Arborist Practice

Arboriculture Glossary

Definitions of the 50 most-tested arboriculture terms for the ISA Certified Arborist exam, grouped by domain.

Independent study reference. We are not affiliated with the International Society of Arboriculture. For definitions used on the exam itself, the authoritative source is the current ISA Arborists' Certification Study Guide. Use this page to refresh concepts, not as a substitute.

Tree Biology

Cambium
A thin layer of actively dividing cells between the bark and the wood that produces new xylem (wood) on the inside and new phloem (inner bark) on the outside.
CODIT
Compartmentalization of Decay in Trees. A model developed by Dr. Alex Shigo describing four walls (barriers) a tree creates to resist the spread of decay after wounding.
Compartmentalization
A tree's natural defense process (described by the CODIT model) in which it walls off injured or decayed tissue to prevent the spread of pathogens.
Mycorrhizae
Symbiotic associations between fungi and tree roots that enhance the tree's ability to absorb water and mineral nutrients, especially phosphorus.
Reaction Wood
Abnormal wood formed in leaning stems to correct orientation—compression wood in conifers (underside) and tension wood in hardwoods (upper side).

Pruning

Branch Bark Ridge
A raised line of bark in the branch crotch formed where branch tissue and trunk tissue meet, used as a guide for proper pruning cut angles.
Branch Collar
The swollen area of tissue at the base of a branch where it connects to the trunk or parent branch. Pruning cuts should preserve this structure.
Crown Reduction
Reducing the overall size of a tree's crown by cutting branches back to lateral branches large enough to assume the terminal role (at least one-third the diameter of the removed branch).
Three-Cut Method
A technique for removing large branches safely: (1) undercut partway, (2) top-cut farther out to remove the branch, (3) final cut at the branch collar.
Topping
An improper and harmful pruning practice that indiscriminately cuts branches back to stubs, destroying the tree's natural form and creating decay and hazard.

Soil Management

Bulk Density
The mass of dry soil per unit volume, including pore space. High bulk density indicates compacted soil with poor root penetration.
Cation Exchange Capacity (CEC)
A measure of the soil's ability to hold and exchange positively charged nutrient ions (cations) such as calcium, magnesium, and potassium.
Compaction
The compression of soil particles that reduces pore space, limiting water infiltration, gas exchange, and root growth.
Field Capacity
The amount of water remaining in soil after excess water has drained away by gravity, representing the ideal moisture level for root uptake.
Wilting Point
The soil moisture level at which water is held so tightly by soil particles that plant roots can no longer extract it, causing wilting.

Tree Risk

Girdling Root
A root that encircles the trunk or other roots, constricting the flow of water and nutrients and potentially weakening the tree's structural stability.
Hazard Tree
A tree with structural defects that could cause all or part of the tree to fail and strike a target (person, property, or activity).
Likelihood of Failure
The probability that a tree or tree part will fail within a defined time period, based on species, condition, and structural defects.
Resistograph
A diagnostic tool that measures the resistance of wood to a thin drill bit, detecting internal decay, cavities, and cracks.
Risk Rating
A categorization (low, moderate, high, extreme) of overall tree risk based on the combined likelihood of failure, impact, and consequences.

Diagnosis and Treatment

Anthracnose
A group of fungal diseases that cause irregular brown or black lesions on leaves, twigs, and sometimes fruit, often during cool, wet spring weather.
Dutch Elm Disease
A lethal vascular wilt disease of elms caused by the fungus Ophiostoma novo-ulmi, spread primarily by elm bark beetles.
Emerald Ash Borer (EAB)
An invasive wood-boring beetle (Agrilus planipennis) from Asia that attacks and kills North American ash trees by destroying the phloem and cambium.
Integrated Pest Management (IPM)
A science-based approach to managing pests that combines cultural, biological, mechanical, and chemical methods to minimize economic, health, and environmental risks.
Oak Wilt
A lethal vascular disease of oaks caused by the fungus Bretziella fagacearum, spread by sap-feeding beetles or through root grafts between adjacent trees.

Identification and Selection

Cultivar
A plant variety that has been selected and propagated for specific desirable characteristics such as form, color, disease resistance, or size.
DBH (Diameter at Breast Height)
The standard measurement of tree trunk diameter taken at 4.5 feet (1.37 m) above the ground on the uphill side.
Drip Line
The imaginary line on the ground directly below the outermost tips of a tree's branches, often approximating the extent of the root system.
Hardiness Zone
A geographically defined area based on average annual minimum temperature, used to determine which tree species are likely to survive in a given location.
Right Tree Right Place
A planting philosophy that matches a tree species' mature size, environmental tolerances, and growth habits to the specific conditions and constraints of the planting site.

Installation and Establishment

Balled and Burlapped (B&B)
A harvesting method in which the tree's root ball is dug with soil intact and wrapped in burlap for transport and planting.
Container-Grown
A tree grown in a pot or container at the nursery. These trees may develop circling roots that need correction at planting.
Planting Depth
The proper depth at which a tree should be planted, with the root flare at or slightly above the final grade.
Root Flare
The area at the base of the tree's trunk where the roots begin to spread outward. This should be visible at the soil surface after planting.
Transplant Shock
Stress experienced by a tree after transplanting due to root loss, resulting in slowed growth, leaf scorch, or dieback during the establishment period.

Trees and Construction

Critical Root Zone (CRZ)
The area of soil around a tree, usually calculated as 1 foot of radius per inch of trunk diameter (DBH), essential for tree survival and stability.
Pneumatic Excavation
A method of removing soil using compressed air (e.g., Air Spade, Air Knife) that displaces soil particles without damaging roots.
Root Barrier
A physical panel or fabric installed in the soil to redirect root growth away from foundations, sidewalks, or utility lines.
Tree Preservation Plan
A document that outlines specific measures to protect trees during construction, including CRZ delineation, fencing, and monitoring schedules.
Tree Protection Zone (Construction)
A fenced area around a tree, typically based on the CRZ, within which no grading, trenching, material storage, or vehicle traffic is permitted.

Safe Work Practices

ANSI A300
A series of American National Standards for tree care operations, covering pruning, fertilization, cabling/bracing, lightning protection, and other practices.
ANSI Z133
The American National Standard for Arboricultural Operations — Safety Requirements, covering safe work practices for tree care operations.
Minimum Approach Distance (MAD)
The closest distance an unqualified worker or conductive object may approach an energized power line, as specified by ANSI Z133 and OSHA.
OSHA
The Occupational Safety and Health Administration, the federal agency that sets and enforces workplace safety and health standards, including those for tree care.
Personal Protective Equipment (PPE)
Safety gear worn by arborists including hard hat, eye protection, hearing protection, gloves, chaps, and high-visibility clothing.

Urban Forestry

Canopy Cover
The percentage of ground area covered by the vertical projection of tree crowns, a key metric for urban forest health and goals.
Carbon Sequestration
The process by which trees absorb CO2 from the atmosphere and store it as carbon in biomass and soil, helping to offset greenhouse gas emissions.
Heat Island Effect
The phenomenon where urban areas experience higher temperatures than surrounding rural areas, partly mitigated by shade and transpiration from urban trees.
i-Tree
A suite of free software tools developed by the USDA Forest Service that quantifies the structure, function, and value of urban and community trees.
Tree Inventory
A systematic survey and database of individual trees in an urban area, recording species, size, condition, and location for management planning.

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