Access to Neighbouring Lands Act 1992: Permits access to neighbouring land if the neighbour has refused access and works (eg. to trees) on the owner's land are necessary but would not be possible (or would be substantially more difficult to carry out) without such access.

Adaptive growth: The growth of wood in the vicinity of a structural defect, tending to compensate for any mechanical weakness. See Reaction wood.

 Adventitious roots: Roots which arise out of order, ie. not from the growing tip. They can arise for example from stems (as in stem cuttings), on the underside of branches in contact with the ground or in the crown of mature trees. See Ancient yew.

 Adverse possession: Occupation of land with the intention to possess it, without the consent of the owner. May lead to ‘squatters’ rights’, governed by the Land Registration Act 2002.

Air spade: Equipment providing a jet of compressed air which assists in the non-destructive excavation of roots, typically for exploratory purposes.

 Amenity value: The environmental and landscape benefits of trees as opposed to their commercial value for timber. Many of these benefits are intangible or difficult to measure, and are prominent in urban, roadside and residential settings.

 Ancient tree: A tree, much of whose value resides in its exceptional antiquity, so that the conservation of the tree to maximize its longevity is generally paramount.

 Ancient woodland: In England and Wales, a site that has been wooded since 1600 AD and is therefore unlikely to have been planted, typically with diverse plants and animals.

 Ancient yew: Yews are thought to be the oldest surviving trees in Britain, perhaps over 2000 years old. Many are found in churchyards but far pre-date the establishment of the Christian church. Adventitious roots typically grow downwards from the crown through the decayed centre of the trunk and become visible over a timescale of centuries as the tree hollows, eventually helping to support the tree.

 Area of Outstanding Natural Beauty (AONB): A national designation, regulated by the Countryside and Rights of Way Act 2000, of which there are 35 in England. In AONBs, the conservation of the natural beauty of the landscape is given ‘great weight’ in development control decisions.

 Arboricultural Implications assessment: That part of the BS 5837 (2005) procedure that evaluates the tree-related constraints to development, often with the aim of arriving at practical options to mitigate them or to inform the provisional design.

 Arboricultural method statement: That part of the BS 5837 (2005) procedure that sets out how site works should be carried out near trees to avoid accidental damage, typically including a risk assessment, recommendations for pre-development tree works, warning signage, restrictions on vehicular and pedestrian access, various other precautions, sequencing, compliance etc. Complements or incorporates the Tree protection plan.

 Arboriculture: Management of individual trees or groups of trees primarily for their amenity value.

 Arboriculturist: Specialist competent to give advice, in U.K. preferably a professional member (M.Arbor.A.) or fellow (F.Arbor.A.) of the Arboricultural Association (www.trees.org.uk), and having the Association’s recommended level of professional indemnity insurance.

 Arborist: Alternative term for tree surgeon.

Aspect: Of a sloping site, the compass direction in which it faces.

Avenue: A tree-lined way or approach, the trees typically even-aged and evenly spaced to give a striking landscape feature.

Bark exudate: Gummy, tarry or sugary deposits on bark, brittle if dry, indicating that the bark is dead, dying or injured. May be associated with disease, physical injury, root damage, sun scorch or drought.

Bark inclusion: See included bark.

Basal area: Cross-sectional area of the main stem of a tree measured (usually over bark) 1.3m above the ground (ie. at 'breast height').

Basal stocking: A mass of epicormic shoots arising from close to the base of a tree, often in response to pruning, damage or decline in the crown. They tend to be vigorous, forming a jungle if untended, and may reduce the vigour of the crown if unchecked.

Beaufort scale: A 12-point scale of wind speed defined by qualitative effects including the movement and fall of branches and trees. Large branches are in motion in Force 6 (strong breeze), whole trees are in motion in Force 7 (high wind) and twigs break off in Force 8 (gale), while the breakage of larger branches and the uprooting of whole trees occurs at Force 9 (strong gale), Force 10 (storm) and above. Structurally defective branches and trees are expected to fail at lower wind speeds than those envisaged in the scale.

 Bleeding (from bark): Watery flow, often leaving a stain, associated with disease, bark injury or root damage. Note that rainwater draining from a fork or hollow may leave a stain not associated with any defect. 

Bleeding (from wood): Flow of sap, typically from pruning wounds. Many genera (including maple, birch, hornbeam, walnut, laburnum, magnolia and poplar) are prone to bleeding if pruned in late winter or spring. Probably more unsightly than damaging, and avoided by pruning these genera in late summer or autumn.

 Bracing: The support of one (weaker) part of a tree by another (stronger) part by means of aerial rods, cables or straps, in order to make the tree safe without having to reduce or remove the weaker part.

 Branch collar: The swelling at the base of a branch, to be left intact in any pruning. See Flush cut.

 Branch ridge: The roughened area of bark in the acute angle of a branch union, or where a branch joins the main stem, to be left intact in any pruning. See Flush cut.

 Broadleaf tree: A botanical definition embracing trees not belonging to the conifers and their allies. Characterized by leaves that are broad (in relation to conifers), and which are usually deciduous. Sometimes called ‘hardwoods’.

 Brown Tail Moth: A cause of defoliation in spring in rosaceous trees, particularly hawthorn and cherry. The caterpillars hatch in late summer and overwinter in conspicuous silky white tents (as do other moth larvae), usually found at the tips of branches. The caterpillars’ hairs cause irritation to the skin and eye.

 B.S. 3998. Tree work: The British Standard regulating some of the technical operations carried out by tree surgeons, including pruning, wound treatment, bracing etc.

 B.S. 8206-2 (1992). Lighting for buildings: Code of practice for daylighting: The British Standard describing how much daylight and sunlight people should be able to get in their houses.

 B.S. 5837 (2005) Trees in relation to construction: Recommendations: The British Standard governing proposals for development. Its aim is to avoid undue conflict with trees. It envisages a step-wise procedure consisting of Tree survey, Tree constraints plan and Arboricultural implications assessment, and where necessary an Arboricultural Method statement and Tree protection plan.

