was on or adjacent to the land of the tree owner at the time of the injury, under the

occupiers’ Liability acts of 1957 or 1984.

The duty owed Is a duty to take reasonable care for the safety of those who may come

within the vicinity of a tree. The courts have endeavoured to provide a definition of what

amounts to reasonable care in the context of tree safety, and have stated that the standard

of care is that of “the reasonable and prudent landowner”. The tree owner is not, however,

expected to guarantee that the tree is safe. The owner has to take only reasonable care

such as could be expected of the reasonable and prudent landowner. The duty owed under

the tort of nuisance is also owed by a tree owner to the occupier of neighbouring land. The

duty, however, is no different to the general duty owed under the tort of negligence.

The courts have not defined the standard of inspection more precisely than the standard of

“the reasonable and prudent landowner”. It has been recognised that this test sounds

simpler than it really is: “it postulates some degree of knowledge on the part of landowners

which must necessarily fall short of the knowledge possessed by scientific arboriculturists

but which must surely be greater than the knowledge possessed by the ordinary urban

observer of trees or even of the countryman not practically concerned with their care”. In

individual cases, the courts have sought to apply this general standard to the facts of each

case. However, there is no clear and unambiguous indication from the courts in regard to the

extent of the knowledge about trees a landowner is expected to bring to tree inspection in

terms of type and regularity of inspection. Generally, the courts appear to indicate that the

standard of inspection is proportional to the size of and resources available (in terms of

expertise) to the landowner. It is of note that the HSE states in the HSE sector information

minute Management of the risk from falling trees (HSE 2007), that: “for trees in a frequently

visited zone, a system for periodic, proactive checks is appropriate. This should involve a

quick visual check for obvious signs that a tree is likely to be unstable and be carried out by

a person with a working knowledge of trees and their defects, but who need not be an

arboricultural specialist. In general terms, a landowner must identify those trees which might,

if they fell, pose a risk to people or property. He should then inspect such trees and identify

any obvious defects in the trees. If the landowner does not have sufficient knowledge of

trees to enable him to identify such obvious defects, he should engage someone who has.

Having identified a defect, the landowner (if sufficiently knowledgeable), or someone with

appropriate knowledge and expertise, should assess the risk posed by the defect and take

appropriate action, which might mean further monitoring of the defect, pruning of the tree or

felling of the tree.

3. Setting

This detached home has several trees and many shrubs within the curtilage. Some are on

the boundary with other landowners and also adjacent to buildings which are occupied as a

dwelling. This tree is in the front garden overhangs the neighbour’s house. As the owner of

the land, the owners have responsibility and a duty of care. The owners are successful

business men and women, but have no specific arboricultural knowledge and therefore are

regarded as lay people. The owners enjoy their garden and the trees in it. As well as

providing colour, shade and ornamental interest, they give them some privacy from the road

and neighbouring properties. They understand the contribution that their trees make to the

wider environment, in terms of the “pleasant leafy neighbourhood” and how this increases

the value of their home. They don’t want to fell the tree. As reasonable and prudent

landowners responsible for trees, they are able to recognise and understand the significance

of obvious visual defects and be able to carry out their own inspection that may result in

needing to obtain further advice. They have recognised the lower bark defect that has

occluded, they have catalogued the history of limb loss on two separate occasions, they

have pointed out a cavity on the northern side of the tree and have noted fungi on the

ground around the tree at certain times of the year. They use a recommended tree surgeon

they believe is capable of providing such advice and undertaking any work required. The

council have inspected the tree and in conjunction with a tree surgeon, they have agreed to

a reduction in the crown, to reduce the risk the tree poses to its owner and neighbours.

