Here is the full definition of ‘tree’ from the A-Z of tree terms: A companion to British arboriculture. Terms in bold face have their own entries in the A-Z.
The definition of ‘tree’ is a composite of tree species, tree form and tree size.
One overall definition (from the blue book) is: ‘A perennial plant with a self-supporting woody main stem, usually developing woody branches at some distance from the ground and growing to a considerable height and size.’
This definition has the three main elements in general form: a woody perennial (tree species), ‘having a self-supporting main stem usually developing branches at some distance from the ground’ (tree form), and ‘growing to a considerable height and size’ (tree size).
An unmistakable tree conforms to all three criteria, while a particular individual may conform to only two or one. For instance, a tall beech hedge has tree species and tree size but not tree form; tree seedlings and bonsai have tree species and tree form but not tree size; and numerous kinds of plants such as palms and tree ferns have tree size and approximate to tree form but are not regarded as tree species.
A stunted clonal jungle of a freely suckering species, such as white poplar on an exposed coastal site, has tree species but not tree form or tree size; a tall grass such as wild cane (Gynerium sagittatum), which reaches a height of 14m, has one criterion of tree size but not tree species or tree form; while a piece of broccoli approximates to tree form but does not have tree species or tree size.
Thus, in deciding to what extent an individual is a tree, a description of the context is likely to be helpful including attributes such as estimated mature size (if juvenile), growth habit, lifespan, collective character (if in a hedgerow, shelterbelt, woodland etc.), whether fallen or defective, and so on.
Alternatively, trees can be left undefined. In Bullock v. Secretary of State 1980 40 P&CR 246, a tree was judged to be ‘anything that ordinarily one would call a tree’ and similarly a tree preservation order protects ‘anything that may ordinarily be termed a tree’ (National planning practice guidance paragraph 36-012-20140306). Presumably, in this layman’s view, tree size and tree form would be more influential than (the possibly unknown) species.
The distinction between trees and shrubs may be immaterial. According to The Plant Health (Forestry) Order 2005, ‘tree’ means ‘a living tree or shrub, or a living part of a tree or shrub, at any stage of growth’. See shrub, thicket, jungle, Trees and shrubs online.