Glossary

The natural process by which a tree sheds its leaves, fruit, or branches. Controlled by hormones like ethylene and auxin.

A detailed tree risk evaluation using specialized equipment such as resistograph, sonic tomograph, or aerial inspection to quantify defects.

Roots or shoots arising from an unusual location on the plant, such as roots growing from a stem or trunk.

A mechanized device such as a bucket truck used to elevate workers to the canopy for tree work, regulated by OSHA standards.

Emergency procedures used to retrieve an injured or incapacitated climber from a tree, a required competency for all climbing arborists.

A cluster of soil particles bound together, creating structure that influences water movement, aeration, and root growth.

Soil deposited by flowing water, typically found in floodplains, often fertile with mixed particle sizes.

A leaf arrangement where one leaf is attached at each node, alternating sides along the stem.

A condition lacking oxygen, often occurring in compacted or waterlogged soils, which inhibits root respiration and promotes harmful microbial activity.

A flowering plant that produces seeds enclosed in an ovary (fruit). Includes most broadleaf trees such as oaks and maples.

The American National Standards for tree care operations, including pruning, fertilization, and support systems.

A group of fungal diseases causing irregular dead blotches on leaves, defoliation, and twig dieback in many hardwood species.

The phenomenon where the main central stem grows more strongly than lateral branches, controlled primarily by auxin produced at the shoot tip.

A professional trained in the science and art of planting, caring for, and maintaining individual trees.

A written document prepared by a qualified arborist assessing tree condition, risk, or the potential impact of construction activities.

A plant hormone that promotes cell elongation, controls apical dominance, and stimulates root initiation. Produced primarily in shoot tips.

The soil or amended material used to fill the hole around the root ball when planting a tree.

A method of transplanting trees where the root ball is wrapped in burlap and often secured with wire for transport and planting.

A method of transplanting trees without soil around the roots, typically done during dormancy for smaller stock.

A tree risk assessment conducted by walking around the tree and visually inspecting it from ground level on all sides.

An insect whose larval or adult stage tunnels into the wood or bark of trees, potentially disrupting vascular tissue and weakening structure.

A method of installing utilities beneath tree roots by drilling horizontally underground to avoid root damage.

A ridge of bark that forms at the junction of a branch and trunk, indicating the boundary between branch and trunk tissue.

The mass of dry soil per unit volume, including pore space. High bulk density indicates compaction and poor root growth conditions.

The diameter of a nursery tree trunk measured at 6 inches above the soil line (or 12 inches above for trees over 4-inch caliper).

A polysaccharide produced by trees to seal sieve plates in phloem and as a wound response, helping compartmentalize damage.

A thin layer of actively dividing cells between the bark and the wood that produces new xylem and phloem each growing season.

A localized dead or diseased area on the bark of a trunk or branch, often caused by fungi or bacteria.

The upper layer of branches and foliage in a tree or forest, also called the crown.

The percentage of ground area covered by the vertical projection of tree crowns, a key metric in urban forestry.

Cation Exchange Capacity. A measure of the soil's ability to hold and exchange positively charged nutrient ions, higher in clay and organic soils.

The primary structural carbohydrate in plant cell walls, providing rigidity and strength to wood and other plant tissues.

The process of binding metal ions (especially iron and manganese) in a soluble organic complex, making them available for plant uptake.

The green pigment in plant cells responsible for absorbing light energy used in photosynthesis, located in chloroplasts.

Yellowing of leaf tissue due to insufficient chlorophyll production, often caused by nutrient deficiency, high pH, or root damage.

Roots that grow in a circular pattern around the inside of a container, which can lead to girdling and instability if not corrected at planting.

The smallest mineral soil particle (<0.002 mm), with high surface area and CEC, contributing to nutrient retention but potentially poor drainage.

The removal of dead, dying, diseased, crowded, weakly attached, and low-vigor branches from the crown of a tree.

The rope used by an arborist to ascend, descend, and position in a tree. Must meet ANSI Z133 specifications.

Two or more stems of nearly equal size arising from the same point, often with included bark, creating a structural weakness.

Compartmentalization of Decay in Trees. Alex Shigo's model describing how trees wall off decay into four compartment walls.