4. Risk Assessment

As previously mentioned, tree failure that causes harm is a relatively rare occurrence, so the

risk associated with living among trees is quite low. Nevertheless, it is impossible to maintain

trees free of risk; some level of risk must be accepted to experience the benefits that trees

provide. The National Tree Safety Group, which is a partnership of organizations in the

United Kingdom, has drafted a guidance document that identifies five key principles for tree

risk management. This provides a foundation for balancing tree risk and the benefits the

trees provide. Trees provide a wide variety of benefits to society and

• Trees are living organisms and naturally lose branches or fall

• The risk to human safety is extremely low

• Tree owners have a legal duty of care

• Tree owners should take a balanced and proportionate approach to tree safety

management

The benefits trees provide to those living and working in the urban environment increase as

the size of the trees increase. As a tree gets older and larger, however, it is also more likely

to shed branches or develop decay or other conditions that predisposes the tree to failure. In

assessing and managing trees, we should strive to strike a balance between the risk that a

tree poses and the benefits that individuals and communities derive from trees. Before a tree

risk assessment takes place, it is important to determine if the possible degree of risk

justifies the time and expense to perform tree inspection and assessment. Many trees are

located where the consequences of failure are minor or negligible. In urban and developed

areas where people, property, and activities could be injured, damaged, or disrupted, the

consequences of tree conflict or failure may be significant or severe. Decisions on whether a

tree inspection is required or what level of assessment is appropriate should be made with

consideration for what is reasonable and proportionate to the specific conditions and

situations.

As previously stated, it is impossible to maintain trees free of risk. Some level of risk must be

accepted to experience the benefits that trees provide. Fortunately, tree failure is an

infrequent occurrence. Serious damage, injury, or death from tree failure is rare. Tree

failures during normal weather conditions are sometimes predictable and preventable.

However, any tree, whether it has visible weaknesses or not, will fail if the forces applied

exceed the strength of the tree or its parts. For example, hurricane-force winds, heavy snow,

or freezing rain can break solid, defect-free trees.

Tree risk assessors often must perform risk assessments with limited information about the

structural condition of the tree and the environment that affects it. There is typically a

considerable level of uncertainty associated with tree risk assessment due to our limited

ability to predict natural processes (e.g., rate of progression of decay, response growth),

weather events, traffic and occupancy rates, and potential consequences of tree failure.

A primary goal of tree risk assessment is to provide information about the level of risk posed

by a tree over a specific time period. This is accomplished in qualitative tree risk assessment

by first determining the categories for likelihood and consequences of tree failure. These two

factors are determined by:

1. Evaluating the structural conditions that may lead to failure; the potential loads on the tree;

and the trees’ adaptations to weaknesses—to determine the likelihood of failure.

2. Evaluating the likelihood that a tree or branch could strike people or property or disrupt

activities.

3. Assessing the targets’ values and potential damage—to estimate the consequences of

failure.

The likelihood of a tree failure impacting a target is the first of two key components of risk.

While it is nearly impossible to precisely determine the likelihood of an event, tree risk

assessors must evaluate and categorize the likelihood of failure and the likelihood of the tree

impacting a target. In tree risk assessment, targets are people, property, or activities that

could be injured, damaged, or disrupted by a tree failure. Targets include people, buildings,

animals, infrastructure, vehicles, landscape structures, and other property that may be

damaged or harmed by a tree failure. The most important situations to assess are locations

where people are frequently present. The greatest risk for injury from tree failures is in

places where many people are unprotected and within the target zone for long periods of

time, especially during storms. Target areas that are often evaluated include streets, parking

areas, footpaths, and play areas. Although damage to structures is possible, the bigger

concern is the people that use them, even if they are not present at the time of the

evaluation. For example, when assessing outbuildings it would be worth checking if planning

permission has been granted for use as a dwelling house.

Targets can be categorized by the amount of time that they are within the target zone—their

occupancy rate. Not all targets are present in the target zone at all times. Occupancy rates

can be classified as constant, frequent, occasional, or rare. Static targets are constant.

Movable and mobile targets can be in any of the four classifications. Occupancy rate is a

primary component in an assessment of the likelihood of impacting a target.

When assessing tree risk, the second key component is the consequence of failure.

