Glossary of Arboriculture Terms

Much of the terminology used by arboricultural consultants is necessary and unambiguous, but often these terms are not encountered elsewhere. So here is a glossary of common terms used in our reports and site visits.

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Abscission

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

Abiotic

Pertaining to non-living agents; e.g. environmental factors.

Absorptive roots

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.

Adaptive growth

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. This helps to maintain a uniform distribution of mechanical stress.

Adaptive roots

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

Adventitious shoots

Shoots that develop other than from apical, axillary or dormant buds; see also ‘epicormic’.

Anchorage

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.

Architecture

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

Axil

The place where a bud is borne between a leaf and its parent shoot.

Bacteria

Microscopic single-celled organisms, many species of which break down dead organic matter, and some of which cause diseases in other organisms.

Bark

A term usually applied to all the tissues of a woody plant lying outside the vascular cambium, thus including the phloem, cortex and periderm; occasionally applied only to the periderm or the phellem .

Basidiomycotina (Basidiomycetes)

One of the major taxonomic groups of fungi; their spores are borne on microscopic peg-like structures (basidia), which in many types are in turn borne on or within conspicuous fruit bodies, such as brackets or toadstools. Most of the principal decay fungi in standing trees arebasidiomycetes.

Bifurcation

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

A term sometimes used to describe pollard heads.

Bottle-butt

A broadening of the stem base and buttresses of a tree, in excess of normal and sometimes denoting a growth response to weakening in that region, especially due to decay involving selective delignification.

Bracing

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

Branch:

• 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

The raised arc of bark tissues that forms within the acute angle between a branch and its parent stem.

Branch collar

A visible swelling formed at the base of a branch whose diameter growth has been disproportionately slow compared to that of the parent stem; a term sometimes applied also to the pattern of growth of the cells of the parent stem around the branch base.

Brown-rot

A type of wood decay in which cellulose is degraded, while lignin is only modified.

Buckling

An irreversible deformation of a structure subjected to a bending load Buttress zone. 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

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

Canker

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

Canopy species

Tree species that mature to form closed woodland canopy.

Cleaning out

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

Compartmentalisation

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.

Compression strength

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

Compressive loading

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

Condition

An indication of the physiological vitality 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

Area based on the Root Protection Area (in square metres) to be protected during development, by the use of barriers and/or ground protection.

Crown/Canopy

The main foliage bearing section of the tree, these terms are interchangeable.

Crown lifting

removal of limbs and small branches to a specified height above ground level.

Crown thinning

The removal of a proportion of secondary branch growth throughout the crown to produce an even density of foliage around a well-balanced branch structure.

Crown reduction/shaping

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

Crown reduction/thinning

Reduction of the canopy volume by thinning to remove dominant branches whilst preserving, as far as possible the natural tree shape.

Deadwood

Branch or stem wood bearing no live tissues. Retention of deadwood provides valuable habitat for a wide range of species and seldom represents a threat to the health of the tree. Removal of deadwood can result in the ingress of decay to otherwise sound tissues and climbing operations to access deadwood
can cause significant damage to a tree. Removal of deadwood is generally recommended only where it represents an unacceptable level of hazard.

Decurrent

In trees, a system of branching in which the crown is borne on a number of major widelyspreading limbs of similar size (cf. excurrent).
In fungi with toadstools as fruit bodies, the description of gills which run some distance down the stem, rather than terminating abruptly.

Defect

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

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

Dieback

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

Disease

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

Distal

In the direction away from the main body of a tree or subject organism (cf. proximal).

Dominance

In trees, the tendency for a leading shoot to grow faster or more vigorously than the lateral shoots; also the tendency of a tree to maintain a taller crown than its neighbours.

Dormant bud

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

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

DBH (Diameter at Breast Height)

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.

Endophytes

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

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

Excrescence

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

Excurrent

In trees, 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 (cf. decurrent).

Felling licence

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

Flush-cut

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

Girdling root

A root which circles and constricts the stem or roots possibly causing death of phloem and/or cambial tissue.

