ISA Certified Arborist Exam Glossary

A plant problem caused by a nonliving factor, such as drought, flooding, temperature extremes, compaction, nutrient imbalance, chemical injury, or mechanical damage. It does not spread from plant to plant, though it often predisposes trees to pests and disease.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 5: Diseases and Disorders, NC State Extension

In IPM, the level of pest population or damage at which treatment becomes justified, based on economic, aesthetic, or plant-health tolerance. Below the threshold, monitoring continues without control measures.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 8: Integrated Pest Management (IPM), NC State Extension

Retrieving an injured, ill, or incapacitated worker from a tree or aerial lift. Rescuers must assess the situation, protect themselves first, and use a quick, safe method, so crews need training, equipment, and an emergency plan in advance.

Safe Work Practices · Source: National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service

A flowering plant whose seeds develop enclosed within an ovary that ripens into a fruit. Broadleaf trees such as oaks, maples, and magnolias, as well as palms, are angiosperms.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

Control exerted by a growing shoot tip, mainly through the hormone auxin, that suppresses the growth of buds and branches below it. Removing the tip releases those lateral buds, which is why heading cuts trigger sprouting.

Tree Biology · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

The last cut in felling, made from the opposite side of the trunk from the notch and stopped short of it so a strip of hinge wood remains to control the fall.

Back Cut: definition, exam questions and common mistakes →

Safe Work Practices · Source: 29 CFR 1910.266 - Logging operations, Occupational Safety and Health Administration (OSHA)

Field-grown nursery stock dug with a ball of soil around its roots, wrapped in burlap and twine and often held in a wire basket. Once the tree is set in the hole, wrapping and basket are removed from at least the upper root ball.

Installation and Establishment · Source: Tree Planting Steps (CMG GardenNotes #636), Colorado State University Extension

A dangerous felling failure in which the trunk splits vertically upward from the back cut before the hinge can work, so part of the trunk rises violently. It is most likely in heavy, forward-leaning trees and can strike the operator.

Back Cut: definition, exam questions and common mistakes →

Safe Work Practices · Source: Chainsaw Safety: Preventing Common Tree Felling Accidents (Circular 1243), University of Georgia Cooperative Extension

Nursery stock sold with no soil around its roots, usually handled while dormant. It is light and inexpensive, but the exposed roots must be kept moist and planted promptly, spread out in a natural pattern.

Installation and Establishment · Source: How to Plant Trees and Shrubs, The Morton Arboretum

In the CODIT model, the strongest boundary (Wall 4), produced by the cambium after an injury. It divides wood that existed when the tree was wounded from wood grown later, usually keeping decay out of the new growth rings.

Tree Biology · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

The standard two-part scientific naming system: the genus followed by the specific epithet, as in Quercus alba. Names are italicized with the genus capitalized, avoiding the confusion caused by regional common names.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

A plant problem caused by a living organism, such as a fungus, bacterium, virus, nematode, insect, or mite. Infectious diseases of this kind can spread to other plants when susceptible hosts and favorable conditions are present.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 5: Diseases and Disorders, NC State Extension

The raised line of bark in the crotch of a branch union, formed as the trunk and branch expand against each other. A proper removal cut starts just outside this ridge and angles away from the trunk.

Branch Collar: definition, exam questions and common mistakes →

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

The swollen ring of tissue where a branch joins the trunk, formed by overlapping trunk and branch wood. Proper removal cuts are made just outside it, leaving it intact so the tree can close the wound and resist decay.

Branch Collar: definition, exam questions and common mistakes →

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

The weight of dry soil in a given volume, pores included, usually reported in grams per cubic centimeter. Higher values signal less pore space, so bulk density is a standard measure of soil compaction.

Soil Management · Source: Soil Health – Bulk Density/Moisture/Aeration (Guides for Educators), USDA Natural Resources Conservation Service

Installing supplemental support in a tree, such as steel or synthetic cables between limbs and threaded rods (braces) through weak unions, to limit limb movement and take load off weak unions. It lowers, but does not eliminate, failure risk and needs periodic inspection.

