ArborViews Australia - Consulting Arborists

Glossary of Arboricultural Terms

absorbing roots fine, fibrous roots that take up water and mineral; most absorbing roots are within the top 30 cm of soil
adventitious shoots and roots that develop other than at their normal positions of origin
aeration provision of air to the soil to alleviate soil compaction and improve its structure
age class young – less than 20% of life expectancy
mature – 20% to 80% life expectancy
over-mature – greater than 80% of life expectancy
allelopathic effect effect caused by chemical substances produced by some plants that inhibit the growth and development of other nearby plants
bifurcation natural division of a branch or stem into two or more stems or parts
bracket fruiting or spore producing body of wood decay fungi, forming on the external surface of the trunk or branch
cambium thin layer of cells that produces phloem on the outside and xylem on the inside
canopy converging crowns of two or more trees
chlorotic leaves turning pale green, yellow or white from lack of chlorophyll, usually due to nitrogen deficiency
cleaning / clean out in pruning – the selective removal of dead, dying, diseased, damaged, broken and defective branches
codominant similar in size and importance, usually associated with trunks or scaffold branches; arising from a common junction and lacking a normal branch union
compaction (soil) compression of the soil, often as a result of vehicle or heavy equipment, that breaks down soil aggregates and reduces soil volume and total pore space, especially the macropores
condition overall state of the tree; refers to health, vigour and structurerated as excellent, good, fair, poor or dead
crown the part of the tree comprising the total amount of foliage
DAB Diameter Above Buttress – trunk diameter measured immediately above the root buttress
DBH Diameter at Breast Height; trunk diameter measured at 1.4 metres above ground level
decay (n.) an area of wood that is undergoing decomposition;(v.) decomposition of organic tissues by fungi or bacteria
decline gradually diminishing health or condition of a tree
decurrent the form of a tree with no central leader but with structural scaffold branches forming the basis of a spreading crown, compare with excurrent
defect (structural) internal or external points of weakness that reduce the stability of the tree
desiccation drying out, or dehydration, of part of a tree – usually roots or leaves.
epicormic arising from a latent or adventitious bud
evapotranspiration the process through which plants release water to the surrounding air, dissipating ambient heat
excurrent the form of a tree with a central leader and symmetric, vertical crown, compare to decurrent
failure structural collapse in part or full of part of a tree – roots, trunk or branches – often leading to the whole tree or part of the tree falling
fastigate having clusters of vertical branches, appearing as a single columnar form
form the shape and symmetry of a tree
hazard a condition that predisposes a tree to failure
health freedom from pests, diseases, ailments, stress –measured as excellent, good, fair, poor or dead
heartwood inner xylem, consisting of dead cells, does not transport water and minerals
included bark bark that becomes imbedded in a union between branches,  a branch and stem, or co-dominant stems
leader dominant upright stem, particularly on excurrent trees
live crown ratio ratio of the height of the crown containing live foliage to the height of the tree
lopping cutting of branches or stems between branch unions (this practice is generally unacceptable).
occlusion Growth processes where would wood develops to enclose the wound face be the merging of the wound margins concealing the wound
phloem conductive tissue immediately beneath the bark; transports food materials throughout the tree
phototropic the tendency of a tree to grow towards light
reactive soils soils with high clay content that expand and shrink due to changes in moisture levels
risk a combination of the potential for tree failure and the likely consequences if failure does occur
root crown area where the main root joins the plant stem, usually at or near ground level
sapwood outer part of the xylem that transports water and minerals
scaffold branches permanent or structural branches; arising from the trunk
Structural Root Zone (SRZ) the area around the tree, usually within 3 to 4 metres from the trunk, in which the structural roots are situated, and which must be protected during construction.
structural roots large, woody tree roots that anchor and support the trunk and crown; roots characterised by secondary thickening and relatively large diameter giving form to the root system and functioning in anchorage and support
structure construction and arrangement of parts (roots, trunk, branches) – rated as excellent, good, fair or poor
target person, object or structure that could be injured or damaged in the event of tree or branch failure
topping cutting the main trunk to reduce the height of a tree (this is an unacceptable practice)
Tree Protection Zone (TPZ) an area around a tree that is protected by a physical barrier from negative impacts, usually from construction activities
vigour capacity to grow, and to resist disease, ailments, pests, stress –categorised as normal, high low and dormant.
xylem the wood – inside of the cambium layer; transports water and dissolved mineral nutrients from the roots to other parts of the tree; provides strength in trunk and branches


The definitions included in this glossary have been adapted from the following:

Australian Standards Committee CS/90, Horticulture (1996), AS 4373-1996 Pruning of Amenity Trees. Standards Australia, Homebush, NSW.

Draper B.D. and Richards P.A. (2009), Dictionary for Managing Trees in Urban Environments, Institute of Australian Consulting Arboriculturists (IACA), CSIRO Publishing, Collingwood, Victoria, Australia.

International Society of Arboriculture (2005), Glossary of Arboricultural Terms, ISA, Champaign, IL.

Lonsdale, David (1999), Principles of Tree Hazard Assessment and Management. TSO, London, England.

Matheny, Nelda P. & Clark, James R. (1994), A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas, (2nd Ed), International Society of Arboriculture, Savoy, IL.