Tree work words: tree biology and plant health

Tree biology words decide how a climber or ground crew reads what is in front of them: where the wood is living, where a branch joins, how a wound closes. This page lists the words a crew meets, with the meaning each named source gives.

Every meaning is written in our own words from the page named in its row. A word that none of those pages defines is not listed. The wider A to Z list is the tree work glossary.

Words and meanings

Term	Meaning	Source
Cambium	Penn State's youth guide describes a layer of cells that adds new inner bark and new wood each year. Washington State's list describes dividing cells that turn into phloem on the outside and xylem on the inside, which thickens stems and roots.	Penn State youth tree guide, glossary; Washington State University list, Woody Plant Vocabulary
Cork cambium	In the Washington State list, the dividing tissue that builds the tough protective outer bark.	Washington State University list, Woody Plant Vocabulary
Bark	The QTRA glossary applies the word to everything outside the vascular cambium, phloem included. Penn State's youth guide describes a protective layer made of inner and outer bark.	QTRA glossary of tree terms, PDF with 1999 extracts; Penn State youth tree guide, glossary
Phloem	Penn State's youth guide places it in the inner bark, where tubes move sugar made in the leaves down to the branches, stem and roots. It says phloem cells that die join the outer bark.	Penn State youth tree guide, glossary
Xylem	In the Washington State list, the tissue that moves water and minerals upward and also supports the tree. It grows inward from the cambium and turns into wood once it stiffens with lignin.	Washington State University list, Woody Plant Vocabulary
Sapwood	In the Washington State list, the outer wood that is still moving water and minerals along the stem. The QTRA glossary calls it living xylem.	Washington State University list, Woody Plant Vocabulary; QTRA glossary of tree terms, PDF with 1999 extracts
Heartwood	Washington State describes wood that has changed inward from sapwood, still gives strength, and holds chemical defence against decay organisms. It adds that some trees never form it. Penn State's youth guide says it is not living wood.	Washington State University list, Woody Plant Vocabulary; Penn State youth tree guide, glossary
Growth ring	In the Washington State list, the bands of xylem seen when a stem, branch or root of some trees is cut across. In temperate zones each band usually marks one year of growth.	Washington State University list, Woody Plant Vocabulary
Ray	In the Washington State list, tissue that runs sideways across xylem and phloem and handles transport, storage, strength and defence.	Washington State University list, Woody Plant Vocabulary
Lignin	In the Washington State list, a substance in some cell walls that thickens and strengthens them, which makes decay and pest damage less likely.	Washington State University list, Woody Plant Vocabulary
Meristem	In the Washington State list, tissue whose cells keep dividing. It is found in root tips, buds, cambium, cork cambium and latent buds.	Washington State University list, Woody Plant Vocabulary
Crown, canopy	The upper part of a tree made up of branches, twigs and leaves, in Penn State's youth guide. The QTRA glossary describes it as the part that carries most of the foliage.	Penn State youth tree guide, glossary; QTRA glossary of tree terms, PDF with 1999 extracts
Leader	In the Washington State list, the main upright stem at the top of a tree, which holds sway over part of the crown by keeping side branches in check.	Washington State University list, Woody Plant Vocabulary
Excurrent and decurrent	Washington State says an excurrent tree has one central leader and a cone-shaped crown, and a decurrent tree has a round or spreading crown. The QTRA glossary describes decurrent as a crown carried on several wide limbs of similar size.	Washington State University list, Woody Plant Vocabulary; QTRA glossary of tree terms, PDF with 1999 extracts
Apical dominance	In the Washington State list, the top bud holding back growth of the side buds.	Washington State University list, Woody Plant Vocabulary
Branch collar	Washington State describes the zone where a branch meets another branch or the trunk, built from vascular tissue of both overlapping. Colorado State Extension says the yearly growth rings of trunk and side branch overlap there, and that in lumber the same spot is the knot.	Washington State University list, Woody Plant Vocabulary; Colorado State University Extension, tree growth and decay
Branch bark ridge	In the Washington State list, a raised line of bark along the top of a branch union, pushed up as trunk and branch grow against each other.	Washington State University list, Woody Plant Vocabulary
Branch protection zone	In the Washington State list, tissue at the base of a smaller branch, inside the trunk or parent branch, that has changed chemically and physically so discoloration and decay spread more slowly into the trunk.	Washington State University list, Woody Plant Vocabulary
Codominant trunks	Colorado State Extension's publication uses the word for neighbouring trunks of similar size. It says no branch collar forms between them and links such unions to the main storm damage in snowy climates.	Colorado State University Extension, tree growth and decay
Aspect ratio	The size relationship between a trunk and a side branch, in Colorado State Extension's publication. A one-to-one union is described as more prone to failure in wind and snow than a one-to-three union.	Colorado State University Extension, tree growth and decay
