Glossary of Terms

Useful Glossary of Arboricultural Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Abscission (Tree physiology)

The shedding of a leaf or other short-lived part of a woody plant, involving the formation of a corky layer across its base. Trees can shed twigs in this way.

Absorptive roots (Tree anatomy)

Non-woody, short-lived roots, generally having a diameter of less than one millimetre, the primary function of which is uptake of water and nutrients.

Access facilitation pruning (Pruning operation)

One off tree pruning operation, the nature and effects of which are without significant adverse impact on tree physiology or amenity value, which is directly necessary to provide access for operations on site.

Adaptive growth (Biomechanics / risk)

In tree biomechanics, the process whereby the rate of wood formation in the cambial zone, as well as wood quality, responds to gravity and other forces acting on the cambium to maintain a uniform distribution of mechanical stress.

Adaptive roots (Biomechanics / risk)

The adaptive growth of existing roots or the production of new roots in response to damage, decay or altered mechanical loading.

Adventitious shoots (Tree physiology)

Shoots (other than from apical, axillary or dormant buds) that develop on trees

Anchorage (Biomechanics / risk)

The system whereby a tree is fixed within the soil, involving cohesion between roots and soil and the development of a branched system of roots which withstands wind and gravitational forces transmitted from the aerial parts of the tree.

Arboricultural Association (Organisation / industry)

The largest professional institution in the UK for the amenity tree care professional. Its purpose is to advance the study of arboriculture and promote the sustainable management of trees in areas where people live, work and play for the benefit of society.

Arboricultural Impact Assessment (Planning / BS5837)

A report, recommended by British Standard 5837:2012 (Trees in relation to design, demolition and construction – Recommendations), that identifies and evaluates the likely effects of a proposed development on existing trees, and sets out recommendations to avoid, minimise, or mitigate those impacts.

Arboricultural Method Statement (Planning / BS5837)

A document, recommended by British Standard 5837:2012 (Trees in relation to design, demolition and construction – Recommendations), that defines how construction or site works will be carried out to avoid damage to trees being retained, particularly within or near their root protection areas.

Arboriculture (General arboriculture)

The science and art of managing trees, shrubs and other woody plants for amenity, environmental and social benefit.

Arboriculturist (Organisation / industry)

A person who has, through education, training and experience, gained expertise in the care, management and assessment of trees, including their relationship with the built environment.

Architecture (Tree structure)

In a tree, a term describing the pattern of branching of the crown or root system.

Asian longhorn beetle (Pest / disease)

The larvae of this native of China and southeast Asia feeds on broadleaved trees, weakening them by its boring activities.

Axil (Tree anatomy)

The point where a bud forms between a leaf and the stem it grows from.

Bark (Tree anatomy)

A term usually used to describe all the tissues on the outside of a tree, beyond the cambium, including the inner bark (phloem) and outer protective layers; sometimes the term is used just to mean the outermost layer.

Bifurcation (Tree structure)

A term referring to a tree fork in the trunk giving rise to two roughly equal diameter branches. These forks are a common feature of tree crowns.

Bolling (Pruning / form)

A term sometimes used to describe pollard heads of trees.

Bottle-butt (Biomechanics / risk)

An unusually wide swelling at the base of a tree trunk and its buttress roots, often indicating a growth response to weakness in that area, sometimes caused by internal decay.

Bracing (Support systems)

The use of rods or cables to restrain the movement between parts of a tree.

Branch (Tree structure)

Primary: A first order branch arising from a stem. Lateral: A second order branch, subordinate to a primary branch or stem and bearing sub-lateral branches. Sub-lateral: A third order branch, subordinate to a lateral or primary branch, or stem and usually bearing only twigs.

Branch bark ridge (Tree anatomy)

The raised arc of bark tissues that forms within the acute angle between a branch and its parent stem. It marks the natural boundary between the branch and the stem, and helps guide correct pruning cuts so the tree can seal the wound effectively and reduce the risk of decay.

Branch collar (Tree anatomy)

A visible swelling formed at the base of a branch whose diameter growth has been disproportionately slow compared to that of the parent stem. The branch collar contains specialised tissues that help the tree seal over a wound after pruning. Making cuts just outside the branch collar allows

the tree to close the wound more effectively and reduces the risk of decay spreading into the main stem.

Brown-rot (Fungi / decay)

A type of wood decay in which cellulose is degraded and lignin is modified.