Bulk density: The mass of a unit volume of oven-dry soil (including particles and pores). Typical values range from 1.0 to 1.8 g cm-3, depending on soil type and degree of soil compaction. Roots will rarely enter a light-textured soil if its bulk density exceeds 1.7 to 1.8, or a heavy-textured soil if its bulk density exceeds 1.5 to 1.6. See soil texture.

California Bearing Ratio (CBR): Resistance pressure, typically of road-building sub-base, measured by a penetrometer and expressed as a ratio of the resistance pressure of a standard material (crushed rock). Measures load-bearing capacity and helps in the specification of the surface. The standard test is set out in BS 1377-4 1990 Methods of test for soils for civil engineering purposes Part 4: Compaction-related tests. See soil penetrometer.

 Canker: An area of dead or malformed bark caused by a pathogen. See girdle.

 Canopy: A term used for the crown of a tree to emphasize its spreading or enclosing character.

 Cavity: A void in a tree trunk, branch or root that may or may not be open to the exterior, generally created by decay. Over many years the wound may become entirely grown over (occluded) while the decay progresses within.

 Cellulose: The structural material of plant cell walls, consisting of a filamentous polymer of glucose. Stiffened by lignin to form wood.

 Circular 11/95 - The use of conditions in planning permissions: Sets out the policy that planning conditions should only be imposed where they are relevant to the development to be permitted, enforceable and reasonable.

 Circular 36/78 – Trees and forestry: Consolidates advice on trees and forestry to local planning authorities, which have various powers to plant and protect trees and a duty to make provision for them when granting planning permission.

 Clay: Soil particles of very small size (<0.002mm). The particles are often plate-like and can bind to each other and to water, so that the clay is coherent and plastic when wet, hard when dry. Clay soils are often slow to drain and, depending on their mineralogy, may be shrinkable. See Soil texture.

 Common law: That part of English law embodied in decisions of the courts, developed since the thirteenth century. May be more flexible than statutory law since it is amenable to development according to changing social circumstances, but may also be uncertain if no case has been decided on a particular point.

 Compartmentalization: A form of defence in woody plants, in which barriers resistant to invasion by pathogens or wood decay fungi are laid down while the wood is living (sapwood), and which continue to act passively once the wood is incorporated into heartwood.

 Compensation: Structural defects may be strengthened by the continued radial growth of new wood (which may be adaptive), especially in young-mature and mature trees (see Maturity), rendering the defect progressively less significant.

 Compression wood: The kind of reaction wood found in conifers, which is strong in compression and characterized by a high lignin content, laid down on the underside of the affected part (such as a branch) to help support it.

 Compressive stress: A pushing (squashing) force, as is exerted longitudinally on the upper side of a beam or the leeward side of a tree trunk bending in the wind. See tensile stress.

Conifer: A botanical definition embracing trees with cones (ie. seeds not formed within ovaries), mostly with needle-like or scale-like leaves and mostly evergreen. Sometimes called ‘softwoods’.

 Conservation Area: An area recognized in the Town & Country Planning Act (1990) in which, among other things, prior notice has to be given to the Local Planning Authority of any proposed tree works.

 Constraints Plan: That part of the BS 5837 (2005) procedure that identifies the tree-related constraints to development.

 Construction exclusion zone (CEZ): Part of a development site from which all pedestrian and vehicular movements are excluded by protective fencing, typically to ensure the wellbeing of trees during site works. Part of the Tree protection plan.

 Coppice: The shoots arising from the stump of a tree which give rise to a second- (or subsequent) generation multi-stemmed tree.

 Coronet cuts: Crown reduction by cuts and purposeful tears and fractures, intended to simulate storm damage.

Crown: The branches, twigs and foliage of a tree, considered collectively. The shape of the crown may be columnar, pyramidal, narrow, rounded, spreading, flat-topped etc. See unbalanced crown.

 Crown subsidence: Branches, especially if spreading, tend gradually to subside under their own weight, and may eventually reach ground level in large open-grown trees. The process is rapid in heavy-cropping fruit trees.

 Crown thinning, crown reduction and crown raising: Crown thinning removes branches from the crown without reducing the extent of the crown. Crown reduction decreases the extent of the crown without decreasing its density. Crown raising increases the headroom to the base of the canopy by removing lower branches. See pruning and topping.

 Crown cleaning: The removal of dead, dying, damaged or diseased wood from the crown of a tree.

 Countryside and Rights of Way Act 2000: Increases the protection for wildlife to include reckless as well as intentional harm. This means that ignorance resulting in injury to a protected species is no longer a defence.

 Cultivar: A plant selected for specific characteristics (whether useful or ornamental) which are distinct, uniform and stable and which are retained when the plant is propagated by appropriate means.

dbh: See diameter at breast height.

Deadwood: In the growth and development of a tree, branches compete with each other and weaker branches are eventually suppressed and die. The deadwood is then liable to fall (sometimes called ‘natural pruning’). Deadwood develops naturally, largely in the inner and lower crown, of all trees that are mature and unmanaged. Deadwood in the outer crown of a mature tree signifies a decline that may either be progressive or due to some prejudicial episode in the past (root damage, summer drought, insect infestation etc.) from which the tree has since recovered.

 Decay: The progressive softening of wood caused by specialized fungi. The fungus typically enters through wounds in the roots (root rots), main stem or branches (butt and stem rots) and can then extend internally, over a timescale of years or decades, longitudinally for several metres. External signs of decay include the presence of fungal fruiting bodies near the affected part, and cavities. Decay confined to heartwood does not affect the life processes of a tree, only its mechanical strength and stability.

 Deciduous: Leaves are lost in winter, as opposed to evergreen.

