Tree Condition & visual tree inspection

Ever wondered what is considered when deciding if a tree is healthy, in an appropriate location, has a potential for causing damage or has an extremely high amenity value? Or what we are looking at when we visit your property to quote or create a tree rept. We at Liam McGough Tree services want to provide you with the best most up to date information to help you make informed decisions. Tree Condition – Visual Tree Assessment

Visual Tree Assessment Summary:

Law requires a Duty of Care – Occupier’s Liability Act 1957/1984: Owner/occupier must take all reasonable steps to prevent reasonably foreseeable harm befalling anyone s/he could reasonably expect to be affected – by their acts or omissions
Value versus Risk – must be a foreseeable problem with a predictable target with reasonable action taken considering all this – no target = no risk
Two courses of action – Modify the tree, (reduce, brace, prop, monolith, fell); modify the target (move the bench, fence off tree, divert footpath, barrier plant)
Visual Tree Assessment – concept suggested by Claus Mattheck – based on certain principles:
The Axiom of Uniform Stress
- Tree is a self-optimising mechanical structure
- It is as strong as it needs to be
- Seeks to have uniform stress over the whole structure
- Economic use of material as possible
- Can only do two things – grow more wood in heavily loaded areas; cease production of new wood in less heavily loaded areas
The Law of the Minimal Lever Arm
- Tree will seek to minimise forces of leverage exerted on it
- Leverage exerted by wind and gravity
- Tree will try to be in a state of equilibrium that requires minimum effort to maintain
- Adapts shape and reinforces parts as required in accordance with the Axiom of Uniform Stress
- Reduction of leverage is done through adding strength where required, or flexing, bending or ultimately snapping. Trees combine the two
- Biological and mechanical requirements also change a tree’s growth through external stimuli i.e. geotropism, phototropism, hydrotropism, thigmotropism, (touch) and chemotropism]

Visual Tree Assessment Principles

It starts as you approach the tree – body language – safe to walk underneath?
Shape – leaning (rectified?) – spaces in crown – Stag’s horns – no buttresses (buried rootplate) – bulges – loose bark – abnormal leaf colour – cracking – cavities – fibre buckling – ribs – Bottle butt – included bark – decay fungi – ivy – bleeding etc - Biological and Structural problems
3 stages of VTA – Visual for obvious symptoms or defects – outside the tree; Further examination if required – inside the tree; Measurement of remaining parts of tree – t/R ratio – how strong is the remaining wood?

Decay Detection Equipment In Order of Use:

Stage 1: Eyes/ Ears / Binoculars: Biological indicators – Foliage size, colour, distrucution – Upper Crown extension growth, vigour, fungal bodies. Mechanical indicators – Branches hazard beams, end loading, subsiding, fibre buckling – Stem splits, cracks, ribs, bulges, ‘bottle-butt’, fungi, exudates – Ground level soil heave, cracks, compaction, waterlogging

Stage 2:

Mallet: Areas of loose bark, differences in sound indicating cavities
Probe: Explore extent of cavities – determines wet or dry
Resistograph: measures changes in torque required to drive a 3mm micro-drill into tree at constant speed – measures changes through cavities or changes to wood structure
- Advantages: Relatively cheap; only a small hole is made in the tree; can detect cavities and different strengths of wood; can detect trends of strength through the tree cross-section
- Limitations: Only measures changes where physically placed; invasive nature could lead to initiation of latent decay fungi or present entry point; limited to length of probe; values are not related to breaking strength; tendency for probe to deviate
Shigometer: Measures the different resistances to an electric charge passing through sound / decayed wood or cavities
- Advantages: Allows mapping of decay zones
- Limitations: Provides no information about the strength of the wood; Only effective when wood is at or above fibre saturation
Thermal imagery: detects changes in temperature associated with decayed wood – produces instant colour image of the tree highlighting areas for concern
Pulling Test: applies a measured load to the tree via a winch and measuring the degree of deformation in terms of fibre stretching and change of stem angle. Values are compared to ‘standard’ data for species and sizes
Stress wave timer (i.e. Picus): Measures the travel of sound waves through a tree – the slower the waves travel the greater the decay
- Advantages: No limit to size of tree; Useful for detecting butt-rott; damage confined to outer wood
- Limitations: Expensive; only detects decay at point of measurement; insertion of probes causes damage to tree; unable to detect certain types of decay at an early stage (i.e. Kretzshmaria deusta); requires professional analysis of data – not enough data currently available to draw accurate conclusions; values not related to breaking strength
Fluorometer: Chlorophyll Fluorescence measured to assess the health and vitality of a tree, and results drawn can indicate certain nutrient deficiencies or simply an overall measurement of the tree’s photosynthetic processing, leading to an assessment on health – useful to assess high vigour stock from nurseries more suitable for street planting for example – non-invasive!