BS5837:2012 (Planning / BS5837)

A British Standard Institute publication, BS 5837 (2012) – Trees in Relation to Design, Demolition and Construction. The British Standard that sets out the arboricultural process that should be followed wherever trees or hedges are being considered in relation to development.

BS 5837 Tree Survey (Planning / BS5837)

A tree survey carried out in accordance with BS5837:2012 (Trees in relation to design, demolition and construction – Recommendations), recording the location, size, condition, quality and category of trees on or near a site to inform the design and planning process.

The survey is typically presented as a tree schedule and accompanied by a Tree Constraints Plan (TCP), which shows the trees’ positions, crown spreads and root protection areas to guide site layout and design.

Buckling (Biomechanics / risk)

An irreversible deformation of a tree structure subjected to a bending load.

Buttress zone (Tree anatomy)

The region at the base of a tree where the major lateral roots join the stem, with buttress-like formations on the upper side of the junctions.

Cambium (Tree anatomy)

Layer of dividing cells producing xylem (woody) tissue internally and phloem (bark) tissue externally.

Canker (Tree pathology)

A persistent lesion formed by the death of bark and cambium due to colonisation by fungi or bacteria.

Chalara dieback of ash (Pest / disease)

A serious disease caused by a fungus (Hymenoscyphus fraxineus) which causes leaf loss, crown dieback and often death of the tree.

Cleaning out (Pruning operation)

The removal of dead, crossing, weak, and damaged branches, where this will not damage or spoil the overall appearance of the tree.

Compartmentalisation (Tree physiology)

The confinement of disease, decay or other dysfunction within an anatomically discrete region of plant tissue, due to passive and/or active defences operating at the boundaries of the affected region.

Competent Arboriculturist (General)

An arboricultural consultant who has the appropriate education, training and experience to carry out tree-related assessments and provide reliable advice, and who understands the standards, legislation and best practice relevant to the work being undertaken.

Compression fork (Biomechanics / risk)

An acute angled fork that is mechanically optimised for the growth pressure that two or more adjacent stems exert on each other.

Compression strength (Biomechanics / risk)

The ability of a material or structure to resist failure when subjected to compressive loading; measurable in trees with special drilling devices.

Compressive loading (Biomechanics / risk)

Mechanical loading which exerts a positive pressure; the opposite to tensile loading.

Condition (Tree condition)

An indication of the physiological vitality or condition of the tree. Where the term ‘condition’ is used in a report, it should not be taken as an indication of the stability of the tree.

Construction Exclusion Zone / CEZ (Planning / BS5837)

The area around the tree from which access is prohibited for the duration of a construction project. Within the CEZ retained trees or area identified for future landscaping are kept clear of any ground works, storage or potentially harmful activity.

The CEZ is based on the Root Protection Area around a tree calculated during the BS 5837 tree survey and arboricultural impact assessment process.

Crown / Canopy (Tree structure)

Terms to describe the main foliage bearing section of the tree.

Crown lifting / Crown raising (Pruning operation)

The removal of the lowest branches, or preparation of lower branches for future removal, to increase clearance beneath the crown.

Good practice avoids removing large branches directly from the trunk, as this can create large wounds and lead to decay or structural weakness. On mature trees, crown lifting should be limited to smaller secondary branches or reduction rather than full removal where possible.

It is commonly used to improve light levels or access, but should normally be limited to no more than 15% of the live crown and retain at least two-thirds of the tree’s total height. Specifications should refer to a fixed clearance height e.g. crown lift to 2.5m above ground level.

Crown reduction / shaping (Pruning operation)

A specified reduction in crown size whilst maintaining, as far as possible, the natural tree shape.

Crown reduction (Pruning operation)

The reduction in height and/or spread of the crown (the foliage-bearing parts) of a tree. It may be carried out to reduce mechanical stress on branches or the whole tree, improve its suitability to the site, or reduce shading and light loss.

The work should retain the main framework of the crown and a significant proportion of the leaf-bearing structure, leaving a similar, though smaller, overall form without aiming for symmetry for its own sake. Cuts should be kept as small as possible and generally not exceed 100 mm in diameter unless necessary.

Reductions should be specified by clear measurements, reflecting the intended finished size, and may also refer to the length of growth to be removed for clarity.

Crown thinning / Crown thin (Pruning operation)

The removal of a proportion of secondary and smaller (tertiary) branches throughout the crown to produce an even density of foliage and a well-balanced branch structure, typically in broadleaved species. Crown thinning does not alter the overall size or shape of the tree.

