Aspects of BS5837:2005Julian Forbes-Laird

BA(Hons), MICFor, MEWI, M.Arbor.A, Dip.Arb.(RFS)Chartered Arboriculturist

Director & Principal Consultant,Forbes-Laird Arboricultural Consultancy Ltd

BS5837:2005 keypoints

• ‘Arboriculturist’ is defined to exclude pseudo-arbs!• Tree surveys must now only be undertaken by arbs• Tree grading system (Table 1) designed to reduce

disagreement, & to offer realistic tree retention by weeding out poor quality trees

• TPZ1 (Root Protection Area, RPA) is defined in m²• TPZ1 is rarely a circle: its final shape must be

determined by an arboriculturist• Tree protection fencing has improved from invisible to

intimidating!• Much more detail in relation to special engineering

measures for inserting development into the RPA

Part 1:Tree grading

Importance of the tree grading process

“At the heart of the 5837 process is the differentiation as to quality of a site’s existing tree stock. From this one simple (?) exercise all else naturally follows. For this reason, if the tree grading is misapplied the end result can only be bad planning”

Grey’s Law:‘Any sufficiently advanced incompetence is

indistinguishable from malice’

The tree grading system(Table 1)

• Arboriculturists only need apply…• A development site TS undertaken by a non-arb has not been carried out

in accordance with the provisions of BS5837• LPAs can & should refuse to accept non-arb surveys• All trees should be assessed objectively, ignoring any extant layout

information• Condition/longevity hurdle for each grade• Removal grade trees should be identified first• Retention trees are considered via a logical cascade through A-C• The three primary values of retention trees are separated into sub-

categories• Sub-categories are required to be listed in data schedule• Overall improvement to transparency of decision-making process

Why ‘cascade chart’?

• The purpose of the title ‘cascade chart’ is to focus attention on how each tree or group should be assessed:

• Assuming that the tree/group does not get assigned to the R-category, the surveyor should start with the presumption that each tree/group is graded A, then test its observed attributes against the strict criteria for this category

• If the tree fails the A-grade tests, then it should be assessed against the criteria for B

• If it fails again, then it is a C

R for removal

• Trees for removal are considered first• This mirrors what surveyors actually do• Trees that cannot be retained are factored out,

allowing more focus on the remainder• Dead trees are included in the R category because

they are not be a material consideration in the planning process

• Notes in the text explain that R category trees may be retained if site layout considerations allow, & that retention of dead trees may be desirable, safety considerations permitting

Applying the R grade

• Less than 10 years remaining contribution

• Should be removed in relation to existing context (i.e. regardless of development)

• Trees that are in a state of imminent collapse & those exposed by loss of same

• Dead or severely declining trees

• Trees of very low quality of disbenefit to other nearby trees of better quality

A, B, C: Trees to be considered for retention

Three longevity bands as qualifying ‘hurdles’:• > 40 for A grade• > 20 for B grade• > 10 for C grade (< 10 = R)• Determining the likely remaining longevity (= retention

span) of a tree may not always be easy, but it can & must be done for the assessment to have integrity

• e.g. consider the implications of advising a client to severely constrain a site layout so that a mature silver birch can be retained, when it only has 10 more years before decline sets in

Structure of the subcategories

These are designed to reflect the three principal though differing ways in which a tree may have value:• Because in and of itself it is a tree• Because it makes a contribution to the landscape• Because it has qualities that transcend a purely arboriculturalunderstanding

Trees can qualify in more than one SC, it is not intended that they accrue value by doing soVeteran trees should be graded A3 (though may also fall into A1 and/or A2)

What is a ‘group’?

