ISA Exam Glossary: Core Terms You Must Know

ISA exam questions use precise technical language, and misreading one term can cost you a question even when you understand the concept underneath it. The highest-value words to lock in first cut across multiple domains at once: CODIT, cation exchange capacity (CEC), the sign-versus-symptom distinction, and the shall versus should language running through both ANSI Z133 and ANSI A300.

Note: For the master overview, see our Complete ISA Certified Arborist Exam Study Guide.

If you don't know the word, you can't answer the question, no matter how many years you've spent in a bucket truck. The ISA Certified Arborist exam draws its vocabulary from the ten domains that make up the test, so this list follows the same structure, domain by domain, with a real definition behind every term.

Tree Biology & Anatomy

Biology is 10% of the exam on its own, but these terms resurface constantly in diagnosis, soil, and risk questions too.

• Meristem: Tissue of actively dividing cells, the source of all new growth. Apical meristems extend length at the tips; the cambium adds girth.
• Cambium: The thin layer of dividing cells between xylem and phloem. It adds new xylem inward and new phloem outward each year, which is how a trunk lays down one growth ring per season.
• Xylem: Vascular tissue carrying water and minerals up from the roots. Wood is xylem: the outer, conducting rings are sapwood, and the inner rings are heartwood.
• Phloem: Vascular tissue carrying sugars down from the leaves. It sits just outside the cambium as the inner bark, which is why girdling damages it first.
• CODIT: Compartmentalization Of Decay In Trees. A tree walls off injury behind four boundaries rather than healing it; Wall 4, new wood grown after the injury, is the strongest.
• Reaction wood: Wood a leaning tree grows to reposition itself. Hardwoods grow tension wood on the upper side to pull the stem upright; conifers grow compression wood on the underside to prop it up.
• Apical dominance: The terminal bud's suppression of the lateral buds below it. Remove the leader and that suppression lifts, often triggering a flush of competing shoots.
• Mycorrhizae: Symbiotic fungi that colonize fine roots and extend a tree's reach for water and nutrients, in exchange for a share of its sugar.

Soil Management

The domain where experienced arborists lose the most points, since soil chemistry rarely comes up in daily fieldwork.

• Cation Exchange Capacity (CEC): A soil's ability to hold positive nutrients like calcium and potassium against leaching. Clay and organic matter carry high CEC; sand carries almost none.
• Soil texture: The fixed proportion of sand, silt, and clay in a soil, set by parent material. It drives drainage and can't be meaningfully changed on site.
• Soil structure: How soil particles aggregate into crumbs or blocks, with pore space between them. Unlike texture, structure builds over years, and compaction can destroy it in an afternoon.
• Compaction: Collapse of pore space, macropores first, under foot or equipment traffic. It suffocates roots by cutting off oxygen and is easy to misdiagnose as disease.
• Infiltration and percolation: Infiltration is water entering the soil at the surface; percolation is that water moving downward through the profile afterward. A crusted surface blocks infiltration; a compacted subsoil layer blocks percolation.
• Chlorosis: Yellowing between the leaf veins while the veins stay green, usually from high soil pH locking up iron or manganese rather than a true nutrient deficiency.

Pruning (ANSI A300)

The single largest domain at 16% of the exam, and most of its vocabulary comes straight from ANSI A300.

• Branch collar: The swollen trunk tissue at a branch's base that anchors compartmentalization. The final cut lands just outside it; cutting into it removes the tissue the tree needs to wall off the wound.
• Included bark: Bark folded inward between two stems instead of solid wood-to-wood contact. A structurally weak union, prone to splitting under load.
• Codominant stems: Two stems of similar diameter competing for the leader position. The union often develops included bark; the standard fix on a young tree is subordination, reduction cuts that let one stem take over instead of removing one outright.
• Crown cleaning, thinning, raising, and reduction: The four ANSI A300 pruning methods. Cleaning removes dead or weak wood; thinning reduces density for light and airflow; raising removes lower branches for clearance; reduction cuts back to a lateral to decrease height or spread.
• Topping: Cutting back to stubs or undersized laterals instead of the branch collar. It removes major photosynthetic capacity at once and triggers weak, fast watersprouts. Never acceptable.
• Lion tailing: Stripping interior foliage and leaving growth only at branch tips. It concentrates weight on a longer, whippier lever arm and raises the risk of wind-snap.
• Three-cut method: Undercut, top cut, then final cut just outside the branch collar, used on any limb heavy enough to tear bark if removed in one pass.

