guideline for planting a diverse urban forest wherein a single species should make up no more than 10% of the tree population, a single genus no more than 20%, and a single family no more than 30%.
12-strand rope
for arborist ropes, a braided rope consisting of 12 strands. Most are coreless. There are two types of 12-strand construction: a tight braid that is not easily spliceable, used for climbing and rigging lines; and a loose, easily spliceable braid, commonly used for slings.
16-strand rope
for arborist ropes, a braided rope that has a 16-strand, load-bearing cover and a filler core that is not significant in load carrying.
24-strand rope
for arborist ropes, a braided rope that has a 24-strand cover and is available in double-braided or kernmantle construction.  
3-strand rope
rope construction in which three strands are twisted together in a spiral pattern. Contrast with braided rope and kernmantle rope.
7-strand, common-grade cable
steel-cable construction in which seven strands are twisted together in a spiral pattern. Used to add supplemental structural support to trees. Terminated by wrapping onto itself.
guideline proposed by Alex Shigo that states in 90 percent of trees conflicting with utility lines, three pruning cuts can achieve 90 percent of the required clearance.
Abiotic Disorder
plant malady caused by nonliving, environmental, or man-made agents (contrast with biotic disorder).
Abscisic Acid (ABA)
plant hormone that stimulates stomatal closure and promotes seed and bud dormancy. Inhibits shoot growth but may promote root growth. Plays a role in wounding responses and in abscission of leaves and fruit.
leaf or fruit drop induced by hormonal changes.
Abscission Zone

Area at the base of the petiole where cellular breakdown leads to leaf and fruit drop

Absorbing Roots
fine, fibrous roots that take up water and minerals. Most absorbing roots are within the top 12 inches (30 cm) of soil.
taking up (contrast with adsorption).
rate of change of the velocity of an object. Acceleration is a vector quantity (see vector and velocity).
Acceptable Risk

The degree or amount of risk that the owner, manager, or controlling authority is willing to accept

Acceptable Risk Threshold

The highest level of risk that does not exceed the owner/manager’s tolerance

Access line
1) second climbing line hung in a tree in case it is needed to reach a victim in an emergency.
2) climbing line installed in a tree to gain access to, but not used to work, the tree.
Access Route
defined entrance and exit route for a property during construction, tree work, or landscape operations.
physiological adaptation process of plants and other living organisms to a climate or environment different from their native environment or where originally grown.
having a pH less than 7.0 (contrast with alkaline).
state or quality of being acid (contrast with alkalinity; see pH).
characteristic fruit of plant species belonging to the Quercus genus.
group of soil bacteria resembling fungi. Actinomycetes play a role in the decomposition of organic matter and the release of mineral elements.
pertaining to carabiners and snaps, the number of distinct motions for the gate to be opened. A nonlocking carabiner is single action, requiring one movement for the gate to be opened.
Action Threshold
pest population or plant damage level that requires action to prevent irreversible or unacceptable physiological and/or aesthetic harm (see economic threshold).
term describing leaves that taper sharply at their tip (apex).
disorder or disease that occurs suddenly or over a short period of time (contrast with chronic).
genetic ability of plants and other living organisms to adjust or acclimate to different environments.
force of attraction between two different materials; in soils, it is the attraction of water molecules to soil particles.
Adjustable Balancer
adjustable rigging sling used to balance tree limbs being removed in rigging operations.
in close, tight proximity. Pressed close to or lying flat against something. Appressed.
adhesion on contact of the molecules of gases, dissolved substances, or liquids to the surface of solids or liquids with which they are in contact (contrast with absorption).
Advanced Assessment (Level 3):

An assessment performed to provide detailed information about specific tree parts, defects, targets, or site conditions. Specialized equipment, data collection and analysis, and/or expertise are usually required

arising from parts of the root or stem and having no connection to apical meristems.
Adventitious Branch

Branch arising from a stem or parent branch and having no connection to apical meristems.

Adventitious Bud
bud arising from a place other than a leaf axil or shoot tip, usually as a result of hormonal triggers.
Adventitious Root

Roots arising from roots or stems and having no connection to apical meristems

Adventitious Shoot
shoot that develops from an adventitious bud.
provision of air to the soil to alleviate soil compaction and improve its structure.
Aeration System
set of holes or trenches created in the tree’s or other plant's root area to improve oxygen availability to the roots and gas exchange with the atmosphere. Aeration systems sometimes include an underground arrangement of porous pipes or tubing.
Aerial (airborne) Patrol
inspection undertaken from a plane or helicopter; often used by electric utilities to monitor tree growth near electric transmission lines.
Aerial Bundle Cable (ABC)
system of reducing the physical hanging space of aboveground electrical conductors.
Aerial Device (aerial lift device)
1) a device, with booms and a bucket, for elevating a worker to the proximity of a tree’s crown; may be mounted on a truck or other vehicle or be towed or self-propelled (also known as a mini-lift).
2) a self-propelled device such as a scissor lift, with an elevating platform but without side reach for placing a worker in the proximity of a tree’s crown; may be used for hedges or interior trees. In the United Kingdom, such devices are called a mobile elevated work platform (MEWP).
Aerial Inspections

Inspection of the upper tree parts not readily accessed from the ground; typically done by climbing or from an aerial lift.

Aerial Patrol

Overflights of utility right-of-way, large areas or individual trees in a defined area to record the location of trees that are likely to fail or cause harm

Aerial Rescue
method of bringing an injured worker down from a tree or aerial lift device.
Aerial Roots
aboveground roots. Usually adventitious in nature and sometimes having unique adaptive functions.
pleasing to the senses, visually or otherwise. Artistic.
Agents of Change
Environmental, social, and public factors that influence the dynamics of the vegetation resource over time.
1) close cluster or mix of small particles of soil and/or organic matter of varying size that are bonded together.
2) sand, gravel, or small rocks in soil; and/or sand, gravel, or small rocks used under paved surfaces.
3) clusters of flowers or fruits that appear as a single unit. 4) individual tree crowns that form a canopy.
Air Excavation Device, Air Excavator
device that directs a jet of highly compressed air to excavate soil. Used within the root zone of trees to avoid or minimize damage to tree roots or near underground structures such as pipes and wires. May also reduce hazards associated with excavation near pipes or wires.
Air quality
presence and concentration of pollutants in the air 
Air Terminal
uppermost point of a tree lightning protection system.
Aircraft Cable
steel wire rope of various constructions designated by the number of strands in the rope and the number of wires in each strand; commonly used in cabling (in Australia and New Zealand) due to its high strength.
chemical compound that, in water solution, reacts as a base by forming hydroxyl ions (OH-). Commonly referred to as "metal hydroxides."
having a pH greater than 7.0 (contrast with acid).
state or quality of being alkaline (contrast with acidity; see pH).
two parallel rows of trees, usually of the same species, form, and age, often having canopies that have grown together.
substances produced naturally by plants as part of a defense against pests and other plants. May adversely affect the growth and development of other plants.
chemical effect or inhibition of growth or development of plants that is induced by allelochemicals.
pertaining to bud or leaf arrangement, one leaf or bud at each node, situated at alternating positions along the stem. In this arrangement, the leaves are not directly across from each other (contrast with opposite and whorled).
Alternate Host
one of a number of separate obligate hosts to the different life stages of certain pathogens, such as rusts, which must alternate between hosts.
American National Standards Institute (ANSI)
a private non-profit organization that oversees the development of voluntary consensus standards by accredited representatives of government agencies, industry, and other stakeholders (see ANSI A300, ANSI Z133, and ANSI Z60.1).
Amon-eye Nut
specialized nut used in cabling trees that has a large eye for attaching a cable to a threaded rod.
Ampere (amp, A)
1) measure of electrical current flow through an electrical conductor; the flow of one volt through a wire with a resistance of one ohm.
2) the current draw or power rating of electrical power tools and appropriately sized power cords and sources and circuit breakers. Important to understand for efficient and safe use of such tools.
without—or with a restricted supply of— air. Process that occurs in the absence of oxygen.