Notes in the text explain that the term ‘group’ is intended to identify:

1. Aerodynamic cohesion of unified or closely adjacent crowns (companion shelter)2. Visual unity (avenues, screens, etc)3. Cultural importance (parkland, wood pasture, etc)

• Where trees occur as groups, they should be assessed as such: very oftenthey stand or fall together

• Individual assessment of trees within groups may still be desirable (e.g.identifying line of least resistance for inserting a crossover)

Part 2:The RPA

Identifying the RPA (Table 2)

• Root protection requirements for each tree are based on a radius derived from a multiplier of 12 x stem dia. @ 1.5m AGL for single stemmed trees, or 10 x stem dia above basal flare for multi-stemmed/low forking trees

• The radial measure is converted by the formula

∏ (r²) = A• This gives an area in m², notionally centred

on the tree stem

Identifying the RPA – cont’d

However, the actual shape of the RPA is rarely a circle: it should be determined & plotted on the Tree Protection Plan by an arboriculturist, taking account of:

• Disturbance tolerance of tree (species, age class, condition, presence of other trees)

• Known factors predisposing tree towards asymmetrical rooting

• Soil type & structure, topography & drainage• A 20% offset is available but only for individual open-

grown trees

Plotting the RPA – worked examples

Tree protection distances compared1991 (size ranges) 2005 (examples)Dia. mmTPZ rad. m Dia. mmRPA rad. m< 350 4.5 300 3.6

350 4.2350-750 6.0 500 6.0

750 9.0>750 8.0 900 10.8Max (OM) 12.0 1250 15.0

In other words, smaller trees get less, but larger ones get more

2007 development land value in the SE of England: the 707m/sq RPA of a 1250mm dia. tree = £160K

Part 3:Standard practice: applying 5837

Strategy

• Inherent within BS5837:2005 is a five-stage process

• This process is intimately woven throughout the development cycle

• This process is not new to 2005

• It is what ‘Engaged Consultants’ have been doing for some time

• And it is what competent TOs require

Arboricultural Stage 1

Tree survey & preliminary constraints advice• Including Tree Survey & Constraints Plans• Tree survey data & TSP become appendices to AIA

(AS3)• TCP is internal advice to design team & not for LPA

consumption• TCP should include preliminary retention/removal

advice• In simple terms, this is based on:

R = removeA = retainB = considerC = ignore

Arboricultural Stage 2

Design Review• Evaluation of arboricultural implications of emerging site layout• Design tested against tree protection requirements in relation to four

zones:Zone 1 RPA (barriers, ground protection & special engineering)Zone 2 Crown protection zone (working space & AFP)Zone 3 Avoiding PDTR (setback distances & shading analysis)Zone 4 Protection/remediation of areas for structure planting

• Arboriculturist works within design team to develop best fit scheme• This is based on an ongoing dialogue between the disciplines involved• The objective is to deliver site viability based on defensible and practical

levels of tree retention• Cramming 100 trees / 100 into a site and damaging them all a little is

less good than keeping 20 trees with adequate separation distances and proper protection

The Four-zone protection system(after Nicholson)

Zone 1: root protection

Zone 2: crown protection

Zone 3: Influence fromshading & dominance

Zone 4: area for new planting

Arboricultural Stage 3

Preparation of supporting documentationArboricultural Implications (Impact) Assessment, demonstrates that

the trees have been properly considered by• An analysis of the tree retention/removal balance, plus• Information on how retention trees will be protected

Typical Appendices:1. Tree Survey data2. Tree Survey Plan3. Tree Retention/Removal Plan4. Tree Protection Plan5. Indicative Arboricultural Method Statement(s), and

sometimes…6. Shading analysis

Tree friendly foundation detail

Example shading analysis using Arborshadow

Extract from shading analysis

Arboricultural Stage 4

Securing discharge of planning conditions

• Arboriculturist works with design team to resolve outstanding details

• Tree friendly solutions & ongoing design review

• Typically relating to site infrastructure & build process, e.g. drainage, services, ext. lighting, construction management

• Often includes preparation of detailed TPP & AMS

• Frequently, AS4 overlaps with AS5

AS4 TPP/AMS

AMS: The devil in the detail• Even with full applications, the development process in the UK

commonly relies on many details of site infrastructure being resolved by conditions

• BUT the principle of material consideration becomes undermined if trees agreed for retention are then removed to facilitate condition discharge

• So: where the ‘leave it to condition’ process has potentially adverse consequences for tree retention, TOs & consultants should resist it

