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Having taken into account the habit features, the next important character to assess in eucalypts is the type of bark. It pays to think in terms of the growth processes. Each year there is an increment of living bark that results in the continual expanding girth of the tree. In all species the outermost layer dies each year. In about half of the species this dead layer completely sheds, exposing a new layer of living bark, and the process continues year after year. These are known as the smooth barks. The dead bark may be shed from these trees in large slabs, in ribbons, or in small flakes. Invariably the newly exposed living bark is relatively smooth and brightly coloured but this fades with weathering. Often the dead bark comes off in pieces at various times of the year such that the trunk is mottled depending on the amount of time the newly revealed patches of bark are exposed to weathering.

  A curious but easily recognised bark type is the minnirichi which is restricted to Western Australian species (apart for E. orbifolia which occurs in South Australia as well). This bark is rough on first appearance and on close inspection is seen to be formed of partly shed longitudinal strips that curl outwards, initially exposing pale or greenish underbark. The older attached strips turn deep red on aging. In one minniritchi species, in particular, the lower bark becomes thick and fibrous while only the upper bark is typical minnirichi.

In many species the smooth bark is uniform over the whole trunk in both texture and colour, e.g. E. mannifera and E. salmonophloia. In others the bark is mottled, while in a few species, particularly the red gums and the grey gums, the newly exposed smooth bark can be brilliant orange or yellow, fading to greys, the surface texture of which becomes granular with age.

E. cladocalyx of South Australia and E. diversicolor of Western Australia show these characters to some extent suggesting an ancient common origin of these various groups.

  In the ribbon gums the long strips of dead bark are imperfectly shed and hang conspicuously in the crown, particularly around the trunk.

In great contrast are the remaining half of the eucalypts, the rough barks, in which the outer annual increment of dead bark simply dries out, leaving the natural fibres which do not shed and which accumulate year after year. These may remain loosely intertwined as in stringybarks, e.g. E. macrorhyncha and in peppermints, e.g. E. radiata, or become infused with gum exudates which harden, resulting in the ironbark, e.g. E. crebra, E. paniculata or the compacted types of rough bark, e.g. E. smithii and E. elata.

The ironbarks only occur in northern Australia and eastern Australia but some species from south-western Western Australia have very hard rough bark that is thinner than that of the ironbarks to which they are entirely unrelated, e.g. E. decipiens and E. indurata.

Assessing rough bark type is one of the hardest part in identifying eucalypts. The rough bark may cover the whole trunk and branches, or it may shed from the branches, or from the trunk only, but to certain characteristic heights up the trunk. Consequently we refer to species as being wholly rough-barked or partly rough-barked, half-barked, or with rough bark only at the base (black butt). There will be a range of variation in the bark between trees of the same species. This is illustrated by E. decipiens which is divided taxonomically into three subspecies diagnosed by the type of rough bark. Therefore, because there are so many different types of rough bark, defined by their texture, colour and persistence on the trunk, we must conclude that bark, because of the variability and imprecision of the descriptive terms, is a feature of only medium reliability for identification purposes.

Types of rough bark