Detailed examination of the elements or structure of something

1) study of the structure and composition of plants and other living organisms.
2) structure and composition of plants and other living organisms (contrast with morphology, physiology, and taxonomy).
1) in cabling, hardware to which support cable is affixed.
2) in rigging, the point at which the rigging system is secured or where friction is controlled.
Anchor Block
in a mechanical advantage system that employs pulleys, the stationary block in a block and tackle system.
Anchor Force
sum of the forces acting on the anchor point(s) in a rigging system.
Anchor Hardware
hardware to which the cable termination is affixed in a cabling or guying installation.
Anchor Hitch
knot commonly used to attach a line to a piece of hardware. Anchor bend.
Ancient Tree
a tree that has passed beyond maturity and is old or aged in comparison with other trees of the same species (see champion tree and veteran tree).

Plant with seeds borne in an ovary. Consists of two large groups: monocotyledons (grasses, palms, and related plants) and dicotyledons (most woody trees, shrubs, herbaceous plants, and related plants) (contrast with gymnosperm)

plant with seeds borne in an ovary. Consists of two large groups: monocotyledons (grasses, palms, and related plants) and dicotyledons (most woody trees, shrubs, herbaceous plants, and related plants) (contrast with gymnosperm).
ion that carries a negative charge (contrast with cation).
1) a plant living only one year (contrast with biennial and perennial).
2) anything that recurs yearly, such as an assessment, inspection, program, or treatment.
Annual Rings

Rings of xylem that are visible in a cross section of the stem, branches, and roots of some trees. In temperate zones the rings typically represent one year of growth

Annual Rings
rings of xylem that are visible in a cross section of the stem, branches, and roots of some trees. In temperate zones, the rings typically represent one year of growth and are sometimes referred to as annual rings.
acronym for American National Standards Institute.
in the United States, industry-developed, national consensus standards of practice for tree care.
in the United States, industry-developed, national consensus safety standards of practice for tree care.
ANSI Z60.1
in the United States, industry-developed, national consensus standards for nursery stock.
red or purple pigment responsible for those colors in some parts of trees and other plants.
group of fungal diseases of trees that affect the leaves, stems, flowers, or fruit, causing spotting, blotching, or necrosis.
plant growth regulator that inhibits the action of the plant hormone gibberellin, which, among other things, regulates cell elongation.
substance applied to the foliage of plants to reduce water loss (transpiration).
Anvil-type Pruning Tool
hand pruning tool with a sharp, straight blade that cuts against a flat metal surface (anvil).
the tip, end, or angular limit of an object; for example, the tip of a leaf or shoot.
group of sap-sucking insects of the Hemiptera order.
having to do with the tip of a leaf or stem.
Apical bud
bud at the tip of a twig or shoot (see terminal bud)
Apical control
inhibition of lateral buds, decreasing from the top down, by apical buds over many seasons, resulting in trees with an excurrent growth form
Apical dominance
condition in which the terminal bud inhibits the growth and development of the lateral buds on the same stem formed during the same season
Apical meristems
growing points at the tips of shoots and roots
free spaces in plant tissue. Includes cell walls as well as intracellular spaces (contrast with symplasm)
1) placing a monetary value on a tree, other plant, other landscaping, including hardscape, or an entire property. 2) a report stating an opinion of appraised value. 3) particularly outside the United States, an evaluation of nonmonetary landscape or plant characteristics.
Approach distances
minimum distances that must be maintained between conductors and other energized equipment and qualified line-clearance personnel or persons other than line-clearance personnel and their bodies or tools. Approach distances vary with qualifications of personnel and with voltages
Appropriate response process (ARP)
method of systematically assessing plant health and client needs to determine which course of action, if any, is recommended
in the context of guidelines, standards, and specifications, that which is acceptable to federal, state, provincial, or local enforcement authorities or is an accepted industry practice
practice and study of the care of trees and other woody plants in the landscape
professional who possesses the technical competence gained through experience and related training to provide for or supervise the management of trees and other woody plants in residential, commercial, and public landscapes
Arborist block
heavy-duty pulley with an integrated connection point (bushing for attaching a rope sling), a rotating sheave for the rope, and extended cheek plates. Used in tree rigging operations.
AS 4373-1996, Pruning of Amenity Trees
in Australia, industry-developed, national consensus standards of practice for pruning.
piece of gear that enables a climber to ascend a rope. Attached to the rope, it will grip in one direction (down) and slide in the other (up) (contrast with descender)
without symptoms
Atmospheric contaminants
Sulfur dioxide, Nitrogen oxides, Ozone, Smog, Particulates
tool that bores holes in wood or other materials while carrying cuttings away from the bored hole (see auger bit, soil auger)
Auger bit
drill bit that bores holes in wood or other materials while carrying cuttings away from the bored hole
in Plant Health Care, the release of beneficial organisms to suppress pest insect or mite populations
plant hormone or substance that promotes or regulates the growth and development of plants. Produced at sites where cells are dividing, primarily in the shoot tips. Auxinlike compounds may be synthetically produced.
Available water
water remaining in the soil after gravitational water has drained and before the permanent wilting point has been reached (compare to field capacity, gravitational water, permanent wilting point, and saturation point)
Axial transport
movement of water, minerals, or photosynthates longitudinally within a tree
point of attachment of a leaf petiole to a stem
Axillary bud
bud in the axil of a leaf. Lateral bud
Axillary shoot
shoot rising at the axil of a leaf
Back cut
cut made on a tree trunk or branch, opposite from and toward the notch, face cut, or undercut, to complete felling or branch removal (contrast with bore cut).
Back feed
process whereby electricity is fed back into and re-energizes distribution lines, usually from a home generator. Voltage can be modified if it passes through a transformer.
Back flow
process whereby water (that may contain pesticides, fertilizers, or other contaminants) is fed back into source pipes or mains through a hose connection or into a water source such as a stream, pond or lake from equipment drawing water from that source.
Back flow preventer
device (often required by law) to prevent back flow by means of an air gap or one-way valve
1) soil or amended soil used to fill the hole when planting a tree. 2) soil, common fill, aggregates, or contaminants in various combinations put back into an excavation. May not be hospitable for tree root growth and function.
single-celled organisms having a cell wall but no organized nucleus. A few species are plant pathogens
pesticides that are used to kill or inhibit bacteria in plants or soil
in rigging, a technique for lowering a tree limb without allowing either end to drop
rigging sling(s), usually with at least one spliced eye and a Prusik to position the load line. Used to rig a tree limb in a balanced configuration
Balled and burlapped (B&B)
tree or other plant dug and removed from the ground for replanting, with the roots and soil wrapped in burlap or a burlap-like fabric (contrast with bare root, container grown, containerized, and in-ground fabric-bag grown; compare with ball and burlap)
Barber chair
dangerous condition created when a tree or branch splits upward vertically from the back cut, slab up.
Bare root
tree or other plant removed from the ground for re-planting without soil around the roots. (Contrast with balled and burlapped, container grown, containerized, and in-ground fabric-bag grown). 2) the harvesting or transplanting of a tree or other plant without soil around the roots.
protective outer covering of branches and stems that arises from the cork cambium
Bark tracing
cutting away torn or injured bark to leave a cleanly cut edge
Barrier Zone

