How a tree works, in plain words

Courses, exams and consulting arborists all lean on a small set of biology words: cambium, xylem, phloem, collar, compartmentalization. This page lists those words and what the sources say they mean. Where two sources describe something differently, the page says so.

The guide on pruning standards covers where the standards for cutting and pruning sit.

The three main parts

The British Columbia Ministry of Forests Tree Book says trees have three main parts: the leaves, the trunk and the roots. The upper part of the tree with the branches is called the crown.

Illinois Extension gives an average split by weight of the tree's parts. It says the average tree is 5 percent fine feeder roots, 15 percent larger transport roots, 60 percent trunk or main stem, 15 percent branches and twigs, and 5 percent leaves.

Roots

The BC Tree Book says roots have two jobs: to anchor the tree to the earth and to absorb water and nutrients from the soil. It adds that fungi live in and on the root cells and help them absorb water and nutrients, and that in return the fungi obtain food from the tree. Illinois Extension calls this a symbiotic relationship with mycorrhizae fungi, and says it increases the absorptive surface of the finer roots by a hundred times or more.

Where the roots are

Several extension pages give the depth of the root system. They do not use identical numbers, because they are describing it in different ways.

Source	What it says	About
Illinois Extension, 2024	Over 90% of root biomass in the upper 18 inches, almost 50% of that in the upper 6 inches	Biomass
Illinois Extension, 2019	99 percent of roots usually in the top 3 feet of soil	Roots
University of Missouri Extension	Most active roots within the top 12 inches, extending beyond the canopy	Active roots
Oregon State Extension	Most feeder roots in the top 12 inches, out to 1 1/2 times the canopy diameter	Feeder roots
University of Wisconsin Extension	Most roots grow horizontally in the top 4 to 15 inches	Roots

The Wisconsin page gives the reason: roots need oxygen, which permeates from the surface of the soil. Illinois Extension says tree root systems are very shallow and wide.

Transport roots and the drip line

Illinois Extension says four to 11 major, woody transport roots start at the base of the trunk and grow horizontally. It says their points of attachment are usually near ground level and show as a marked bulge of the trunk. It says transport roots extend four to seven times beyond the drip line, which it defines as the area beneath the crown that receives runoff from rainwater. It also says new roots form rapidly after injuries.

The terms "root plate" and "root zone" are common in tree work. None of the sources linked here define them.

The trunk, layer by layer

Colorado State Extension and the BC Tree Book describe the layers of a trunk from the outside in.

Layer	What the sources say
Outer bark	The protective outer covering. BC says it protects from fire or insects and insulates from extreme heat and cold
Phloem	The inner bark. It carries sugars made in the leaves through the tree, including down to the roots
Cambium	The layer of active cell division between the bark and the xylem
Sapwood	Younger rings of xylem. Their living cells carry water and store sugars
Heartwood	Older xylem rings no longer carrying water

Cambium, xylem and phloem

Oregon State Extension says the cambium is just inside the bark along the entire length and circumference of the stem, branches and roots. Its handbook on plant parts says xylem tubes conduct water and dissolved minerals, phloem tubes carry food such as sugars, and the cambium continuously produces new xylem and phloem cells. That new tissue is what increases a stem's girth.

The BC Tree Book adds that cambium cells grow more slowly in winter, and that the slower growth produces the annual rings. Colorado State says xylem tissue is the technical name for the wood.

Sapwood and heartwood

The BC Tree Book says that as new layers develop, the inner layers die and become heartwood, and calls heartwood dead wood in the centre of the tree that gives the tree its strength. Colorado State describes it as older rings no longer active in water transport and says heartwood is very susceptible to decay organisms.

Photosynthesis and stored energy

Oregon State Extension says photosynthesis converts carbon dioxide and water into carbohydrates, the basic food of the tree. Dr. Alex Shigo, a USDA Forest Service scientist, wrote that the energy enters as solar radiation and transforms carbon dioxide and water into carbohydrates, the chemical form in which energy is stored. The BC Tree Book says the leaves or needles do this, and that chlorophyll, the chemical that makes leaves green, is found inside cells where chloroplasts absorb the sun's energy.