Defect: Any feature of a tree that is likely to make it less safe (in the case of a structural defect) or otherwise to reduce its health, longevity, landscape prominence or value for any other reason.

 Derelict hedge: A hedge that has lacked maintenance for many years, allowing the hedge plants to grow until they approximate to tree form, becoming variable in size and gappy, particularly at the base. In consequence no longer serving the purpose of enclosure but still marking a boundary.

 Design and access statement: A document justifying the design of a development as to amount, layout, scale, landscaping and appearance (including materials). Needed to support some planning applications, including those in Conservation Areas, AONBs and SSSIs.

 Diameter at breast height (dbh): The diameter of a tree measured at height 1.3m above the ground, measured from the higher side if the ground is sloping.

 Die-back: Death of shoots or roots starting at the extremities. See stag-heads.

 Diffuse light: Light that has been scattered, typically through cloud, as opposed to direct sunlight. Over half the solar radiation reaching the surface of southern England is diffused and, in relation to the points of the compass, is largely non-directional.

 Displacement: Where a tree is planted very close to a structure, the structure may be displaced if unable to withstand the mechanical stresses imposed by the growth of the main stem or roots. If the structure is sufficiently strong the base of the tree and roots become distorted instead. Roots below a surface may displace it upwards.

Distal: In relation to a particular point on a plant, that part towards the extremity, whether of a shoot or root, ie. away from the point of attachment. The opposite of proximal.

 Drift: Deposits since the last ice age that overlie the solid geology, modifying soil charactistics,

Drip-line: The extremity of the crown, as would be indicated by the shedding of rain-water.

 Dutch Elm disease: A fungus (imported in elm timber from Canada) that is fatal to European elms. It is spread by bark beetles, which inoculate their egg chambers with the fungus, making the wood relatively palatable to the larvae. The disease is now permanently established in much of the U.K., and European elms which re-grow as suckers become periodically re-infected.

 Duty of care: The ‘common duty of care’ under the Occupiers’ Liability Act 1957 is: ‘to take such care as in all the circumstances is reasonable to see that the visitor is reasonably safe in using the premises for the purposes for which he is invited or permitted by the occupier to be there’. It is generally taken to imply the need to inspect trees from time to time, although the appropriate thoroughness and frequency of inspection depends on the context.

Dynamic behaviour: The way in which the movement of branches and trees in a high wind increases loading. Unpredictable. See resonant frequency.

 Endemic: Native exclusively to a defined area.

End loading: A stem (or branch) with few branches (or secondary branches) except near its end has high end loading. It can increase the risk of failure because swaying in a wind is poorly damped (see resonant frequency), and the capacity for adaptive growth may be limited.

 Enforcement notice: If a Local Planning Authority considers that there has been a breach of a planning condition providing for the protection of trees, it may serve a notice under the Town and Country Planning Act 1990 to enforce compliance. See Stop notice.

 Epicormic shoots: Shoots arising from the base of a tree, its trunk or main framework branches, from buds dormant more than one season. May be stimulated by pruning (which increases the light reaching the lower part of the tree), or indicative of damage or decline in the upper crown.

 Epiphyte: An organism attached to a plant or otherwise using it for support, including higher plants (such as ivy and orchids), ferns, mosses, lichens etc. Epiphytes may damage the host indirectly but are not nourished by it (through any interference in its life processes), contrary to parasites.

 Evergreen: Foliated throughout the year (although there is a gradual turnover of leaves).

Exposure: A site factor that may limit tree growth and increase the risk of windthrow or storm damage, determined principally by windiness as affected by altitude, aspect and topography. In British upland forestry formalized in a ‘windthrow hazard classification’, in the lowlands inferred by observation. Loosely, the opposite of shelter.

 Fastigiate: A growth habit with branches strongly ascending, like Lombardy Poplar. A common ornamental form.

 Felling license: Required from the Forestry Commission to fell more than 5m3, or to sell more than 2m3 of wood in any one calendar quarter. Exemptions include trees less than 8cm in diameter at breast height (dbh), thinnings less than 10cm dbh and coppice poles less than 15cm dbh. The F.C. normally imposes a provision to re-establish the felled woodland.

Fibre buckling: A form of failure in which the structural fibres in wood are unable to withstand a longitudinal compressive stress. 

 Flush cut: A pruning cut that removes the branch collar and/or part of the branch ridge, slowing the occlusion of the wound or damaging its compartmentalization.

 Fly-tipping: The dumping of waste on land with no license to receive it. Fly-tipping is illegal in England under the Environmental Protection Act 1990 because the waste may be hazardous, cause damage to watercourses or soil, is unsightly and is cleaned up at taxpayers’ expense.

 Footing: The (relatively wide) base of a foundation to spread load and improve stability, introduced in Victorian times.

 Footprint: The outline at ground level of a building when viewed from above. Equivalent to the outer line of the foundations, where present.

 Forestry Act 1967: Creates the requirement for a Felling license, administered by the Forestry Commission.

 Framework: Typically, the main branches (sometimes also called scaffold branches), each of which supports a significant portion of the crown. They largely determine the shape of the tree’s crown depending on their height of origin, orientation etc. There is no precise distinction between framework branches and other lesser branches.

 Friable: Of soil, crumbling easily in the fingers.

Gall: Abnormal growth of leaves, buds, stems etc. in reaction to the presence of an intrusive parasite, often an insect or mite.

 Girdle: Damage that kills the bark all the way round the stem, so that the distal part dies.

 Gley: Soil with impeded drainage that readily becomes waterlogged, generally at some depth below the surface, denying oxygen to this part of the soil and resulting in characteristic bluish or grey colours. Gleyed soil is unfavourable for root growth and may restrict rooting depth.

Grafting: Complementary cuts are made in a rootstock and scion, which are then bound together so that they fuse and, after some growth, function as one.

Graft union: The point at which two identities, the rootstock and scion, are united.  Often indicated after some years by a bulge near the base of the tree or an abrupt change in bark characteristics. May be weak if the two parts are incompletely compatible.