Material should be removed evenly throughout the crown and not exceed the specified amount, generally no more than 30%. It is commonly carried out to increase light penetration, reduce wind resistance and weight (though not necessarily leverage), and is often not a one-off operation, particularly in species prone to prolific epicormic growth.

Deadwood (General arboriculture)

Branch or stem wood that contains no living tissue. Retaining deadwood provides valuable habitat for many species and rarely affects the health of the tree.

Removing deadwood can allow decay to enter otherwise sound tissues, and accessing it may cause additional damage during climbing operations. It is therefore generally recommended that deadwood is only removed where it presents an unacceptable level of risk.

DBH – Diameter at Breast Height (Tree measurement)

Stem diameter measured at a height of 1.5 metres (UK) or the nearest measurable point. Where measurement at a height of 1.5 metres is not possible, another height may be specified.

Decurrent (Tree form)

A system of branching in which the crown is borne on a number of major widely spreading limbs of similar size.

Defect (Biomechanics / risk)

In relation to tree hazards, any feature of a tree which detracts from the uniform distribution of mechanical stress, or which makes the tree mechanically unsuited to its environment.

Delamination (Biomechanics / risk)

The separation of wood layers along their length, visible as longitudinal splitting.

Dieback (Tree condition)

The death of parts of a woody plant, starting at shoot-tips or root-tips.

Disease (Tree pathology)

A malfunction in or destruction of tissues within a living organism, usually excluding mechanical damage; in trees, usually caused by pathogenic micro-organisms.

Distal (Tree structure)

In the direction away from the main body of a tree or subject organism.

Dominance (Tree physiology)

The tendency for a leading shoot to grow faster or more vigorously than the lateral shoots, often maintaining a taller crown than its neighbours.

Dormant bud (Tree physiology)

An axial bud which does not develop into a shoot until after the formation of two or more annual wood increments; many such buds persist through the life of a tree and develop only if stimulated to do so.

Dysfunction (Tree physiology)

In woody tissues, the loss of physiological function, especially water conduction, in sapwood.

Endophytes (Tree pathology)

Micro-organisms which live inside plant tissues without causing overt disease, but in some cases capable of causing disease if the tissues become physiologically stressed, for example by lack of moisture.

Epicormic shoot (Tree physiology)

A shoot having developed from a dormant or adventitious bud and not having developed from a first year shoot.

Excrescence (Tree condition)

Any abnormal outgrowth on the surface of a tree or other organism.

Excurrent (Tree form)

A system of branching in which there is a well-defined central main stem, bearing branches which are limited in their length, diameter and secondary branching.

Fastigiate (Tree form)

Having upright, often clustered branches.

Feathered tree (Tree form)

A tree with balanced lateral shoots down to near ground level.

Felling licence (Legal / planning)

In the UK, a permit to fell trees in excess of a stipulated number of stems or volume of timber.

Flush-cut (Pruning operation)

A pruning cut which removes part of the branch bark ridge and/or branch collar.

Habit (Tree form)

The overall growth characteristics, shape of the tree and branch structure.

Haloing (Tree management)

Removing or pruning trees from around the crown of another (usually mature or post-mature) tree to prevent it becoming suppressed.

Hazard beam (Biomechanics / risk)

An upwardly curved part of a tree in which strong internal stresses may occur without being reduced by adaptive growth; prone to longitudinal splitting.

Heartwood / false-heartwood / ripewood (Tree anatomy)

Sapwood that has become dysfunctional as part of the natural aging processes.

Heave (Soils / trees)

A term mainly applicable to a shrinkable clay soil which expands due to re-wetting after the felling of a tree which was previously extracting moisture from the deeper layers. May be used to describe the lifting of pavements and other structures by root diameter expansion and also the lifting of one side of a wind-rocked root-plate.

Horse chestnut leaf miner (Pest / disease)

The potential weakening of horse chestnut trees caused by the caterpillars of the moth Cameraria ohridella ‘mining’ the leaves for food.

Included bark (Tree anatomy)

Bark of adjacent parts of a tree (usually forks, acutely joined branches or basal flutes) which is in bark-to-bark contact.

Increment borer (Assessment tool)

A hollow auger, which can be used for the extraction of wood cores for counting or measuring wood increments or for inspecting the condition of the wood.

Infection (Tree pathology)

The establishment of a parasitic micro-organism in the tissues of a tree or other organism.

Internode (Tree anatomy)

The part of a stem between two nodes.