Tree Risk · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

A patch of killed bark and underlying cambium on a trunk or branch, often sunken, cracked, or discolored. Fungal or bacterial infection, insect injury, frost or sun damage, or mechanical wounds can cause it, and large cankers can girdle stems.

Diagnosis and Treatment · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

The percentage of a defined area, such as a city or neighborhood, covered by tree crowns when viewed from above. Communities measure it to track change over time and often set canopy cover goals in urban forest plans.

Urban Forestry · Source: Urban & Community Forestry: A Guide for Local Governments in Washington, Washington State Department of Natural Resources

A soil's ability to hold and exchange positively charged nutrient ions (cations), such as calcium, magnesium, and potassium, on negatively charged clay and organic matter. Higher CEC means greater nutrient-holding and buffering capacity.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

A chainsaw safety device that quickly stops the chain from turning. It is activated manually by pushing the front hand guard ahead with the left wrist or, on many saws, automatically by the inertia of a kickback.

Safe Work Practices · Source: Chainsaw Safety: Preventing Common Tree Felling Accidents (Circular 1243), University of Georgia Cooperative Extension

Abnormal yellowing of leaves that should be green, because too little chlorophyll is being made. Common causes include iron or manganese deficiency (often tied to high soil pH), root damage, and poor drainage; in interveinal chlorosis the veins stay green.

Diagnosis and Treatment · Source: Extension Gardener Handbook: Glossary, NC State Extension

A root that grows around the inside wall of a nursery container instead of outward. Such roots should be cut or shaved off at planting, because if left they can keep circling and later girdle the trunk.

Installation and Establishment · Source: How to Plant Trees and Shrubs, The Morton Arboretum

A model of how trees respond to wounding: instead of replacing damaged tissue, they form chemical and anatomical boundaries that wall off injured wood and limit the spread of discoloration and decay.

Tree Biology · Source: Tree Growth and Decay (CMG GardenNotes #611), Colorado State University Extension

Paired or multiple upright leaders of roughly equal size that fork from one point and compete for dominance. Their union forms no branch collar, often traps included bark, and tends to split, so structural pruning subordinates one stem while the tree is young.

Codominant Stems: definition, exam questions and common mistakes →

Pruning · Source: Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension

A two-way communication protocol between workers aloft and on the ground: a command such as "stand clear" is given, and an acknowledging response such as "all clear" must be received before material is cut or dropped.

Safe Work Practices · Source: Solutions for Tree Care Hazards (OSHA 3940), Occupational Safety and Health Administration (OSHA)

A leaf whose blade is divided into separate leaflets on a common stalk, as in ash and walnut. A bud sits at the base of the whole leaf but not at each leaflet, which distinguishes it from a twig of simple leaves.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

The shelf-, hoof-, or bracket-shaped fruiting body of a wood-decay fungus growing on a trunk, root flare, or branch. A conk is a sign that decay is present inside, often extensively.

Tree Risk · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

How serious the outcome would be, whether injury, property damage, or disruption, if a failing tree or part hit a target. It depends mostly on the target's value, along with the size of the part, how far it falls, and any protection.

Tree Risk · Source: Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative

The soil area around a tree containing the roots it most needs to stay upright and healthy; injuring roots there sharply raises the risk of decline or failure. It is usually sized from trunk diameter, but formulas differ among standards and jurisdictions.

Critical Root Zone (CRZ): definition, exam questions and common mistakes →

Trees and Construction · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

Pruning that targets dead, dying, diseased, broken, or weakly attached branches, and often water sprouts, to lower risk and improve health. Mature trees need this type of pruning more often than any other.

Pruning · Source: Pruning Mature Shade Trees (CMG GardenNotes #615), Colorado State University Extension

Pruning off lower limbs so that people, vehicles, buildings, signs, or views have room beneath the crown. Enough live crown should remain for trunk taper and health; USDA Forest Service guidance keeps live branches on at least two-thirds of tree height.

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

Selectively removing small live branches so those left are spread evenly along limbs and through the crown. It lets in more light and air and lightens limb weight while keeping the tree's natural shape. Stripping inner branches from limbs produces lion-tailing.