Consequences are a function of the value of the target and the amount of injury, damage, or

disruption (harm) that could be caused by the impact of the failure. The amount of damage

depends on the part size, fall characteristics, fall distance, and any factors that may protect

the target from harm. Consequences of a tree failure may be considered “minor” for targets

of relatively low value or for easily repaired structures, such as outbuildings and fences, At

the opposite end of the spectrum, a tree failure that could lead to severe injury or a fatality,

or that disrupts a primary power line, poses potentially “severe” consequences. When

evaluating consequences it is essential to consider the size of the tree or branch that could

fail, and how it could impact a target. Generally, a small branch has less potential to cause

damage than a large branch.

Small branch failures do occur more frequently, and even a small branch could cause

personal injury, a power outage, or a traffic accident. Branches less than 2.5 cm (one inch)

in diameter are not usually considered in most tree risk assessments. The minimum size

branch that should be considered by the tree risk assessor may be specified in the scope of

work. In some low-use areas, the minimum branch diameter of concern may be as large as

10 cm (four inches) in diameter. For example, if a four-inch diameter branch falls on a house

from a height of 3 m above the roof, the degree of damage would be low, and no injury to

people inside would be expected. If the same size branch were to fall from near the top of a

large tree with no branches in between to slow it down, more extensive damage could occur.

If lower branches of the tree would slow or stop the fall of the falling branch, the anticipated

damage would be less. In estimating how much damage could occur from a tree failure, the

arborist should consider the relative amount of force with which it is likely to strike the target.

A falling tree or branch will gain speed as it accelerates toward the ground. So, in general,

the higher the distance from which a branch falls, or the greater the distance from the tree to

the target, the greater the force that the tree or branch will have at the point of impact.

Where the arborist is considering the potential of whole tree failure, the distance from a tree

trunk to a well-built, two-storey house, a tree that fails may simply lean against the house,

causing minor damage. On the other hand, if the distance is such that the tree can

accelerate significantly before the trunk strikes the house, damage may be much greater. If

there are lower branches in the tree that are likely to slow or stop the fall of the trunk,

damage may be lessened. In this example, the lower branches serve to protect the target.

Large diameter, wide-growing branches that are low on the trunk may also affect the fall

pattern of a tree. If the branches contact the ground well before the trunk, the fall may be

slowed or stopped, or the tree may roll.

Most tree risk assessment reports include a rating of risk posed by the tree. In a qualitative

tree risk assessment, assessors can use a matrix to help categorize risk. The risk category

is then compared to the level of risk that is acceptable to the client. If the risk category

defined for the tree exceeds the level of acceptable risk, mitigation options should be

presented. The likelihood of a tree failure impacting a target and the consequences of the

failure are the factors to consider when categorizing tree risk. The likelihood of a tree failure

impacting a target is determined by considering two additional factors. First is the likelihood

of a tree failure occurring within a specified period of time. The likelihood of tree failure is

determined by examining structural conditions, defects, response growth, and anticipated

loads. Second is the likelihood of the failed tree or branch impacting the specified target.

Impact may be the tree directly striking the target, or it may be a disruption of activities due

to the failure. These two factors are evaluated and categorized using a matrix to estimate the

likelihood of the combined event: a tree failure occurring, and the tree impacting the

specified target. The likelihood of that combined event is then compared with the expected

consequences of a failure impacting the target to determine a level of risk.

Judgment about the significance of defects, conditions, and response growth can be guided

by the information available in various professional resources, as well as through species

failure profiles, site conditions, and tree risk assessor experience. It is essential to consider

all of the compounding factors, as well as any response growth in the tree, which may have

compensated for the condition. When more than one defect or condition is present in a tree,

the impact of the combination must be considered. Not all conditions and defects have a

significant impact on tree structure. Assessing each condition with regard to its likelihood of

failure or level of risk will help discern the significance of each condition relative to the entire

tree. Tree failures usually occur when there is a critical combination of tree defect(s),

conditions, and contributing environmental factors, such as wind, rain, freezing rain, or snow.

