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The Wet Tropics of Queensland was declared a World Heritage property
in 1988 following the concerted efforts of a conservation movement
dating back to the 70's. The movements objectives was promulgated
by publications of Prof. Len Webb and Geoff Tracey which highlighted
the conservation significance of the unique vegetation in this
area. About one million hectares - 70% of the Queensland's rainforest
- is in the Wet Tropics.
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The region where we have chosen to base our headquarters
boasts Australia's richest and most luxuriant rainforests.
Here, the mean annual temperature exceeds 24 degrees C.
and the average annual rainfall is 2800mm (but can be as
high as 10,000mm).
Australia's two million hectares of rainforest covers 0.3%
of the continent, yet are home to half its native plant
and animal species. The greatest biological diversity in
terrestrial Australia is found in the wet tropical rainforests.
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More
than 200 vertebrate species live here. Most depend on rainforest
for survival. About a quarter are found nowhere else in
Australia. These forests are strongly linked with the Gondwanan
rainforests. They contain the most diverse group of primitive
flowering plant families (13 out of a world total of 19)
and primitive Marsupials (pouched animals) such as the Musky
rat and the Lumholtz's tree kangaroo.
Apart
from being species-rich, Wet tropical rainforest is the
only place in Australia with a recognised Aboriginal rainforest
culture, dating more than 10,000 years.
On a global scale,
Australian rainforests represent less than 1% of the world's
rainforests yet protect about 5% of all rainforest species.
Rainforests once covered all the earth's tropical lands.
Most were in South and Central America, South-East Asia
and West & Central Africa. Smaller patches were in Australia
India and Pacific Islands. About half of the world's rainforests
are believed to have been destroyed by human activity in
the last century or so.
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Today, rainforest covers less than 10 percent of the earth's land
surface. As the earth's most complex and diverse ecosystem, rainforest
is home to more than half the world's known plant and animal species
and is a valuable source of cultivated and ornamental plants and
drugs. We have a global responsibility to conserve our remaining
forests.
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Definition
of 'Tropical Rainforest'
There are many colloquial names given to the
'tropical rainforest'. In Australian colonial times the rainforest
was described as the 'scrub' or 'brush'. In the recent past, the
most commonly used name has been 'jungle'. For many people this
probably covers what we will later determine as the tropical rainforest.
The term evokes an image of something a little more tangled than
the real thing, and it is probably a more appropriate word for the
edge of the rainforest rather than the main vegetation. In the last
few decades, however, the words 'rain forest' have actually replaced
the old fashioned 'jungle'. The words "rain" and "forest" are more
of a technical term, and were originally used by a German botanist
over a hundred years ago; "tropischer Regenwald" (Wolf and Prance
1998). Today, there are many definitions of 'tropical rainforest',
especially for scientists. These vary around the world, depending
on the classification schemes used. |
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One of the most useful of those schemes is the Holdridge
pyramid. This is particularly popular in the Americas where
it originated, and is often used to distinguish between
different types of 'rainforest' in parts of the Neotropics,
such as Central America (Wolf and Prance 1998).
Rainforest in this scheme is actually divided into 'moist-
forest', 'wet- forest', and 'rainforest'. It uses the factors
of water and temperature, and their variations, to determine
the type of vegetation. Within Australia, a system designed
by Specht is commonly used. It is based on canopy cover
and the height and form of the dominant vegetation. In this
classification, tropical rainforest comes under the general
vegetation type 'closed forest' (Adam 1997). So even when
we scientifically define 'tropical rainforest' we run into
difficulties.
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For example, what
the public would consider 'tropical rainforest' in Australia may
be called ' 'closed forests' by Australian scientists, but would
be considered merely as a 'moist-forest' or 'wet-forest' by American
ecologists. Ultimately then, we must remember that definitions and
classifications are all artificial constructs.
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Conditions
required for Tropical Rainforest
There are several conditions required for a
tropical rainforest to grow. Primarily, there needs to be light,
warmth, and moisture. In addition, there are factors that are limiting
and need to be absent. |
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Other conditions, such as soil type, are often secondary
consideration. The tropics receive a lot of sunlight. This
is due to their location on the larger, bulging equatorial
region of the globe. Here, there is little variation in
the amount of sunlight during the day throughout the year.
The time of the sun setting and rising does not vary much
more than about an hour every day. This constant sunlight
results in high temperatures. Further, the temperature does
not vary much during the year. On the constantly shaded
rainforest floor, the temperature will vary even less.
Visitors to rainforest regions are often shocked by just
how much it rains. After all, by definition this is an essential
requirement for the forests survival and how it got its
name. Not only does it rain a lot, but needs to do so most
of the time; that is, the rainfall generally needs to be
there throughout the year.