The collection of all trees and associated vegetation in and around a community, managed for the benefit of residents.

The compression of soil particles that reduces pore space, limiting water infiltration, gas exchange, and root growth.

Decomposed organic matter used as a soil amendment to improve structure, water retention, nutrient content, and microbial activity.

A leaf divided into two or more leaflets attached to a common stalk (rachis), as in ash, walnut, and hickory.

A cone-bearing tree, typically evergreen with needle-like or scale-like leaves, belonging to the division Pinophyta.

In tree risk assessment, the severity of the outcome if a tree or tree part fails and strikes a target.

A tree grown in a pot or container at the nursery, allowing year-round planting but requiring attention to circling roots.

The area of soil around a tree where roots essential for stability and health are located, typically a radius of 1 foot per inch of trunk diameter.

Selective removal of dead, dying, diseased, or broken branches from the crown of a tree.

Removing lower branches of a tree to provide vertical clearance for pedestrians, vehicles, buildings, or lines of sight.

Reducing the height or spread of a tree's crown by pruning back to lateral branches large enough to assume the terminal role.

Pruning to improve the structure, form, and appearance of trees that have been topped, severely headed, or storm-damaged.

Selective removal of live branches to reduce crown density, increase light penetration and air movement.

A cultivated plant variety selected and propagated for desirable characteristics such as form, flower color, or disease resistance.

A plant hormone that promotes cell division, delays senescence, and stimulates lateral bud growth, often counteracting auxin dominance.

Diameter at Breast Height. The standard measurement of tree trunk diameter taken at 4.5 feet (1.37 m) above ground level.

The practice of removing dead branches from a tree to reduce risk and improve appearance without harming the tree.

The decomposition of wood by fungi and other organisms, reducing structural integrity. Trees compartmentalize decay via CODIT.

Trees and shrubs that lose their leaves seasonally, typically in autumn in temperate climates.

A progressive loss of vigor in a tree characterized by reduced growth, dieback, and eventual death, often caused by multiple stress factors.

A structural flaw or weakness in a tree that may predispose it to failure, such as decay, cracks, or included bark.

A flowering plant with two seed leaves (cotyledons), net-veined leaves, and vascular bundles arranged in a ring. Most broadleaf trees are dicots.

Progressive death of twigs and branches starting from the tips inward, often a symptom of stress, disease, or root problems.

A plant species having male and female reproductive structures on separate individual plants, such as ginkgo or holly.

The imaginary line on the ground directly below the outermost tips of a tree's branches, approximating the root zone.

A pruning cut that reduces a branch back to a lateral branch at least one-third the diameter of the branch being removed.

A fleshy fruit with a hard inner layer (endocarp) enclosing a seed, such as cherry, peach, or walnut.

The environmental and economic benefits provided by trees, including air purification, carbon sequestration, stormwater management, and cooling.

Excessive foliage concentration at the tips of branches due to improper pruning such as lion tailing, increasing failure risk.

A single layer of cells in the root that forms a selective barrier (Casparian strip) controlling water and mineral movement into the vascular system.

Shoots that develop from dormant or adventitious buds on the trunk or main branches, often as a stress response.

The outermost cell layer of young roots, stems, and leaves, providing protection and controlling gas and water exchange.

The time required for a transplanted tree to regenerate enough roots to sustain growth without supplemental irrigation, typically 1-3 years.

A gaseous plant hormone that promotes fruit ripening, leaf abscission, and stress responses. Elevated levels can indicate tree decline.

A plant that retains its leaves throughout the year, replacing them gradually rather than shedding them all at once.

The breakage of a tree or tree part such as a branch, stem, or root system, potentially causing damage to a target.

Equipment and practices used to prevent falls from height during tree work, including harnesses, lanyards, and anchor systems.

The process of cutting down a standing tree using directional cuts (notch and back cut) to control its fall.

The amount of water remaining in soil after gravitational drainage has ceased, representing the upper limit of plant-available water.

Soil or other material placed over existing grade near a tree, which can suffocate roots by reducing oxygen availability.