Tree Risk Management

Nearly every question in this domain traces back to one distinction: hazard versus risk.

• Hazard versus risk: A hazard is the potential to cause harm, a defect on its own. Risk adds a target: likelihood of failure, likelihood of impact, and severity of consequences, combined.
• Target: Anything a tree failure could damage: people, vehicles, structures, utilities. Target occupancy, how often something is present underneath, factors directly into the risk rating.
• The three risk factors: Likelihood of failure, likelihood of impact, and consequences, each rated on its own before combining into an overall risk rating.
• TRAQ (Tree Risk Assessment Qualification): A specialization built on top of ISA Certified Arborist status, covering formal, standards-based risk assessment. It requires certification first and renews on its own cycle.
• Levels of assessment: Level 1 is a limited visual screening for large populations. Level 2 is a full ground inspection, the standard for one tree of concern. Level 3 adds tools like resistance drilling when a visual inspection can't answer the question.

Safe Work Practices (ANSI Z133)

Nearly all of this domain's vocabulary comes from a single document: the ANSI Z133 safety standard.

• Shall versus should: Shall means mandatory, no situational exception. Should means recommended, a strong suggestion but not a citable violation if skipped. Exam questions often hinge on which word governs the scenario.
• Minimum Approach Distance (MAD): The distance an unqualified worker must keep from an energized line, roughly 10 feet for common distribution voltages. A "qualified" line-clearance arborist, meaning documented electrical training, can work closer.
• Kickback: The violent upward motion of a chainsaw's bar when its upper nose quadrant contacts wood or another object. A chain brake helps, but keeping the nose clear is the real prevention.
• Bight: The loop described by a rope or cable under tension, such as on a chipper's winch line. Standing inside it means a snapped line or shifted load moves through the space you occupy.
• Helmet classes: ANSI Z89.1 electrical ratings for arborist helmets: Class E (20,000 volts, required near energized conductors), Class G (2,200 volts, general work), and Class C (no electrical protection at all).

Diagnosis, Pests & Diseases

This domain rewards a process more than memorization, and the process runs on a handful of specific terms.

• Sign versus symptom: A sign is evidence of the causal agent itself, a fungal fruiting body, frass (insect boring debris), or the insect. A symptom is the tree's reaction: wilting, dieback, chlorosis.
• Biotic versus abiotic: Biotic disorders come from living agents: insects, fungi, bacteria. Abiotic disorders come from non-living causes like compaction, drought, or chemical injury, and get ruled out first in urban settings.
• Integrated Pest Management (IPM): A threshold-based approach: cultural controls first, biological controls second, chemical controls only once a real threshold is crossed and the first two aren't enough.
• Plant Health Care (PHC): The proactive side of tree care, right tree in the right place, proper soil and water management, aimed at preventing problems before they start. IPM is the reactive piece that runs inside it.
• Epinasty: Downward curling or twisting of leaves and shoots, most often a symptom of herbicide exposure rather than obvious chemical burn.

Tree Protection & Construction

The smallest domain on the exam, and the most procedural. Two acronyms carry most of the weight.

• Critical Root Zone (CRZ): The minimum volume of roots and soil a tree needs to survive, a biological fact that doesn't change based on a construction schedule.
• Tree Protection Zone (TPZ): The area fenced off on a job site, normally sized at least as large as the CRZ, using the rule of one foot of radius per inch of trunk diameter.
• DBH (Diameter at Breast Height): Trunk diameter measured at 4.5 feet above grade, the standard reference point for sizing a protection zone or running an appraisal.
• Drip line: The ground directly beneath a tree's outermost canopy, a rough field proxy for the root zone since roots typically extend well beyond it.

Identification & Selection

You don't need hundreds of species memorized. You need this vocabulary and a filtering process.