Guying

A form of artificial support with cables for trees with a temporarily inadequate anchorage.

Habit

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

Hazard beam

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

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

Heave

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; also the lifting of pavements and other structures by root diameter expansion; also the lifting of one side of a wind-rocked root-plate.

High canopy tree species

Tree species having potential to contribute to the closed canopy of a mature woodland or forest.

Incipient failure

In wood tissues, a mechanical failure which results only in deformation or cracking, and not in the fall or detachment of the affected part.

Included bark (ingrown bark)

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

Increment borer

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

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

Internode

The part of a stem between two nodes; not to be confused with a length of stem which bear nodes but no branches.

Lever arm

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

The hard, cement-like constituent of wood cells; deposition of lignin within the matrix of cellulose microfibrils in the cell wall is termed Lignification.

Lions tailing

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 endloading.

Loading

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

Along the length (of a stem, root or branch).

Lopping

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

Mature Heights (approximate):

• Low maturing – less than 8 metres high
• Moderately high maturing – 8 – 12 metres high
• High maturing – greater than 12 metres high

Microdrill

An electronic rotating steel probe, which when inserted into woody tissue provides a measure of tissue density.

Minor deadwood

of a diameter less than 25mm and or unlikely to cause significant harm or damage upon impact with a target beneath the tree.

Mulch

Material laid down over the rooting area of a tree or other plant to help conserve moisture; a mulch may consist of organic matter or a sheet of plastic or other artificial material.

Mycelium

The body of a fungus, consisting of branched filaments (hyphae).

Occluding tissues

A general term for the roll of wood, cambium and bark that forms around a wound on a woody plant (cf. woundwood).

Occlusion

The process whereby a wound is progressively closed by the formation of new wood and bark around it.

Pathogen

A micro-organism which causes disease in another organism.

Photosynthesis

The process whereby plants use light energy to split hydrogen from water molecules, and combine it with carbon dioxide to form the molecular building blocks for synthesizing carbohydrates and other biochemical products.

Phytotoxic

Toxic to plants.

Pollarding

The removal of the tree canopy, back to the stem or primary branches. Pollarding may involve the removal of the entire canopy in one operation, or may be phased over several years. The period of safe retention of trees having been pollarded varies with species and individuals. It is usually necessary to repollard
on a regular basis, annually in the case of some species.

Primary branch

A major branch, generally having a basal diameter greater than 0.25 x stem diameter.

Primary root zone

The soil volume most likely to contain roots that are critical to the health and stability of the tree and normally defined by reference to BS5837 (2005) Trees in Relation to Construction Recommendations.

Priority

Works may be prioritised, 1. = high, 5. = low.

Probability

A statistical measure of the likelihood that a particular event might occur.

Proximal

In the direction towards from the main body of a tree or other living organism (cf. distal).

Pruning

The removal or cutting back of twigs or branches, sometimes applied to twigs or small branches only, but often used to describe most activities involving the cutting of trees or shrubs.

Radial

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

Rams-horn

In connection with wounds on trees, a roll of occluding tissues which has a spiral structure as seen in cross-section.

Rays

Strips of radially elongated parenchyma cells within wood and bark. The functions of rays include food storage, radial translocation and contributing to the strength of wood.

Reactive Growth/Reaction Wood

Production of woody tissue in response to altered mechanical loading; often in response to internal defect or decay and associated strength loss (cf. adaptive growth).

Removal of dead wood

Unless otherwise specified, this refers to the removal of all accessible dead, dying and diseased branchwood and broken snags.

Removal of major dead wood

The removal of, dead, dying and diseased branchwood above a specified size

Respacing

Selective removal of trees from a group or woodland to provide space and resources for the development of retained trees.

Residual wall

The wall of non-decayed wood remaining following decay of internal stem, branch or root tissues.

Root-collar

The transitional area between the stem/s and roots.

Root-collar examination

Excavation of surfacing and soils around the root-collar to assess the structural integrity of roots and/or stem.

Root protection area (RPA)

An area of ground surrounding a tree that contains sufficient rooting volume to ensure the tree’s survival. Calculated with reference to BS5837 (2005).

