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.
Melaleucas
Melaleucas sometimes grow alongside eucalypts, but
they tend to dominate in areas which eucalypts avoid: low lying
areas which are flooded in the wet season. Most species of
melaleucas need some moisture below the surface so are often to be
found where a clay subsoil exists below a sandy surface; areas
where melaleucas grow should generally be avoided by drivers who do
not wish to become bogged. Areas dominated by eucalypts are better
drained and often sandy.
Some melaleucas, such as Melaleuca
quinquenervia thrive in swampy areas where they grow, along
with pandanus, in almost permanent water. However, other melaleucas
are more at home in drier areas which are flooded only in the wet
season. The thickly layered, papery bark protects the trees from
moisture loss and from fire. Interestingly the name
‘melaleuca’ is derived from the Greek words melanos,
meaning ‘black’, and leucos, meaning
‘white’. The first specimens described by Europeans had
probably been recently burnt and the white branches were contrasted
against a black trunk. Paperbark is a commonly used name but the
name ‘teatree’ refers to both melaleucas and to members
of the Leptospermum genus.
The bark of melaleucas has traditionally been put
to many uses by indigenous people, notably as roofing material for
shelters. The aromatic leaves yield essential oils with germicidal
properties. M. cajuputi is the source of cajuput oil which
has many useful properties, notably as a powerful anti-spasmodic
stimulant. However, the presence of a harmful compound, cineole, in
many melaleuca oils limits them to external use.
The broad-leaved paperbark (Melaleuca
viridiflora) occurs across northern Australia. Depending on
conditions, it may be a small straggly shrub or a straight trunked
tree up to 18m tall. It is very adaptable, growing on steep
mountain slopes on the edge of the Wet Tropics right through to
swamps and coastal plains and most areas between. It can be a
scattered shrub in a woodland, or a dominant species in
shrubland
This tree has the largest and coarsest leaves and the largest
flowers (stamens 2cm long) of all melaleucas. It flowers heavily
with cream to yellowish-green ‘bottlebrush’ flowers
(viridiflora means ‘green flowers’) which
attract bees, butterflies, birds and flying foxes. The flowers are
occasionally red.
Acacias
Acacias, also known as wattles, belong to the largest genus of
flowering plants in Australia. One of the most widespread plant
groups, they can be found growing in well-drained rainforest
conditions but their main stronghold is the very driest parts of
the Australian continent where rainfall is inadequate for
eucalypts. Throughout the savanna woodlands, acacias may be found
as an understorey or on shallow, gravely or sandy soils which are
unsuitable for the more dominant eucalypts and melaleucas. They
sometimes form quite large communities.
Acacias are well adapted to drought, poor soils and fire. In
extremely dry times, acacias simply stop growing, and wait for
rain. When it falls, water is directed by the branches towards the
base of the trunk.
Most species, as adults have no leaves. They start life with
small fern-like leaves (right) but as the little seedlings mature
the leafstalks to which they are attached become enlarged and
flattened. They outgrow the little leaves which eventually drop
off. For the rest of its life, the plant will rely on these tough,
flattened leaf stalks, called phyllodes, to perform as leaves. The
advantage is that they lose less water than normal leaves.
Phyllodes generally have several main veins, running parallel to
the edge, but no midrib vein. Tough, low in food value and
containing unpalatable chemicals, they are unattractive to
leaf-eating animals. At the base of each phyllode is a small gland
which often produces nectar. This attracts ants, which act as an
extra deterrent to herbivorous insects.
Acacias are legumes, so they have nodules in their roots
containing bacteria which fix nitrogen from the atmosphere. This
allows them to thrive in nutrient poor soils. Many of the seeds
have an attachment, known as an elaiosome (below), which is rich in
protein and oil. This is attractive to birds which digest the
elaiosome, and pass the seeds. It also appeals to ants which
‘plant’ many seeds by carrying them underground into
their nests. The hard, water-resistant coat of these seeds can only
be fractured by high temperatures or abrasion which means that they
often germinate at the best times — when fire has
produced a nutrient-rich ash bed.
Acacia flowers are tiny but cluster together in ball-shaped or
elongated groups. Their colour varies from cream to gold. One
acacia species (A. purpureipetala), west of Herberton in
Queensland, has purple flowers.
Australian versus African acacias
Acacias are found in all continents except Europe and
Antarctica. They are particularly widespread in Africa —
but close examination shows that these plants are very different,
having evolved survival strategies which best suit conditions in
their respective homelands.
Few Australian acacias have leaves. The phyllodes are
unattractive to herbivores. African acacias all have leaves, which
seem designed for browsing herbivores. (The spines on African
acacias are designed to protect the developing leaf buds, not the
leaves themselves, which fall to the ground if not eaten.)
Australian acacia seeds are sown by ants and birds, attracted by
the elaiosome (not by the dry cast-off pods); in Africa, large
mammals distribute their seeds after feeding on the nutritious pod
(carob).
Australian acacias have bacteria in their roots which fix
nitrogen; African ones provide a shady canopy which attracts
mammals which provide nitrogen in the form of droppings.
In Australia, pods and litter around the acacias create intense
fires which can kill the trees but provide good conditions for
seeds to germinate and grow. In Africa, mammals attracted to the
trees remove grass and keep fires mild.
