Eucalypts are almost a defining feature of Australia. They are
the dominant tree of the developed areas of the country, although
only sparsely represented in the driest regions. There are over
800 species which have adapted to nearly every environment. In EUCLID
we include (because of its close affinities to Eucalyptus)
the long-standing genus Angophora, which is exclusive to
eastern Australia excluding Tasmania.
Eucalypts must have been known from the early 16th century when
the Portuguese colonised Timor. There are at least two indigenous
species, E. alba and E. urophylla on the island. Following
the Portuguese occupation, it is probable that eucalypts were established
from seed in Brazil which was colonised about the same time, although
records are too hazy to confirm this. Eucalyptus came into
recorded history in 1788 when the French botanist, L'Héritier
de Brutelle, published Eucalyptus obliqua, the well known
Messmate of widespread distribution in the wetter regions of the
south-east of the continent and Tasmania and Kangaroo Island. This
species was named from a specimen collected on Bruny Island south
of the island of Tasmania by one of the botanists on Captain James
Cook's third voyage in 1777.
Evolution and distribution
species are likely to have evolved from rainforest precursors in
response to great changes in the landscape, soils and climate of
the continent. No point of origin is possible to determine but it
is assumed to have been on the Australian landmass from which several
species have migrated probably by land bridges to islands north
of the continent.
One species, E. deglupta, is distributed as far as the
island of Mindanao, in the southern Philippines which places one
eucalypt naturally in the northern hemisphere. However, the genus
is now cultivated world-wide in tropical and temperate countries
and in some places has become naturalised.
Eucalypts are now of great importance commercially in other countries,
particularly South Africa, China, India and Brazil and to a lesser
extent in central and northern Africa and in Mediterranean countries.
They have many advantages apart from the timber and fibre which
are the basis of huge paper industries. Eucalypts are also notable
for their oils, use in lowering water tables, horticulture, shade
and simple ornamentation, largely for the colourful flowers in many
Innumerable books have been published on eucalypts. Some of these
are packed with all aspects of information, others concentrate on
the more spectacular flowering species while others specialise in
identification. Identification has always been regarded as difficult
but this has been largely due to the lack of instruction on the
botanical characteristics. Understanding the eucalypt plant is a
vital element in attempting the identification process.
It is a fact that, to the uninitiated, most Eucalyptus
species tend to look the same. People may think of well known wattle
species and other plant groups and make interesting comparisons
with eucalypts. Consider the Cootamundra Wattle (Acacia baileyana)
and Golden Wattle (Acacia pycnantha) of eastern Australia,
which are so clearly different with their greatly contrasting types
of foliage. Only a few eucalypts, on the other hand, have very odd
leaves and these are mainly the more bizarre but beautiful ornamentals
from Western Australia.
In eucalypts there is a striking array of juvenile or seedling
leaf types from opposite and completely connate pairs of leaves
(e.g. E. uncinata), to crowded and spirally arranged short
linear leaves (e.g. E. brockwayi), to disjunct petiolate
ovate leaves (many species). What
can be equally confusing is the change from seedling to juvenile
to sapling to adult leaves that takes place in the majority of species.
When identifying a eucalypt, this must be remembered. Attention
is drawn to this in the descriptions that accompany every species
Some species never, or seldom, develop true adult leaves
in the mature crown but instead retain their juvenile leaf phase
where the leaves are commonly glaucous and rounded. This condition
is rare in eastern Australian species but is notable in E. risdonii
an endemic to Tasmania and in E. cinerea of New South
Wales and Victoria. In Western Australia many more species have
the glaucous crown, probably the most spectacular being the glaucous-leaved
E. macrocarpa which produces large red flowers.
The species that produce the colourful flowers,
however, are not restricted to those with juvenile crowns, particular
examples being the brilliantly flowered E. ficifolia, E. erythrocorys
and E. caesia which all flower with apparently adult leaves.
