More Arboricultural Terms

Aerial inspection A more thorough investigation than is possible from ground-level of apparent defects in the tree’s canopy detected as a result of a Visual Tree Assessment (VTA – see below), which may be carried out from a rope and harness, ladder, mobile elevating work platform or other suitable equipment to allow the inspector to examine the defect from close quarters.  It is important that the inspector is competent to both carry out the practical task and to analyse and determine the significance of any defects found.
AFAG Arboriculture And Forestry Advisory Group – the body charged by the HSE with producing industry best practice guidance for the forestry and arboriculture industries.
Allelopathic A term used to describe plants which exude toxins into the environment around them, preventing the germination of other plants and killing smaller specimens of competing plant species
Apical The leading shoot/tip of a branch or root which lengthens as a result of primary growth.
Base That section of a tree stem from ground level to 1m.
Basal  Referring to the section of the stem at, and immediately above, ground level.
Bifurcation A union of two upright scaffold limbs that may or may not incorporate included bark.  Such unions may be described as tension* or compression* forks.
Bolling The swollen area on the stem of a ‘pollard’ tree (see below) to which the canopy is cut back on a regular cycle.  This area should be maintained intact at each cutting.
Branch stub Section of a branch, distal to the branch collar, which has been left on the tree following pruning.  It is bad pruning practice as it may increase growth of epicormic shoots and / or the likelihood of decay fungi invading the stem or scaffold from which the branch arises.
Brashing Removal of (usually dead) branches from the lower stems of plantation-grown conifers.
Burrs Swollen areas of disrupted tissue on the stem and sometimes also scaffolds. Often occurring on oak and elm, they pose no problem for the health or structural strength of the affected tree.
Buttress roots The flared area where the roots merge with the stem base.
Butt sweep/ swept Where the base of a tree emerges from the root flare at an angle for a short distance before turning abruptly to the vertical. It indicates that the tree fell over when young but subsequently stabilised itself.
Cambium A layer of actively dividing cells lying just under the bark which forms wood to the inside and bark to the outside.
Canker  Disruption of the bark and underlying tissues, usually caused by a fungal or bacterial organism, leading to decay and / or death of affected stem(s)
Canopy/crown The limbs and branches of a tree from above the stem or bole.
Cavity A void, with an external opening, within a tree stem or scaffold which represents an advanced stage of decay caused by fungal organisms.
Cellweb A load support system, produced by Geosynthetics Ltd., comprising a 3-dimensional mattress of interconnecting, perforated cells and a Mot Type 1 granular material as infill, laid over a layer of geotextile separation fabric.
Cobra Brace A flexible and non – invasive cable – bracing system for supporting mechanically weak branch structures within the crown of a tree.
Compression fork A non-shape optimised branch union, often associated with included bark, which is considered a structural defect.
Conservation or Fracture pruning Making pruning cuts in such a way as to encourage tearing and splitting of the branch below the pruning point to create the illusion that the branch has broken naturally and also to expose a larger amount of underlying tissue to encourage the production of adventitious shoots which will re-form a more compact and stable canopy.
Coppice The practice of cutting trees close to ground level without killing them.  New stems form  from the resultant epicormic* shoots.  (*See below)
Crossing/Abrading stems In trees with dense canopies, it is not unusual for stems or branches to come into permanent contact with one another; wind action can cause such limbs to chafe, which may eventually do sufficient damage to cause one or both of them to fail at that point.
Crotch Junction of two branches or of a stem and a branch.
Crown die – back An accumulation of dead twigs and small branches at the periphery of the canopy, often associated with impaired root-function.
Cultivar A named clonal variety of a species vegetatively propagated to preserve some special feature such as form, or leaf colour, texture or shape.
Dominant A tree which is significantly larger than adjacent specimens in woodland; smaller specimens may be suppressed as a result.
Disrupted bark A change in the normal pattern of bark for the species, which may indicate fungal or bacterial infection (e.g. cankering), localised, increased mechanical stresses or simply genetic variation.
Epicormic Shoots arising from dormant buds which, when very dense on tree stems, can make tree inspection difficult, as they may mask defects.  Having only a shallow attachment to the underlying tissues, they can become unstable as they lengthen.
Exudation(s) Fluids, often dark and smelly, emanating from the bark, may indicate the presence of internal decay.
Formative Pruning Pruning carried out on a tree to improve its shape/structure and to remove any dead, damaged, or diseased material, including climbing plants and any made materials, e.g.  Cable supports.
Girdled/girdling Objects (typically fence-wire and tree-ties) placed or wrapped around a tree stem become tighter as the growing stem expands and may eventually ‘strangle’ the tree, causing death of all tissues above the point of constriction.
Grafted  tree One produced in the nursery by attaching a shoot from a particularly desirable form or
species to a rooted stem (‘rootstock’), often of a different species.
Harp branch A largely horizontal lateral branch* that has one or more upright / vertical branches growing from it at 90°.  A phenomenon frequently seen in large, old conifers.
