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.
Palms
Corypha palms (Corypha utan), left,
sometimes grow in extensive groves, where they are subjected to
seasonal flooding. Reaching 20–25 metres in height, these
palms may be 40 or 50 years old before they flower for the first
and only time. They then die. The fan-shaped fronds begin to fall
before this occurs so the multi-branched flower stalks, which can
be up to 5 metres in length, appear at the top of a largely
leaf-less trunk. The flowers are tiny but there are millions of
them. Fruits take about 18 months to mature.
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There are about 20 species of Livistona palms in Australia. They
are about the most drought resistant of all Australian palms and
can be found growing, as single specimens or in groups, in open
woodlands, alongside eucalypts. Some flourish in open sunny
situations or as part of the understorey in open forest where they
are subjected to, and survive, frequent fires. Others prefer wetter
areas. They can even be found right in the driest centre of the
continent where rocky areas ensure some permanent moisture. These
are thought to be relicts, dating back to times when the climate
throughout the continent was much wetter and palms would have been
much more common.
Grass trees
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Grass trees (Xanthorrhoea sp) flourish in dry places.
With their characteristic mops of spiky foliage, often
fire-blackened trunks and long flower spikes, they are a common
sight in tropical woodlands. Older plants may have branched trunks.
Grass trees have a reputation for growing slowly but some specimens
have been known to reach heights of nearly 2 metres in just 20
years, so the ages of large specimens may sometimes be exaggerated.
The roots of young plants can contract, pulling the stem below
ground level. It may be many years before it appears above ground
to form a trunk. Grass trees are well adapted to fire. The leaves,
which are rich in resin, are highly flammable and fire seems to
stimulate flowering. A long spike, covered with tiny flowers, grows
up from the crown at a rate of 4–10 cm a day, eventually
reaching lengths of three or four metres. These flowers produce
large amounts of nectar which attracts birds, insects and small
possums. Resin is produced in the leaf base. It was used
traditionally to attach spear heads to shafts and was an important
trade item; analysis of resin on ancient spear heads has been used
to work out trade routes. Hundreds of tonnes of the resin were
exported to Germany and the USA in the nineteenth century. It was
used in varnishes, metal lacquers, wood stains and explosives. The
flower spikes make good lightweight spears and fire sticks. To see
a recent list of research findings on grass trees
click here .
Cycads
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Cycads of various species are to be found in the understorey of
tropical woodlands. These are ancient 'dinosaur' plants which date
back 230 million years, long before flowering plants evolved.
Separate male and female plants produce cones, not flowers. These
plants are able to increase the temperatures of their mature cones
up to 17deg. above the local air temperature. This usually happens
in the afternoon and may serve to attract pollinating insects
(usually weevils) by enhancing the odours of the plant. It may also
help the male cones to shed pollen. Cycads have unique root
structures, which look like corals. These contain symbiotic
blue-green algae/bacteria which are able to fix nitrogen from the
air, thus allowing these plants to thrive in poor soils. Studies in
Western Australia have shown that cycads there can fix up to 35 kg
of nitrogen per hectare per year. Cycads in tropical woodlands are
often burnt but respond with a crown of fresh leaves. Fire also
seems to stimulate production of seeds: a comparison of burnt and
unburnt areas showed that cycads in the former produced over seven
times more seeds. All parts of the cycad are poisonous, the toxins
including mercury which is obtained from small quantities in the
soil. (For more on cycads, see Tropical Topics No. 59.) To
see a recent list of research findings on cycads
click here .
Grevilleas
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Although there are more than 200 species of grevilleas in
Australia, they are poorly represented in the tropics when compared
with the southern part of the continent. However, they can form
stands where conditions are suitable. Grevillea flowers are rich in
nectar and attract a large number of birds and insects. Most
flowers have a series of long, protruding stalk-like 'styles'.
Before the bud opens, the sticky end of this style collects pollen
from the surrounding stamens. When the bud opens it is ready, in a
prominent position, to deposit the pollen on visiting honeyeaters,
which then carry it to another plant. Dryander's grevillea (G.
dryandri) is a low shrub which is widespread across northern
Australia. Its flowers vary in colour from cream to pink and red.
The bushman's clothes peg, (G. glauca) is a species found in
Queensland. Its common name comes from the very hard, woody fruits
which split partially to create ideal emergency clothes pegs, or
paper clips. They are roughly circular with a diameter up to 4 cm.
To see a recent list of research findings on grevilleas
click here.