In contrast, it is clear
to the average observer that in south-eastern Australia, nearly
all Eucalyptus species have green leaves of roughly similar
size and fairly inconspicuous white flowers. Only two species in
south-eastern Australia, E. sideroxylon
and E. leucoxylon, can have
strongly coloured flowers. A few tropical species have brilliantly
coloured flowers, e.g. E. miniata, E. phoenicea, E.
ptychocarpa and E. cadophora subsp. pliantha.
So the problems of identification in EUCLID for south-eastern Australian
species usually fall back on the less conspicuous and accessible
but highly diagnostic characters, often ones that may be less relevant
in other plant groups. In Western Australia, however, if the tree
or mallee is in flower, identification is made easier.
The first assessment of a eucalypt will obviously be made on approach
to a tree or forest. An important decision that has to be made is
to recognise whether the trees are cultivated, or in a natural stand.
If cultivated, they could be from anywhere in Australia and the
identification cannot take into account the
regional break-up in EUCLID.
The observer also needs to take into account other aspects of
the specimen, viz. the height of the plant, the number of stems
or trunks, the colour of the crown, the overall appearance of the
crown to determine if it is composed of juvenile or adult leaves,
general size of the leaves (very small, e.g. E. parvula or E.
kruseana, or very large, e.g. E. globulus) and the type
of bark, basically, whether rough or smooth. It is worth mentioning
here, that if a photographic record is wanted, trees should always
be photographed in early morning or late afternoon, although in
winter, photography is usually successful at most times of the day
if the sun is out. This timing is to ensure that side light is used
such that the trunk or stems are in sunlight. Crown colours can
be distorted or misinterpreted by top light near the middle of the
Inspection of specimens
The "internal" features of the eucalypt plant are more
reliable than the "external" features. They are relatively
protected from the elements and from various forms of predation.
They are the parts that require handling and close inspection or
even dissection, as opposed to mere observation. Specimens for study
may be obtained in several ways from a living tree. Sampling mallees
is usually easy because the leaves and flowering structures are
often at about head height and no sophisticated methods of collection
are needed. For most trees, however, a
weighted length of rope can be thrown
over a low branch which can then be broken off with a sharp
tug and pulled to the ground for close inspection of the parts.
For tall trees it is a curious fact that the flowers and fruits
are small and scarcely visible to the unaided eye, e.g. E. regnans.
Then the canopy needs to be inspected with binoculars and a useful
branch selected. If it is above rope-throwing height, the branch
may be reached with the use of a shanghai
by shooting a lead weight attached to a fine, light line over the
branch and then attaching a thicker, stronger rope to one end of
this line and then pulling this line up over the branch. For large
branches which are often dropped for commercial seed collecting,
high-powered rifles are used. Often the smallest trees or mallees
have the largest buds and fruits, e.g. E. pyriformis. These
plants are the easiest to sample, examine and assess.
The whole process of identification begins in the field with broad
external assessment and ends with microscopic examination. The characters
in this sequence of investigations have reliabilities that vary
from very low to high and finally absolute. Only experience allows
the user to weigh up these relative values and apply them with confidence.
In summary it might be said that the number of opercula on the
developing flower bud is of absolute reliability, staminal inflexion
and ovule row numbers of high reliability, bud numbers, flower colour
and bark type of medium reliability, leaf colour of low reliability,
bark colour of very low reliability. External features are very
susceptible to seasonal and intra-population variability.
When choosing a specimen for identification there are some things
to be avoided. For example always choose "typical" leaves
on the specimen for assessment, avoiding the largest and the smallest.
Similarly avoid using fruit that are lying on the ground, especially
if in a mixed eucalypt species stand, for they may not belong to
the tree under which they are lying. When looking around for juvenile
leaves to assess then make sure they belong to the tree or mallee
you are investigating if there is any doubt do not use them.
A mixed species stand may produce a variety of juvenile leaves.
Time spent looking at both adult and juvenile growth in a stand
will be very rewarding.
If an identification is proving difficult then growing of seedlings
may be a help in resolving it. Obviously this slows down the process
but valuable information can be obtained from observing seedling
growth firstly the shape of the cotyledons and secondly whether
the leaves become disjunct early in growth or persist as opposite
for many pairs. The shape of seedling leaves, whether they are stalked
or stalkless and other leaf features can help also.