Tree Crown Thinning: definition, exam questions and common mistakes →

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

A cultivated variety chosen for desirable features, such as form, color, or disease resistance, and normally reproduced by cuttings, grafting, or other vegetative means so offspring match the parent. Its name appears in single quotes: Acer rubrum 'October Glory'.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

Describes a tree or shrub that loses its entire set of leaves every year, usually ahead of winter or a dry season, and leafs out again later. Most temperate broadleaf trees and a few conifers, such as larch and baldcypress, are deciduous.

Tree Identification and Selection · Source: Extension Gardener Handbook: Glossary, NC State Extension

A flaw in a tree's form or wood, such as decay, cracks, cankers, weak or included-bark unions, dead branches, or root damage, that makes the tree or one of its parts more likely to fail.

Tree Risk · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

An identification tool built from pairs of contrasting statements about features such as leaves, buds, or bark. Choosing the statement that fits at each step leads, pair by pair, to the plant's name.

Tree Identification and Selection · Source: Extension Gardener Handbook: Glossary, NC State Extension

Touching a live electrical conductor, for example an overhead power line, with any part of the body. Current can flow through the worker to ground, causing electrocution or severe burns.

Safe Work Practices · Source: Electricity and Tree Care Work: A Deadly Combination (OSHA 3861), Occupational Safety and Health Administration (OSHA)

A model showing that infectious disease requires three things at once: a susceptible host, an organism able to infect it, and conditions that favor infection, lasting long enough. Altering any one side, such as by planting resistant species, can prevent or reduce disease.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 5: Diseases and Disorders, NC State Extension

The circle on the ground traced directly under the outer edge of a tree's branches. It is a handy reference for root protection, but roots usually spread well past it, so it often understates the area needing protection.

Trees and Construction · Source: Extension Gardener Handbook: Glossary, NC State Extension

The area beneath and around overhead tree work where cut branches, tools, or debris may fall. It should be marked, kept clear, and entered only after clear communication with the worker aloft.

Safe Work Practices · Source: Tree Care Work: Falls and Falling Object Hazards (Hazard Bulletin HB-3731), Occupational Safety and Health Administration (OSHA)

A planned and cleared path for moving quickly away from a tree being felled, angled diagonally back from the intended line of fall (commonly about 45 degrees) and never directly behind the tree.

Safe Work Practices · Source: 29 CFR 1910.266 - Logging operations, Occupational Safety and Health Administration (OSHA)

Describes a plant that keeps living foliage year-round because it sheds older leaves gradually rather than all at once. Most conifers and some broadleaf trees, such as southern magnolia and American holly, are evergreen.

Tree Identification and Selection · Source: Extension Gardener Handbook: Glossary, NC State Extension

A tree form with one dominant central trunk and smaller, subordinate side branches, giving a conical or pyramidal crown, as in most conifers and sweetgum. It contrasts with the decurrent form, whose competing branches make a rounded, spreading crown.

Tree Identification and Selection · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

The water content a soil retains once free drainage after a thorough soaking has largely stopped. Water held between this level and the permanent wilting point is the portion plants can use.

Soil Management · Source: Extension Gardener Handbook: Glossary, NC State Extension

A pruning cut made tight against the trunk, removing the branch collar or cutting inside the branch bark ridge. It wounds trunk tissue, enlarges the wound, and makes decay of the stem more likely.

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

A knot, such as the prusik or Blake's hitch, tied around a rope so that it grips when loaded but can be slid along the rope when unloaded. Climbers use friction hitches to ascend, descend, and adjust their position.

Safe Work Practices · Source: National Tree Climbing Guide (2015 Electronic Edition), USDA Forest Service

A root that wraps around or presses across the trunk base or other major roots, often hidden at or just below the soil surface. As both enlarge it constricts the trunk and weakens the tree; deep planting and uncorrected circling roots are common causes.

Installation and Establishment · Source: Girdling Roots: To Cut or Not to Cut, Penn State Extension

Raising or lowering the soil level around an existing tree. Fill smothers roots by limiting air and water movement into the soil, while cuts remove roots near the surface; both are major causes of construction-related decline.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

A seed plant whose seeds are not enclosed in an ovary but develop exposed, usually on the scales of cones. Conifers such as pines, spruces, and firs, along with ginkgo, are gymnosperms.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

Cutting a stem back to a bud, to a stub between buds, or to a side branch too small to assume the lead. The cut triggers clusters of weakly attached sprouts, so it is avoided on most landscape trees.

