Aerial inspection: An inspection of the tree by means of gaining access to the

upper crown by ropes or elevated platform

Arborist: A professional in the practice of Arboriculture

Arboriculture: The study, cultivation and management of trees.

Bacterial Canker (Pseudomonas syringae): An airborne bacterial plant pathogen that typically infects Prunus (Cherry) species. It causes sunken areas of dead bark and small holes in leaves, called ‘shothole’. It kills the living layer of the tree (cambium) and starves the tree. Kills the trees it infects.

Bracing: A process used by arborists to provide additional structural support to a valuable tree. It can involve adding metal and/or rubber cables between a trees major limb to help prevent possible damage or failure. Might include metal pins inserted laterally though the limbs. Trees develop a dependency to bracing so the support can never be removed. Will require adjustment over the coming years.

Branch forks: Where two branches “fork” off in different directions. Also called ‘Crotches’.

Branch Union: Where a branch meets the trunk.

Brown rot: A fungal disease in woody plants

Bulges: A type of body language exhibited by a tree under regular excessive load. The remaining sound wood is squashed by the weight of the tree and protrudes outwards.  A symptom of creeping compression failure. Occurs when there is significant heart rot and minimal sound wood.

Cavities: An internal opening usually surrounded compensation growth, where a tree has concentrated growth to negate the effects of a particular wound or damage.

Co-dominant (Co-dom.): Two or more, generally upright, stems of roughly equal size and vigour competing with each other for dominance. Where these arise from a common union the structural integrity of that union should be assessed.

Compression: A force exerted on one side of a leaning tree, in the example of compression it would be the underside of branches or a lean.

Conservation Area: A tract of land awarded protected status

Construction Exclusion Zone (CEZ): These zones are created to protect roots and canopies from inadvertent damage by construction activity and are set up beyond all RPA’s. They are usually fenced off by protective fencing throughout the entire construction phase. No works are permitted in these zones other than minor landscaping works (such as removal of hard surfaces and replacement with soft landscaping). Where practicable the entire Root Protection Area and the area beneath the tree canopy shall be treated as a Construction Exclusion Zone.

Cross-sectional flattening: Failure mechanism of hollow tree trunks. Occurs in trees (think scaffolding pole) which buckle under load. A cardboard tube will demonstrate this principle well. Hold it upright, squeeze it in the middle and apply downward force to the top.

Crown Reduction: The removal of excess weight in a top-heavy tree, ideally in a gradual manner (progressive reduction e.g facilitating retrenchment). Usually required to mitigate risk in a valuable tree with extensive basal decay. Lowers centre of gravity and wind loading. Trees will require a pruning maintenance plan to manage to the new growth that develops at the new apex points.

Dead wood/Deadwooding: Branches that remain on the tree albeit dead i.e. without functioning cambium. The removal of potentially hazardous dead limbs (good practice). Total removal is not necessary. Deadwood in a standing tree is an important ecological asset. Best practice finds a balance between making a tree safe and retaining as much deadwood as possible (aesthetic choices aside).

Decay detection equipment: Equipment or techniques such as a fractometer or PiCUS tomograph that can detect and decay on the inside of a tree decay pattern The pattern shown and identified by certain types of pathogens causing decay.

Etiolated: Long thin stem/trunk. Light seeking. Poor height to width ratio possible slenderness hazard. Term usually applied to trunks. See Phototrophic.

Epicormic growth: Vitaly important for older trees that are starting to retrench (loose the crown). Allows the tree to develop low down branches. See Retrenchment. Usually many shoots growing from the trunk often with a different foliage colour to the crown. Shoots growing from dormant buds possibly activated by: age, damage to the tree, changes in light levels, sudden environmental change, thinning, crown dieback, decay, heavy pruning, root death, cold, changes in the water table or other stressors.

Failure: Most failures (trunk or branch) are initiated by wind loading, others occur due to static loading from snow or ice, or from developing a phototrophic/etiolated (light seeking/elongated) unsustainable form. Often there is also decay present.

Failure Potential: The chance of a failure happening.

Fluting: Protuberant vertical ribs of wood, common to genera such as Betula (Birch), Robinia (Locust), Ficus (Fig) and Metasequoia (Dawn Redwood). Flutes become more pronounced with tree age.

Force Majeure: Unforeseeable circumstances e.g. weather events.

Fungal decay bracket: A type of fungal fruiting body that looks like a bracket. Many fruit bodies do not present like this.

Gummosis: Leaking sap is typically a symptom of a larger problem, such as pathogen infections like Bacterial Canker, see above.

Halo Thinning: The removal of younger competing trees from the immediate area surrounding a selected premium quality tree so that it can receive the light and space needed to thrive. Common technique in forestry.

Hazard Rating System: The ISA’s (International Society of Arboriculture) recognised system for rating trees and their hazards.

Heartwood: The inner most wood of the tree.

Inosculation: A natural phenomenon in which trunks, branches or roots of two trees grow together. It is biologically similar to grafting and such trees are referred to in forestry as gemels, from the Latin word meaning “a pair”. Co-dominant trees can display similar limb wrapping/girdling when the dominant stems are so close together that the tree, over time, puts down new rings around both stems.

Lion-Tailing: The poor practice of removing all or most of the secondary and tertiary branches from primary or scaffold limbs leaving most of the foliage at the tips of the branches. Leads to increased branch breakout.

Keystone Species: A Keystone Species is an organism which has a unique and vital role in any given ecosystem. The input of these organisms to their respective ecosystems is greater than most other organisms of their kind.

Local planning authority: The LPA is empowered by law to exercise planning functions for a particular area.

Monolith: A tree reduced to its main stem with minimal or no branches. Good ecological practice as an alternative to removing a tree to ground level. Often not aesthetically pleasing.

Natural flare: A normal flare or taper at the base of the tree where it meets the roots

Occlusion rib: A form of body language to describe a strip or crack in a tree that has sealed itself with callous wood.

Pathogen: An often-virulent disease, virus or organism that can cause harm e.g. Phytophthora sp./ssp. (The Plant-Destroyer), Armillaria mellea (Honey Fungus), Hymenoscyphus fraxineus (Ash dieback).

Phototrophic: Light seeking. Long slender trunks or over long branches that are approaching appoint where loading is unsustainable and branch breakout or stem snap will probably occur. Term usually applied to branches. See Etiolated.

PiCUS: An fairly non-invasive piece of decay detection equipment that uses sonar.

Progressive reduction: Staged pruning work to reduce system shock but aiming to facilitate retrenchment in a less-radical manner to outright retrenchment pruning. Step 1: mitigate risk, no more than 15% of crown mass removed from main loading limbs only. Step 2: The following autumn or spring, formative pruning and crown cleaning. Step 3: further reduction of upper mass as necessary. All favouring lower limbs and epicormic growth.

Pruning wound: A wound on the tree caused by previous pruning.

Restricted Activity Zone (RAZ): It is not always practicable to create a CEZ over the entire RPA. This is because access may be required, or some works may be proposed within the RPA. In such circumstances a RAZ is created where limitations are placed on construction activity. Ground protection measures may be specified or the RAZ may be fenced off throughout part of the construction phase.

Retrenchment: The natural process of old or ancient trees as they lower their centre of gravity by large limb loss.

Retrenchment Pruning: An aggressive reduction that removes a large volume of the upper crown but retaining much of the main trunk and largest scaffold limbs. Lowers centre of gravity. Can greatly reduce the risk of root plate or trunk failure in old or ancient trees that are top heavy. It can also send a tree into rapid decline due to loss of photosynthetic area.

Root decay/rot:  The main cause of rot is poorly drained or overwatered soil. Soggy conditions prevent root respiration (gas exchange and nutrient uptake) and impede the plant from absorbing what it requires to live (unless it has aerenchyma cells, which facilitate plants breathing under water). As the oxygen-starved roots die and decay, the rot often spreads, even if the drainage problems have been dealt with. Weakened roots are more susceptible to soil fungus, which eat the plant matter (decay). Notorious fungus include but are not limited to: Armillaria mellea (Honey Fungus) and Phaeolus schweinitzii (Dyers Mazegill).

Root protrusions: Roots that protrude out from the ground surface.

Root Protection Area (RPA): This is a theoretical area of ground around a tree where the roots are likely to proliferate. Ground disturbance in this area should be minimised in order to avoid significant impact on tree health. For single stem trees the RPA is a standard 12 times the diameter of the trunk. This is equivalent to a circle with a radius of 15m or a square with approximately 26m sides.

Root System: Water and nutrient gathering part of the tree doubling as structural support for the above ground mass. For most trees it extends to around 1 meter below the surface (excluding tap roots) and to about 1.5 times the height of the tree in all directions if not restricted.

Sabre/Sabreing/Sabre Trunk: The reaction wood of a leaning tree will often push or pull the tree upright after a root plate shifting wind event, correcting its direction of growth. Referred to as sabre trees as they look like sabres.

Scaffold Stems: Main branches supporting most of the crown (not the trunk).

Scaffold Structure: Main branch area.

Slime Flux: Black with a sickeningly sweet smell. It is caused by many species of bacteria, such as yeast and other microorganisms. These different microorganisms infect the sap, consuming it as food and depleting the oxygen in the heartwood. The wood then turns a darker colour than the rest of the wood. This discoloration is called “wetwood”. Once these anaerobic (oxygen-less) conditions are created in the wetwood the bacteria start to ferment the sap, producing methane gas (foul odour) and increase the acidity (pH) in the tree. The resulting pressure forces the fermented sap, now called “slime flux”, out of the tree in an ooze which can congeal on the surface of the trunk. The infected oozing sap can kill other plants that are growing around the base of the infected tree.

Soft rot: A type of fungal or bacterial disease in plants that leads to a loss of capacity of the organism to hold itself upright. Damage tends to be gradual (not in the case of cinder fungus).

Sound Wood: Fully functional in both load bearing (tension or compression) and water and nutrient distribution (xylem and phloem). No or minimal decay. A tree can be hollow like a scaffolding pole and still have enough sound wood to have a lower failure risk than a much younger tree with no decay. This is due to older trees being naturally over engineered. Perhaps as much as 4.5 times their normal wind loading range. Trees like Oaks and Yews generally only fail to decay organisms over very long time periods and are usually able to adapt with reaction wood to developing rot pockets into great age.

Shear Crack: Trees have tension and compression aspects. Sometimes due to wind loading there is excessive strain put on a trunk and the tension wood and compression aspects can vertically slide against one another, initially forming a crack. Shear cracks can lead to spectacular tree failure, very dramatically referred to as a shear bomb due to how loud the failure can be. Link. Shear is also common in the torsion fibres of wind twisted trees. See Torsion.

Target: Any infrastructure a tree or a part of a tree could fall on in any direction to 1.5 times its height. Includes pedestrian areas, roads, pavements, and anything else created by or used by humans.

Tension: A force exerted on one side of a leaning tree or the topside of branches.

Thinning: Increasing light and air penetration into a crown by pruning selected branches. Often done to reduce loading stress on long limbs that are not overly phototrophic.

Topography: The arrangement of the natural and artificial physical features of an area.

Torsion (twist): The way in which the helical grain of the tree grows. It can be twisted or untwisted due to wind (rarely catastrophically). Can lead to shear events when wind load changes and the tree untwists.

Tree Preservation Order: A TPO is made by the Local Planning Authority in order to protect specific tress in a particular area from deliberate damage and destruction

Tree evaluation form: A form used to assist in the inspection of trees created by the International Society of Arboriculturalists.

Visual Tree Assessment: The current best practice method of evaluating structural defects and stability in trees devised by Claus Mattheck (Mattheck, et al., 2015).

Widowmaker: The term “widowmaker” is a morbid reminder for people working of living near woodland to avoid situations that can both cause death or serious injury. The short definition of the term can be translated into the phrase: any loose overhead debris such as limbs or treetops that may fall at any time. Widowmakers are extremely dangerous and present a continual hazard.

Windthrow: Trees uprooted or broken by wind forces.

Zone of potential failure: The area in which the tree or any part of it can cause damage to surrounding structures.