Stage 3: To determine strength of remaining wood, if remaining wall has a t/R ratio of 0.3 or more, with a full crown:

Increment borer: Extracts a 5mm core from remaining wall.
Fractometer: measures bending and breaking strength of remaining material, values compared against examples from sound timber of that species.
- Advantages: Modest cost; directly measures stiffness and strength; accurate map of strong and weak zones; differences between strength and stiffness can be stated.
- Limitations: damages sound wood, perhaps allowing extension of decay .

Pests and Diseases:

Stress: When a system is pushed close to the limits it is designed for (recoverable) i.e. temporary wilting point.

Strain: When a system is pushed beyond those limits (not recoverable) i.e. permanent wilting point.

Trees are a ‘generating system’ – create new material in response to stress providing they have reserves – contain and move on, but injury will always be there (Humans / animals are ‘regenerating system’ cells can repair and heal.

CODIT – compartmentalisation of decay in trees – in response to injury:

1/ Strengthens existing barriers

2/ Creates new barrier

3/ Continues to grow

Walls

‘VISO’

Response – ‘VARC’

Wall 1

Vertical (easiest way for pathogens to travel – xylem / phloem)

Vessels – Blocked with parenchyma (process is called tyloses)

Wall 2

Inwards

Annual Rings – Phenolic compounds, tannic acid (fungally resistant materials)

Wall 3

Sideways

Rays – same as above plus suberin

Wall 4

Outwards

Cambium – annual growth rings

Helping CODIT:

Formative pruning when tree is vigorous
Pruning to branch collar
Minimal pruning
Pruning in summer
Awareness of species response

Biotic Stress Factors: Mammals – man, voles, rabbits, hares, deer, birds (woodpecker boring, marking, bullfinch nipping buds), insects (Biting – consumes a part of the tree; sap-suckers tap into phloem for sugary sap i.e. aphids), bacteria (exist within vascular system, exude methane – methane expands up to 4 times and pushed yeasts and exusion through lenticels), fungi (exist in or around cells, steal energy and compromise health and structure when taking what they need in the case of parasitic fungi)

Abiotic Stress Factors: Non-living – Extremes of weather, soil conditions, chemicals / pollution, mechanical damage; sunlight

Stress occurs when the tree’s living processes are impaired or damaged. i.e. the tree’s ability to carry out photosynthesis, transpiration, respiration, translocation, storage and reproduction. Stress is a reversible condition. Unresolved stress becomes strain. Strain is an irreversible condition, resulting in death of part of, or the whole tree, depending on severity and the part of the tree affected. Biotic and abiotic factors are distinguished in classic plant pathology as ‘parasitic’ and ‘non parasitic’: essentially ‘living’ and ‘non-living’, although not all living agents are parasitic. Weeds competing for available water for example, will lead to stress for the tree, although they are not taking their sustenance directly from the tree.