With the exception of sudden branch drop, calm-day tree failures are very rare and usually

result from extreme defects. Most tree failures occur when wind speed exceeds the seasonal

norm for the site. In discussing likelihood of failure, a time period should be referenced to put

the likelihood in context. Often, the time period is the inspection interval (the time

recommended for the next inspection); however, some inspectors base all assessments on a

one-year time interval. Either method is acceptable, as long as the time period is specified

and is reasonable. This time period should not be considered a “guarantee period” for the

risk assessment. The assessment states the conditions found at the time of the inspection,

weather, and activities in and around the tree can have a significant impact on tree condition

and the likelihood of failure.

5. Definitions

The likelihood of tree failure can be categorized using the following guidelines:

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 period.

Possible —failure could occur, but it is unlikely during normal weather conditions within the

specified time period.

Probable —failure may be expected under normal weather conditions within the specified

time period.

Imminent —failure has started or is most likely to occur in the near future, even if there is no

significant wind or increased load.

The second factor to be considered is the likelihood of the failed part impacting the target. To

estimate this likelihood, the arborist should attempt to determine the occupancy rate of any

targets within the target zone, and any factors that could affect the failed tree as it falls

toward the target. Likelihood of impacting a target can be categorized using the following

guidelines:

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 that is fully exposed to the assessed tree, or an

occasionally used site that is partially protected by trees or structures. Examples include a

rarely used public footpath 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 are 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 or 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

Likelihood of

Impacting

Target Very

low

Likelihood of

Impacting

Target Low

Likelihood of

Impacting

Target Medium

Likelihood of

Impacting

Target High

Imminent Unlikely Somewhat likely Likely Very likely

Probable Unlikely Unlikely Somewhat likely Likely

Possible Unlikely Unlikely Unlikely Somewhat likely

Improbable Unlikely Unlikely Unlikely Unlikely

Table 1. The matrix used to estimate the likelihood of a tree failure impacting a

specified target.

6. Consequences

Consequences are estimated based on the value of the target and the harm that may be

done to it. The consequences depend on the part size, fall characteristics, fall distance, and

any factors that may protect the risk target from harm. The significance of target values—

both monetary and otherwise—is subjective and relative to the client. Values should be

assessed from the client’s perspective. Consequences of failures can be categorized using

the following guidelines:

Negligible consequences are those that involve low-value property damage or disruption

that can be replaced or repaired, and do not involve personal injury. Examples of negligible

consequences include:

• a small branch striking a fence

• a medium-sized branch striking a shrub bed

• a large part striking a structure and causing low monetary damage

• disruption of power to landscape lighting

Minor consequences are those involving low to moderate property damage, small

disruptions to traffic or a communication utility, or very minor injury.

Examples of minor consequences include:

• a small branch striking a house roof from a high height

• a medium-sized branch striking a deck from a moderate height

• a large part striking a structure and causing moderate monetary damage

• short-term disruption of power at a service drop to a house

• temporary disruption of traffic on a neighborhood street

Significant consequences are those that involve property damage of moderate to high

value, considerable disruption, or personal injury. Examples of significant consequences

include:

• a medium-sized part striking an unoccupied new vehicle from a moderate or high height

• a large part striking a structure and resulting in high monetary damage

• disruption of distribution primary or secondary voltage power lines, including individual

services and street-lighting circuits

• disruption of traffic on a secondary street

Severe consequences are those that could involve serious personal injury or death, damage

to high-value property, or disruption of important activities. Examples of severe

consequences include:

• injury to a person that may result in hospitalization

• a medium-sized part striking an occupied vehicle

• a large part striking an occupied house

• serious disruption of high-voltage distribution and transmission power lines

• disruption of arterial traffic or motorways

7. Tree Risk Rating

Tree risk assessment reports typically include a rating of risk. A risk matrix (Table 2) is a

means of combining ratings of likelihood and consequence factors to determine a level or

rating of risk. The matrix approach was selected for use in this report because of its broad

acceptance by the ISA, ease of use, and effective application for rating risk. This matrix was

designed specifically for the evaluation of risk posed by tree failures. The limitations

associated with using a matrix include the inherent subjectivity associated with the selection

of both the likelihood and consequence factors, and the lack of comparability to other types

of risk assessed using other means.