Lever arm (Biomechanics / risk)

A mechanical term denoting the length of the lever represented by a structure that is free to move at one end, such as a tree or an individual branch.

Lignin (Tree anatomy)

The hard, cement-like constituent of wood cells.

Lion’s tailing (Pruning defect)

A term applied to a branch of a tree that has few if any side-branches except at its end, and is thus liable to snap due to end loading.

Loading (Biomechanics / risk)

A mechanical term describing the force acting on a structure from a particular source; e.g. the weight of the structure itself or wind pressure.

Longitudinal (Tree structure)

Along the length of a stem, root or branch.

Lopping (Pruning operation)

A term often used to describe the removal of large branches from a tree, but also used to describe other forms of cutting.

Pathogen (Tree pathology)

A micro-organism which causes disease in another organism.

Photsynthesis (Tree physiology)

The process whereby plants use light energy to produce carbohydrates from carbon dioxide and water.

Pollarding (Pruning operation)

The removal of the tree canopy back to the stem or primary branches, usually on a cyclical basis.

Primary root zone (Planning / BS5837)

The soil volume most likely to contain roots critical to the health and stability of the tree.

Pruning (Pruning operation)

The removal or cutting back of twigs or branches.

Radial (Tree anatomy)

In the direction of the radius of a circular object such as a tree stem.

Reactive growth / Reaction wood (Biomechanics / risk)

Production of woody tissue in response to altered mechanical loading.

Residual wall (Biomechanics / risk)

The wall of non-decayed wood remaining following decay of internal tissues.

Rib (Biomechanics / risk)

A ridge of wood that has developed because of locally increased mechanical loading.

Ring-barking / girdling (Tree damage)

The removal of a ring of bark and phloem around the circumference of a stem or branch, usually resulting in death of tissues above the damage.

Root collar (Tree anatomy)

The transitional area between the stem and roots.

Root collar examination (Assessment)

Excavation around the root collar to assess structural integrity.

Root Protection Area (Planning / BS5837)

An area of ground surrounding a tree that contains sufficient rooting volume to ensure the tree’s survival.

Root zone (Tree anatomy)

Area of soil containing absorptive roots of the tree

Sapwood (Tree anatomy)

Living xylem tissue.

Selective delignification (Fungi / decay)

A type of white-rot decay in which lignin is degraded faster than cellulose.

Shedding (Tree physiology)

The normal loss of leaves, bark or other parts.

Snag (General arboriculture)

A portion of a cut or broken stem extending beyond a growing point.

Soft-rot (Fungi / decay)

A type of decay where fungi degrade cellulose within the cell walls.

Structural roots (Tree anatomy)

Roots that contribute significantly to structural support.

Subsidence (Soils / trees)

Sinking of soil due to moisture loss, sometimes caused by tree roots.

Taper (Tree structure)

Change in girth along the length of a stem or branch.

Target (Risk assessment)

Persons or property that might be harmed by tree failure.

Target canker (Tree pathology)

A perennial canker containing concentric rings of dead tissue.

Torsional stress (Biomechanics / risk)

Mechanical stress caused by twisting forces

Tree Constraints Plan (Planning / BS5837)

A plan prepared in accordance with BS5837:2012 (Trees in relation to design, demolition and construction – Recommendations) that shows the position and extent of existing trees and their constraints, including crown spreads and root protection areas, to inform the layout and design of a development site.

Tree Impacts Plan (Planning / BS5837)

A drawing that illustrates the predicted effects of a proposed development on existing trees, based on BS5837:2012 (Trees in relation to design, demolition and construction – Recommendations), and typically produced as part of an Arboricultural Impact Assessment (AIA). It shows how layout proposals, construction activities and site changes may affect retained trees, including any resulting constraints or required mitigation measures.

Tree Preservation Order – TPO (Planning / BS5837)

A legal order made by a local planning authority to protect a tree or group of trees that provide public amenity, making it an offence to carry out works such as pruning or felling without prior consent.

Tree Protection Plan – TPP (Planning / BS5837)

A plan, prepared in accordance with BS5837:2012 (Trees in relation to design, demolition and construction – Recommendations), showing the trees to be retained and the measures required to protect them during development, including root protection areas, protective fencing, and any necessary ground protection or special working methods.

BS 5837 Tree Survey (Planning / BS5837)

Vascular wilt (Tree pathology)

A disease affecting water-conducting tissues.

Veteran tree (General)

A tree which, because of its age, size and condition, is of exceptional biodiversity, cultural or heritage value, and is treated in planning policy as an irreplaceable habitat.