Growth stresses: Naturally-occurring compressive or tensile stresses in the woody parts of a tree which effectively increase its strength.

 Ground beam: A beam at or near ground level supporting a wall and transferring the load to foundation piles or piers.

 Habit (growth habit): Giving a tree its characteristic form, for example owing to the stoutness and orientation (fastigiate, ascending, spreading, pendulous, weeping etc.) of a tree’s branches.

 Ha-ha: A trench forming the boundary of a garden with vertical inner side typically faced with stone and sloping outer side, making in effect a sunken wall designed not to interrupt the view from garden to countryside beyond, and to be invisible until closely approached.

 Hanger: Dead branch fallen from the crown but caught by, and resting on, branches lower down. May be liable to fall.

 Hazard: Something with a capacity to cause damage or injury. See Terms associated with tree risk.

 Health and Safety at Work Act (1974): Legislation primarily obliging employers to make the workplace safe. Outside the workplace the analogous obligation, for example on householders, is the duty of care. 

 Heartwood: The inner wood of a branch or main stem which is typically coloured (in contrast to the pale outer sapwood). It is formed naturally as the inner sapwood dies, becoming largely non-functional except for its mechanical role in helping to support the tree. Not present in branches/stems below a certain size and in some species.

 Heave: Upward movement of ground and affected structures. Tree-related heave typically occurs when a building is built on shrinkable soil, and at the time of building the soil is already very dry owing to water abstraction by existing trees. Once the trees are removed, the soil gradually re-hydrates and swells, and the building is heaved upwards, typically creating cracks. There are many other causes of cracks in buildings. Heave can also refer to the lifting of pavements, surfaced driveways etc. by the radial growth of underlying roots.

 Hedgerows Regulations 1997: Under this legislation, an amendment to the Environment Act 1995, field hedges of native species cannot be removed without permission.

Herbaceous: Not woody

 High hedge: Under the Anti-social Behaviour Act 2003, a hedge may be so high as to cause a nuisance (by adversely affecting the complainant’s reasonable enjoyment of his property). A ‘high hedge’ has to be evergreen or semi-evergreen to be within the definition of the Act.

 Highways Act 1980: It is an offence to allow trees to interfere with the use of a highway or footpath. The Highway Authority (HA) has the power to serve notice on a tree owner to make trees safe or to increase visibility and/or lighting for users of the highway. If the notice is not complied with, the HA can do the work and recover the cost. The HA can also restrict planting, or alternatively grant a license to plant and maintain vegetation in the highway.

 Honeydew: A sticky deposit, largely of sugar-water, found on trees and objects in their vicinity, excreted by sap-sucking insects feeding in the crown. It can be troublesome on pavements, motor vehicles, windows and gardens.

 Hoo: A promontory or point of vantage (ie. not confined to a coastal feature in this definition). The form used in east England of the word ‘hoe’ (as in ‘Plymouth Hoe’).

 Horse chestnut bleeding canker: Attributed to a bacterium that kills the bark, resulting in a conspicuous black bark exudate. Branches and trees, including young trees, can be completed girdled and killed, or there may be remission, resulting in local areas of dead bark.

 Horse chestnut leaf miner: A moth whose larvae feed within horse chestnut leaves, causing brown patches of regular shape and giving a premature autumnal appearance. Infestations do not usually lead to a noticeable decline in tree health, possibly because the damage occurs late in the season.

 Included bark: Areas of bark on adjacent parts of a tree, typically on the inner faces of a narrow fork, which become grown over to occupy part of the internal joint. The bark-to-bark contact is weaker than the more usual woody union.

 Injunction: A court order specifying actions to prevent future harm, as opposed to a claim for damages which arises from harm already caused.

 Ivy: An epiphyte (not a parasite), often growing on trees. It is not directly harmful but may be so extensive as to compete for water and light, and can increase the risk that a tree is blown over. A profuse growth of ivy near ground level makes it difficult to observe stem and branch defects in the overgrown parts, or to measure main stem diameter accurately.

 Jungle: An impenetrable thicket of woody vegetation consisting of a very high density of small-diameter stems, typically created by the suckering of understorey and pioneer species such as hazel and blackthorn  

 Landslip: Movement of soil down a slope, not necessarily suddenly.

 Leader: The topmost vertical shoot of a tree, present if the tree has strong apical dominance, characteristic of young trees and conifers. Trees with a rounded crown have no leader.

 Lever arm: The length of a structure free to move at one end (such as a branch) and the distribution of weight along that length, affecting loading. 

 Light interception: Trees may cast shade which is beneficial or excessive, they may be valued near houses for reasons unrelated to lighting (shelter, privacy etc.), and some lose their leaves in winter when the sun is low. Guidance relies on such details of context. 

 Lightning damage: If lighting passes through the water column within a tree, the water vapourizes and the affected part explodes. The upper crown or whole tree may be destroyed, or plates of bark blown off. Damage may be confined to a part of the cross-section only, resulting in an elongate wound. A struck tree may also be killed without sustaining physical damage.

 Lignin: The complex constituent of wood, made from phenolic precursors, that makes it stiff.

 Loading: The mechanical stresses imposed by the weight, orientation etc. of trees and branches in relation to tree architecture. See Terms associated with tree risk.

 Loam: A soil whose soil texture is not dominated by one particle size, generally advantageous for soil fertility.

 Local Government (Miscellaneous Provisions) Act 1976: This Act gives the Local Planning Authority power to serve notice on a tree owner to make safe any tree thought to be in imminent danger of causing damage or injury. If necessary the LPA can enter the owner’s property to do the work and can then recover reasonable costs.

 Local Planning Authority: The body responsible for administering planning guidance and legislation in a particular area, typically the local District/Borough Council or Unitary Authority.