Abiotic agents (including results of human in/activity) are:

Drought
Flooding / waterlogging / perched water table
Frosts (early / late)
Extremes of heat and cold
Wind – including coastal salt laden
Hail / snow / ice
Lightening
Chemicals – road salt, herbicide
Mechanical damage - strimmer / mower
Vehicular damage
Compaction / capping
Dust deposits
Nutrient deficiencies
Extremes of pH
An excess of nutrients, gases and other pollutants in the soil
Poor aftercare – irrigation, weed control, ties not adjusted / removed
Fire
Poor hygiene

Biotic agents:

Mammals – including man
Birds
Biting insects
Sucking insects and mites
Nematodes
Fungi
Bacteria
Mycoplasmas and spiroplasmas
Viruses
Parasitic plants (e.g. Viscum album – strictly a hemi-parasite)
Climbing plants (e.g. Hedera, Lonicera)
Weed competition

Stress is often cumulative; stress from whatever cause will often lead to further stress from another cause. In this way stress caused by abiotic agents will often be instrumental in stress caused by biotic agents. A tree already weakened by whatever cause is more likely to be attacked.

Examples of how these two factors are linked include the following:

1. Exposed sapwood / heartwood - Caused by:

Mechanical damage, pruning, wind, snow, lightening, fire - Leading to:
Attack by airborne bacteria such as Erwinia amylovora, and fungi such as Ganoderma, and Laetiporus

2. Exposed sapwood - Caused by:

As above - Leading to
Attack by fungi such as Polyporus and bacteria as above

3. Stressed branches / stems - Caused by:

Root damage or death associated with compaction , flooding, gases in the soil, weed competition, herbicides - Leading to:
The development of fungi such as Piptoporus and Daldinia

4. Waterlogged conditions - Caused by:

Drainage operations (or lack of), excessive rainfall, compaction, perched water table - Leading to:
An increased risk of Phytophthora due to increased levels of ethanol, as a bi-product of anaerobic respiration

5. Prematurely removed leaves - Caused by:

Strong winds - Leading to:
Ideal entry points for the airborne bacteria of Xanthomonas populi

6. Neglecting to disinfect pruning tools between infected trees - Leading to:

The spreading of infection such as Erwinia amylovora from one tree to another

7. A cool wet spring will increase the prevalence of Taphrina deformans, since the over-wintering fungal spores will infect new leaves and shoots more readily.

8. Hot dry summers are linked with the spread of Cryptostroma Corticale, (as are Grey Squirrels who it is thought may help to spread the fungal spores from one tree to another under their claws.)

9. Certain conifer stumps left untreated after felling, such as Pinus sylvestris, will allow infection by the spores of Heterobasidion annosum.

10. Dense Ivy growth in the crown of a tree, or large areas of Mistletoe may add significant weight to the tree and also increase sail area, therefore leading to an increased likelihood of wind damage.

11. The activity of heartrot fungi which hollow out the stem of a tree would also ultimately increase the chance of failure as a result of wind.

12. Rainsplash will spread Fireblight and Bacterial Canker.

13. Wind will carry fungal spores and insect vectors. Insect vectors will carry fungal spores e.g. Scolytus scolytus.

14. Failure to clear fallen leaves, though not a stress factor in its own right will encourage re-infection by Rhytisma acerinum.

15. Transplanting shock is a contributory stress factor in the development of Vertcillium Wilt.

16.‘Pot-bound’ stock, if the roots are not teased out or removed, is likely to ultimately be unstable and prone to windthrow.

17. Overthinning or excessive crown reduction on a thin barked tree, such as Beech, may lead to sun -scorch, death of cambium.

18. Beech trees affected by Cryptococccus fagisuga, Felted Beech Scale, are more likely to be colonised by Nectria fungi.

19. Mulch mats, particularly those made from black polythene, can cause stress by overheating the rootzone of newly planted trees and they also provide an ideal, protected habitat for voles, allowing them to damage the bases of newly planted trees without detection.

20.Tree shelters can sometimes create an excessively humid environment around recently planted trees, thereby inhibiting oxygen levels around the stem of the tree required for respiration and leading to a higher incidence of mildew, which inhibits photosynthesis. They also stimulate weed growth, and ant nest construction.

21. Deep snow may enable rabbits to reach over spirals / shelters and cause damage.

22. Woodpeckers excavating moth and beetle larvae may cause sufficient structural damage to small limbs to render them unstable in strong winds.

23. Compaction can lead to the debilitation or death of myccorhizal fungi which are believed to inhibit the spread of Armillaria into root tissue.