• AMS prepared under AS3 (pre-condition) are only as good as the detail of the application which they support

• As a rule, any construction proposed within the RPA should be fully detailed up at application stage: this is the only way to ensure successful tree protection

Devilish example• Under clause 11.6 5837 gives advice on ‘perched’

foundations for use within the RPA• In practice, this solution can elevate the structure by

ca. 350mm• This will usually have a knock-on effect on ridge

height, which is frequently frozen at the application stage

• Thus leaving foundation detail to conditions expecting a perched solution to be adopted is doomed to failure

• The detail of any foundations within the RPA must be enshrined in the application itself

Development is a specialist subjectFor an arb to contribute successfully to the design and build process, he orshe requires an intimate knowledge of the development process and how itcan impact on trees• E.g. concrete: C35 concrete (typically used for piles) has a pH of ca.

>12.5 when wet (enough to chemically burn off skin), with adjacent soil alkalinity levels being likely to rise to over 10 pH due to leachate released during the curing period: typically 25-50% of the 150l per ton of C35’s water load is lost into the ground

• Trees’ ability to extract nutrients from the soil ceases at around pH8.4; higher pH factors are simply toxic

• 10 no. 300mm x 6m piles comprises = 8.4T of C35, giving run-off of ca. 440L

• To avoid the arbortoxic effects of curing leachate, piles in (and arguably adjacent to) the RPA should either be of the stainless steel screw type or, if concrete, should be sleeved for the first 2500mm below ground

Arboricultural Stage 5Implementation• LPAs are increasingly using a planning condition

requiring arboricultural site monitoring• This can be offered to LPAs/Inspectors as a security

blanket• Arb site monitoring includes:1. Checking correct alignment & construction of tree

protection fencing & ground protection (if applicable)2. Ensuring compliance with AMS3. Overseeing arboricultural operations such as AFP4. Responding to emerging queries from site agent etc• And don’t forget: once the thing is built, post-

development tree inspection & aftercare for new planting may be needed

Remember: development sites are hostile territory…

Part 4:Case Study - UCBG

The scenario

World-class plant research facility including new home for the Darwin Herbarium

To be inserted into historic setting including pristine arboretum & listed building

Big project: e.g. muck-away for basement required five 8-wheelers per hour, six hours per day, five days per week, for four months

Proposed tree retention

• The decision-making process included an assessment of the botanical importance of trees within development zone

• Rare young trees of good form were relocated• Difficult but balanced decisions were taken to

derive the final tree retention / removal balance• FLAC promoted the scheme through the

planning process against fierce opposition from the LPA tree officer

Heras fencing protecting yew hedge

Detail of corner bracing

Double height fencing protecting Carpinus carolinensis

Ground protection 1

Ground protection 2

Example access facilitation pruning

Part 5:Battle-testing BS 5837:2005

Notes from a recent Public Local Inquiry

Background

• JFL instructed by third party objectors to attack Appellant’s proposal in relation to a beech tree

• Objectors could not afford PoE route, so JFL retained as an advocate

• Appellant’s arboricultural expert was an AARC, instructed after the scheme was refused consent

• Tree agreed as A1 by all parties• RPA calculation: 725 x 12 x 8.7 x ∏ = 238m²• Existing radial crown spread 9m towards site

Proposal

• Development proposed at 4m SD to stem centre• Lateral crown reduction proposed to 3.5m from

stem• Proposed crown to building SD 500mm• Shape of RPA 8m x 30m• 50% of RPA shown under adjacent footway/road• 70% of RPA on land under third party control

(highway authority/neighbour)• No information provided on foundation design,

proposed finished levels & utility corridors: all would be left to conditions

Before…

During…

After…

Weaknesses in BS5837:2005

• No definition of ‘open grown tree’• No aspect ratio to RPA• No rider on off-site RPA limits• No limits to access facilitation pruning or

amenity/quality correlation• No prohibition against cyclical pruning to facilitate

minimal separation distance• No recommendation that complex design details are

unsuitable for reservation

JFL is a technical editor for BS5837(2010)…