Pruning · Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension

The older, central wood that no longer conducts water and has no living cells. Often darkened by deposited compounds, it still gives structural support but has no living defenses against decay organisms.

Tree Biology · Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension

The strip of uncut wood left between the notch and the back cut when felling. It keeps the tree attached to the stump as it falls and steers it in the intended direction, so the back cut must stop short of it.

Back Cut: definition, exam questions and common mistakes →

Safe Work Practices · Source: 29 CFR 1910.266 - Logging operations, Occupational Safety and Health Administration (OSHA)

Bark trapped inside a tight, V-shaped union as two stems, or a branch and stem, grow pressed together. Because sound wood cannot form across the union, the attachment is weak and prone to splitting.

Included Bark: definition, exam questions and common mistakes →

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

Touching an object, such as a branch, tool, rope, or vehicle, that is itself touching a live electrical conductor. Current can pass through that object and the worker to ground.

Safe Work Practices · Source: Electricity and Tree Care Work: A Deadly Combination (OSHA 3861), Occupational Safety and Health Administration (OSHA)

A decision-making approach that keeps pests below damaging levels by combining accurate identification, monitoring, action thresholds, and compatible cultural, mechanical, biological, and chemical controls, using pesticides only when needed and as selectively as possible.

Diagnosis and Treatment · Source: Integrated Pest Management and Plant Health Care (CMG GardenNotes #101), Colorado State University Extension

A non-native organism whose arrival in an ecosystem harms, or is likely to harm, the economy, the environment, or human health. Emerald ash borer, which has killed vast numbers of ash trees, is a prime example.

Urban Forestry · Source: What are Invasive Species?, National Invasive Species Information Center, USDA National Agricultural Library

A pre-work talk, repeated whenever conditions change, in which the person in charge covers the job's hazards, the procedures to be used, special precautions, control of energy sources such as power lines, required PPE, and each worker's role.

Safe Work Practices · Source: 29 CFR 1910.269 - Electric power generation, transmission, and distribution, Occupational Safety and Health Administration (OSHA)

A sudden, violent movement of a running chainsaw's guide bar up and back toward the operator, typically when the upper nose of the bar (the kickback zone) contacts wood or another object, or the chain is pinched in the cut.

Safe Work Practices · Source: Chainsaws - Kickback (OSH Answers fact sheet), Canadian Centre for Occupational Health and Safety

An assessor's judgment of how probable it is that a tree or one of its parts will break or uproot during a set period, based on defects, expected loads, tree condition, species traits, and site factors. ISA categories range from improbable to imminent.

Tree Risk Assessment: definition, exam questions and common mistakes →

Tree Risk · Source: Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative

How likely it is that a failing tree or branch would actually hit a target, considering the probable direction of fall, the kind of target, how often it is present, and any shelter, such as a structure, in between.

Tree Risk Assessment: definition, exam questions and common mistakes →

Tree Risk · Source: Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative

The quickest level of tree risk assessment (Level 1): a visual check of individual trees or whole populations, such as a walk-by or drive-by survey, to spot obvious defects that make failure probable or imminent.

Tree Risk · Source: Urban Forestry for Local Government, Texas A&M Forest Service

Over-thinning that strips the inner side branches and foliage along limbs, leaving tufts of foliage only at the branch ends. It shifts weight outward, reduces taper and wind damping, and raises the risk of branch failure and sunscald.

Lion Tailing: definition, exam questions and common mistakes →

Pruning · Source: Lion Tailing and its Impact on Tree Health (MF3551), Kansas Forest Service, K-State Research and Extension

The nearest a worker, together with any tool or material being held, may get to an energized or grounded electrical conductor or object. Required distances grow with voltage and are larger for workers who are not electrically qualified.