A chemical and anatomical barrier formed by the cambium present at the time of wounding in response to wounding. Inhibits the spread of decay into xylem tissue formed after the time of wounding. Wall 4 in the CODIT model (contrast with reaction zone)

Basal bark application
application of herbicides, usually mixed with penetrating oil, to the lowest 12 to 18 inches (30 to 45 cm) of the main stem(s) of unwanted vegetation.
Basal rot
decay of the lower trunk, trunk flare, or buttress roots. Also called butt rot.
Basal Swelling

Increased wood growth in the area near or where the trunk and roots come together

Basic Assessment (Level 2)

Detailed visual inspection of a tree and surrounding site that may include the use of simple tools. It required that a tree risk assessor inspect completely around the tree trunk looking at the visible aboveground roots, trunk, branches and site.

device, often relying on balls or rollers, to reduce friction between mechanical parts such as a wheel, or a pulley sheave and an axle, or a shaft and case connecting rod (contrast with bushing).
Beer knot
knot commonly used to join two ends of a piece of tubular webbing to create a loop (see water knot).
common name given to some insects of the Coleoptera order.
means of securing or slowing a climbing line by using wraps around a cleat, carabiner, or other device.
Belay device
piece of equipment used to provide friction for belaying a climber.
type of knot used to join two rope ends together (contrast with hitch).
Bend radius
radius of an object around which a line passes.
Bend ratio
ratio of the diameter of a branch, sheave, or other object to the diameter of the rope that is wrapped around it.
Bending Moment

A turning, bending or twisting force exerted by a lever, defined as the force (acting perpendicular to the lever) multiplied by the length of the lever (see moment).

Bending strength
maximum fiber stress wood incurs just before it cracks or breaks when force is applied laterally.
Beneficial organisms
insects and other organisms that promote plant health or assist in the control of pest populations.
a wall or mound of dirt that directs, diverts, or holds water; screens objectionable views or reduces objectionable noise; or provides additional rooting volume for trees or other plants.
Best management practices (BMP)
best-available, industry-recognized courses of action, in consideration of the benefits and limitations, based on scientific research and current knowledge.
Better half hitch
half hitch(es), typically two or more, added to other knots for additional security. Also called “finishing with a half hitch.”
plant living two years. Usually grows the first year, then flowers and fruits the second year (compare to annual and perennial).
natural division of a branch or stem into two or more stems or parts.
curve or arc in a rope between the working end and the standing part.
capable of being broken down by natural, organic processes and reabsorbed into the environment.
biological diversity in an environment as indicated by number of different species of plants and animals.
Biogenic volatile organic compounds
Pollutants, produced by tree, that can contribute to the formation of ozone and carbon monoxide
Biological control
method of managing plant pests or weeds through the use of natural predators, parasites, or pathogens.
Biomechanics (tree biomechanics)

The study of the action of forces on living trees

Biorational control product
management product or pesticide formulated from naturally occurring plant extracts, microbes, or microbial by-products that poses very low risk to nontarget organisms and has limited environmental persistence.
Biorational pesticide
pesticide formulated from naturally occurring plant extracts, microbes, or microbial byproducts that poses very low risk to nontarget organisms. (2) pesticide that has limited environmental persistence and poses very low risk to nontarget organisms.
pertaining to living organisms.
Biotic agent
a living organism capable of causing disease.
Biotic disorder
disorder caused by a living organism (contrast with abiotic disorder).
double pinnate (contrast with palmate and pinnate).
expanded body of a leaf. (2) sharp, cutting part of a tool.
Blake’s hitch
friction knot climbers use, sometimes in place of the tautline hitch or Prusik knot.
flow of sap from plant wounds, injuries, or pathogen invasion. (2) flow of blood from a human or animal wound.
Blended fertilizer
mix of slow-release and soluble fertilizers, with the percentage of water-insoluble nitrogen (WIN) listed on the label.
any disease or disorder, regardless of the causal agent, that kills young plant tissues.
heavy-duty pulley used in rigging. Designed for dynamic loading. (2) casing enclosing one or more parallel pulleys.
Block and tackle
system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.
method of using an arborist block to rig down trunk sections (contrast with butt-hitching). In some countries, to remove a trunk by cutting and dropping small pieces at a time.
method of ascending a tree using a climbing rope.
main trunk of a tree below the branches, usually used in reference to a tall tree whose first branch is high off the ground.
1) post on which wraps can be taken with a rope to tie it off or to provide friction for control.
2) a sturdy post used to protect vulnerable areas or objects from vehicles.
1) lag- or machine-threaded cable anchor or bracing rod; used with a nut and washer in supplement support systems in trees.
2) a machine-threaded fastener used with a nut and washer in various equipment or structures.
Bonding conductor
conductor that connects a tree support cable or metal conduit to a lightning protection system.
art of pruning and managing tree growth through root pruning to maintain a miniature size.
long, movable arm of an aerial device or crane.
Border zone
a section of a transmission or pipeline right-of-way that extends from the wire or pipe zone to the right-of-way edge. The border zone is managed to promote a low-growing plant community of forbs, tall shrubs, and low-growing trees below a specified height (e.g., 25 ft or 7.5 m).
Bore cut
using the tip of a chain saw to cut into or through the middle of a piece of wood. Back-cut technique in which the hinge is established by plunge cutting through the stem, then cutting back away from the hinge. Plunge cut (contrast with back cut).
Botanical pesticide
pesticides derived from plants.
compounds (pesticides, oils, etc.) made from plants.