Root collar	The QTRA glossary spells it root-collar and places it where the stem or stems turn into roots.	QTRA glossary of tree terms, PDF with 1999 extracts
Buttress root	In the Washington State list, roots at the base of the trunk that help hold the tree up and spread out mechanical stress.	Washington State University list, Woody Plant Vocabulary
Sinker root	In the Washington State list, a root that grows down, anchors the tree and takes in water and minerals.	Washington State University list, Woody Plant Vocabulary
Absorbing root	In the Washington State list, fine roots that take in water and minerals, with most of them in the top 12 inches of soil.	Washington State University list, Woody Plant Vocabulary
Girdling root	In the Washington State list, a root that wraps around part or all of the tree or other roots and squeezes the vascular tissue.	Washington State University list, Woody Plant Vocabulary
Root protection area	In the QTRA glossary, a planning tool that marks a national minimum area around a tree judged to hold enough roots and soil volume to keep it viable.	QTRA glossary of tree terms, PDF with 1999 extracts
Epicormic shoot	A shoot that grows from a resting or adventitious bud rather than from a first-year shoot, in the QTRA glossary. The Washington State list defines a watersprout as an upright epicormic shoot above the root graft or soil line.	QTRA glossary of tree terms, PDF with 1999 extracts; Washington State University list, Woody Plant Vocabulary
Photosynthesis	In the Washington State list, how green plants turn light, water and carbon dioxide into glucose.	Washington State University list, Woody Plant Vocabulary
Respiration	In the Washington State list, how plants use oxygen to release energy from sugars and starches.	Washington State University list, Woody Plant Vocabulary
Transpiration	In the Washington State list, water lost as vapor through the stomata of leaves.	Washington State University list, Woody Plant Vocabulary
Stomata	In the Washington State list, tiny openings on the underside of leaves, each between two guard cells, that let gases pass and control water loss.	Washington State University list, Woody Plant Vocabulary
Mycorrhizae	In the Washington State list, a partnership between certain fungi and plant roots.	Washington State University list, Woody Plant Vocabulary
Deciduous and evergreen	In the Washington State list, a deciduous plant drops all its leaves on a schedule set by its genes and the climate, and an evergreen does not lose all its foliage in one year.	Washington State University list, Woody Plant Vocabulary
CODIT (walling off decay)	In the Washington State list, the tree's own defence of building chemical and physical barriers that limit the spread of disease and decay organisms. Penn State Extension describes the walls a tree builds around a wound and uses CODIT as the name for the process.	Washington State University list, Woody Plant Vocabulary; Penn State Extension, spread of decay in trees
Reaction zone	Colorado State Extension's publication describes cells near damage that change chemically to seal it off. In most species the zone shows as darker wood.	Colorado State University Extension, tree growth and decay
Woundwood	Wood with lignin that forms from callus tissue within months of a wound, in Penn State Extension's article. It is the growth seen closing over a wound on a trunk or where a branch was removed.	Penn State Extension, spread of decay in trees
Reaction wood	In the QTRA glossary, wood a tree grows when the mechanical load on it changes, often because of an internal defect or decay.	QTRA glossary of tree terms, PDF with 1999 extracts
Adaptive growth	In the QTRA glossary, how wood laid down by the cambium responds to gravity and other forces, which helps keep mechanical stress evenly spread.	QTRA glossary of tree terms, PDF with 1999 extracts
Vigour and vitality	The QTRA glossary says vigour is how much carbohydrate goes into growth, and vitality is a measure of physiological condition.	QTRA glossary of tree terms, PDF with 1999 extracts
Stress (plant)	In the QTRA glossary, a state where one or more body functions fall outside their best range, for instance through lack of water, poor nutrition or temperature extremes.	QTRA glossary of tree terms, PDF with 1999 extracts
Diameter at breast height (DBH)	Stem diameter measured at 1.5 metres in the UK, or the nearest measurable point, in the QTRA glossary.	QTRA glossary of tree terms, PDF with 1999 extracts

Reading these sources

Four sources carry most of the page. A Washington State University list of woody plant vocabulary gives the working definitions of tissues, roots and branch parts. Colorado State University Extension and Penn State Extension describe branch unions and how decay spreads. A glossary hosted by QTRA, which incorporates extracts from a 1999 UK government publication, supplies the arboricultural terms. A Penn State youth guide supplies a few plain definitions.

Sources do not always agree on one word. Heartwood is dead wood that supports the tree in the Penn State youth guide, and wood that gives chemical defense against decay in the Washington State list, so the rows say which source says what. The wider story of how trees grow is on the tree biology guide.

Words for pests, disease signs and symptoms are on tree work words: pests and diseases. Words for decay and risk are on tree work words: tree risk and decay.