Root zone

Area of soils containing absorptive roots of the tree/s described. The Primary root zone is that which we consider of primary importance to the physiological well-being of the tree.

Sapwood

Living xylem tissues.

Secondary branch

A branch, generally having a basal diameter of less than 0.25 x stem diameter.

Selective delignification

A kind of wood decay (white-rot) in which lignin is degraded faster than cellulose.

Shedding

In woody plants, the normal abscission, rotting off or sloughing of leaves, floral parts, twigs, fine roots and bark scales.

Silvicultural thinning

Removal of selected trees to favour the development of retained specimens to achieve a management objective.

Simultaneous white-rot

A kind of wood decay in which lignin and cellulose are degraded at about the same rate.

Snag

In woody plants, a portion of a cut or broken stem, branch or root which extends beyond any growing-point or dormant bud; a snag usually tends to die back to the nearest growing point.

Soft-rot

A kind of wood decay in which a fungus degrades cellulose within the cell walls, without any general degradation of the wall as a whole.

Spores

Propagules of fungi and many other life-forms; most spores are microscopic and dispersed in air or water.

Shrub species

Woody perennial species forming the lowest level of woody plants in a woodland and not normally considered to be trees.

Sporophore

The spore bearing structure of fungi.

Sprouts

Adventitious shoot growth erupting from beneath the bark.

Stem/s

The main supporting structure/s, from ground level up to the first major division into branches.

Stress

In plant physiology, a condition under which one or more physiological functions are not operating within their optimum range, for example due to lack of water, inadequate nutrition or extremes of temperature.

Stress

In mechanics, the application of a force to an object.

Stringy white-rot

The kind of wood decay produced by selective delignification.

Storm

A layer of tissue which supports the fruit bodies of some types of fungi, mainly ascomycetes.

Structural roots

Roots, generally having a diameter greater than ten millimetres, and contributing significantly to the structural support and stability of the tree.

Subsidence

In relation to soil or structures resting in or on soil, a sinking due to shrinkage when certain types of clay soil dry out, sometimes due to extraction of moisture by tree roots.

Subsidence

In relation to branches of trees, a term that can be used to describe a progressive downward bending due to increasing weight.

Taper

In stems and branches, the degree of change in girth along a given length.

Target canker

A kind of perennial canker, containing concentric rings of dead occluding tissues.

Targets

In tree risk assessment (with slight misuse of normal meaning) persons or property or other things of value which might be harmed by mechanical failure of the tree or by objects falling from it.

Topping

In arboriculture, the removal of the crown of a tree, or of a major proportion of it.

Torsional stress

Mechanical stress applied by a twisting force.

Translocation

In plant physiology, the movement of water and dissolved materials through the body of the plant.

Transpiration

The evaporation of moisture from the surface of a plant, especially via the stomata of leaves; it exerts a suction which draws water up from the roots and through the intervening xylem cells.

Understorey

A layer of vegetation beneath the main canopy of woodland or forest or plants forming this.

Understorey tree species

Tree species not having potential to attain a size at which they can contribute to the closed high canopy of a woodland.

Vascular wilt

A type of plant disease in which water-conducting cells become dysfunctional.

Vessels

Water-conducting cells in plants, usually wide and long for hydraulic efficiency; generally not present in coniferous trees.

Veteran tree

A loosely defined term for an old specimen that is of interest biologically, culturally or aesthetically because of its age, size or condition and which has usually lived longer than the typical upper age range for the species concerned.

White-rot

A range of kinds of wood decay in which lignin, usually together with cellulose and other wood constituents, is degraded.

Wind exposure

The degree to which a tree or other object is exposed to wind, both in terms of duration and velocity.

Wind pressure

The force exerted by a wind on a particular object.

Windthrow

The blowing over of a tree at its roots.

Wound dressing

A general term for sealants and other materials used to cover wounds in the hope of protecting them against desiccation and infection; only of proven value against fresh wound parasites.

Woundwood

Wood with atypical anatomical features, formed in the vicinity of a wound.

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