Veteran Tree Expert (General)

A suitably qualified and experienced arboriculturist with specialist knowledge in the identification, assessment and management of veteran trees, typically demonstrated through recognised training and certification such as the VetCert scheme, which provides accreditation in veteran tree survey and management.

Vigour (Tree physiology)

The rate and extent of a tree’s growth, reflecting how actively it is producing new shoots, leaves and woody tissue under current conditions.

Vitality (Tree condition)

A measure of a tree’s physiological condition and ability to sustain normal functions, assessed by indicators such as leaf size and colour, shoot extension, and overall crown condition, relative to what is typical for the species and age.

White-rot (Fungi / decay)

A form of decay in which lignin and cellulose are degraded.

Wind exposure (Biomechanics / risk)

The degree to which a tree is exposed to wind.

Wind pressure (Biomechanics / risk)

The force exerted by wind on an object.

Windthrow (Biomechanics / risk)

The uprooting or overturning of a tree by wind forces, often involving failure of the root–soil system.

Woundwood (Tree anatomy)

Wood with altered structure that is produced by a tree around a wound, helping to close the wound and protect underlying tissues.

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Tree surveys for planning & construction

Need to demonstrate a well-considered approach to trees? We offer practical advice on how to minimise or circumvent potential arboricultural impacts. Our expert BS5837 compliant guidance makes all the difference in achieving best possible outcomes.

Tree health &
PiCUS sonic tomography

Delving deeper to pinpoint the cause of tree decline can be the difference between losing a tree and retaining it. We use cutting edge, non-invasive PiCUS technology to identify internal decay and assess declining trees.

Tree protection & the law

Carrying out work on Tree Preservation Order (TPO) trees or trees in Conservation Areas requires formal approval from the local council. Our former Tree Officer experience means we know the TPO process well. We combine this with qualified tree expertise to deliver the best possible outcome.

Tree safety & risk assessment

By taking reasonable care to consider tree risk and acting on recommendations, tree owners demonstrate they have met their health and safety obligations and legal duty of care. Rest assured, we are licensed QTRA tree risk assessors.

Tree subsidence, mortgage & insurance

Mortgage lenders companies may ask a homebuyer for a tree report. Insurers may request tree investigations to determine why a tree failed or caused damage. We are suitably qualified Arboriculturists with expertise in this field.

Historic parkland & veteran tree advice

We provide specialist advice on the management and restoration of historic parkland and formal parks and gardens. We offer expert advice on veteran tree management accredited through the VetCert veteran tree expert award scheme.

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Our full-service solution starts with us discussing your project and tree concerns.

We then provide guidance tailored to your exact requirements.

With a strong strategy in place, we swiftly take your project from start to finish.

Technology for Smarter Tree Surveys

Technology underpins all of our tree surveys and arboricultural services so we can deliver efficient workflows, accurate data, and outputs you can rely on.

Using GIS mapping and differential sub-metre GPS, we provide precise, clearly presented information to support confident decision-making.

Our technology-led approach ensures our findings are robust, defensible, and aligned with current standards and planning requirements.

As specialists we have tools such as PiCUS sonic tomography which allow us to detect and map internal tree decay, which supports decisions on tree retention or removal. We also operate high-pressure Air Spade technology for non-destructive root investigationsto inform subsidence and construction projects.

Working Together for Your Trees

We work as an extension of your team - collaborating closely with landscape architects, ecologists, engineers, planners and designers to ensure our tree advice is fully integrated into the wider project.

If your project requires it, we can also bring together a tailored team of trusted associates, including specialists in archaeology, heritage and landscape architecture to create a seamless, multi-disciplinary approach from the outset.

I have worked with Enviroscope for many years and I highly recommend the excellent service that they provide, particularly where projects have required a bespoke solution to retain trees within development proposals.

S.S.
Associate Director – Landscape Architect
Tetra Tech

Glossary of Terms

Useful Glossary of Arboricultural Terms

Useful Guides

Glossary of Terms

FAQs

Tree surveys for planning & construction

Tree health &
PiCUS sonic tomography

Tree protection & the law

Tree safety & risk assessment

Tree subsidence, mortgage & insurance

Historic parkland & veteran tree advice

Good Tree Care begins with Quality Advice

Technology for Smarter Tree Surveys

Working Together for Your Trees

I have worked with Enviroscope for many years and I highly recommend the excellent service that they provide, particularly where projects have required a bespoke solution to retain trees within development proposals.

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