• Genus and species: The two-part scientific name for every tree: genus capitalized, like Quercus for oak, and specific epithet lowercase, like rubra for red, both italicized. A cultivar follows in single quotes, not italicized.
• Gymnosperm versus angiosperm: Gymnosperms are cone-bearing trees, mostly conifers, with seeds exposed on the cone. Angiosperms are flowering plants, covering essentially every broadleaf tree, with seeds enclosed in fruit.
• Opposite versus alternate branching: Whether buds emerge directly across from each other (opposite) or staggered (alternate). Most trees are alternate; the common opposite genera are remembered with the mnemonic MAD Horse: Maple, Ash, Dogwood, Horsechestnut.
• Right tree, right place: Matching a species to its site, hardiness zone, mature size, soil volume, and pH, before it goes in the ground. Most long-term tree failures trace back to a bad match at planting, not poor upkeep later.

Installation, Establishment & Urban Forestry

Two of the exam's smaller domains, grouped here because their vocabulary rarely overlaps with the rest of the test.

• Root flare: Where a trunk widens into its first structural roots. It should sit at or slightly above grade; nursery stock often buries it, so it must be found by hand, not assumed from the visible soil line.
• Stem girdling root: A root that grows across the trunk instead of away from it. As both thicken, it compresses tissue beneath the bark, cutting off flow years before the crown shows decline.
• Volcano mulching: Mulch piled in a cone against the trunk instead of a wide ring pulled back from the bark. It traps moisture and invites decay and rodent damage.

How These Terms Connect Across Domains

None of this vocabulary stays in one lane. CODIT explains why the branch collar matters so much in pruning, and hazard-versus-risk in tree risk management is biology and site conditions applied to one question: what happens if a defect gives way.

Not sure you're even eligible to sit the exam yet? The eligibility and application guide covers the experience requirement and how to apply.

Frequently Asked Questions

What does CODIT stand for?

Compartmentalization of Decay in Trees. It describes how a tree walls off damaged tissue behind chemical and physical boundaries instead of healing the wound. Trees compartmentalize; they never regenerate the wood that was already damaged.

What is cation exchange capacity (CEC)?

A soil's ability to hold positively charged nutrients like calcium and potassium against leaching. Clay and organic matter have high CEC; sandy soils have low CEC and lose nutrients quickly to irrigation or rain.

What is the difference between a sign and a symptom?

A sign is evidence of the causal agent itself, a fungal fruiting body or an insect. A symptom is the tree's response, such as wilting or chlorosis. A symptom alone doesn't tell you the cause.

What is the difference between hazard and risk?

A hazard is the potential to cause harm, on its own. Risk adds a target: the likelihood of failure, the likelihood of impact, and the severity of the consequences, combined. A dead tree in an empty field is a hazard but barely a risk.

What is the difference between shall and should in ANSI standards?

Shall means mandatory, no situational exception. Should means recommended, a strong suggestion, not a citable requirement. Both ANSI Z133 and ANSI A300 use this distinction, and exam questions often hinge on which word governs the scenario.

What is the difference between CRZ and TPZ?

The critical root zone (CRZ) is the biological minimum of roots and soil a tree needs to survive. The tree protection zone (TPZ) is the area fenced off on a job site, usually sized at least as large as the CRZ.

What is the difference between biotic and abiotic disorders?

Biotic disorders come from living agents: insects, fungi, bacteria. Abiotic disorders come from non-living causes like compaction, drought, or mechanical injury, and they're more common on developed sites, which is why they get ruled out first.

What does DBH mean?

Diameter at breast height, a tree's trunk diameter measured at 4.5 feet above grade. It's the standard reference point for sizing a tree protection zone, running an appraisal, or tracking a tree in an inventory.

What is TRAQ, and is it the same as ISA certification?

TRAQ, Tree Risk Assessment Qualification, is a specialization built on top of ISA Certified Arborist status, focused on formal tree risk assessment. It isn't a substitute for certification, and you need the base credential before the TRAQ course will accept you.

How many technical terms do I actually need to know for the ISA exam?

There's no official count, but terminology carries more weight than its own domain suggests, since the same core terms, CODIT, CEC, sign versus symptom, resurface across multiple domains at once. Prioritize the cross-cutting vocabulary in biology, soil, and risk before narrower, single-domain terms.