24. Fire damage is linked with the development of Daldinia concentrica

25. Excessive use of farmyard manure and other organic mulches and their association with higher levels of carbon dioxide in the soil due to the activity of bacteria, reducing respiration, also stimulates the production of Phytophthora zoospores

26. Urban trees stressed by excessive pruning, root damage / severance etc are more prone to exploitation by Eucalypterus tiliae and Pulvinaria regalis.

Fungal strategies: Heart Rot

Enter through wounds
Specialist fungi – exist in harsh environment; little oxygen; high in lignin and tannin – Ganoderma Spp, Laetiporus sulphureus

Sapwood Exposed:

Opportunists – land on exposed sapwood / wounds
Polyporous squamosus

Sapwood intact:

Or ‘latent decay strategists’
No need for exposure – remain dormant in transport system of tree until stress
Piptoporus betulinus / Daldinia concentrica

Active parthenogenesis:

An invade healthy cells and attack cambium
They take in their own oxygen supply so can exploit environments without oxygen
Quick death is usual
Armillaria mellea / Heterobasidion annosum

Saprophytic

Colonisation of deadwood
Desiccation tolerant
Bjerkandera adusta / Coriolus versicolor

Types of decay:

White Rot

Caused by fungi that prefer lignin – the ‘glue’ in the tree
Compressional strength removed, good tensile strength as cellulose still present
- Type 1: Selective Delignification
  - Early stage – lignin broken down in middle lamella or secondary wall
  - Later stage – individual cells become separated from matrix – vessels left so wood is in strands
  - Result is ductile fracture (like breaking a bread roll – breaks more gradually and at higher bending loads)
- Type 2: Simultaneous Rot
  - Lignin and cellulose and hemicellulose broken down at approx same rate
  - Brittle fracture
  - Inonotus hispidus

Soft Rot

Hyphae tunnel up and down secondary wall
Acts as a more benign form of brown rot
Brittle Fracture eventually
Kretzshmaria deusta

Brown Rot

Cellulose gone
Poor tensile strength but still compressional as lignin still present
Brittle fracture – like a biscuit will just snap – little warning
Caused exclusively by Basidiomycetes and mostly polypores (brackets)
Mostly brown rot in conifers – rarely get white rot

Planting Site: Indicated problems, symptoms and appropriate solutions:

1) Poor soil

Problems:

Compaction, pedestrian and vehicular, therefore poor water and air percolation
Surface capping, therefore poor water and air percolation
‘Perched’ water table – localised impermeability – ‘plough pan’ / ‘iron pan’
Impermeable surface coverings leading to reduced soil quality
Lack of topsoil – limited rooting depth
Low levels of nutrients, or excesses of certain nutrients
Presence of toxins on some reclaimed industrial sites e.g. arsenic, or roadsides e.g salt
Low in organic matter, and living organisms
Poor texture / structure – inability to ‘hold’ water
Extremes of pH – e.g. high, due to excessive amounts of cement rubble
Poor drainage - waterlogging – reduced oxygen - anaerobic respiration
Lack of mycorrhizae because trees not present on site before
Excessive weed growth / competition – conditions favouring deep rooted perennials

Symptoms:

Poor rate of growth – incremental and extension
Chlorotic, possibly leading to necrotic, foliage – leaves small
Wilting of foliage – possible early leaf abscission
Low levels of vigour, leading to susceptibility to other agents of disease / decay, such as insects, fungi, bacteria etc
Crown dieback
Excessive or uncharacteristic epicormic growth
Unusually prolific fruit / seed production
Brightly coloured fruits (Nitrogen deficiency)
Death of tree
Surface puddling / cracking
Lack or sickliness of other vegetation around tree
Foul smelling soil

Solutions:

Remove and replace
Cultivation to at least 600mm prior to planting
Decompaction using mole plough, ripper etc – on large planting sites
Correct storage of removed topsoil pending its return to the site – i.e. shallow heaped, not compacted, sown with a clover crop to increase levels of nitrogen
Installation of correct drainage
Mound planting
Use of ‘improved’ soils at time of planting e.g. ‘Amsterdam’ soil
Grilles around newly planted trees in areas frequented by pedestrians, or trees in raised beds
Adhere to BS 5837, NJUG Vol. 4 – avoid compaction / contamination
Avoid use of aggregates with ‘fines’ around trees
Amelioration – sand to open clays, OM to bind sand
Incorporation of water retentive gels
Organic (well rotted) or artificial fertilisers (slow release)
Introduce earthworms
Avoid use of additives excessively high in N – e.g. chicken manure
Import good quality, weed free top soil and ‘install’ correctly - i.e. avoid compaction or leaving air pockets, at the two extremes
Selection of appropriate species for prevailing conditions – use of nitrogen fixers, pioneer trees, -those tolerant of wet conditions, particular pH etc.
Air injection / spading around existing trees
Radial trenching
Mulch to reduce compaction and gradually improve nutrient levels
Addition of Mycorrhizal spores – leaf mould or proprietary mixes
Don’t clear away leaf litter
Reduce competition for available water and nutrients – spray weeds with systemic herbicide, avoid a strict mowing regime where trees are in a grass sward

2) Pollution

Problems:

Road salt
Coastal salt laden winds
Gaseous atmospheric pollution – e.g. Carbon monoxide
Particulate pollution – e.g. smoke particles
Build up of dust deposits on demolition / construction sites
Spillage of toxic substances – e.g. diesel, oils, cement etc
Leakage of underground services – e.g. domestic gas
Presence of methane – associated with landfill sites
Leachates from landfill sites
Agricultural substances – slurry, silage effluent
Inappropriate (negligent or malicious) use of herbicides
Application of fungicides to amenity grass areas, killing Mycorrhizae
Excessive application of artificial fertilisers
Fly-tipping / thoughtless disposal of harmful substances – car batteries
Light pollution from street lamps

Symptoms:

‘Scorched’ foliage
Physical presence of causal agent on or around the tree
Lack of or death of surrounding vegetation
Chlorotic or necrotic spots or entire leaves
Blocked stomata impeding photosynthesis and transpiration and therefore promoting wilt
Dieback of aerial parts
Death of cambium leading to areas of loose bark
Rank smelling soils
Reduced soil oxygen leading to reduced respiration, root death, crown dieback, and ultimately death of tree
Increased susceptibility to biotic agents of disease / decay
Stunted growth
Smaller, sparser foliage
Early abscission
Unseasonal growth encouraged by artificial light, possibly vulnerable to frost damage

Solutions:

Maintenance and repairs of offending services
Effective emission controls
Compliance with BS 5837 – toxic substances on development sites – washing off dust deposits from foliage
Correct capping of landfill sites – continuous and deep enough (min 1m)
Venting of gases
‘Flushing’ of trees affected by road salt through irrigation
Minimal application of salt – correct calibration, training of operatives
Impermeable surface coverings, but on a highly permeable sub-base to allow lateral gaseous diffusion and water percolation
Uses of alternative methods / substances (non-toxic)
Use of tolerant species i.e. those able to resist pollution(Platanus spp.) and those better able to absorb and assimilate (Tilia spp.)
Use of salt tolerant species – Q. ilex, Pinus spp., H. rhamnoides
Use of species better able to perform anaerobic respiration – Salix spp.
Avoid drainage / run-off from hard surfaces towards bases of trees
Use mulching / mulch mats where possible as an alternative method of weed control around trees
Application of appropriate herbicides around trees (i.e. contact or systemic rather than residual) at appropriate rates and frequencies, by trained operatives
Careful positioning of street lights in relation to trees, and vice versa

3) Services

Problems

Leak of toxic substances at or below ground level
Root damage during installation / maintenance works
Stem and branch damage through careless impact by machinery
Stem damaged by leaning materials / securing plant / machinery overnight
Larger services acting as root barriers – restriction of available rooting area
Services installed within the top 600mm – direct competition for space
Overhead services contacting aerial parts of tree
Arcing from electricity cables
Absorption of residual herbicide by neighbouring trees through their excessive or inappropriate use, to keep areas such as sub-stations clear of weed growth
‘Unprofessional’ pruning of trees to clear overhead services i.e. not in accordance with industry best practice, BS 3998
Safe access for maintenance pruning of trees, or their removal to below ground level

