From Tropical Topics newsletter, No. 71,
December 2001, produced by Stella Martin at the Queensland
Environmental Protection Agency. Download the PDF to read the whole
issue.
Eucalypts, melaleucas and acacias dominate the dry
tropical woodlands. The first two are particularly common in areas
where there is a reliable annual wet season, eucalypts favouring
the drier situations and melaleucas preferring those which are
seasonally flooded. Acacias predominate further south, in arid
areas where drought may last for years.
Ubiquitous eucalypts
Eucalypt trees are synonymous with Australia, although a number
of species also grow on islands to the north where they would have
spread at times of lower sea levels.
Buds
The name Eucalyptus comes from two Greek words, eu
, which means 'well' and kalyptus which means 'covered'.
This is a reference to the unusual flower buds which are covered
with a little cap (or operculum) which is actually formed from the
fused petals and sepals. When the cap comes off, the numerous
stamens unfurl around the central pistil, or female
part — eucalypt flowers have no petals as such.
Gum nuts
Eucalypt fruits are also distinctive. After fertilisation, the
cup-like base of the flower dries, enlarges and becomes a woody
fruit, the gum nut. The roof of the ovary splits, in most species,
into hard woody valves, which might poke out in the form of points,
or may sink below the rim, out of sight. The various shapes of
these gum nuts, along with size and shape of the buds, can help
with identification.
Common eucalypts
Stringybarks (Eucalyptus tetrodonta) and woollybutt
(E. miniata) are the most widespread tree species in
northern tropical woodlands, occurring over Cape York Peninsula,
the Top End and the Kimberley. Although Eucalyptus
tetrodonta (left) is often called the Darwin stringybark it is
common right across northern Australia. Woodlands dominated by this
species cover almost 37 percent of Cape York Peninsula. It can form
dense patches. The stringybark has a long tap root and does best in
deep, sandy soils. It can grow to 30m, but its height depends on
the depth of the soil. The name tetrodonta is composed of
the Greek words tetra (four) and odontos (teeth)
refers to the four prominent projections below the cap of the bud
which later form four 'teeth' on the seed capsule. To see a recent
list of reseach findings on E. tetrodonta click here .
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Woollybutt flowers
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Darwin woollybutt (E. miniata) also grows well
beyond the Northern Territory and is commonly found associated with
E. tetrodonta right across the northern woodlands. Growing
to about 15-25m, this tree has earned its common name from the
particularly spongy and fibrous bark which grows a quarter to
half-way up the trunk. Above this, the bark is smooth and white.
Flowers are a stunning orange with bright yellow-tipped stamens and
are responsible for the scientific name: miniatus is the
Latin word for 'flame'. The barrel-shaped, ribbed gum nuts, grow to
6cm in length and almost as wide. They are popular in flower
arrangements. Native bees often nest in branches which have been
hollowed out by termites, providing 'sugarbag' — a
traditional source of honey. To see a recent list of research
findings on Eucalyptus miniata click here .
Bark
Eucalypts are commonly referred to as gum trees, but these trees
with typically smooth barks are only part of the story. Other
eucalypts have quite different barks.
In gum trees , the bark, or outer, dead layer of
tissue is shed annually, exposing the new, inner bark below. This
is often highly coloured and may be covered with a white powder. In
some species, old bark remains attached in dangling strips.
Rough-barked species, by contrast, retain the outer dead bark
allowing it to accumulate year after year. As the girth of the
trunk increases, this bark splits longitudinally. The different
patterns created depend on the varying nature and lengths of the
fibres composing the bark.
Stringybarks (top right) have a thick, spongy, fibrous
bark which can be pulled off in long strips.
Ironbarks (middle right) have a solid, thick, heavy bark
with deep longitudinal furrows. It is often impregnated with sap
(gum) from the tree, known as kino, which makes it very hard. It is
generally very dark — grey or black.
Bloodwood (Corymbia sp) bark (bottom right) is
often tessellated, or chequered. This is because the short fibres
in the bark break into small plates which may be hard and woody,
spongy or loose and flaky. The trunk may be patterned with
different coloured plates. A blood-red sap (kino) leaks from wounds
in the bark. To see a recent list of research findings on the
Corymbia species click here.
Box eucalypts have a firm short-fibred bark with narrow
longitudinal lines or tessellations. It is relatively thin and may
wear off, leaving a mottled appearance. These trees are named after
the European box trees due to the similar pattern of interlocking
grains in the timber.
Separated eucalypts
In 1995 all the bloodwoods were separated from the
Eucalyptus genus into their own genus, Corymbia .
There are now 550 recognised Eucalyptus species and 130
species of Corymbia. Bloodwoods are better represented in northern
Australia than in the south. They are present, and sometimes
dominant, in the northern eucalypt woodlands.
Deciduous eucalypts
Although they are known as evergreen trees, in northern
Australia a number of eucalypt species respond to the seasonally
dry conditions by dropping their leaves. This is unusual in
eucalypts. However, it enables certain eucalypt species to compete
effectively in shallow soils. On deeper soils evergreen species
dominate. The degree to which a tree loses its leaves may be
related to the situation in which it grows. For instance, poplar
gum (E. platyphylla), found in eastern Queensland, may drop
all its leaves in a dry situation but just some of them in a
well-watered position.