The reproductive structure of a fungus, such as a mushroom, conk, or bracket, often indicating internal decay in a tree.

An abnormal growth on a plant caused by insects, mites, fungi, bacteria, or nematodes, usually not life-threatening to the tree.

A taxonomic rank grouping closely related species. The first part of a binomial scientific name, always capitalized and italicized.

A plant hormone that promotes stem elongation, seed germination, and flowering. Works in concert with auxin and cytokinin.

A root that grows around the trunk or another root, potentially restricting the flow of water and nutrients.

A simple sugar produced during photosynthesis that serves as the primary energy source for tree metabolism and growth.

Raising or lowering the soil level around a tree, which can damage roots by burial, exposure, or altered drainage patterns.

A plant that produces seeds not enclosed in an ovary, including conifers, cycads, and ginkgo.

The process of gradually acclimating a plant to outdoor conditions before transplanting, reducing transplant shock.

A tree or tree part with the potential to cause harm to people or property. In risk assessment, hazard combines a defect with a target.

A pruning cut made between nodes, leaving a stub that promotes dense, weakly attached epicormic growth. Generally not recommended.

The central, non-living wood of a tree trunk, typically darker in color, providing structural support.

A distinct layer of soil with specific characteristics (O, A, B, C horizons), each differing in color, texture, and composition.

An organism, typically a plant, that provides sustenance for a parasite or pathogen.

Stable, dark, decomposed organic matter in soil that improves structure, water retention, and nutrient availability.

A suite of free software tools from the USDA Forest Service used to quantify the ecosystem services and values of urban trees.

An evaluation of how proposed construction or development activities may affect trees on or near a site.

Bark that is trapped between two branches or between a branch and trunk, creating a weak attachment point.

The process by which water enters the soil surface and moves downward through pore spaces.

The pathogen material (spores, mycelium, bacteria) that can initiate an infection when it contacts a susceptible host.

A pruning cut made between two nodes (buds or branches). Equivalent to a heading cut and generally not recommended.

Integrated Pest Management. A systematic approach combining biological, cultural, physical, and chemical controls to manage pests with minimal environmental impact.

International Society of Arboriculture. The professional organization for arborists worldwide that administers the Certified Arborist credential.

The sudden upward or backward thrust of a chainsaw when the upper portion of the bar tip contacts an object, a leading cause of chainsaw injuries.

A short rope or strap used to secure an arborist to a tree or structure, providing work positioning or fall arrest.

A branch that grows from the side of a parent branch or trunk.

The downward movement of dissolved nutrients through the soil profile by percolating water, potentially moving them below the root zone.

The dominant, upward-growing stem of a tree, also called the central leader or terminal.

The inclination of a tree from vertical, which may be natural growth or indicate root failure or soil movement.

A porous area in the bark of stems and roots that allows gas exchange between internal tissues and the atmosphere.

A complex polymer deposited in cell walls of xylem tissue that provides rigidity, waterproofing, and resistance to decay.

In tree risk assessment, the probability that a tree part will fail and impact a target within a given time period.

A tree risk assessment performed from a single vantage point or passing viewpoint, providing a basic level of evaluation.

The improper practice of removing interior branches and foliage, leaving growth concentrated at branch tips. Increases failure risk.

Forces acting on a tree or tree part, including self-weight, wind, ice, snow, and the dynamic forces from movement.

An ideal soil type containing a balanced mixture of sand, silt, and clay, providing good drainage, aeration, and nutrient retention.

Safety procedures for ensuring that equipment is properly shut off and cannot be restarted before maintenance or repair is completed.

Large pore spaces in soil that allow rapid drainage and air movement, often created by roots, earthworms, or soil structure.

A comprehensive document guiding the long-term management, planting, and maintenance of an urban or community forest.

Plant tissue containing undifferentiated cells capable of active division, found at root and shoot tips.

Small pore spaces in soil that hold water against gravity through capillary forces, important for plant water supply.

Actions taken to reduce tree risk, such as pruning, cabling, bracing, target removal, or tree removal.

A flowering plant with one seed leaf (cotyledon), parallel leaf veins, and scattered vascular bundles. Palms are the most common monocot trees.