 Lopping: A kind of pruning in which the branches of a tree are decapitated. Not regarded as good practice. See Crown thinning …

 Magnitude: The size, weight, height etc. of a hazard affecting its potential to cause damage or injury. See Terms associated with tree risk.

 Maturity: Young-mature trees are growing vigorously and their crowns have not yet reached full size. Mature trees have full-sized crowns, this size being determined by the context. Late-mature trees often have accumulated defects in the aerial part which may or may not be significant, while senescent and veteran trees are in a definite decline associated with lessening crown volume.

 Method statement: A written description of how a particular task or activity should be carried out, to ensure that a particular standard is reached, to take account of any associated risk etc. See Arboricultural Method Statement.

 Mistletoe: A semi-parasite, having green leaves for photosynthesis but growing into the host to obtain water and mineral nutrients. Capable of reducing growth, causing deformity or killing the host in a heavy infestation, but advantageous to wildlife.

 Monolith: A tree reduced to its main stem (ie. without branches), sometimes left on felling a dead or dying tree (in appropriate contexts). It is left primarily for its wildlife value (decay fungi, wood-eating insect larvae and their predators including woodpeckers).

 Narrow crotch: Where two branches arise at an acute (narrow) angle, as when both branches are close to the vertical. The union is relatively weak if there is included bark.

 Native: Present in a defined region for a certain amount of time, for example in Britain since the last Ice Age or since the English Channel was flooded, or present without having been brought by humans.

 National House Building Standards (NHBS) Chapter 4.2: ‘Building near trees’: Gives guidance on the depth of foundations in relation to soil type and proximity of trees.

 Naturalized: Introduced (as opposed to native) species that sustain themselves naturally and may be invasive.

Negligence: The failure to take reasonable (foreseeable) action to deal with tree-related hazards so as to reduce the risk of injury or damage. Owners of trees have a duty not to be negligent.

 NJUG 10: ‘Guidelines for the planning, installation and maintenance of utility services in proximity to trees’: This document, by the National Joint Utilities Group, gives guidance on the location of service trenches in relation to existing trees and areas of new planting.

 Noxious weeds: Weeds subject to legislation that may require measures to be taken to control their spread: Spear thistle, Weeping (Field) thistle, Curled dock, Broadleaved dock, Common ragwort, Giant hogweed, Japanese knotweed.

 Nuisance (Common law): Private nuisances are said to be of three kinds: (i) encroachment on a neighbour’s land, as by the growth of a tree across a boundary (although not all such encroachments are a nuisance), (ii) direct physical injury to a neighbour’s land, as by damage to walls and buildings, especially by roots, and (iii) interference with a neighbour’s quiet enjoyment of his land, applicable for example to High hedges.

 Nuisance (Statutory): The Local Planning Authority has the power to bring a nuisance to an end under the Environmental Protection Act 1990, usually by serving an abatement notice, so that the person affected by the nuisance does not have to take action in the courts. The scope of this kind of nuisance is uncertain but may relate primarily to health (since the predecessor to the 1990 Act was the Public Health Act 1936).

 Oak processionary moth: A cause of defoliation, especially in oak. Eggs are laid in the outer crown of the host tree in late summer and hatch the following spring. The larvae feed on the leaves and often move head-to-tail in a ‘procession’, hence their common name. They make communal silken webbed nests, as do other moth larvae, and their hairs are a severe irritant.

 Occlusion: The continued radial growth of new wood which gradually grows over stem and branch wounds and stubs. These can become completely overgrown if the tree has sufficient vigour and the wound is not excessively large.

 Occupancy: The extent to which the target area of a hazard contains targets, which may be variable if the targets are not fixed (people, animals, vehicles etc.). See Terms associated with tree risk.

 Occupiers Liability Act (1957, 1984): Under this Act, occupiers are obliged to take reasonable steps to ensure that their premises (including woodland) are reasonably safe. See Duty of care.

 Open-grown: A tree grown with little or no competition, so that its roots and crown spread without constraint. An imprecise definition since virtually all trees suffer some competition for water and nutrients, for example from the herbaceous vegetation near the base of the tree, and trees' root systems can interpenetrate without evidence of any adverse effect. In fact, such 'competition' could be advantageous overall. See root graft.

 Parasite: An organism that exploits another, eg. for food, to the prejudice of the host. Parasites may kill their hosts, be pathogenic or have little significant effect.

 Pathogen: A kind of parasite that causes disease.

 Parkland: A traditional agricultural land-use, typically permanent pasture with occasional open-grown specimen trees planted primarily to enhance a prospect and to provide shelter and shade to livestock.

 PassivHaus: A building whose interior climate is maintained without active heating and cooling systems, involving efficient insulation and airtightness, and optimized building form and orientation. Pioneered in Germany.

Penetrometer: Instrument for measuring resistance to pressure. A resistance in soil of 1.4 to 2.0 MPa is sufficient to restrict or prevent root growth.

 Phoenix regeneration: The renewal of the aerial part of a tree by growth from near the base, for example from epicormic shoots. Occurs naturally in some old trees, in others may be encouraged by management to increase longevity.

Picus tomograph: Ultrasound technique for estimating the extent of internal decay non-destructively. Results are perturbed by flutes in the stem, as often occur close to ground level in larger trees.

 Pile: A deep slender foundation element installed by driving, augering or casting concrete into a bored hole. Foundation loads are transmitted to piles by means of a ground beam.

 Pier: A squat concrete or masonry column constructed below ground level used to support a ground beam.

 Pioneer: A species adapted to colonizing bare ground or disturbed sites, making it easier for other plants to colonize in a succession.

Pipe theory: The speculative proposition that ‘All the branches of a tree at every stage of its height when put together are equal in thickness to the trunk’. Since this regards the trunk as a collection of pipes that divide at a branch union, some going one way and some the other, it is taken to refer to an equivalence of cross-sectional area.

 Planning condition: Planning consent is sometimes granted subject to conditions, for example that trees lost in a development are to be replaced.