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Finally, if one is travelling
through a tropical rainforest they will notice another condition;
very high humidity, resulting from the combination of the above
two factors of heat and water. For many people, it is this high
humidity that makes the tropical rainforest an uncomfortable experience.
In addition to the above, there are certain 'limiting' conditions
that cannot occur if a tropical rainforest is to develop. The temperature
must not drop so low as to allow frost to appear (Winter and Atherton
1987). 'Moisture stress' cannot occur for prolonged periods; if
a dry season lasts for a period of a few months every year, then
rain forest will not survive and a monsoon forest will develop (Whitmore
1999), and in drier situations still, an open woodland or savanna
will grow.
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There are other, secondary conditions that may also determine
the development of a tropical rainforest. One of these is
soil type. If the soil is deeper and more fertile, rainforest
may able to develop in areas with less rain or lower temperatures
that would normally limit its growth.
In fact, this is the situation in large parts of the Eastern
and Southern Amazon rainforest of South America (Whitmore
1999). Following on from this and contrary to popular belief,
rainforests do not limit themself to poor soils. Rather,
they can grow in wet and warm conditions despite poor soils.
Some of the most diverse and luxuriant rainforests in the
world grow on rich soils, such as the rain forests on the
volcanic soils in areas of Costa Rica, Indonesia and here
on the Atherton Tablelands of Australia.
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| Tropical Rainforests
of the world Tropical rainforests should thus develop in areas where
these conditions are met around the world. These rainforests are,
by definition, within the tropics, the region bordered by the tropic
of cancer in the Northern Hemisphere and the tropic of Capricorn
in the south. |
| Map - Rainforests
of the World...missing |
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Looking at the map, we can see that there are several
distinct tropical rain forest regions in the world. These rainforest
regions correlate with the main biogeographical realms (see any
biology text for a map of the biogeographical realms, e.g. Beck
et al 1991). There is Neotropical rainforest (Central America
and South America), Afrotropical rainforest (mainly central and
western Africa), Oriental rainforest (mainly Southeast Asia) and
Australasian rainforest (New Guinea and North-east Australia).
About half of the world's tropical rainforests are in the Neotropics
(Whitmore 1999). The smallest major zone is comprised in the Australasian
tropical rainforest. Although this and the Southeast Asian regions
are quite close, they are distinct regions of tropical rainforest.
Characteristics of a Tropical Rainforest
Now that we have seen where rainforests can
grow, we need to be able differentiate between them. While some
species of flora and fauna are shared between rainforests types,
most are unique to that particular region, therefore we don't
usually differentiate tropical rainforests on particular species
composition.
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However, the species of plants in these rainforests are
under the influence of the same environmental pressures
and thus evolve developing similar features. These characteristics
can then be used to recognise a particular rainforest type
ie. Tropical, Sub-Tropical, Temperate, Cool-Temperate etc.
For convenience these characteristics can be divided into
two sections; general structural characteristics of the
rainforest, (e.g. a multi-layered, closed canopy, a predominance
of vines and epiphytes), and more specific physiognomic
features, (e.g. buttress roots, cauliflory, and drip tips).
More detail about these features are covered below.
There are also the more subtle ecological features such
as spatial complexity and diversity, and while such features
are important to the definition of what a tropical rainforest
is, they are not so immediately obvious and are not covered
in these brief notes.
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The structure of the Tropical
Rainforest
The tropical rainforest consists of a multi-layered canopy.
The main canopy consists of an undulating 'sea' of tree
crowns with heights of between 25 and 45 metres in lowland
tropical rainforest (Whitmore 1999).
Every now and then, taller trees poke out above all else.
These are known as 'emergents' (Johns 1999). Below the main
canopy may be many different levels or strata of trees.
These may be at different stages of growth, depending on
local patches of disturbance. The very bottom floor of the
undisturbed rainforest is relatively open with little undergrowth.
Where there are gaps in the canopy caused by disturbance,
there is an explosion of tangled plants.
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Climbers
Climbers can include various types of plants.
They are defined by their common method of climbing other plants
to reach sunlight without using their own support. They are unified
by their method of growth, and different climbers are not necessarily
closely related. Generally, the greener shorter-lived climbers are
known as 'vines', and the longer-lived woody vines are known as
'lianas' although this terminology varies with language and geography
and is not really scientific. Climbers such as vines and lianas
are one of the most obvious aspects of this ecosystem. While vines
are found all over the world, over 90% of species are found in the
tropics. In Australia, this importance is reflected in the classification
system where some rainforests are known as 'vine forests'. In some
areas, there may be almost as many climber species or individuals
as there are trees. Once secure, the lianas may develop large woody
stems as, to use a jungle cliché, 'big as a man's thigh'. Barring
collapse, some of these woody climbers may be as long-lived as many
of the trees of the forest.