Symbiotic fungi that colonize tree roots, greatly enhancing water and nutrient absorption in exchange for carbohydrates.

The death of cells or tissue in a localized area, often appearing as brown or black discoloration on leaves, bark, or roots.

A local law or regulation governing tree removal, protection, or planting, often including penalties for non-compliance.

Material derived from living organisms in various stages of decomposition, critical for soil health, structure, and nutrient cycling.

Occupational Safety and Health Administration. The federal agency regulating workplace safety, including standards for tree care operations.

A leaf shape or venation pattern where lobes or veins radiate from a single point, resembling an open hand, as in maples.

Thin-walled living cells in wood and bark that store starch and other reserves, and play a role in compartmentalization.

A microorganism (fungus, bacterium, virus, or nematode) capable of causing disease in a host plant.

Partially decomposed organic matter that accumulates in waterlogged, anaerobic conditions. Acidic and highly water-retentive.

The downward movement of water through soil under the influence of gravity, following infiltration at the surface.

A layer of cells inside the endodermis of roots from which lateral roots originate.

The outer protective covering of stems and roots that replaces the epidermis, consisting of cork, cork cambium, and phelloderm.

A measure of soil acidity or alkalinity on a scale of 0-14 (7 is neutral). Most trees prefer slightly acidic soil (pH 6.0-6.5).

The inner bark tissue that transports sugars produced by photosynthesis from leaves to other parts of the tree.

The process by which plants convert light energy, water, and CO2 into glucose and oxygen using chlorophyll.

A leaf arrangement or venation pattern with leaflets or veins arranged on both sides of a central axis, like a feather.

A pruning system involving the repeated removal of all new growth back to permanent branch stubs (pollard heads) on a regular cycle.

The portion of soil volume occupied by air and water rather than solid particles, critical for root growth and function.

Personal Protective Equipment. Safety gear required for tree work, including hard hat, eye protection, hearing protection, chainsaw chaps, and gloves.

Horizontal rows of parenchyma cells in wood that transport and store materials radially between the bark and pith.

The level of risk remaining after mitigation measures have been implemented on a tree.

A sticky organic substance produced by conifers and some hardwoods as a defense mechanism against insects and pathogens.

The ability of a plant to prevent or limit infection or damage by a pest or pathogen through physical or chemical mechanisms.

The metabolic process in which plants break down glucose to release energy (ATP) for growth and maintenance, consuming oxygen and releasing CO2.

The narrow zone of soil directly surrounding and influenced by plant roots, teeming with microbial activity.

The use of ropes, pulleys, and friction devices to control the descent of cut branches or trunk sections during tree removal.

A strip of land reserved for public use such as roads and utilities, where trees may require clearance pruning or removal.

The overall categorization of tree risk (low, moderate, high, extreme) based on the combination of likelihood of failure and consequence.

A physical or chemical barrier installed to redirect root growth away from infrastructure such as sidewalks and foundations.

A condition where roots have filled a container and begun circling, potentially leading to girdling roots after planting if not corrected.

A structure built over the root zone to distribute loads and protect roots from compaction during construction.

The area at the base of the trunk where it transitions to the root system, visible as a widening at ground level.

The deliberate cutting of roots, either to prepare a tree for transplanting or to sever roots threatening infrastructure.

A harness worn by a tree climbing arborist that supports the body and allows hands-free work positioning in the tree.

A winged seed (fruit) adapted for wind dispersal, characteristic of maples, ashes, and elms.

The largest mineral soil particle (0.05-2.0 mm), providing good drainage and aeration but low nutrient and water retention.

The living, outermost wood of a tree trunk that actively transports water and minerals (xylem).

A primary structural branch that forms the framework of the tree's crown.

The biological aging process in plants leading to the deterioration and eventual death of cells, organs, or the whole organism.

A leaf margin with sharp, forward-pointing teeth, like the edge of a saw.

A medium-sized mineral soil particle (0.002-0.05 mm), finer than sand but coarser than clay, feeling smooth when wet.

A leaf with a single, undivided blade attached to the stem by a petiole, as in oaks and maples.

The relative proportions of sand, silt, and clay in a soil, which determines its physical properties and behavior.