Leans characterized by the top of the tree bending over more than the lower trunk, creating a curve

loop knot used to form single or double end line loop(s) in a rope, often to attach items to the rope.
Bowline on a bight
knot used to form two loops in the standing part of a rope.
Box cable system
tree cabling system that forms closed polygons. Used to join together more than three stems (contrast with direct cable system and triangular cable system).
Brace Rod (ridge brace)

Metal rod used to support weak sections of crotches of a tree

installation of metal rods through portions of a tree for supplemental support.

The fruiting body of a decay fungus (see conk)

modified leaf, usually growing just below the flower petals.
Braided rope
rope construction in which the strands are woven together in a diagonal pattern (contrast with 3-strand rope).
stem arising from a larger stem. A subdominant stem. Pith in true branches has no connection to the parent stem.
Branch angle
the angle, typically less than 90 degrees, formed in the union between stem and branch.
Branch arrangement
orientation and distribution of branches along a trunk.
Branch bark ridge
raised strip of bark at the top of a branch union, where the growth and expansion of the trunk or parent stem and adjoining branch push the bark into a ridge.
Branch collar
area where a branch joins another branch or trunk that is created by the overlapping vascular tissues from both the branch and the trunk. Typically enlarged at the base of the branch.
Branch protection zone
chemically and physically modified tissue within the trunk or parent branch at the base of a smaller, subordinate branch that retards the spread of discoloration and decay from the subordinate stem into the trunk or parent branch.
Branch taper
natural, gradual reduction in the diameter of a branch from its point of origin to the tip (see taper).
Branch union
point where a branch originates from the trunk or another branch. Fork. Crotch.
Breach of Duty (of care)

Failure to take reasonable care to avoid injury or damage to a person or property in a situation where the law imposes a duty of care.

Breaking strength
force at which a new piece of equipment or rope fails under a static load.
Bridge graft
method to repair a semi-girdled to completely girdled trunk in which scion wood is grafted above and below the trunk injury to reconnect the trunk vascular cambium.
Broadcast fertilization
application of fertilizer over the soil surface (contrast with drill-hole fertilization and liquid fertilization).
Brown rot
fungal wood rot characterized by the breakdown of cellulose. Contrast with soft rot and white rot.
Brown Rot

Fungal wood rot characterized by the breakdown of cellulose (contrast with soft rot and white rot)

term describing the brown appearance of dead foliage, usually following the application of herbicide.
Buck strap
strap used in tree climbing, often employed for ascending trees with climbing spurs. Similar to a work-positioning lanyard.
cutting of a tree trunk or log into shorter, manageable sections.

Failure mode characteristic of collapsing under compressive stress

small lateral or terminal protuberance on the stem of a plant that may develop into a flower or shoot. Undeveloped flower or shoot containing a meristematic growing point.
Bud trace
vascular connection extending from the base of latent buds inward to near the pith. Grows in length with each annual increment and appears as a thin, continuous line when viewed in longitudinal section.
Buffering capacity
ability of a soil to maintain (i.e., resist change in) its pH.

Swellings on branches, trunk, or root flares: often caused by new tissue formed as a response to movement and that reinforces the wood structure at the weak area.

Bulk density
mass of soil per unit volume. Often used as a measure of compaction.
Bull rope
large diameter rope used in rigging.
Buntline hitch
simple hitch commonly used to attach a line to a piece of hardware.
an abnormal swelling of a tree trunk characterized by swirling wood grain and meristematic tissue. Wood with these structures is prized for woodworking (ontrast with gall).
strong, coarsely woven cloth made from fibers of jute, flax, or hemp. (2) a burlaplike fabric made of synthetic fibers. Both are used for containing soil in a root ball (ee balled and burlapped).
1) metallic tube or lining, without moving parts, to reduce friction between mechanical parts such as a pulley sheave and an axle; also the nonrotating sheave used to increase the bend radius for the sling attachment (contrast with bearing).
2) a device, often made of rubber or plastic, to reduce vibration or wear between mechanical parts.
3) a fitting used to connect plumbing parts of different diameters (such as in spray equipment).
Butt Rot

Decay of the lower trunk, trunk flair, or buttress roots

method of lowering pieces when the rigging point is below the work, traditionally without the use of a block. Contrast with blocking.
tying off a limb at the butt (larger) end for rigging.
Butterfly knot
knot that can be used to form a loop in the standing part of a line.
Buttress Root

Roots at the trunk base that help support the tree and equalize mechanical stress.

Bypass pruner
a tool that pushes a sharpened blade through a small-diamoeter stem, past an unsharpened hooked or curved blade. Also called secateurs, pruning shears, or hand pruners (contrast with anvil-type pruning tool).
Cabling (flexible bracing)

Installation of steel or synthetic cable in a tree to provide supplemental support to weak branches or branch union.


Undifferentiated tissue formed by the cambium, usually as the result of wounding (contrast with woundwood)


Thing layer(s) of meristematic cells that rive rise (outward) to the phloem and (inward) to the xylem, increasing stem and root diameter


localized diseased area on stems, roots, and branches. Often shrunken and discolored.

Carbon dioxide
atmospheric gas produced by animals during respiration and used by plants during photosynthesis to produce sugars; an important greenhouse gas.
Carbon sequestering
storage of carbon in plant tissues

Open or closed hollow within a tree stem, branch or root usually associated with decay


Complex carbohydrate found in the cellular walls of the majority of plants, algae, and certain fungi.

Center of Force

The point toward or from which a force acts; the resultant point of aggregate forces


The person or organization for whom professional services are rendered. Usually, the owner or manager responsible for the trees.


Instrument used for measuring the height of a tree or other structure.


Acronym for compartmentalization of Decay in Trees (see compartmentalization)

Codominant Stem (trees in a stand)

forked branches nearly the same size in diameter, arising from a common junction and lacking a normal branch union

Column of Decay

Wood decay inside a tree that extends longitudinally up and down a stem or through a branch. 


Natural defense process in trees by which chemical and physical boundaries are created that act to limit the spread of disease and decay organisms (see CODIT)


In mechanics, the action of forces to squeeze, crush, or push together any material or substance (contrast with tension)

Compression Crack

Fracture caused by compressive stress.