 Plant: On development sites, used to refer to site vehicles such as dumpers, diggers and cranes.

 Pollard: The decapitation of a tree at a certain height above ground level, in species adapted to this treatment. Traditionally above the browsing height of farm animals, now common in urban contexts. Re-growth typically develops from near the point or points of decapitation. Once a tree is pollarded it is common to re-pollard periodically to avoid over-dense crowns and high loadings on the (often complex) union between framework and re-growth. Alternatively, in a lapsed pollard, this complex architecture may be accepted indefinitely.

 Post-development pressure for tree works: When new development is so close to a tree or trees that the residents are anxious about tree hazard, shading etc., and put pressure on their Local Planning Authority for works (felling or pruning) to mitigate the anxiety.

 Probability: The likelihood that a hazard eventuates. See Terms associated with tree risk.

 Proof of evidence: A document provided by a witness to a legal representative or Inspector, setting out what the witness intends to say in advance of a hearing.

 Protection of Badgers Act 1992: It an offence to kill badgers or interfere with a badger sett.

Proximal: In relation to a particular point on a plant, that part towards the point of attachment, whether of a shoot or root, ie. away from the extremity. The opposite of distal.

Pruning: The cutting off or cutting back of shoots or branches from a tree, whether to direct growth (formative pruning), make safe, to remove an obstructing or diseased part, to increase longevity (veteran trees), to maintain productivity (fruit trees) etc.

Quantified tree risk assessment (QTRA): A scheme of tree risk assessment with emphasis on quantifying the component probabilities in tree risk.

Raft: A foundation (usually on soft ground) consisting of an extended layer of reinforced concrete in the form of a slab (ie. unsupported by strip foundations).

 Reaction wood: Atypical wood that grows in a region of the tree which experiences high mechanical stresses (in a leaning tree, at a branch origin, around a wound etc.). Reaction wood is relatively strong either in compression or tension to resist these stresses.

Resistograph: Technique for estimating the extent of internal decay by means of a small-diameter drill. The resistance to the drill bit is continuously recorded, low resistance suggesting advanced decay. Not entirely non-destructive since a tree’s natural defences are breached, potentially allowing the ingress of decay.

Resonant frequency: The natural frequency of swaying fo a tree or branch, the amplitude of which may be exaggerated or damped by gusts of wind depending on the interval between gusts. More or less damped by branches of different sizes, each with their own resonant frequency.

 Resupinate: The growth habit of some fungi of spreading as a layer over the host surface rather than forming a bracket or toadstool.

Root flare: The outwardly curving base of a tree where it joins the roots, often distinguishable as individual root buttresses.

Root graft: The roots of neighbouring trees of the same species or cultivar grow adjacent to each other and eventually fuse naturally (see graft union), allowing the exchange of water and nutrients between them.

 Root plate area: The area needed by a tree’s root system to keep the tree stable; broadly, that part of the root system displaced when a tree is uprooted.

 Root Protection Area (RPA): The (usually) circular area around the base of a tree, expressed in square metres (m2), that contains sufficient rooting volume to ensure the future well-being of the tree in the event of nearby soil disturbance (as on a development site). RPA radius is the radius of the circle in metres (m). The RPA is a common constraint to development when space is limited. It is calculated according to guidelines in BS 5837 (2005).

 Rootstock: In grafted trees, that part below the graft union that gives rise to the root system. See scion.

 Roundel: A group of trees planted as a landscape feature in parkland, popular in the 18th C. Often composed of native deciduous trees 'nursed' by an interplanting of conifers, which were eventually thinned or removed.

Safe: Not significantly weakened by any visible defect. A ‘safe’ tree is not absolutely safe, but its design allows for strength well beyond the loadings normally encountered (Safety factor).

Safety factor: Ratio between breaking stress and average stress, estimated in undefective trees to be in the region of 4.5 (not dissimilar to the safety factors built into man-made structures).

 Sail area: The area of a tree’s crown that intercepts wind, broadly varying with wind direction, leafiness etc.

 Sand: Mineral soil particles in the size range 0.06mm to 2mm (from fine to coarse). Sand feels gritty. Sandy soils are often deep and free-draining, but are likely to dry out quicker than soils with finer mineral particles (clay or silt).

 Sapwood: The outer wood of a branch or main stem which is typically pale (in contrast to the often coloured inner heartwood). It is living and its functions include transport within the plant (of water etc.), the storage of food reserves (especially in the dormant season) and actively resisting infection, as well as contributing to the mechanical support of the tree. 

 Schedule of works: A written description of tree works, the usual basis for soliciting quotations and establishing a tree work contract with a tree surgeon. Sometimes called a ‘specification’.

 Scion: In grafted trees, that part above the graft union that gives rise to the aerial part of the tree, typically desired for fruit, ornament etc.

Semi-natural woodland: Woodland of native species whose character is determined both by human influence and by evidence that the site supported pre-existing natural woodland.

 Senescent: The late stage of a tree’s life characterized by a decline in crown and root volume.

 Settlement: Downward movement of soil under load, especially when compressed by newly-built foundations.

 Shaw: A strip of woodland usually between 5m and 15m wide, commonly forming a boundary or lining a road. Shaws may be remnants of larger areas of woodland or may have developed from unmanaged hedgerows.

 Shear stress: The force tending to make one part of a solid slide past another. Resistance to shearing increases with the pressure on the shearing faces, so that the shear strength of soil resisting the fall of a tree increases with the weight of the tree.

Shelter: A site factor tending to enhance tree growth, characterized by low windiness.

 Shelterbelt: A linear planting of trees, typically perpendicular to the prevailing wind, whose purpose is to provide shelter, primarily a reduction in wind speed. The calming effect can extend up to 50 times the height of the belt to leeward, 10 times the height to windward, and is best developed when the belt is uniformly partly porous to air, minimizing turbulence.