Safe Work Practices · Source: 29 CFR 1926.968 - Definitions (Electric Power Transmission and Distribution), Occupational Safety and Health Administration (OSHA)

A layer of material, such as wood chips or shredded bark, spread over the root zone to hold soil moisture, buffer temperature swings, and cut competition from weeds and turf. Organic mulch also builds soil organic matter; keep it pulled back from the trunk.

Installation and Establishment · Source: Extension Gardener Handbook: Glossary, NC State Extension

Mutually beneficial associations between certain soil fungi and fine roots. The fungi extend the root system's reach and improve water and nutrient uptake, while receiving sugars from the tree.

Tree Biology · Source: Vegetative Plant Parts (EM 9904), Oregon State University Extension Service

The wedge-shaped cut made in the side of a tree facing the intended direction of fall. Together with the hinge it guides the fall; common types are the open-face, conventional, and Humboldt notches.

Safe Work Practices · Source: 29 CFR 1910.266 - Logging operations, Occupational Safety and Health Administration (OSHA)

How much of the time a target, such as people or vehicles, is present within the area a failing tree or part could strike, rated from rare to constant. It is a key input to the likelihood of impact.

Tree Risk · Source: Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative

A pattern in which two leaves, each with its bud, attach at the same node on opposite sides of the twig. It is less common than alternate arrangement, so it quickly narrows identification; maples, ashes, and dogwoods are examples.

Tree Identification and Selection · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

Gear a worker wears for protection against job hazards. For arborists it typically means head, eye, hearing, hand, leg (chainsaw-resistant chaps or pants), and foot protection, chosen for the task and inspected before use.

Safe Work Practices · Source: Personal Protective Equipment - Overview, Occupational Safety and Health Administration (OSHA)

The inner-bark tissue that carries sugars and other products of photosynthesis from the leaves to stems, roots, and other living tissues, where they are used or stored. Wounds that girdle it can starve the roots.

Tree Biology · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

The process by which leaves and other green tissues capture light energy and use carbon dioxide and water to make sugars, releasing oxygen. These sugars fuel growth and are stored, largely as starch.

Tree Biology · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

One of the regions on the USDA Plant Hardiness Zone Map, which divides areas by their average annual extreme minimum winter temperature in 10°F bands, with 5°F half zones. It is used to judge which trees and other perennials can survive local winter cold.

Tree Identification and Selection · Source: 2023 USDA Plant Hardiness Zone Map, USDA Agricultural Research Service

A holistic, preventive approach to caring for trees and landscapes that applies IPM principles. It emphasizes keeping plants healthy through good selection, siting, and cultural care, plus regular monitoring, so problems are prevented or caught early.

Diagnosis and Treatment · Source: Integrated Pest Management and Plant Health Care (CMG GardenNotes #101), Colorado State University Extension

A site meeting before work starts that brings together the owner, contractors, designers, and arborist to walk the protection zones, review tree protection requirements, and agree on rules and penalties for violations.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

Specialized wood formed in leaning stems and branches that helps support them or bend them back toward vertical. Conifers form compression wood on the lower side; hardwoods form tension wood on the upper side.

Tree Biology · Source: Wood Handbook: Wood as an Engineering Material (FPL-GTR-190), Chapter 3: Structure and Function of Wood, USDA Forest Service, Forest Products Laboratory

Removing the end of a stem or limb where a living side branch can take over as the new leader; that branch should usually be at least one-third the diameter of the removed portion. It is the proper way to reduce height or spread.

Crown Reduction: definition, exam questions and common mistakes →

Pruning · Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension

A cut that takes a branch off entirely where it joins the trunk or parent limb, placed just outside the branch collar. Also called a thinning cut, it opens the crown with little effect on height.

Pruning · Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension

Actions that reduce tree risk, either by treating the tree (pruning, support systems, site improvements, or removal) or by managing the target (moving it or restricting access). Removal is generally considered only after other options.

Tree Risk · Source: Tree Risk Management (FNR-475-W), Purdue Extension, Forestry and Natural Resources

The overall level of tree risk, commonly low, moderate, high, or extreme, found by combining likelihood of failure, likelihood of impact, and consequences in rating matrices. It is weighed against the owner's risk tolerance to guide mitigation.