Compression Wood

Reaction wood in gymnosperms, and some angiosperms, that develops on the underside of branches or leaning trunks and is important in load bearing (contrast with tension wood).


The summary and result of a risk assessment.


Fruiting body or nonfruiting body (sterile conk) of a fungus. Often associated with decay (see bracket)


Outcome of an event

Consequences of failure

Personal injury, property damage, or disruption of activities due to the failure of tree or tree part.

Constant Occupancy

A target is present at nearly all times, 24 hours a day, 7 days a week.

Corrected Lean

Tree lean characterized by a leaning lower trunk and a top that is more upright as a result of self-correction; sweep

Council of Tree and Landscape Appraisers (CTLA)
organization that produces the Guide for Plant Appraisal. It is composed of one representative from the American Society of Consulting Arborists, American Society of Landscape Architects, Associated landscape Contractors of America, Association of Consulting Foresters of America, International Society of Arboriculture, and Tree Care Industry Association.

Separation in wood fibers; narrow breaks or fissures in stems or branches. If server, may result in tree or branch failure


Process of degradation by microorganisms.

Decay-detection Device

An instrument or tool developed to detect decay in tree parts.


The breakdown or separation of a substance into simpler substances.


An imperfection, weakness, or lack of something necessary. In trees, defects are injuries, growth patterns, decay, or other conditions that reduce the tree’s structural strength

Definite Indicator

An indicator that decay is definitely present.

Degree of Harm

The amount or extent of injury, damage, or disruption


The length of a straight line through the center of a circle

Diameter Tape

A measuring tape scaled such that when it encircles a tree trunk, the diameter can be read directly


Wood response of a tree to microorganisms, including bacteria and non-decay-causing fungi, resulting in dead, darkened wood with little strength


A delay or interruption of progress or continuity.

Dominant (trees in a stand)

The tree or trees in a stand that are typically larger in height (taller), diameter, and crown spread that all the adjacent trees.


Wind resistance

Drip Line

Imaginary line defined by the branch spread of a single plant or group of plants

Drive-by (assessment)

Limited visual inspection from only one side of the tree, performed from a slow-moving vehicle. Also, may be called a windshield assessment.

Duty of Care

Legal obligation that requires an individual to apply reasonable actions when performing tasks that may potentially harm others


Study of how objects move under the action of forces.


Portion of an annual right (growth ring) that forms during spring, characterized by large-diameter cells and thin walls. Also called springwood (contrast with latewood).

Edge Tree

A tree on the edge of a stand, growing under conditions of light and exposure different from those prevailing within the stand.

Environmental benefits
improvements to the natural world (e.g., air and water quality) provided by the urban forest.

The body of moral principles or values governing a group or individual’s conduct


Occurrence of a particular set of circumstances. In tree risk assessment, a tree or tree part falling and impacting a target.

Extreme (risk rating)

Defined by its placement in the risk rating matrix (Matrix 2); failure is imminent with a high likelihood of impacting the target, and the consequences of the failure are severe.

Failure (of tree or tree part)

Breakage of stem, branch, or roots, or loss of mechanical support in the root system.

Failure mode

Location/manner in which failure could or has occurred; for example, stem failure, root failure or soil failure.

In the planning process, the evaluation of program of process effectiveness, incorporation of changes in community values, and response to new information.

Elongated, tapering, thick-walled cell that provides strength to wood


A long, narrow opening or split

Flexure Wood

Response growth triggered by the continued flexing of a tree stem or branch


Any action or influence causing an object to accelerate/decelerate. Calculated as mass multiplied by acceleration. Is a vector quantity.

Forest Stand

A group of trees in a wooded setting

Freeze-thaw Crack

Frost crack; vertical split in the wood of a tree, often near the base of the bole, caused by internal stresses and low temperatures

Frequent Occupancy

The target zone Is occupied for a large portion of the day or week.

Frost Crack

Vertical split in the wood of a tree, often near the base of the bole, caused by internal stresses and low temperatures

Fungal Fruiting Structures

The reproductive structures of a fungus (conks, brackets, mushrooms).

Fungus (pl. Fungi)

Group of organisms from the kingdom Fungi, including yeasts, molds, mushrooms, and smuts. Typically multicellular, saprophytic, or parasitic and lacking vascular tissue and chlorophyll. Reproduces vegetatively and by various types of spores borne in fruiting bodies. 

Girdling Root

Root that encircles all or part of the tree trunk or the tree’s other roots, constricting the vascular tissues and inhibiting secondary growth and the movement of water and photosynthates.

Grade Changes

A topographic alteration to the surface of the round


The force that attracts a body toward the center of the earth

Greenhouse effect
rise in temperature that the earth experiences because certain gases in the atmosphere trap energy from the sun.
Greenhouse gasses
gasses in the atmosphere, including water vapor, carbon dioxide, nitrous oxide, and methane, that trap energy efficiently, and protect air and water quality.
Vegetated place
Ground-Penetrating Radar (GPR)

A nondestructive device that uses radar pulses to image the subsurface


1) A steel or synthetic-fiber cable between a tree or branch and an external anchor (another tree, the ground, or other fixed object) to provide supplemental support.

2) A steel cable between a utility pole and external anchor (another pole, the ground, or other fixed object, which may sometimes be a tree) to keep the pole upright. Guys act in tension (contrast of prop)


Plants with exposed seeds, usually within cones (contrast with angiosperm)

Hand Pull Test

A load test that involves installing a line in a tree, and then pulling and releasing the line several times to move the tree or branch. Most commonly used in a pre-work inspection but can be used as part of an advanced tree risk assessment.


Personal injury or death, property damage, or disruption of activities.


situation or condition that is likely to lead to a loss, personal injury, property damage, or disruption of activities; likely source of harm. In relation to trees, a hazard is the tree part(s) identified as a likely source of harm.

Hazard Tree (synonym, hazardous tree):

A tree identified as a likely source of harm


Wood that is altered (inward) from sapwood and provides chemical defense against decay-causing organisms and continues to provide structural strength to the trunk. Trees may or may not have heartwood (contrast with sapwood)

Heartwood Rot

Any of several types of fungal decay of tree heartwood, often beginning with infected wounds in the living portions of wood tissue. Also called heart rot.

High (likelihood of impact)

The failed tree or tree part is likely to impact the target. This is the case when there is a constant target, with no protection factors, and the direction of fall is toward the target.

High (risk rating)

Defined by its placement in the risk rating matrix (Matrix 2); consequences are significant, and likelihood is very likely or likely, or consequences are severe and likelihood is likely

organic chemical compounds containing hydrogen and carbon atoms. Hydrocarbons from fossil fuels are linked to pollution and global warming

Study of the properties, distribution and effects of water on the Earth’s surface, underground, and the atmosphere.