Symptoms:

Death of parts of, or whole trees due to root damage, removal, death
Tree instability i.e. soil cracks, root heave, through death or removal of roots
Ensuing biotic attack once tree has been weakened
Trees inappropriately pruned leading to a loss of amenity value
Abrasion to stem / branches
Scorch / burn damage to branches
Complete failure of tree – windblow due to root severance
Evidence of recent trenching

Solutions:

Better liaison between those responsible for tree maintenance and service providers.
Efficient maintenance of services
Use of flexible / seamless pipes / ducts
Compliance with BS 5837 and NJUG Vol. 4 – i.e. measures to minimise root damage within the Precautionary Area – hand digging, thrust boring etc.
Trenching along radii of roots if unavoidable, to minimise damage
Adherence to the recommendations of BS 5837 with regard to service installation on development sites
Separation of trees and underground services i.e. service strips and dedicated tree planting areas elsewhere
Trees in ‘containers’
Use of root deflectors / barriers at time of planting
All services installed below the depth of most root activity i.e. below 600mm
Phasing out of overhead services
Avoid interaction of trees and overhead services by either not planting or using only low growing species
Maintain ‘wayleaves’ in wooded areas

4) Poor planting and aftercare

Problems:

Planting pit too small – roots distorted
Glazed sides to planting pit
Topsoil not separated from subsoil
Roots allowed to dry out
Roots removed completely at time of planting to make the job easier / quicker
Damaged (or girdling) roots not removed at time of planting – xylem vessels not ‘re-opened’ – ‘pot-bound’ effect continues
Rough handling of rootballs leading to their break up and consequent root dessication
Planted too deep / too shallow
Backfill not sufficiently consolidated – leaving voids, or over compacted – excluding water and oxygen
Ties too loose / too tight / not sufficiently flexible / too low in relation to the top of the stake
High staking / stakes left in situ for too long
Lack of weed control or use of an unsatisfactory method i.e. excessive use of herbicide, strimmers causing stem damage
Lack of, or inappropriate irrigation
Poor, or no pest control (where needed)

Symptoms:

Poor growth (incremental and extension) / survival rates
Instability as a result of girdling roots / no roots
Exposed roots
Death of parts / whole tree
Abrasion or ‘strangulation’ damage to stem
Swellings above / below ties – ties ultimately becoming ‘included’
Base ring-barked – strimmer, rabbits, voles
Instability after stake removal
Excessive weed growth around base
Water puddling around base

Solutions:

Detailed planting specs. / method statements
Trained staff
Sides and base of pit forked to encourage root development
Square pits rather than round to discourage root girdling
Avoidance of ‘pot-bound’ stock – better specification / checking procedures
Bare roots kept covered at all times – polythene bags
Unplanted trees kept out of direct sunlight
Rootballed trees handled with care
__________Plant to ‘nursery line’
Low staking, ideally left for only 18 months
Use of flexible ties at the correct height (25mm from top of stake)
Ties adjusted / removed promptly
Implement the correct type of pest control / guards / shelters
Use strimmer guards / mulch to avoid strimming
Correct application of suitable herbicides or alternative methods, such as mulch or mulch mats
Weed control for a minimum of three years
Frequent irrigation with smaller quantities, rather than too much too infrequently, leading to wastage of water and leaching away of nutrients

In conclusion, many of the symptoms of ill-health displayed by trees could be attributable to a number of different causal agents. On occasions different damaging agents will act sequentially or simultaneously. In order to make the correct decision to deal with an existing problem, or prevent it from reoccurring in the future, it is necessary to accurately determine the cause, through adequate investigation, and then take the appropriate action.

