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
Many woodland trees, such as eucalypts, are hardwoods, but there
are softwoods among them, notably the kurrajongs. They are often
found on rocky outcrops. They have links to rainforest trees and
most have ancient Gondwanan origins.
Boab trees (Adansonia gregorii ) are distinctive
features of the Kimberley region of Western Australia (left). They
are also found in northwest Northern Territory. The immense,
swollen trunks on older trees can measure over 15 metres around.
Radio-carbon dating of a related species in Africa indicated an age
of 1000 years. Given their slow growth rate and the immense size of
some trunks, large Australian boabs are also likely to be very old.
The large white flowers, which are pollinated by hawkmoths, appear
on the spreading branches of the tree when it has dropped its
leaves. The name boab is a shortened version of the African
'baobab'.
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Boabs are a puzzle. In spite of their trunk shapes, they are not
related to Australian 'bottle trees' (Brachychiton sp.). Their
closest rellies are six species in Madagascar and one on the
African mainland. It has been suggested that seeds, or even entire
trees, arrived in Australian after floating across the Indian
Ocean, but this theory runs into trouble because there are no
closely related species in Madagascar, unless they have since
become extinct. Alternatively, these trees may have been in
Australia since before the break-up of Gondwana. If that is the
case, however, why have they not spread further, since they are
well adapted to drought conditions? Possibly the superior root
systems of eucalypts have given them a competitive edge. Fire,
however, may play a part. Studies have shown that although young
boabs can resprout successfully from the roots after a fire, they
bear very few fruits. On the other hand, in areas where overgrazing
has led to a reduction of fires, boabs are spreading rapidly. To
see a recent list of research findings on boabs click here .
Kurrajongs
Most (29 of 31) of the trees belonging to the Brachychiton
family are endemic to Australia. They are found growing in a
variety of situations from rainforest to woodlands but all seem to
have evolved to cope with drought conditions. They tend to drop
their leaves in the dry season and several species have swollen
stems for water storage. Many of the species which grow in tropical
woodlands are commonly known as kurrajongs.
The northern kurrajong (B. diversifolius) is found across
the north. Its leaves vary enormously as it grows. Simple leaves on
young seedlings go through about six distinct changes, becoming
more lobed as the tree grows. This tree spends much of its early
life developing a huge, deep tap root.
The red-flowered kurrajong (B. paradoxus) (left) has, as
the name suggests, beautiful red flowers which form clusters along
the branches when the tree is leafless. Some of the flowers are
male and some are female. This tree occurs in northern Northern
Territory and Queensland.
Three species of kapok trees and bushes (Cochlospermum
spp) are found across the north, in open-woodland. They grow
particularly well on rocky hill slopes and ridges. These trees drop
all their leaves during the dry season but become covered at this
time with bright yellow flowers. These result in large, green,
barrel-shaped fruits up to 8 cm long which, as they mature, become
brown and split to release masses of silky hairs, embedded with
seeds (right). Each seed has a little parachute and can be
dispersed by the wind. The kapok has been used as
cottonwool for stuffing cushions and in Aboriginal body
decoration.
Serious chemicals
With their spreading crowns of bright green leaves, groups of
Cooktown ironwood (Erythrophleum chlorostachys), right,
stand out from the crowd. In spite of its name, this tree is common
throughout northern Australia, not just around Cooktown, growing on
well drained soils in association with bloodwoods, stringybarks and
other eucalypts. It is a highly poisonous tree, known in parts of
Western Australia as 'camel poison'. All parts of the tree are
toxic, even dry leaves, although it seems that little corellas can
eat the seeds. Suckering shoots are particularly dangerous.
Domestic animals have been poisoned by leaves accidentally trapped
in the side of trucks which were transporting them; 50 grams of
leaves is enough to kill. Do not tie your horse to this tree!
On the plus side, the hard timber, which is one of the densest
of any Australian tree, is resistant to termite attack. It was
widely used by early settlers for railway sleepers and fence posts.
Traditionally, Aboriginal people have made good use of the hard
timber. Infusions of the leaves have been used to cure scabies
while burning green leaves repel mosquitoes and sand flies. To see
a recent list of research findings on E. cholorstachys
click here .
The tree produces creamy green flowers in spikes which
eventually turn into flat, brown, woody, seed pods up to 14 cm
long.
Heart-leaf poison bush (Gastrolobium grandiflorum) which
is found across northern Australia, contains the poison 1080
(monofluoroacetic acid). This is extremely poisonous to many
introduced animals. There are 27 species of Gastrolobium sp in
Australia. All are thought to be poisonous and 18 are known to
contain toxic amounts of 1080. All except heart-leaf poison bush
are found in south-western Western Australia (where the seven
species of Oxylobium plants containing 1080 also occur). It seems
that native mammals and birds in this region are not killed by the
toxin, having evolved alongside these plants. For that reason 1080
is often used for fox-baits in Western Australia because it is
considered safe for native animals.
Heart-leaf poison bush is a straggly, multi-stemmed bush which
grows to about 2 metres. Leaves are grey-green, often with a notch
at the tip. The large flowers are red and 'pea-shaped'; a
shrub in full flower makes a spectacular display.
Matchwood, or turpentine (Erythroxylum ellipticum ) is a
fairly unremarkable small tree found across the north but it has
some notorious cousins — related species in South
America are the source of cocaine. None of the Australian species
produce this, but do contain some interesting compounds. The timber
is very durable, burns when wet and yields a beautiful red
heartwood which is popular with wood turners.
Fire!
Along with drought, fire is one of the greatest trials facing
vegetation in tropical woodlands, and trees living in savanna lands
had to evolve strategies for surviving it long before humans
entered the equation with firesticks and matches. The thick barks
of many trees are a good defence, while smooth ones give little for
the fire to catch on to. Most sclerophyll trees have special
epicormic buds protected deep below the bark which will sprout if
the main foliage is badly damaged. Woody swellings around the
roots, lignotubers, can also shoot if necessary and some
species (such as Darwin stringybark) produce extensive root
suckers. Fire-sensitive trees may avoid the problem by creating
dense stands and dropping abundant leaves to suppress the growth of
fuel (grass).
Many woodland species have woody pods which protect the seeds
from fire, although the heat often stimulates them to open and
release their seeds, at the appropriate moment, into a nutritious
ash bed. Some hard seeds, such as acacias, need heat to stimulate
germination. However, recent experiments have shown that it may be
the smoke (possibly the ammonia in the smoke rather than the
heat) which triggers germination in some species.