Heave The result of soil re-wetting following a period of severe moisture deficit.  Often occurring when trees are removed from shrinkable soils causing the ground to swell, or ‘heave’.
Heave(d) Swelling of soil surface close to the base (and on the top side) of a leaning tree, indicating that the roots have moved and that the tree may be unstable.
Heritage tree A tree which has one or more of the following attributes: great age or size for the species; rarity; exceptional/unusual form and/or attractiveness; original introduction into the country; original named variety; great ecological value; significant cultural/historic associations. The term ‘heritage’ tree is generic and includes other designations, such as ‘veteran’ (see below) and ‘ancient’.
Honeydew A sticky, sugary liquid excreted by aphids and formed from the sap of plants on which they are feeding.
Included bark union(s) Upright limbs, forking at acute angles, with a tendency to force one another apart as their diameters increase due to secondary thickening. Branch-loss often results.
Ivy (Hedera helix) Woody plant which sometimes climbs into trees to gain more light.  If its growth is sufficiently rampant, it may out-grow its host and kill it through shading or cause breakage through its added weight.  It may also hide defects in the tree and make inspection difficult.
Lateral branch A branch extending more or less horizontally from an upright stem.
Leader(s) The shoot(s) forming the apex of a tree.
Lopping The internodal pruning of either single, several or all stems, scaffolds, and / or branches of early-mature and mature trees, which causes large wounds, tissue die-back, extensive decay and production of epicormic shoots.  If the developing epicormic shoots are not subsequently removed on a regular basis, they are likely to become unstable as a result of their weak attachment to decaying stems, scaffolds, and / or branches.  Lopping should not be confused with Pollarding (see below).
Maiden A tree that has not been pollard or coppice managed.
Mast The fruits or nuts of the beech which in some years (known as ‘mast years’) are produced in great quantities.
Monitor Where the specified action is to ‘monitor’ a condition or defect identified in the ‘condition’ column, this indicates that a higher level of inspection is required than that which is recommended for trees in general.
Monolith A stable tree stem, perhaps with short stubby limbs, which has been created or retained primarily as wildlife habitat.
Mulch A material, usually (but not necessarily) organic, laid down over the soil surrounding a tree to create better rooting / growing conditions for the tree.
Natural regeneration Trees which have arisen from the germinating seeds of existing trees with no human involvement.  The locations of the resultant trees will be hap-hazard, rather than planned, as would be the case with planted trees.
Necrosis / Necrotic Dead or dying plant tissue, identified as such by its unhealthy colour / appearance.
Phoenix A tree which has formed a new, upright canopy after having fallen over.
Occluded Where a wound is completely covered by the formation of wound - wood, it is said to be “occluded”.  At this point decay processes within the wound will normally cease.
Picus A technical, non - invasive instrument, which detects and maps internal decay in trees by means of sonic tomography.
Poaching The effect of the passage of the hooves of herbivores across tree root systems causing ground surface erosion, sub surface compaction and the physical removal of bark from surface roots – can also be caused by pedestrian and mechanical traffic.
Pollard(ing) An ancient system of managing trees to produce repeated crops of small-diameter timber at a height where browsing animals cannot reach the new shoots.  It involves the removal of the entire canopy of a young tree back to the main stem, usually at a height of around 3 metres. The resultant new growths are removed cyclically when they have developed sufficiently to provide usable timber.  If the process is begun early in the tree’s life, and re-cutting is performed regularly, the resultant wounds are small, decay is kept to a minimum and new shoots are not allowed to develop to a point at which they may become unstable (cf. ‘Lopping’ and ‘Topping’ ).  As a result, ‘pollarded’ trees, if properly maintained throughout their lives, can live far beyond the normal life-span for their species.
Recumbent Descriptive term for a tree-stem which is lying on the ground – it may or may not be the result of partial root-plate failure.
Re-generation The uncontrolled, natural colonisation of trees and other plants which occurs in the absence of heavy grazing pressure and human activity (e.g. mowing).
Ring-barking A situation in which an area of bark has been removed from the stem of a tree which completely encompasses the stem.  Death of the parts of the tree above the ring-barked area will result.
Root flare / Buttresses / Collar Swollen area at the base of the tree where the stem merges with the roots at the soil surface.
Root-plate The area of soil and roots around a tree which is up-turned when a tree is up-rooted - usually by wind action.
Root-stock In trees produced by ‘grafting’ or ‘budding’ to produce a particular form of plant (eg; weeping) which will not come true from seed. The part of the tree below the point of grafting / budding is called the root-stock and will have a different form from the plant material (‘scion’) grafted / budded onto it.  If growth is allowed to develop from the root-stock it will alter the desired appearance of the tree as the root-stock is normally more vigorous than the scion. 
Root-sucker Aerial stem arising from the root system at a distance from the parent tree, which, if not
removed, is likely to grow into a full-sized tree.
Root-zone / Area The area of soil around a tree in which the anchoring and feeding roots of a tree are found.  This area may extend to a distance away from the tree much greater than the height of the tree.
Sail-effect The canopy of a tree, especially when in full leaf, acts like the sail of a ship, gathering the wind and transferring the wind-loading through the twigs and branches into the stem and roots where it is normally dissipated harmlessly into the soil.  An external mechanical loading such as this sets up internal stresses in the tree; if the loading is severe, it may cause a structural weakness present in any part of the tree to fail.
Sap-wood The outermost regions of woody branches, roots and stems responsible for the transportation of water and nutrients throughout the plant.
Scaffold branch One of the major branches which form the main structural framework of a tree’s crown.
Secondary thickening Increase in diameter of stems, branches and major roots caused by the laying down of concentric, annual rings of wood.
Self-set Trees which have arisen naturally as seedlings, rather than having been deliberately planted: as a result they are often inappropriate in species and / or location.
Shoot extension growth During each growing season in temperate climates the growth buds on a woody plant burst and expand as a woody shoot.  The remains of the bud which initiated this growth remain visible for several years as a swollen area on the twig.  At the end of the growing season this growth stops and a new terminal bud is formed.  The length of the shoot produced in each of several successive growing seasons can therefore be measured and this gives an indication of the vitality of the tree and shows any changes that have occurred (cf Apical).
Silvicultural thinning Removal of a stated proportion of (normally smaller or poorer quality) trees in woodland to improve the development of the remainder by providing more space and light.
Single/singling The act of cutting to the ground all the stems of a multi-stemmed tree except the strongest/straightest one.
Soil-moisture deficit The status of water in a soil between ‘Field Capacity’ and ‘Permanent Wilting Point’; this phenomenon is best illustrated in soils with a high clay content.
Split(s) Stems, scaffolds and branches may develop internal, longitudinal cracks as a result of excessive loading.  Such defects may be on one side only or right through the affected part.  They may or may not be associated with decay.  Even where decay is not present, internal splits can be very unstable and are generally regarded as a more significant structural weakness than decay alone.
Sporophore Fungal fruiting body, which in wood-decaying species is often bracket-shaped. It is the part of the fungus which is most easily observable. It may be woody and perennial or produced annually, when it is normally soft and easily destroyed by frost.
Stag-heading A late stage in crown dieback (see above), when large dead branches protrude from the top of the canopy.  It may indicate root death and possible instability.
Stool The swollen base of a coppice from which the new shoots arise after cutting.
Stub Portion of branch left attached to stem / scaffold following incorrectly performed pruning or branch breakage- usually leading to decay / production of epicormic shoots (see above).
Stump grind The process in which a mechanical grinder is employed to convert woody stumps and roots into a woodchip-like material.  This type of operation is often used to grind stumps below the level of the surrounding substrate to facilitate re-surfacing works.
Sub-dominant Term used to describe an individual in a group of trees which has not kept pace with the growth of the most vigorous trees in the group but whose growth is not entirely suppressed (see below) by them.
Sucker Strong shoot, similar to an epicormic shoot, but arising from the roots.
Tear-out wound Wound resulting from a stem or branch becoming detached from the main stem at its junction but leaving no stub (see below).  Extensive decay is likely to result.
Tension fork A shape-optimised branch union usually a wide stem or branch fork, which is structurally strong (cf. included bark union or compression fork).
Topping The intermodal pruning of stems and / or upright scaffolds and branches of early-mature and mature trees which causes large wounds, tissue die-back, extensive decay and production of epicormic shoots.  If the developing epicormic shoots are not subsequently removed on a regular basis they are likely to become unstable as a result of their weak attachment to decaying stems, scaffolds and branches.  Topping should not be confused with Pollarding (see above).
Translocated The mode of action of a type of herbicide which is absorbed by plants and moves through their conductive tissues to kill the roots.
Underpruning Pruning taking the form of raising, by removal of lateral branches or limbs, the lower canopy on one side of a tree to allow the safe passage of conductors beneath a tree’s upper canopy.
Veteran tree Chiefly classified thus for the richness of its habitat, particularly its dead-wood habitat; frequently (but not necessarily) a tree which is exceptionally old for its species.
(Visual Tree Assessment)
A ground-based investigation looking for tree defects based on the principle that a tree is a self-optimising structure, which attempts to maintain even stress over its entire surface by preferentially adding wood to overloaded areas (weak points).  This additional wood shows up as abnormal bulges whose significance the VTA inspector is trained to determine through comparison with a normal (undamaged) tree. 
U-shaped union Wide stem or branch fork, which is structurally strong (cf. Included bark union).
Wind-snap Where tree branch or stem is broken by wind action (cf Windthrow).
Windthrow Where whole tree is uprooted by wind action.
The process and result of trees failing at the root collar or edge of root-plate in high winds, leaving them lying horizontally often with exposed root plates.
Witches’ brooms Conspicuous, abnormally dense, large or small clusters of live or dead twigs in an otherwise healthy tree crown, usually caused by a species of the Taphrina fungus, but may also be caused by mites or viruses.
Wound-wood The tissues which develop around the edges of tree wounds and which may eventually close (occlude) the wound.


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