Helliwell System

Various occasions when it may be necessary to carry out an amenity evaluation of individual trees:

When considering if the serving of a Tree Preservation Order (TPO) would be appropriate – a TPO should only be used if it is considered “expedient in the interests of amenity”, therefore ‘amenity’ must be assessed
Before confirming a TPO which has been served under a 201 notice as an emergency procedure, perhaps before having time to adequately assess amenity value – to confirm sufficient amenity value actually exists
When there is an objection to the serving of a TPO on the grounds that it is not expedient in the interests of amenity – for the above reason
When an application to carry out work to, or remove a TPO’d tree is received – in order to establish whether the proposed work / removal is justifiable in the light of the tree’s value, or whether three is of such value the application should be refused
When a 211 Notice is received in respect of a tree in a Conservation Area – in order to establish if the tree is worthy of protection with a TPO
When reviewing TPO’s – for example to see if trees in an ‘Area TPO’ are worthy of protection as individuals / groups
Where there has been unauthorised work to, or the destruction of a TPO’d tree – in order to decide on an appropriate financial penalty, should the matter be referred to the Crown Court, where the fine can be unlimited and the value of the tree is something that will be taken into account
Where a tree (not necessarily TPO’d) has been damaged / destroyed by a third party, and the owner of that tree is seeking compensation
When recommending appropriate action to abate the risk posed by a hazardous tree – a particularly high value may indicate that the ‘target’ rather than the tree should be modified
During the course of a mortgage report – the value of the tree may well influence any recommendations to reduce or remove the risk of future tree related structural damage i.e. address the structure rather than the tree
During the course of a subsidence investigation – for the same reason as above
When deciding / approving the routing of underground or overhead services – a more expensive installation route may be justified if the trees that will be damaged by the cheaper option, are of a demonstrably high value
When classifying trees for retention on a site of proposed development – as part of a BS5837 survey
When deciding appropriate financial penalties / replacement costs for trees damaged by service providers / developers
When calculating ‘added value’ to a site for sale, as development or amenity land
When bidding for a budget for the maintenance of a given population of trees – in order to justify certain expenditure
When deciding whether to brace or prop as opposed to branch removal, if there is perceived to be a chance of failure. A very high amenity value may justify the installation of artificial support to maintain that amenity value.

Monetary value in the Helliwell System:

Most evaluation procedures for amenity trees are in monetary terms because virtually all of the above examples have financial implications, whether that be for development value, financial gain through the contravention of a TPO, appropriate penalties for such action, or comparing the value of a tree with the cost of structural repairs, or service installation. Money is the common denominator between arboriculture and other professions.

Factors that are taken into account when using the Helliwell System:

The Helliwell System of amenity tree valuation is based on various factors, each needing to be assessed and awarded an appropriate ‘score’. The final figure, resulting from the multiplication of these scores, is then multiplied by the appropriate conversion factor (increasing annually in line with the Retail Price Index) and a final financial value arrived at.

The factors are as follows:

1) Size

this is measured as the area of the tree as seen from one side
if this varies from one side to another, an average should be taken
the height should be measured with a clinometer, and
multiplied by the average crown spread
the height of any clear stem is omitted

Obviously the larger a tree is, the greater is its amenity value is likely to be, therefore the range of scores is from 0.5 for a tree of between 2 and 5 sq.m., and 8 for a tree over 200sq.m. Trees with a size less than 2sq.m. are deemed to have no significant amenity value, and therefore score zero.

2) Life expectancy

guidelines are given as to the average life expectancies of trees commonly encountered
the actual age of the tree in question is estimated (possibly using Mitchell’s system)
this is subtracted from the expectancy, to give the length of the tree’s remaining contribution as an amenity
trees with less than 2 years expectancy are given a zero score
other factors are taken into account, such as
- structural defects
- pests and diseases
- unfavourable location

3) Importance in Landscape

Takes into account the prominence of the tree in relation to
- landform
- buildings / structures
- other trees
- the number of people who can see it
trees in a public area will generally score higher than trees in a private setting
trees that cannot be seen from any public vantage point will score zero

4) Presence of other trees

the ‘visual area’ is decided on – not always easy
sometimes it is clearly defined e.g. a town square
if not, recommendation is for 50 hectares
it is as viewed from above, and calculates
the percentage of the area covered by other trees
the greater the percentage of other trees present, the lower the score for the tree being valued

5) Relation to setting

as a general rule, the best is the largest tree/s the available space will contain
a small tree in a large space will look insignificant
a large tree in a small space will be overwhelming
totally unsuitable trees will score zero
those that are particularly suitable scoring the most

6) Form or Shape

this is concerned solely with visual effect, not safety or longevity
it does not necessarily need to be ‘natural’ to be good
damaged or mutilated trees will not score well

CAVAT – Capital Assessment Value for Amenity Trees

The Full Method involves five steps, and sets of key variables:

Basic value / unit value x size;

The basic value is calculated using trunk area as the key measure of size. The trunk area is calculated in the standard way, by using the measured trunk diameter or circumference, and converted to give the radius.