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Different species
of climbers have developed different parts of their anatomy to climb
up. Which part of the plant has evolved this is in a large part
determined by the phylogeny of the plant. The climbing 'rattan palms'
have several methods; there are hooks on the undersides of the leaves
and on the stalks of many species. Many rattans in the genus Calamus
found in the Daintree have developed a tendril, which normally carries
the inflorescence (the flowers) and fruits into a climbing appendage.
It is light and armed with recurved hooks. Any sort of disturbance,
such as a slight wind, may end with the light tendril hooking onto
nearby vegetation and thus giving the plant a foothold into the
surroundings, enabling it to climb. The vigorously growing 'Supplejack
vine' Flagellaria indica climbs by using it's leaves. The ends of
the leaves have elongated and curl around to catch onto the surrounding
vegetation.
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Epiphytes
Epiphytes are another obvious component
of rainforests. They are plants that grow on other plants,
without connections to water and nutrients on the forest
floor. Visitors often mistake them for bird's nests, tree
parasites, or part of the tree. However, they are not usually
taking anything from the tree, and are merely using the
host to get closer to the light. It
is not uncommon to see a large number of both species and
individuals on the one tree. Many can grow to a size where
they are larger in diameter than the tree that supports
them. 'Epiphyte' is a growth form and an ecological term,
and many different unrelated plants have evolved this habit.
However, many of the more obvious epiphytes in the rainforests
are ferns. This is probably due in a large part because
of the fact that fern spores are so light, and the slightest
breeze can uplift the dust-like spores into the canopy.
Other epiphytes in Australia can include many different
species of orchids.
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As epiphytes do
not have connections to the ground, they need to collect their
own water and nutrients. Many epiphytes do this by having
broad leaves that form a base and then collect debris raining
down form the canopy. This also traps much of the water, and
soon the plants has it's own little garden. Ferns such as
the Birds' Nest Fern have a bucket like shape with large long
and upright leave channelling debris and water into a central
bucket. |
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Leaves
The canopy of the rainforest is usually closed,
with plants retaining much of their leaf cover throughout the year.
Fully deciduous trees are uncommon and may only account for a few
percent of the tree species in the tropical rainforest. Rainforest
leaves themselves share certain morphological features that are
adaptations to the environmental pressures that they all have in
common. They are usually leathery, a nice glossy green, and up to
90% may possess the distinctive 'drip tip' (Johns 1999). This is
an extension of the end of the leaf, generally believed to have
the effect of running the excessive moisture off the leaf. Leaf
size in the best developed tropical rainforest is relatively large,
being generally described as 'mesophyll' (Whitmore 1999) or even
larger. Of course, this is a generalisation, and leaf size also
varies depending on its position in the canopy. |
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In the Australian tropical rainforest, much of the splashes
of colour are provided not by flowers, but by leaves. Leaves
can be yellow, pink, red, purple or almost black. Unlike
in temperate regions, many of these colours are not of leaves
about to fall, but are often flushes of new growth.
Yellow leaves are usually leaves with undeveloped chlorophyll.
However, red or pink leaves are coloured by a pigment called
'anthocyanin'. This pigment reflects red light, which is
normally used in photosynthesis. It is commonly found in
fruits and flowers and is responsible for the red colour
in beetroot (Martin 1990). These bright colours may occur
for several reasons. The presence of the pigment anthocyanin
may be to protect the developing chlorophyll inside the
new leaves from too much light, or high energy or damaging
ultraviolet light.
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It has also been suggested that
these colourful pigments may contain fungicides and other poisons
(Breeden 1999). Another idea put forward involves the idea that
many herbivores prefer soft, fresh growth to the older, tougher
leaves and that the brighter colours of the fresh growth may serve
as a warning that the new leaves are toxic or distasteful (Farrant
1999).Many plants in the family Myrtaceae, such as the 'Pendas'
and the Syzgium spp., have bright red and pink new leaves.
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Cauliflory
Some trees in the rainforest have the
remarkable ability to flower and fruit on the branches and/or
the trunk. These two conditions are called 'ramiflory' and
'cauliflory' respectively. Early explorers, and visitors today,
have mistaken them for a parasite on the plant (Whitmore 1999).
They can make for a spectacular show in the tropical rainforests
and do somewhat resemble cauliflowers. |
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and possums readily pollinate the flowers and disperse the
seed thus allowing the species to flourish. |
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Roots
The roots of many tropical rainforest
trees have many interesting variations. In many trees, the
roots will spread as much horizontally as they do vertically.
Erosion by running water may eventually reveal this complex
network just underneath the ground.
Many trees also develop large flanges
above the ground that flank the tree on either side. These
are called 'buttress' or 'plank roots'. Although some species
are capable of developing them, and some are not, the development
of buttress roots is a reflection of environmental pressures
and not an indicator of species. Reasons suggested for such
development include support and stability, allowance of
air exchange in water-logged soils, and the capture of leaf
litter.
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