Most trees have more than one potential failure mode and may have multiple risk targets.

For example, a tree with excessive root decay may also have several dead branches; the

whole tree could fail from root decay, and dead branches may fail. Similarly, the whole tree

may fall on a house, while the dead branches would fall only on the driveway. When

evaluating individual trees, it is appropriate to evaluate each factor as independent events

and to recommend mitigation options along with estimated residual risks for each factor.

Risk aggregation is the consideration of multiple risks in combination, and is difficult to do

even with complex mathematical analyses. Therefore, the tree risk assessor cannot simply

add or multiply the risk ratings for the individual failure modes to reach a whole tree risk

rating. What the tree risk assessor can do is identify—among all the failure modes and

consequences assessed—the failure mode having the greatest risk, and report that as the

tree risk rating. Assigning a tree risk rating for a tree will always be the higher of several risks

assessed. For example, in a given situation, whole-tree failure may be unlikely, but could

have significant consequences if it occurs; using Table 2, the risk rating is “low.” At the same

time, failure of a dead branch may be very likely, but with minor consequences; the risk

rating is “moderate.” The risk rating may be reported as “moderate,” the higher of the two

ratings. This rating often is presented as the single risk level for the tree, especially when

dealing with limited visual assessments. It is important to note, however, that if measures are

taken to mitigate the highest risk, there still is residual risk associated with that tree,

including the remaining risk factors. The risk rating for that tree may or may not change

based upon the remaining risk factors.

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.

Low . The low-risk category applies when consequences are “negligible” and likelihood is

“unlikely”; or when 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.

Moderate. Moderate-risk situations are those for which consequences are “minor” and

likelihood is “very likely” or “likely”; or when 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 for mitigation than high- or extreme-risk

trees.

High. High-risk situations are those for which consequences are “significant” and likelihood

is “very likely” or “likely,” or when 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 as soon as is practical. The decision for mitigation

and timing of treatment depends upon the risk tolerance of the tree owner or risk manager.

In populations of trees, the priority of high-risk trees is second only to extreme-risk trees.

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. Continuing the example from the sections on

likelihood and consequence: for the house, the risk of a medium-sized, dead branch with a

likelihood of failure an impact rating of “unlikely,” and consequences rating of “minor,” would

result in a risk rating of “low.” For the parked car, the likelihood is “somewhat likely” and the

consequences are “significant,” so the risk is “moderate.” For the driver of the car, the

likelihood is “unlikely” and the consequences “severe,” so the risk is also “low.” Overall, the

tree risk rating would be “moderate,” the highest of these three individual ratings. Whether

the client chooses to mitigate the risk depends on their perception of risk and what level of

risk they find acceptable, as well as the cost, aesthetics, and inconvenience of mitigation.

Likelihood of

Failure and

impact

Consequences

Negligible

Consequences

Minor

Consequences

Significant

Consequences

Severe

Very likely Low Moderate High Extreme

Likely Low Moderate High High

Somewhat likely Low Moderate Moderate Moderate

Unlikely Low Low Low Low

Table 2. Risk rating matrix showing the level of risk as the combination of likelihood

of a tree failing and impacting a specified target, and severity of the associated

consequences.

This system was developed by Mr E. Thomas Smiley who is a Board-Certified Master

Arborist. He is an arboricultural researcher at the Bartlett Tree Research Laboratory

(Charlotte, NC), Mrs Nelda Matheny is president of HortScience, Inc. She is a Board-

Certified Master Arborist and Registered Consulting Arborist and Mrs Sharon Lilly who is the

ISA Director of Educational Goods & Services. She is also a Board-Certified Master Arborist.

8. Steps to Developing a Tree Risk Rating

8.1. Identify possible targets.

This Beech tree is located in a domestic garden close to two outbuildings, one of which is

considered to be newly occupied. Change of use of barn from business use back to

residential use at Lea House, Station Road, Barlaston, Stoke On Trent Staffordshire ST12

9DA was granted planning permission by Stafford Borough Council Ref. No:

12/18094/COU | Received: Fri 14 Dec 2012 | Validated: Wed 27 Feb 2013| Status:

Application Permitted.