Oregon State says water and nutrients move upward in the xylem, and food made in the leaves moves downward in the phloem. Colorado State adds that sapwood cells store sugars. This is the plain-words reason sources keep describing leaves, roots and stored sugars as one connected system.

The branch collar and the branch bark ridge

These two terms come up in every pruning discussion, so it helps to know what the biology pages mean by them.

Branch collar. Shigo calls the swollen base of the branch the collar. Colorado State Extension describes it as the place where the annual growth rings of the trunk overlap the annual growth rings of the side branch, like shuffling a deck of cards. It adds that in lumber, the branch collar is called the knot.
Branch bark ridge. Shigo's 1985 article says a living branch is separated from the trunk by an external feature called the branch-bark ridge and an inner partition of compacted xylem along the angle of the ridge. That is the only description in the sources linked here. The published tree care standards define the term in full; see the pruning standards guide linked above.

How trees respond to wounds

The main idea in this area is that a tree does not heal in the way a person does. Several Forest Service and university sources say it in nearly the same words.

Shigo, in a 1985 article hosted by the USDA Forest Service: trees cannot heal, they make no repairs. Instead they defend themselves from the consequences of injury and infection by walling off the damage. In a word, they compartmentalize.
Penn State Extension, updated November 10, 2024: a wounded tree cannot heal and can only defend itself from the spread of internal decay by walling off the damaged area and giving up those cells.
Kevin Smith, USDA Forest Service Northern Research Station, 2006: compartmentalization is a boundary-setting process that protects the vascular cambium from attack and favors tree survival.

Smith also writes that the challenge for tree care is to favor the biology that supports safe, healthy trees while understanding that all trees die and that all wood rots. Illinois Extension says new roots form rapidly after injuries, which is a separate response from wall-building in the trunk.

CODIT, the four walls

CODIT stands for compartmentalization of decay in trees. Penn State Extension says that in the 1970s, USDA Forest Service Forest Pathologist Dr. Alex Shigo developed the model to teach practitioners such as foresters and arborists. Smith says the concept has helped tree care practitioners interpret patterns of decay in living trees for more than 30 years.

Both Penn State and Colorado State describe four walls, but they do not describe them the same way. Both are shown here.

Wall	Penn State Extension	Colorado State Extension
1	The tree plugs xylem channels near the wound with tyloses or gums	Resistance is very weak up and down inside the xylem tubes
2	Forms in latewood, resisting spread inward toward the center	Walls toward the center are rather weak
3	Forms in living ray cells, resisting circular spread	Ray cell walls are somewhat resistant
4	The barrier zone, made by cambium cells, resists spread outward	Growth rings added after the injury are highly resistant

Colorado State explains why wall 1 is weak: otherwise the tubes would plug, stopping the flow of water, and kill the plant. It says decay moves up to a small degree but readily moves downward, and that the downward movement may be twenty or more feet and can include the root system. It describes the xylem as boxes framed by annual growth rings and ray cells.

Smith writes that the second stage of compartmentalization involves the formation of a barrier zone by the vascular cambium after wounding.

Decay words

A Forest Service publication of the Northern Research Station, a pictorial overview of discoloration and decay drawn from dissecting thousands of trees, uses these terms.

Heartrot. The publication says the old concept of heartrot, the decay of dead heartwood by heartwood-rotting fungi, has been replaced by a more dynamic idea of discoloration and decay in living trees.
Discoloration. Wood discolors before decay occurs. The publication says the discoloration process slows decay, giving the cambium time to grow and add new wood.
Reaction zone. At the boundary between infected, discolored wood and sapwood, a dark margin appears. The publication says this reaction zone retards the spread of the decay-causing pathogen.

For how these words feed into a risk report, see the tree risk assessment guide.

Where it sits in the ISA exam

The ISA Certified Arborist program guide, revised August 2025, lists the exam as 200 multiple-choice questions in 3.5 hours. It puts Tree Biology first among ten domains, at 11% of the questions.

That is the domain share the guide prints. It is not a syllabus. ISA says presenting the credential to potential employers makes a candidate more competitive in the job market. See the certifications guide for what each country asks for.

Tree identification

The sources here are from the United States and Canada. For identifying trees, see the guide on tree identification resources.