Certified Arborist key terms

A tree's internal defense process that uses four walls of wood to slow the spread of decay fungi after wounding. It limits damage but does not heal or remove infected wood.

A swollen ring of tissue at the base of a branch where it joins the trunk. Proper cuts leave it intact because it contains chemicals and wood that help the tree seal the wound.

A raised ridge of bark running along the top of a branch union. It marks the boundary between branch and trunk wood and guides where a pruning cut should be made.

Bark trapped inside a narrow branch union where two stems press together. It creates a weak attachment because wood fibers are not connected across the union, making failure more likely.

Wood formed to support leaning or unbalanced stems. In conifers it is compression wood on the lower side; in broadleaf trees it is tension wood on the upper side, and it behaves differently when cut.

A water-conducting cell in the xylem of broadleaf trees. Vessels are stacked end to end to form tubes that move water and dissolved minerals from roots to leaves.

The inner bark tissue that transports sugars made in leaves to roots, stems, and storage tissues. Damage to phloem can starve roots even when xylem water flow remains intact.

A plant variety selected and propagated by humans for desirable traits such as form, pest resistance, or fall color. Cultivar names are written in single quotes and are not the same as species.

The proportion of sand, silt, and clay particles in soil. Texture strongly affects water holding, drainage, aeration, and how easily roots can grow through the soil.

The arrangement of soil particles into aggregates and pores. Good structure allows air, water, and roots to move through soil; compaction and tilling can break aggregates apart.

The dry weight of soil divided by its volume, including pore space. High bulk density usually means compaction, which reduces root growth, aeration, and water movement.

The amount of water soil holds after excess water has drained away by gravity. It represents the upper limit of water available to roots between irrigation or rainfall events.

The speed at which water enters the soil surface. Slow infiltration increases runoff, erosion, and ponding, while fast infiltration may reduce water available to roots.

A measure of soil acidity or alkalinity on a scale from 0 to 14. It affects nutrient availability and root health, with most trees growing best in slightly acidic to neutral soil.

The ability of soil particles to hold positively charged nutrient ions such as calcium, magnesium, and potassium. Higher capacity generally means soil can store more nutrients for roots.

An element that a tree needs to complete its life cycle. Essential nutrients include macronutrients such as nitrogen, phosphorus, and potassium, and micronutrients needed in very small amounts.

The percentage by weight of nitrogen, available phosphate, and soluble potash in a fertilizer, shown as three numbers such as 10-10-10. It does not list other nutrients or filler.

Yellowing of leaves caused by insufficient chlorophyll. It often signals nutrient deficiency, poor soil drainage, root damage, or unsuitable soil pH rather than a pest problem.

The widened base of a tree trunk where major roots begin. It should be visible above ground after planting; burying it can lead to girdling roots, decay, and decline.

A root that grows around the trunk or another root and presses into the bark. It can restrict water and sugar movement, causing slow decline, dieback, and instability over time.

Stress after planting that may include wilting, leaf drop, slow growth, or dieback. It results from root loss, poor handling, improper planting depth, or inadequate water after transplanting.

Soil used to fill the planting hole around a new tree. The preferred practice is to use the native soil and avoid amendments that create a distinct, root-confining texture difference.

Pruning young or medium-aged trees to develop a strong central leader and well-spaced, well-attached branches. It focuses on future form and strength rather than immediate clearance or size reduction.

Pruning to reduce the height or spread of a tree canopy by cutting branches back to lateral limbs large enough to assume the terminal role. It is not the same as topping.

Removing most small inner branches along a limb and leaving foliage only at the ends. It can cause sunscald, poor taper, excessive sprouting, and weak branch structure.

Cutting a tree's main stem or large branches back to stubs without regard for branch structure. It causes decay, weak regrowth, and stress and is not accepted arboricultural practice.

Installing a flexible steel cable between limbs to limit movement and reduce stress on a weak union. It is a support system, not a repair, and requires inspection and maintenance.

A model showing that disease occurs only when a susceptible host, a virulent pathogen, and favorable environmental conditions occur together. Removing any one factor can prevent or reduce disease.

Tree damage or decline caused by nonliving factors such as drought, heat, frost, flooding, salt, compaction, or herbicide. It is not contagious and often appears in patterns related to site conditions.

A preventive, integrated approach to managing tree health using monitoring, cultural practices, resistant plants, and targeted treatments. It emphasizes diagnosing causes and treating only when justified by a threshold.

The pest population or damage level at which treatment is justified to prevent unacceptable harm. It helps avoid unnecessary pesticide use and guides monitoring-based decisions.

A systematic process for estimating the likelihood of a tree or tree part failing and the consequences to a target. It combines inspection, site evaluation, and sometimes advanced testing.

The probability that a tree or tree part will fail within a stated time period. It considers defects, species, site conditions, weather exposure, and load on the tree.

A person, vehicle, building, or other object that could be injured or damaged if a tree part fails. Risk assessment considers both the likelihood of failure and the value of the target.

The American national safety standard for arboricultural operations. It covers chainsaw use, climbing, electrical hazards, traffic control, and personal protective equipment for tree work.

The sudden backward or upward movement of a chainsaw when the nose of the guide bar contacts an object or the chain pinches. It is a major cause of chainsaw injuries.

The area beneath a tree where limbs and debris are expected to fall during removal or pruning. Only trained workers should enter, and the area must be controlled for safety.

A local law regulating trees on public or private property. It may require permits for removal, protect heritage trees, set planting standards, or require replacement planting.

Selecting a species and cultivar whose mature size, root needs, soil tolerances, and pest resistance match the planting site. This reduces future maintenance, conflicts, and risk.