In reaching its decision the council said that the proposals relate to the reinstatement of

ancillary C3 uses within a range of attached outbuildings to the southwest of Lea House.

This is a period property set within large mature grounds on the southern edge of Barlaston

opposite The Green. The buildings have most recently been used for commercial purposes

by previous owners and are currently being refurbished internally. The building stands on the

southern boundary of Lea House with Barlaston Lea beyond. Approval was granted for a

garage at the neighbouring property which adjoins the boundary otherwise the main dwelling

at Barlaston Lea is well separated (24m) across a surfaced parking area. The site is outside

but adjoins the Barlaston Residential Development Boundary. The site and adjoining

properties to the south of Station Road are within North Staffordshire Green Belt. Protected

trees within TPO 86 of 1981 bound the site, but as the proposal is for a change of use none

would be affected by the proposal.

The plans submitted to and approved by the council highlight to me that the building is not to

be converted into a fully occupied independent dwelling. The proposals indicate that the roof

space adjacent to the tree will be a store also within the first floor will be an art room studio.

8.2. Identify tree part(s) that could strike target.

8.3. Evaluate likelihood for each part to fail.

a. improbable

b. possible

c. probable

d. imminent

The Beech tree subject to this risk assessment can be described as M – Mature; tree in final

third of life expectancy. I wouldn’t at this stage say that it was OM – Over Mature; tree in

decline. This is because there is adequate vigour in the tree, the defects in the bark have

begun to occlude, the canopy is not showing signs of leafing late, the canopy is not

unusually thin and shoot extension seems to be adequate and possibly improving. There is

however a history of branch failure

8.4. Evaluate likelihood of tree/part impacting target.

a. very low

b. low

c. medium

d. high

8.5. For each failure mode, identify likelihood for tree failure impacting a specified

target (Table 1).

a. very unlikely

b. unlikely

c. somewhat likely

d. likely

e. very likely

8.6. For each failure mode, estimate consequences of failure

a. negligible

b. minor

c. significant

d. severe

8.7. For each failure mode, designate the risk (Table 2).

a. low

b. moderate

c. high

d. extreme

9. Recommendation

I would be looking to recommend a crown reduction to alleviate biomechanical stress by

reducing both the leverage and the sail area of the tree, which can allow retention of a tree in

a confined space. It can also be used to create a desired appearance or to make the tree

more suited to its surroundings. Unlike topping crown reduction retains the main framework

of the crown and therefore a high proportion of the foliage-bearing structure, which is

important for the maintenance of vitality. When assessing the suitability of a tree for crown

reduction, particular regard should be paid to the characteristics of the species as well as the

physiological condition of the individual tree. The extent of crown reduction should be

determined on the basis of the risk assessment and on an assessment of the ability of the

tree to withstand the treatment. The general principle is that, following reduction, there

should still be a strong framework of healthy small-diameter branches and twigs (leaf­

bearing structure), capable of producing dense leaf cover during the following growing

season.

The crown should normally be reduced in proportion to its original shape, so as to avoid

altering the balance of the tree as a whole, but the objective should not be to achieve

symmetry for its own sake. The shape of the crown may be altered if there is a specific need

to do so for biomechanical integrity. Within the context of crown reduction, as opposed to

topping, the pruning cuts would normally expose a much smaller proportion of heartwood

than of sapwood and should not exceed 100 mm in diameter.

The specification for crown reduction and/or reshaping should be accurate and clear, so that

the desired result is achieved to reduce the risk from the detected defects in the tree in

relation to the adjacent property. To avoid ambiguity, the specified end result can be stated

that branch-spread which are to remain, will not overhang the neighbouring outbuildings.

The work should comply with the specified removal in BS3998:2010.

Peter Jackson Ba(Hons) DipLA CMLI M.Arbor.A ISA Certified Arborist