Shrinkable soil: Soils that shrink on drying (and swell on re-wetting) are shrinkable. Several kinds of stiff clay have high shrinkability.

 Significant (of defects): Of any tree defect that does not meet accepted standards of safety, and which therefore requires action to mitigate tree risk.

 Silt: Mineral soil particles in the size range 0.002mm to 0.06mm, intermediate in size between clay and sand.

Site of Special Scientific Interest (SSSI): A conservation designation in the U.K. governed in England by the Wildlife and Countryside Act 1981 (as amended by the Countryside and Rights of Way Act 2000), which protects an area of land considered to be of special interest by virtue of its plants, animals or geological features.

Site factors: The soil, climate, topography and vegetation of a site in relation to tree growth. Site factors are numerous and interactive.

Soil: A mixture of mineral particles, often of various sizes due to weathering, roots and other living things, soil organic matter and the associated voids (pores) filled with air and/or water.

Soil aeration: The movement of gases in soil, primarily by diffusion through the soil pores. For example, oxygen diffuses from the atmosphere to the vicinity of the plant root while carbon dioxide diffuses in the opposite direction. The rate of diffusion is related to the proportion of the soil volume that contains air

Soil compaction: An increase in bulk density due to the pressure exerted by animals, vehicles, (locally) by root growth etc. Pore space is reduced, which may also restrict soil aeration, water infiltration and drainage.

 Soil-forming factors (main ones): Climate (especially temperature and rainfall), parent material, topography, organisms and time.

 Soil organic matter: Once-living material from plants (roots, fallen leaves etc.), animals, fungi etc., once it has decomposed to the extent that it is amorphous. The end product, that resists further decomposition, is called humus. High soil organic matter content is associated with good fertility.

Soil poaching: The physical breakdown of soil under load, for example from the passage of heavy animals or vehicles. Soil strength decreases with wetness.

 Soil structure: The aggregation of soil particles into clumps (peds) of various shapes and the associated spaces between them, affecting many properties of soil including its porosity to air and water, and its fertility.

 Soil texture: The size of the mineral particles in the soil, classified (from fine to coarse) as clay, silt, sand, gravel or stones, or some mixture of these to give a characteristic particle size distribution. Sandy soils give a light texture, clayey soils give a heavy texture.

 Sooty mould: A fungus growing on honeydew, which may block light (when growing on leaves) but is not directly harmful to the tree. More prevalent in dry years, when the honeydew is not washed off by rain.

 Sphaeroblast: A spheroid woody, bark-covered lump, characteristic on beech. Arises over many years from the abnormal development of a bud which produces wood without elongating. Not harmful.

Stag-head: The outer branches of a tree that have died and now tend to protrude beyond the present extent of the crown. May be indicative of a progressive decline or of a past injurous episode (for example due to summer drought or insect infestation) from which the tree has since recovered.

 Standard: A tree standing taller than adjoining or surrounding woody vegetation, such as a hedgerow or coppice woodland.

Static integrated assessment (SIA): Method for estimating the resistance of a stem to breakage based on diameter, height, sail area and  the (species-dependent) strength of wood. Leaves out of account the possibility of growth stresses, variation in wood strength due to adaptive growth and the dynamic behaviour of trees in a high wind.

 Static integrated method (SIM): Method for estimating the stability of a tree’s root system by means of a pulling test, with attendant observation of tilt and curvature of the trunk. Commonly used in continental Europe. Assumes that all forms of decay or weakness are equally manifested as a reduction in the stiffness of wood. 

Statutory law: That part of English law created by Acts of Parliament, mostly made since the 1830s.

Stop notice: If a Local Planning Authority considers that there has been a breach of a planning condition providing for the protection of trees, and an enforcement notice has been ineffective, it may serve a notice under the Town and Country Planning Act 1990 to stop site works.

Storm damage: In high winds, individual branches are more liable to fail (under high twisting stresses) than the whole tree. In consequence, many old trees have branch wounds caused by storm damage. A tattered stub of any length may be left, or the branch can be pulled out at its origin, leaving a rough-edged cavity. In this definition the word 'storm' is used loosely to mean any sufficiently high wind to cause branches to break. See Beaufort scale.

 Strimmer injury: Damage to the bark near the base of a young tree, more or less serious depending on extent.

 Strip footing: A footing consisting of a layer of concrete, typically about 150mm thick.

 Stub: That part of a pruned branch protruding beyond the branch collar. It is not good practice to leave stubs since they impede occlusion and are prone to decay.

 Subsidence: Downward movement of ground and affected foundations. Tree-related building subsidence occurs when (a) roots grow under a building, and (b) the soil is shrinkable. The roots dry out the soil under the building, the soil shrinks and the building subsides, typically creating cracks. There are many other causes of cracks in buildings.

Suckers: Shoots arising from the roots of a tree, which can arise surprisingly far from the parent (48m was recorded for one ‘Tree of Heaven’).

 Sudden oak death: A fungus, first identified in California in 2000, with about 40 host species (not just oaks). Depending on the host it may appear as a lethal stem canker (associated with bleeding from the bark) or be confined to the foliage and shoots. In England, the most susceptible tree species appear to be American red oak and beech, while the native oaks are not thought to be very susceptible.

SUDS - SUstainable Drainage Systems. SUDS are designed to manage surface runoff, helping to prevent flooding and pollution, and often include trees, which increase the interception and infiltration of rainwater.

Summer branch drop: The shedding of large limbs, typically those with high end loading from late-mature trees in summer. It can occur in calm conditions. The wood is thought to be weakened by stress fractures (due to drought) or incipient decay, and may follow rain.

Target: A target is the subject of injury or damage within range of a hazard. See Terms associated with tree risk.

 Target area: The area reachable by (eg. within the falling distance of) a hazard. See Terms associated with tree risk.

Tensile stress: A pulling force, as is exerted longitudinally on the lower side of a beam or the windward side of a tree trunk bending in the wind. See compressive stress.