Community Tree Index value / location, in terms of population and use, and accessibility:

There are two operations in step 2. Firstly, the basic value is adjusted to take account of the population density using the Community Tree Index, the second operation is to consider the relative accessibility to the public of the tree in its general locality.

Functional value / functional status;

The CTI value is then reduced according to the surveyor’s expert assessment of the tree’s functionality, i.e. how well it is performing biologically, as against what would be expected of a well-grown and healthy tree of the same species and girth in that location.

Adjusted value / amenity factors, both positive and negative;

The functional value is then adjusted to take into account the surveyor’s assessment of any special amenity factors and also the tree’s appropriateness to the location.

Full value / safe life expectancy.

Finally, the value is adjusted for safe life expectancy

TEMPO – Tree Evaluation Method for Preservation Orders

Assesses trees as to whether or not they are suitable for a TPO
Nationally recognised system, produced by an independent arboricultural consultant
Scores are allocated towards meeting certain criterial needs required to justify a TPO
The scoring produced by the valuation method indicates that any score between 11 to 14 points merits a “defensible TPO”
and that any score over 15 points “Definitely merits a TPO”

Climate Change:

Possible consequences of climate change likely to affect the physiological processes of trees:

longer growing season – increased growth rates, earlier flower production / sexual reproduction, later abscission – shorter dormancy / transplanting bare-roots
warmer winters - reduced winter pest mortality – more sustained pest attack – defences under pressure – seeds requiring winter chilling to break dormancy compromised – germination affected – less need for roadsalt? –more positive affects on processes??
wetter winters – more waterlogging –incidence of Phytophthora etc. so defence again – anaerobic respiration – anchorage compromised
drier summers – increased losses – drought stress – reduced water for all processes – increased pathogen activity – cell collapse / summer branch drop
drought / waterlogging – reduced soil fauna – less recycling of nutrients – photosynthesis, transpiration, translocation, storage
drought / waterlogging – root death, reduced storage – respiration
increased wind – transpirational loss – windthrow / snap- exposed tissues – defence
more extreme precipitation events – more physical damage – windthrow – exposed tissues – defence
more southerly storm track – wind from an unaccustomed direction – no reaction growth
increased incidence of alien pests – no natural predation – cambial / sapwood damage – cell division, translocation affected – structural integrity of stems
increased populations of deer and squirrels – more browsing, gnawing, fraying, compaction – all processes
increased shading – reduced flowering, seed germination
more autumn frost damage- abscission not successfully completed before leaves shed
reduced mycorrhizae –reduced uptake of water and dissolved nutrients – all processes - increase in Armillaria

Measures that could help to retain trees in the urban environment:

subsidence - trees in ‘containers’
subsidence - plant away from structures likely to be affected by subsidence
lack of water - install irrigation systems, greater use of ‘grey’ water
lack of water – porous surface coverings, mulches
lack of water - expand use of lower water demanders
current provenances becoming unviable - introduce ‘tenderer’ species
increased risk from alien pests and diseases - greater import controls – frequency of inspections – tool sterilisation
increased damage from native pests (mammals) – greater population control – culling
increased damage from native pests (insects) – greater use of pesticides
greater co-operation between authorities – regional strategies
increased fear of structural failure – more pro-active pruning, use of lower growing species
higher incidence of wind damage – adopt a ‘short-term’ policy – replace before maturity – trees in groups – mutual protection
heat stress – non-reflective surface coverings
reduced period for planting bare roots – greater use of container grown specimens
enhanced risk of unpredictable tree failure – target ‘control’, exclusion
greater use of species varieties to offset adverse effects, ensure survival