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MANGROVES
The term 'Mangrove' refers to both a type
of plant and a biome/ecosystem/vegetation community.
When used to refer to particular plants,
the term expresses their ability to tolerate the salt-water
inundated conditions on the edge of the sea and the land within
the tidal zone. |
| Only a relatively
small amount of plants have evolved to survive these conditions.
These specifically adapted plants make up the community known
as the 'mangroves'. Although regarded as biomes found in many
tropical regions of the world, mangroves can also be accepted
as an international ecosystem. These communities are very closely
related sharing many plant species and hosting very similar
fauna. They are divided up into different vegetation communities
based on species composition and regional variations. |
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Mangroves in Australia
Mangroves are a very important community in Australia. They are
thought to comprise
22% of the coastline. The communities in Southern Australia
are amongst the highest latitude communities anywhere in the world.
These temperate Australian mangrove communities are often dominated
by just one species (Avicennia marina), and it is in the tropics
where they become speciose and ecologically important. Surveys by
Tomlinson have suggested that the highest diversity of mangrove
species in the world are found in New Guinea and Northern Australia.
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Up to 30 species of true mangroves may be observed
in some areas such as the Wet Tropics of Northern Queensland,
with many more associated or edge flora, such as back-freshwater
and epiphytic species. They are also important communities with
regards to their ecological roles, like hosting juvenile forms
of coral reef, feeding migratory wading birds, and housing a
rich array of intertidal fauna such as crustaceans, molluscs
and birds. Mangroves are also important in the part they have
layed in their development of other Australian plant communities,
both in an evolutionary and ecological context. |
Mangroves develop only in certain areas and under
certain conditions. Vast plains of mangroves can be found in the
top end of Australia where there are extensive seasonal floodplains
and tidal estuaries. They can occur for many kilometres up river
albeit with variations. In flatter areas further inland where there
is high evaporation and water inundation only at higher tides, the
communities are referred to as 'salt marshes'. However where the
water mixes to floodplain fresh, the community changes to freshwater
wetland communities, such as Melaleuca spp. In high rainfall mountainous
areas, such as in the 'Wet Tropics' in North Queensland, the more
sudden topography of the land means there is less of a gradual transition
(except on some bigger rivers such as the Daintree, Bloomfield,
etc). In areas such as at Cape Tribulation there can be a very sudden
division between mangrove forest and tropical rainforest, and thus
giving credence to some classification systems which view these
mangrove communities as a type of rainforest.
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community thus acts as a filter between the land and the sea
and can lead to the development of communities of either side
of their system. Mangroves may lead to stabilization of sediment
to literally make new land. After time, this may lead to invasion
of tropical rainforest from the land- ward side. On the seaward
side the capture of nutrients and the attraction of fauna to
the region may assist in the development of sea-grass communities.
These are communities of the only truly marine flowering plants.
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The conditions that lead to the
development of mangroves can change quickly.
Because the distribution of mangroves is reliant and sensitive to
the present area of coastline and the inundation and reach of the
tide, their distribution can change accordingly. Mangroves are thought
by some to not be a true system, but merely an interface. Their
rapid but changeable colonization possibly never leads to a true
climax community such as a rainforest, but is merely a reflection
of changing conditions. The rising and lowering of the sea levels
in the past, especially in the recent Pleistocene era has significantly
changed the distribution and development of mangroves. These were
quite extensive movements of vegetation communities in the floodplains
of the Alligator river systems in northern Australia.
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Water Dispersal
Scientists have stated that all mangrove
seeds can be water dispersed. These fruits, seeds and seedlings
that are salt water-dispersed have particular features. Part
of the fruit wall is usually fibrous to facilitate buoyancy.
They have to be woody and thick so that they can travel far
and resist seepage in and have to be a reasonable size so
the seed has enough reserves for the slow and possibly long
wait for a suitable site. Depending on the species, mangrove
seeds can be viable anywhere from 35 days to a year.
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Many examples of these can be found along the Daintree coast, for
instance the fruit of Barringtonia spp, common along Coconut beach.
One of the most common woody fruits found on our shores is that
of the 'Looking Glass mangrove' Heritiera littoralis. It has an
attractive oval fruit with a laterally flattened ridge erupting
from the surface. When the fruit is held with the ridge down, many
naturalist observers often think it looks rather like a keel. Yet
sometimes the seed will spin around and float with the ridge up,
and the 'keel' suddenly becomes more like a 'sail'. Seed vivipary
One of the most unusual aspects of seed dispersal in mangroves is
the high incidence of 'vivipary'.
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Most plants experience dormancy at the seed
stage; this allows time for the fruit to fall or be taken
off the parent plant and be dispersed. Then certain conditions
will trigger the continuation of the development process with
germination. However, in some mangrove plants there is no
dormancy period and thus the seed simply grows before it has
left the parent plant. There is no satisfactory explanation
for why this happens. Yet the fact that it happens to this
extreme mainly in many mangrove species, and rarely in other
plants, would suggest there should be an environmental condition
unique to the mangal community that acts as such a strong
selector for this feature. There are of course many unique
features of mangrove communities, including the periodic and
disturbing inundation by water over the substrate.
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One suggestion focuses on this latter feature; dropping
off an already growing seedling may be an adaptation to make sure
that when the seed finds a place to grow and take root, it does
so very quickly, to avoid getting washed away by the next tide.
Well known and common local examples are provided by various species
of Rhizophora and Bruguiera, with their long and distinct, but familiar,
cigar-like seeds, are quite common at Cape Tribulation and along
Marrdja boardwalk.