The basic unit of biological classification, consisting of organisms that can interbreed. The second part of a binomial scientific name.

The variety and relative abundance of tree species in an urban forest, important for resilience against pests and diseases.

The use of stakes and ties to support a newly planted tree until its root system establishes. Should be removed after one growing season.

The central vascular cylinder of a root or stem, containing xylem, phloem, and associated tissues.

A small appendage at the base of a leaf petiole, present in some species, useful for identification.

Microscopic pores on leaf surfaces that open and close to regulate gas exchange and water loss (transpiration).

A tree planted in the public right-of-way, typically between the sidewalk and curb, managed by the municipality.

A flaw in a tree's structure, such as cracks, decay, weak branch unions, or root damage, that increases the risk of failure.

A pruning cut that leaves a stub protruding beyond the branch bark ridge or collar, inhibiting wound closure.

A waxy substance found in cork cells and the Casparian strip that makes cell walls impermeable to water and gases.

A pruning technique that reduces the length of a competing leader or branch to slow its growth relative to the preferred leader.

A visible response of a plant to a disease or stress, such as wilting, discoloration, or abnormal growth.

In risk assessment, a person, structure, or object that could be harmed by a falling tree or tree part.

A pruning cut that removes a branch at its point of origin or back to a lateral branch large enough to assume the terminal role.

A lightweight line thrown over a branch to set a climbing line or rigging point, typically using a weighted throw bag.

The overall physical condition of soil in relation to plant growth, including structure, porosity, moisture, and workability.

The indiscriminate cutting of branches to stubs or lateral branches not large enough to assume the terminal role. Considered harmful and NOT recommended by ISA.

An elongated, thick-walled cell in xylem that conducts water and provides structural support, the primary water-conducting cell in conifers.

The loss of water vapor from a plant, primarily through stomata in the leaves, which drives water uptake from roots.

Stress experienced by a tree after being moved to a new location, caused by root loss and environmental changes.

Tree Risk Assessment Qualification. An ISA credential for arborists trained in systematic tree risk assessment methodology.

The process of determining the monetary value of a tree using methods such as the trunk formula or cost approach.

A systematic recording of tree data including species, size, condition, and location, used to guide management decisions.

A document specifying measures to protect trees during construction, including tree protection zones and construction methods.

A designated area around a tree where construction activities are restricted to prevent root and trunk damage, typically fenced.

Physical barriers such as wooden boards or fencing installed around tree trunks to prevent mechanical damage during construction.

The pressure exerted by water against the cell wall, keeping plant cells firm. Loss of turgor causes wilting.

All trees and associated vegetation within an urban area, including parks, yards, street trees, and natural areas.

The phenomenon where urban areas experience higher temperatures than surrounding rural areas, mitigated by tree canopy and evapotranspiration.

Pruning performed to maintain clearance from power lines and other utility infrastructure, often using directional pruning techniques.

A naturally occurring subdivision of a species with distinct morphological differences, designated in taxonomy with 'var.' abbreviation.

A strand of conducting tissue containing both xylem and phloem, transporting water, minerals, and sugars throughout the plant.

An organism that transmits a pathogen from one host to another, such as bark beetles spreading Dutch elm disease.

A wide, open-ended cell in angiosperm xylem that conducts water efficiently, connected end-to-end to form continuous tubes.

The degree of pathogenicity of a disease-causing organism, or its ability to cause severe damage to the host.

The upper surface of the zone of saturation in the soil where all pore spaces are filled with water.

A leaf arrangement with three or more leaves attached at the same node, encircling the stem.

The drooping of leaves and stems caused by insufficient water, vascular disease, or root damage that prevents water uptake.

The soil moisture level at which a plant can no longer extract water from the soil, causing permanent wilting.

A metal cage used to hold the root ball of a balled-and-burlapped tree. Typically cut away or bent down at planting.

The use of equipment to support an arborist in a stationary position while working in a tree, allowing hands-free operation.

Callus tissue produced by the cambium that grows over and compartmentalizes wounds, sealing exposed wood from decay organisms.

The woody tissue of a tree that conducts water and dissolved minerals upward from the roots.