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Habit

1. Tree - erect single-stemmed woody plant with various crown forms.
The definition of tree deliberately has no upper or lower height limit. If the user finds it difficult to decide whether the specimen is a tree or a shrub it is probably better to avoid using this character. The definition of tree includes the two special categories in common usage only in Western Australia - mallet and marlock (see more below). Note that a tree may have a lignotuber at the base of the trunk and epicormic shoots on the trunk or stems, or lack either or both of these means of vegetative recovery after disturbance such as fire.

Tree

2. Mallee or shrub - a mallee is a woody plant that is multistemmed from ground level and seldom taller than 10 m. In eucalypts a shrub is a low growing and reproductively mature plant, that may be less than 1 m tall, and is usually growing in an extreme environment. There is no clear distinction between mallee and shrub.
A mallee has at the base of the stems a woody structure, the lignotuber, that has numerous dormant buds that enable vegetative recovery after fire or other disturbance. The term mallee is often applied to eucalypts and has wide currency in southern Australia. Shrub is infrequently applied to eucalypts, good examples being E. vernicosa in high mountain areas of Tasmania, E. yalatensis on the Nullarbor Plain and E. surgens atop coastal cliffs at Toolinna Cove in Western Australia. Smaller marlock plants are included here as well as below, e.g. E. mcquoidii which may be reproductive at about 0.4 m tall.

3. Mallet or marlock (only applies to Western Australian species) - a mallet is a tree with a slender trunk with branches steeply angled on it, and lacks both lignotuber and epicormic buds (e.g. E. astringens). A marlock is a single-stemmed shrub or small tree with spreading branches that are densely leafy often almost to the ground, and lacks a lignotuber (e.g. E. platypus). Correctly used the character state mallet or marlock has great discriminating value. Species with mallet habit are also included in the character state Tree above. Marlock, as here defined, is easily understood whilst the plants are relatively small, but from 8 m tall the distinction between marlock, mallet and tree is often unclear. Marlock applies to relatively few species, but some are frequently cultivated e.g. E. platypus, E. conferruminata, growing taller than they do in the wild.