 Tension sling: A root configuration, said to be rare, in which the roots grow round a drain and gain support from it, transferring load to the drain and liable to break it.

 Tension wood: The kind of reaction wood found in hardwoods which is strong in tension and is characterized by a low lignin content, laid down on the upper side of the affected part (such as a branch) to help support it.

Terms associated with tree risk: A hazard has the capacity to cause harm, which may be more or less serious depending on the magnitude of the hazard (for example the weight of a branch and the height from which it would fall). Probability is the likelihood that a hazard eventuates, and is influenced by the loading at potential points of structural failure and the extent and effectiveness of any adaptive growth. A target is the subject of injury or damage within range of a hazard, ie. within the target area. The occupancy of the target area varies if targets are not fixed (people, animals, vehicles etc.). 

Thinning: May refer to crown thinning or tree thinning.

 Topography: Broadly, the description of place; usually confined to landforms.

Topping: A kind of pruning in which the branches of a tree are all decapitated to reduce the tree to a specific height. An indiscriminate form of pruning not regarded as good practice, to which some trees, such as beech, birch and most conifers, are intolerant. See Crown thinning …

 Torsion: A twisting force and common cause of failure of branches in a high wind, giving rise to storm damage.

 Training: To change the shape of a tree by means other than (formative) pruning, typically by tying young branches into a particular position. Commonly applied to young fruit trees when a low, spreading form is desired.

 Tree constraints plan: Site plan showing the tree-related constraints to development as envisaged in BS 5837 (2005). Common constraints are the trees’ Root Protection Areas and crown radii, and the trees’ influence in relation to light interception and post-development pressure for tree works.

 Tree inspection:  A procedure to describe a tree, including its species identity, maturity, health, dimensions etc., and to identify and evaluate any defects. Some obligation to inspect trees periodically is implied in the duty of care.

 Tree preservation order (TPO): An order made by the Local Planning Authority to protect a tree under the Town & Country Planning Act (1990), as amended, usually on the grounds of public amenity. The owner of a TPO tree has to seek prior approval from the LPA for any proposed works to a TPO tree.

 Tree protection plan: That part of the BS 5837 (2005) procedure intended to ensure the wellbeing of trees on a development site through the provision of protective measures, including protective fencing to create Construction exclusion zones, surfaces for pedestrian and vehicular access to protect roots etc. Complements the Arboricultural method statement.

 Tree survey: A survey to establish the dimensions, position, species, age category, condition and context of the tree or trees of interest. A fundamental of tree advisory practice, informing the discussion and recommendations that follow. The aim may be to evaluate tree safety, to help decide on an appropriate course of action or, in relation to development sites, to satisfy the first stage of the procedure envisaged in BS 5837 (2005).

 Tree risk: A composite of (hazard + magnitude + probability + target value + occupancy). Note the broad definition (in contrast to probability, which in this context is defined narrowly). See Terms associated with tree risk.

 Tree risk assessment: There are various approaches or schemes for assessing tree risk. Many give a categorical or numerical score intended as a defensible basis for a course of action, or to justify no action.

 Tree surgeon: A contractor skilled in climbing and performing aerial operations in trees, as is often necessary near buildings, roads etc. Reputable tree surgeons have adequate public liability insurance, are able to work to BS 3998 (1989) and can work to a written Schedule of works as appropriate.

Tree thinning: The removal of some trees from a closely planted group, after some years of growth, to free the remainder from competition, allowing the latter to continue growing and to attain their mature size.

 Trench-fill foundation: A foundation consisting of a trench about 400mm wide filled with mass concrete.

 Trenchless technology: Various techniques involving straight-line bores launched from one pit and received in another, as might be appropriate in installing services beneath a root system.

 t/R ratio: In hollow trees, the ratio of the thickness of sound wood to the radius of the tree. Must be evaluated in relation to the context, but as a generality a ratio lower than 0.3 is associated with an increased risk of structural failure.

 Unbalanced crown: A crown that is much more extensive in one direction than another, often due to competition from the crown of a nearby tree.

 Underpinning: The extension of existing foundations, in depth or width, so that they either rest on more supportive subsoil or distribute their load across a greater area. A relatively drastic and expensive remedy for serious structural movement in buildings. Other remedies include strengthening the structure above-ground, drainage repairs, reducing any influence from nearby vegetation and stabilizing the soil.

 Understorey: Woody plants growing naturally beneath the canopy of a woodland, often of shrubby form and suitable for planting in hedgerows.

 Veteran: A veteran tree is so old as to be declining, but has the additional connotation of a ‘battle-scarred survivor’, and as such it has features which increase the tree’s value as habitat for wildlife.

Visual Tree Assessment (VTA): A systematic approach to tree risk assessment developed by Mattheck & Breloer (The body language of trees: A handbook for failure analysis ISBN 0-11-753067-0), consisting of the diagnosis of defects and the evaluation of their significance from visual signs and the application of biomechanical criteria.

 Water demand (in relation to subsidence): The ability to cause drying of a clay subsoil, which integrates leaf area with the radial and vertical distribution of roots, some tree species having a characteristically wider and/or deeper distribution of roots than others.

 Wildlife and Countryside Act 1981: Provides protection for native wildlife, together with a European directive enacted in the Conservation (Natural Habitats etc.) Regulations 1994, including otter, great crested newt, water vole, dormouse and all species of bats.

Windbreak: Any obstacle to the wind intended to reduce its force, which may consist of a hedge or line of trees. See shelterbelt.

Windthrow: The fall of a tree in a high wind by the slippage of the root plate and breakage of the outer roots, so that the tree is uprooted. 

Roundel

A group of trees planted as a landscape feature in parkland, popular in the 18th C and favoured by Brown and Repton. Often composed of native deciduous species ‘nursed’ by an interplanting of conifers which was eventually thinned or removed.