Tree Risk · Source: Trees: Dendrophobia - The Fear of Trees, University of Kentucky Urban Forest Initiative

The widened base of the trunk where the main structural roots spread away from the stem, also called the trunk flare or root collar. At planting it should be visible at or slightly above grade; burying it is a common error.

Installation and Establishment · Source: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry

Cleanly cutting roots with sharp tools, for example at the edge of a required excavation, rather than leaving them torn by machinery. Clean cuts limit injury, and exposed roots should be covered with soil quickly so they do not dry out.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

The outer, usually lighter-colored band of wood that still contains living cells. It conducts water and stores starch, and because it is alive it can respond actively to wounding, unlike heartwood.

Tree Biology · Source: Wood Handbook: Wood as an Engineering Material (FPL-GTR-190), Chapter 3: Structure and Function of Wood, USDA Forest Service, Forest Products Laboratory

Direct evidence of the causal organism itself or its products on the plant, such as fungal fruiting bodies, ooze, mildew, insects, cast skins, or frass. Signs differ from symptoms, which are the plant's own reactions.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 7: Diagnostics, NC State Extension

Compression of soil, often by vehicles, equipment, or foot traffic, that collapses pore space. Compacted soil has higher bulk density and restricts root growth, water infiltration, drainage, and the oxygen supply to roots.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

A distinct layer within a soil, roughly parallel to the surface, that differs from the layers above and below in properties such as organic matter, color, or clay content. The sequence of horizons (O, A, B, C) forms the soil profile.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

A measure of soil acidity or alkalinity on a logarithmic 0 to 14 scale, with 7 neutral. It strongly affects nutrient availability; for example, iron and manganese become less available to trees in high-pH soils.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

The arrangement of sand, silt, and clay particles into clumps called aggregates or peds, along with the pores between them. Good structure lets air, water, and roots move freely; compaction and working wet soil easily destroy it.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

How much sand, silt, and clay a soil contains, expressed as relative percentages that place it in a textural class such as loam or clay. Texture is essentially permanent and strongly affects drainage, water holding, aeration, and nutrient retention.

Soil Management · Source: Extension Gardener Handbook, Chapter 1: Soils and Plant Nutrients, NC State Extension

Temporarily supporting a newly planted tree with stakes and wide, flexible ties, used only when needed, such as on windy sites or for trees that cannot stand alone. Supports come off once roots anchor the tree, so the trunk can flex and strengthen.

Installation and Establishment · Source: Tree Support Systems (FNR-547-W), Purdue Extension, Forestry and Natural Resources

Pruning, mainly of young and medium-aged trees, to build a strong framework: one dominant leader, well-spaced scaffold branches smaller than the trunk, and no codominant stems or included bark. It reduces future failures and the need for large cuts.

Structural Pruning: definition, exam questions and common mistakes →

Installation and Establishment · Source: Structural Pruning of Young Shade Trees (CMG GardenNotes #613), Colorado State University Extension

A visible change in a plant's own appearance or growth caused by a pest, pathogen, or environmental problem, such as wilting, chlorosis, leaf spots, cankers, or dieback. Symptoms show how the plant is reacting, not necessarily what caused it.

Diagnosis and Treatment · Source: Extension Gardener Handbook, Chapter 7: Diagnostics, NC State Extension

In tree risk assessment, anyone or anything a failing tree or branch could hurt, damage, or interrupt, such as people, vehicles, buildings, or ongoing activities. If nothing could be struck, a failure causes no harm.

Tree Risk Assessment: definition, exam questions and common mistakes →

Tree Risk · Source: Tree Risk Management (FNR-475-W), Purdue Extension, Forestry and Natural Resources

A sequence for removing heavier branches without tearing bark: an undercut partway through the branch some distance out, a top cut that drops the limb's weight, then a final cut just outside the branch collar to remove the stub.

Pruning · Source: Pruning Cuts (CMG GardenNotes #612), Colorado State University Extension

Reducing a tree's size by sawing large live limbs and leaders off between branches or at undersized laterals, without regard to tree structure. It leads to decay, weakly attached sprouts, and stress, and is not an accepted practice.