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| TROPICAL
SAVANNAH WOODLANDS |
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Definitions
The definition of 'Tropical Woodlands' is not straightforward. The
term is used here as an umbrella definition encompassing various biomes
that are sometimes considered separate, but can be difficult to differentiate,
such as tropical grasslands, savannas and tropical open forest. It
is perhaps best seen as a spectrum; at one end the trees cluster together
and it becomes a 'dry forest', while at the other end the trees are
very widely spaced apart and it becomes a grassland. It is almost
a biome defined by default it; the terrestrial habitat of the tropics
where the rainfall is not enough to result in rainforest, but sufficient
to avoid developing into arid grasslands and desert. As vague as all
this sounds, there are factors and features that determine and are
indicative of tropical woodlands and savanna. The rainfall pattern
is usually 'monsoonal', that is; there is an extreme wet season and
an extreme dry season. During the dry season the landscape is usually
effected by fires. The vegetation is comprised mainly of grass with
variously spaced woody plants such as trees and shrubs which do not
have a connecting canopy. The woody plants are often deciduous and
often show adaptations to fire, such as thick, insulative bark. Grazing
animals are usually an obvious and significant part of the biomass,
be it termites or herds of larger animals. |
| However,
part of the problem in defining and recognizing tropical savanna
and woodlands is determining how much of an effect other factors
such as humans, grazing and fire have had in shaping the biome
(Bourliere and Hadley). |
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The
distribution of tropical woodlands
Tropical woodlands and savanna occupies approximately 23 million square
kilometers of the planet, which is about 40% of the land surface of
the tropics (Solbrig 1996). Much of this occurs in the vast African
continent where the drier woodlands are much more expansive than the
rainforest. In Australia, the woodlands and savanna is also greater
in area than the rainforest. |
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| The question of what factors determine
distribution of the tropical woodlands can all be traced back to one
ultimate factor, position on the globe. By the very definition of
their name 'tropical woodlands', they are generally presumed as growing
in the bulging equatorial area between the two latitudinal lines that
are knows as the tropics of Cancer in the north and Capricorn in the
south. At the higher latitudes beyond these lines, the sun is never
directly overhead as it can be within those lines. However this biome/s
often stretches into the subtropics, as in southern Africa. For those
woodlands within the tropics, however, there are some definite tropical
conditions. As in the tropical rain forests, the more directly overhead
sun means there is more direct sunlight and the result is simply more
heat. |
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Additionally, the variation
of both sunlight and thus temperature within the year is minimal,
with a lack of a cold season to act as a limiting factor on
life. However, the temperature difference through the seasons
is large in comparison to the neighbouring biomes of the rainforest,
for there, the canopy and the more consistent cloud cover
dampens and equalizes the temperature somewhat. In this latter
area of virtually non seasonal, high and steady temperature
and high and steady rainfall, tropical rainforest can be expected
to develop. Further out, in the areas where there is virtually
non-seasonal high and steady temperature, but seasonal rainfall,
more open woodlands can be expected to grow. Certain plant
adapt well to these conditions, such as the cabbage tree palms
of Northern Queensland (left).
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| Tropical woodlands
in Australia |
| The biome of tropical
woodlands is very widespread in Australia, They cover roughly
the top third of the continent in a rage arc. In Australia the
international biome of 'tropical woodlands' becomes the regional
ecosystem probably best described as 'Tropical Eucalyptus woodlands'.
In structure, look and 'feel' it is very similar the biome of
tropical woodlands all over the world. And in fact for travellers
comparing tropical woodlands in the Americas, Africa or Asia
they can look very similar. Pictured right is Paperbark (Melaleuca
spp.) woodland in Northern Queensland, another plant community
that has adapted well to the vast range of seasonal rainfall.
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Seasons
To many visitors the tropical Eucalypt woodland can look rather
monotonous. In Australia and Africa it seems to go for mile after
mile unchanging. However, there are dramatic changes both spatially
and temporally. The latter change is fascinating to observe for
the resident over the years, changing from flooding and high green
grass in the wet season, to fires and burnt blackness in the dry
season. The definition of the seasons in the tropical open woodland
can be interpreted in various ways. The seasonal changes of the
tropical woodlands in Australia are visually quite obvious. And
thus we humans often feel the urge to put it all in some sort of
context or a framework of seasons. The reasons for doing this varies
because there are people who study the land and it's biology and
people who live off the land and it's biology.
For the first European explorers coming from a temperate high
latitude environment, where tradition viewed four distinct seasons,
this was the template. However this template did not exactly fit
the North Australian Tropics, as settlement grew the seasons simply
became known as the 'wet' and the 'dry'. These two seasons are retained
today, and roughly correspond to a merely lengthened wet summer
and a dry winter. The establishment of such a dichotomy was undoubtably
a welcome paradigm shift and an ultimate acceptance of the real
tropical world. The vast space of the tropics are often a big contrast
to the smaller varied areas of the temperate regions and the euro-centric
view of life that is so often hard to shake.
Before Europeans settlement, there were off course other people
living in Australia. An example of how they termed the seasons can
be seen by the Aboriginal Kuku-yalanji people in Cape York (Morwood
1995), - Pre-monsoon; the 'storm time' during November and December,
(Jarramali), - The monsoon season; 'proper wet time' from December
through to March (Kamba) - The end of the wet; 'underwater time'
from April to May, (Kabakabada), - Middle dry season; 'cold time'
from June to September, (Buluriji), - Late dry season; 'hot time',
(Wungariji).
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