Imminent (likelihood of failure)

Failure has started or is most likely to occur in the near future, even if there is no significant wind or increased load. The imminent category overrides the stated time frame.


Striking a target or causing a disruption that affects activities   

Improbable (likelihood of failure)

The tree or tree part is not likely to fail during normal weather conditions and may not fail in extreme weather conditions within the specified time frame.

Included Bark

Bark that becomes embedded in a crotch (union) between branch and trunk or between codominant stems. Causes a weak structure.


An organized and systematic examination.

Inspection Frequency

The number of inspections per given unit of time (for example, once every three years)

Inspection Interval

Time between inspection

Interior Tree

A tree within a stand of trees, protected from wind exposure

Land Disturbances

Disruptions to a terrestrial site, community or ecosystem that alters the physical environment.

Land-use History

Actions, events, or changes that have taken place on a site

Land-use Planning
Map and/or document that allocates specific locations in the community for various uses.

Portion of an annual ring (growth ring) that have taken a place on a site


Predominant angle of the trunk from vertical

Legal Precedent

A principle or rule established by a prior court or other decision-making body

Level Arm

The distance between the applied force (or center of force) and the point where the object will bend or rotate.

Level(s) of Assessment

Categorization of the breadth and depth of analysis


Something for which one is responsible. Legal responsibility

LiDar (Light Detection and Ranging)

A remote sensing method that uses laser technology to measure tree size and location in relation to the target of concern

Lighting Protection System

Hardware installed in a tree to conduct the charge of a lighting strike to ground


Organic substance that impregnates certain cell walls to thicken and strengthen the cell to reduce susceptibility to decay and pest damage.


The chance of an event occurring. In the context of tree failure, the term may be used to specify:

1) the chance of tree failure occurring

2) the chance of impacting a specified target

3) The combination of the likelihood of a tree failing and the likelihood of impacting a specified target.

Likelihood Matrix

A tool for categorizing the combined likelihood of a failure impacting a target.

Likelihood of Failure

The chance of a tree or tree part failure occurring within the specified time frame

Likelihood of Failure and Impact

The chance of a tree failure occurring and impacting a target within the specified time frame.

Likelihood of impact

The chance of a tree failure impacting a target during the specified time frame.

Likely (likelihood of failure and impact)

Define by its placement in the likelihood matrix (matrix 1); imminent likelihood of failure and medium likelihood of failure of impact, or probable likelihood of failure and high likelihood of impact


Restraints or factors that restrict the precision, applicability, or extent of something

Limited visual assessment (level 1)

A visual assessment from a specified perspective such as a foot, vehicle, or aerial (airborne) patrol of an individual tree or a population of trees near specified targets to identify conditions or obvious defects of concern.

Live Crown Ration (LCR)

The ratio of crown length to total tree height


1) A general term used to indicate the magnitude of a force, bending moment, torque, or pressure applied to a substance or material.

2) Cargo; weight to be borne or conveyed.

Load Testing

In advanced tree risk assessment, pulling tests to measure or observe the amount of inclination and/or deformation to assess stability

Low (likelihood of impact)

There is a slight chance that the failed tree or tree part will impact the target

Low (risk rating)

Defined by its placement in the risk rating matrix (matrix 2); consequences are negligible, and likelihood is unlikely, or consequences are minor, and likelihood is somewhat likely


A broad-headed hammer made of wood, plastic, or resin used for “sounding” a tree

Management Plan
document that describes how goals are to be accomplished within a defined time frame: includes tasks, priorities, best management practices, standards, specifications, budges and staffing analyses.
Mass Damping

A process by which the amplitude of oscillations is reduced; in trees, motion created by the forces of wind or rigging operations may be reduced through branch movement

Master Plan
Document that describes a community’s urban forest, both public and privately owned, and identifies goals.

 A rectangular array of rows and columns used to facilitate problem solving or decision making

Mechanical Stress

A measure of the internal forces acting within a deformable body; force per unit area


study of forces and their effects on bodies at rest or in motion.

Medium (likelihood of Impact)

The failed tree or tree part could impact the target, but is not expected to do so

Meristematic Tissue

Undifferentiated tissue in which active cell division take place. Found in the root tips, buds, cambium, cork cambium, and latent buds.

Minor (consequences)

 Minor personal injury, low- to moderate-value property damage, or small disruption of activities.


In tree risk management, the process for reducing risk

Mitigation Options

Alternatives for reducing risk

Mitigation Priority

Established hierarchy for mitigation of risks based on risk ratings, budget, resources, and policies

Mobile Target

a target that is in motion or intermittently moving

Moderate (risk rating)

Defined by its placement in the risk rating matrix (Matrix 2); consequences are minor and likelihood is very likely, or likelihood is somewhat likely and consequences are significant or severe


a turning, bending, or twisting force exerted by lever, defined as the force (acting perpendicular to the lever) multiplied by the length of the lever.

Movable target

Target that can be relocated

Multiple Risks

The concept that any tree, part, or failure mode could represent more than one type of risk.

Municipal arboriculture
a specialized field of arboriculture and urban forestry that deals with management of planted and naturally occurring greenspaces on public lands in communities.

 Failure to use reasonable care, resulting in damage or injury to another.

Negligible (consequences)

No personal injury, low-value property damage, or disruptions that can be replaced or repaired.

Neutral Plane

An imaginary plane where there is neither tension nor compression

Nitrogen oxides
gaseous compound that results from high temperatures in internal combustion engines which in turn causes atmospheric nitrogen to combine with oxygen. Nitrogen dioxide (NO2) is one of the major components of acid rain.
Noise attenuation
reduction of sound levels
Occasional Occupancy

The target zone is occupied by people or other targets infrequently or irregularly


seeping or exudation from a tree cavity or other opening. 


A tree that has grown with exposure to wind and other elements from all directions.

Overextended Branch

Branch that extends outside the normal crown area.


The person or entity responsible for tree management

gas formed by photochemical reaction of nitrogen oxides and volatile organic compounds (hydrocarbons) in ultraviolet sunlight and moisture.

Thin walled, living cells essential in photosynthesis, radial transport, energy storage, and production of protective compounds.

small solids suspended in air that are generated by combustion of fossil fuels, construction and demolition, industrial processes, soil tillage and erosion, and complex reactions between sunlight and gaseous pollutants. Associated with respiratory and cardiopulmonary diseases and cancer.

Causal agent of disease. Usually refers to micro-organisms

Patterns of Failure

Common modes of tree failure within a tree species or failure of multiple trees in a contiguous area that share similar site histories or environmental conditions.

Peroxyacetyl nitrate (PAN)
component of smog; causes respiratory problems and eye irritation; toxic to plants at high concentrations.

Plant vascular tissue that transports photosynthates and growth regulators. Situated on the inside of the bark, just outside the cambium. Is bidirectional (transports up and down) (contrast with xylem)

Possible (likelihood of failure)

Failure may be expected in extreme weather conditions, but it is unlikely during normal weather conditions within the specified time frame.

Potential Indicator

An indicator that decay might be present.


any form of water that falls to the Earth’s surface, such as rain, snow, or sleet

Prioritizing Targets

A process for ranking targets according to importance or value.


The measure of the chance of occurrence expressed as a number between 0 and 1 (0-110%), where 0 is impossibility and 1 is absolute certainty. Often expressed as a percentage.

Probable (likelihood of failure)

Failure may be expected under normal weather conditions within the specified time frame.


A stiff, small-diameter rod, stick, or wire that is inserted into a cavity or crack to estimate its size or depth


rigid brace, acting in compression, to support a tree, tree branch, or utility pole. Prop pole (contrast with guy)

Protection Factors

Structures, trees, branches, or other factors that would prevent or reduce harm to targets in the even of a tree failure


Removing branches (or occasionally roots) from a tree or other plant, using approved practice, to achieve a specified objective

Pruning Cycle

In utility and municipal arboriculture, the time scheduled between pruning events that is established as a guideline for providing reasonable clearance between trees and conductors.

Qualitative Tree Risk Assessment

A process using ratings of consequences and likelihood to determine risk significance levels (e.g. extreme, high, medium, or low) and to evaluate the level or risk against qualitative criteria

Quantitative Tree Risk Assessment

A process to estimate numerical probability values for consequences and to calculate numeric values for risk


Distance from the center of the perimeter of a circle. One half of diameter

Ram’s Horn

Inward curling formation of woundwood resembling the horns of a ram

Rare Occupancy

The target zone is not commonly used by people or other mobile/moveable targets


Parenchyma tissues the extend radically across the xylem and phloem of a tree and function in transport, storage, structural strength, and defense.

Reaction Wood

Wood formed in leaning or crooked stems, or on upper or lower sides of branches, as a means of counteracting the effects of gravity (see compression wood and tension wood).

Reaction Zone

Natural boundary formed chemically within a tree to separate damaged wood from existing healthy wood. Important in the process of compartmentalization (contrast with barrier zone).


One or many alternatives that are promoted to achieve a desired outcome, based on professional judgment.

Replacement cost method
method to appraise the monetary value of trees considered replacement with nursery or field-grown stock. Based on the cost of replacement with the same or comparable species of the same size aid in the same place, subject to depreciation for various factors. Contrast with trunk formula method
Reporting (risk assessment reporting)

Presenting the client with a summary statement describing in detail the results of an assessment.

Residual Risk

Risk remaining after mitigation

Resistance-recording Drill

A device consisting of a specialized micro-drill bit that drills into trees and graphs resistance to penetration; used to detect internal differences in the wood, such as decay

Resource Assessment
In urban forestry, an evaluation of trees based on inventory, canopy assessment, and/or sample surveys; may also consider factors affecting the urban forest such as soils, hydrology, water quality, temperature extremes, and wind.
Response Growth

New wood produced in response to loads to compensate for higher strain in outermost fibers; includes reaction wood (compression and tension), flexure wood, such as decay.

Retain and Monitor

The recommendation to keep a tree and conduct follow-up assessments after a stated inspection interval.


Natural process during which an overly mature tree reduces its crown and increases its girth to consolidate resources and increase longevity; the deliberate process of reducing tree height to mimic natural processes.


a root-like aggregation of fungal hyphae.


Longitudinal bulge of response wood growth.


The combination of the likelihood of an event and the severity of the potential consequences. In the context of trees, risk is the likelihood of a conflict or tree failure occurring and affecting a target, and the severity of the associated consequences.

Risk Aggregation

The consideration of risks and combination

Risk Analysis

The systematic use of information to identify sources and to estimate the risk.

Risk Assessment

The process of risk identification, analysis, and evaluation

Risk Categorization

The process of assigning risk and risk factors to categories based on severity or hierarchy.

Risk Evaluation

The process of comparing the assessed risk against given risk criteria to determine the significance of risk

Risk Management

The application of policies, procedures, and practices used to identify, evaluate, mitigate, monitor, and communicate risk

Risk Management Plan
Document that defines a tree risk management policy, describes tree inspection and documentation procedures, and provides and action plan to minimize risk.
Risk Perception

The subjective perceived level of risk from a situation or object, often differing from the actual level of risk

Risk Rating

The level of risk combining the likelihood of a tree failing and impacting a specified target, and severity of the associated consequences.

Risk Rating Matrix

A tool for ranking and displaying risk by assigning ratings for consequences and likelihood

Risk Tolerance

Degree of risk that is acceptable to the owner, manager, or controlling authority

Root Collar Excavation (RCX)

Process of removing soil to expose and assess the root collar (root crown) of a tree

Root Rot

Decay located in the roots; root decay is usually developed from the bottom up, and crown symptoms may or may not be visible.


Organism that lives on and may act to decay dead organic matter


Outer wood (xylem) that is active in longitudinal transport water and minerals

Sapwood Rot

Decay located in the sapwood. Bark and/or cambium may be damaged or dead. Signs of the classification of rot are usually numerous, but small, fruiting bodies along the bark’s surface are common.

Scope of Work

The defined project objectives and requirements.


lines formed where two edges of bark meet at a crack or wound

Secondary Xylem

Xylem produced to the interior of the vascular cambium during secondary growth

Severe (consequences)

Serious personal injury or death, high-value property damage, or major disruption of important activities.


1) in mechanics, the movement of failure of materials, especially laminar material such as wood, by sliding side by side.

2) A tool used to cut small-diameter plant material, including secateurs and snips, as well as long-bladed hand tools and power tools used to cut hedges.

3) To cut; often used to describe cutting foliage or stems to a single plane as in a hedge.

Shear Plane Crack

A crack at the neutral plane between tension and compression stresses

Shell Wall

The remaining solid wood around a cavity or internal wood decay.

Significant (consequences)

Substantial personal injury, moderate- to high-value property damage, or considerable disruption of activities

mix of atmospheric chemical compounds including ozone, peroxyacetyl nitrate (PAN), nitrogen oxides, and hydrocarbons that react chemically when exposed to ultraviolet light
Soft Rot

Decay of plant tissues characterized by the breakdown of tissues within the cell walls (contrast with brown rot and white rot)

Soil Compaction

Compression of the soil, often as a result of vehicle or heavy-equipment traffic, that breaks down soil aggregates and reduces soil volume and total pore space, especially macropore space.

Soil Depth

The vertical extent of soil present below ground

Somewhat likely (likelihood of failure and impact)

Defined by its placement in the likelihood matrix (Matrix 1); imminent likelihood of failure and low likelihood of impact, or probable likelihood of failure and medium likelihood of impact, or possible likelihood of failure and high likelihood of impact.