Crown Reduction: definition, exam questions and common mistakes →

Pruning · Source: What’s Wrong with Topping? (FNR-FAQ-14), Purdue Extension, Forestry and Natural Resources

The loss of water vapor from a plant, mostly through the stomata of its leaves. The resulting pull draws water and dissolved minerals up through the xylem from the roots and helps cool the leaves.

Tree Biology · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension

Stress a tree suffers after planting or transplanting, mainly because much of its absorbing root system was lost, so water uptake cannot keep up with demand. Wilting, leaf scorch, slow growth, and dieback can persist until roots regenerate.

Installation and Establishment · Source: Tree Planting Steps (CMG GardenNotes #636), Colorado State University Extension

A systematic survey that records each tree in an area: where it is, its species and size, its condition, and the care it needs. The data guide planting, maintenance schedules, risk management, and budgets for an urban forest.

Urban Forestry · Source: Urban & Community Forestry: A Guide for Local Governments in Washington, Washington State Department of Natural Resources

A local law that regulates trees, often starting with public trees in parks and street rights-of-way and sometimes extending to private trees. Ordinances typically address planting, maintenance, protection, removal permits, and penalties.

Tree Ordinance: definition, exam questions and common mistakes →

Urban Forestry · Source: Urban & Community Forestry: A Guide for Local Governments in Washington, Washington State Department of Natural Resources

A written plan, prepared before construction, that identifies which trees will be preserved and how: protection zones and fencing, limits on traffic, grading, and storage, special work methods, and who is responsible for compliance.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

A defined area around a tree, fenced off before work begins, where construction activities such as grading, trenching, traffic, and material storage are prohibited. Its size is set by the project arborist or local rules, often based on trunk diameter.

Tree Protection Zone (TPZ): definition, exam questions and common mistakes →

Trees and Construction · Source: Preventing Construction Damage to Trees and Shrubs, The Morton Arboretum

Digging a long, narrow excavation, usually for utilities or footings. Trenches near trees sever many roots, so utilities should be routed away from trees, tunneled beneath roots, or placed as far from the trunk as possible.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

Installing utilities by boring beneath a tree's root system instead of trenching through it. Because most roots grow near the surface, a tunnel passes below them and avoids severing much of the root system.

Trees and Construction · Source: Protecting Trees During Construction (Fact Sheet 7.420), Colorado State University Extension

A long-range strategic plan for a community's trees that sets goals, objectives, and actions, usually informed by inventory and canopy data, to guide planting, maintenance, policy, and budgets over many years.

Urban Forestry · Source: Urban & Community Forestry: A Guide for Local Governments in Washington, Washington State Department of Natural Resources

An urban area that is noticeably warmer than its surroundings because buildings and pavement absorb and re-emit heat and vegetation is scarce. Tree shade and transpiration help cool these areas.

Urban Forestry · Source: What Are Heat Islands?, U.S. Environmental Protection Agency

A thin layer of dividing cells between the bark and the wood. It adds new xylem toward the inside and new phloem toward the outside each year, so it is responsible for growth in diameter.

Tree Biology · Source: Vegetative Plant Parts (EM 9904), Oregon State University Extension Service

Heaping mulch into a deep cone against the trunk. The excess layer can starve roots of oxygen and water and keeps bark constantly moist, inviting decay, pests, and eventual decline.

Volcano Mulching: definition, exam questions and common mistakes →

Installation and Establishment · Source: Mulching Trees and Shrubs (CMG GardenNotes #658), Colorado State University Extension

Paint, tar, or sealant applied over pruning cuts or other wounds. Dressings do not stop decay and can slow wound closure, so they are generally not recommended, except in narrow cases such as limiting oak wilt or Dutch elm disease spread.

Pruning · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

Firm, hardening tissue that forms at the edges of a wound and grows over it to close it. It develops from callus, the softer undifferentiated cells formed first. An even ring around a pruning cut shows the cut was well placed.

Tree Biology · Source: How to Prune Trees (NA-FR-01-95, revised 2012), USDA Forest Service, Northeastern Area State and Private Forestry

The water-conducting tissue that makes up a tree's wood. It moves water and dissolved minerals up from the roots, stores energy reserves, and provides support; each year's new layer forms a growth ring.

Tree Biology · Source: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension