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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.
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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.
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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.
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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
BACK TO EUCALYPT
FEATURES
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