Sonic Assessment

A process of measuring wood density, or other mechanical properties, using an instrument that transmits, receives, and records and velocity of sound or electric waves through wood.


Process of striking a tree with a mallet or other appropriate tool and listening for tones that indicate dead bark, a thin layer of wood outside a cavity, or cracks in wood.

Person or group that has an interest in, or is affected by, an activity or decision.
Standard of care

Degree of care that a reasonable person should exercise in performing duty of care; a measurement used to assess whether an individual acted in a reasonable manner.

Static Pull Test

In advanced tree risk assessment, a load test that measures outermost fiber stain in the stem or branches, and/or inclination at the root flare, in response to a controlled pull.

Static Target

Target that cannot be easily moved

Stormwater runoff
rain or snow melt that runs over the ground

The deformation resulting from a stress measured as a change in specimen length per unit of total length.

Stratifying targets

A process for classifying targets according to importance or value

Strength Loss

Degradation of the ability to withstand mechanical stress.


1) In Plant Health Care, a factor that negatively affects the health of a plant; a factor that stimulates a response.

2) in mechanics, a force per unit area.

Structural Defect

Feature, condition, or deformity of a tree that indicates a weak structure or instability that could contribute to tree failure.

Structural Support System

A device or mechanism providing supplemental support to individual branches and/or entire trees.

Subdominant (trees in a stand)

Understory trees in a stand or forest with growth somewhat restricted by larger nearby trees.


 A waxy substance present in some cell walls.

Sudden Branch Drop (SBD

Sudden, unanticipated failure of tree branch with little or no discernible defect: often associated with long, horizontal branches and warm temperatures.

Sulfur Dioxide
the most abundant gaseous pollutant: results from burning coal and oil. SO2 can be a respiratory irritant. It is one of the major components of acid rain
Suppressed (tree in a stand)

Understory trees in a stand or forest with growth severely restricted by competing nearby trees.

Sustainable Urban Forest
Maintenance of ecological, social, and economic functions and benefits over time. Individual components of the urban forest change over time as trees die and are replaced, but the function is sustained.

Corrected tree lean characterized by a leaning lower trunk and a top that has grown back toward vertical


Change in diameter over the length of trunks, branches, and roots.


People, property, or activities that could be injured, damaged, or disrupted by a tree failure.

Target Management

Acting to control the exposure of targets to risk

Target Value

The monetary worth of something; the importance or preciousness of something.

Target Zone

The area where a tree or tree part is likely to land if it were to fail.

Target-based Options

Risk mitigation actions aimed at reducing the likelihood of impact in the event of tree failure


In mechanics the action of forces to stretch or pull apart any material or substance (contrast with compression)

Tension Wood

A form of reaction wood in broadleaved trees (hardwoods) that forms on the upper side of branches or the trunk of leaning tree (contrast to compression wood)

Time Frame

Time period for which an assessment is defined.


Time period for recommended mitigation.


Use if multiple sensors placed around a trunk or limb to record sound or magnetic waves traveling through the wood, with measurement resulting in a picture of internal density characteristics. Typically used in arboriculture to measure the extent of decay in trees.


The land and water features of an area, including changes in elevation


The action of twisting or being twisted


Elongated, tapering xylem cell adapted for the support and transport of water and elements.

Tree Architecture

The structural form and shape of a tree

Tree Conflict

An interference between the needs of a tree and society or infrastructure.

Tree Growth Regulator

Chemical that can be applied to trees that slows terminal growth by reducing cell elongation.

Tree Inventory
Record of each tree within a designated population; typically includes species, size, location, condition, and maintenance requirements.
Tree Population

A defined set, group, or collection of trees.

Tree Risk Assessment

A systematic process used to identify, analyze, and evaluate tree risk

Tree Risk Evaluation

The process of comparing the assessed risk against given risk criteria to determine the significance of the risk.

Tree Risk Management

The application of policies, procedures, and practices used to identify, evaluate, mitigate, monitor and communicate tree risk.

Tree-based Actions

Risk mitigation actions aimed at reducing the likelihood of tree failure.

Trunk formula method
method to appraise the monetary value of trees considered too large to be replaced with nursery or field-grown stock. Based on the developing a representative unit cost for replacement size and in the same place, subject to depreciation for various factors. Contrast with the replacement cost method.
Unacceptable Risk

A degree of risk that exceeds the tolerance of the owner, manager, or controlling authority

Unlikely (likelihood of failure and impact)

Defined by its placement in the likelihood matrix (matrix 1); possible or probable likelihood of failure and low likelihood of impact, or possible likelihood of failure and medium likelihood of impact, or improbable likelihood of failure with any likelihood of impact rating, or any likelihood of failure rating with very low likelihood of impact.

Urban forestry
management of naturally occurring and planted trees and associated plants in urban areas.
Urban heat island
phenomenon of cities having air temperatures warmer than adjacent rural areas and producing more smog.
Vascular Cambium

Lateral meristem from which secondary xylem and secondary phloem originate (see cambium)

Verbal Report

 Oral report: results of the risk assessment delivered to the client orally.

Very Likely (likelihood of failure and impact)

defined by its placement in the likelihood matrix (Matrix 1): imminent likelihood of failure and high likelihood of impact.

Very Low (likelihood of impact)

The chance of the failed tree or tree part impacting the specified target is remote.


End-to-end, tub=like, water-conducting cells in the xylem of angiosperms.

Veteran Tree

A tree which, because of its great age, size, or condition, is of exceptional cultural, landscape, or nature conservation value.

Visual Assessment

Method of assessing the structural integrity of trees using external symptoms of mechanical stress (such as bulges, reactive growth ect.) and defects (cracks, cavities, etc.)

Walk-by (assessment)

a limited visual inspection, usually from one side of the tree, performed as the tree risk assessor walks by the tree(s).

White Rot

Fungal decay of wood in which both cellulose and lignin are broken down (contrast with brown rot and soft rot)

Wildlife Habitat

An environment suitable for sustaining one or more species of animals.

Wind Exposure

Exposure to the forces of wind

Wind Load

The force on a tree or structure 

Wind Velocity

The speed of the wind


Uprooting and overthrowing of a tree caused by wind

Wood Decay

The process of wood degradation by microorganisms

Work Order

A written document detailing the work to be completed and authorizing performance of contracted work.

Work Plans
Predetermined, orderly means for job completion; identifies what work will be performed when, by whom, to what standards and safety requirements and at what costs.

 Lignified, differentiated tissues produced on woody plants as a response to wounding.

Written Report

A document with text, images, and/or references, delivered in print or electronic form, containing the results of the risk assessment.


Main water- and mineral-conducting tissue in trees and other plants. Provided structural support (contrast with phloem)

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