There is a lot of
talk of the value of trees, woodland, and forests because of the growing
awareness of climate change, but very little attention is given, in our media at least, to ‘Mangrove Swamps.’ These coastal woodlands
grow in saline conditions along sheltered coasts and inlets of a
great many tropical and sub-tropical countries.
There are about
eighty species of Mangrove shrubs and trees. They aren’t all
related to one another. The term refers to their ability to thrive in
salt water and to filter The salt out of their system. They grow
in inter-tidal areas, subject to daily fluctuations in water level,
from inundation as the tide rises, to drying out as it falls,
conditions which would kill most species. They have various types of
roots according to species, from short vertical aerial roots to a
tangle of stilt roots as much as four feet high. These enable the
roots to obtain oxygen while the tree grows in water-logged, oxygen-poor mud.
These Mangrove
Swamps, Forests or Mangels, form a unique ecosystem. The tangle of
roots slow down erosion, blanket the effects of storm surges and
tsunamis, and trap silt washed from the interior countryside. The
highly organic mud acts as a very important carbon sink, and the roots act as a
nursery for great numbers of fish and other marine organisms.
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| Mangroves at high water showing the aerial stilt roots. |
Blue carbon is the term for carbon captured by the world's ocean and coastal ecosystems. Sea grasses, mangroves, salt marshes, and other systems along our coast are very efficient in storing CO2. These areas also absorb and store carbon at a much faster rate than other areas, such as forests, and can continue to do so for millions of years. The carbon found in coastal soil is often thousands of years old. When these systems are damaged or disrupted by human activity, an enormous amount of carbon is emitted back into the atmosphere, contributing to climate change.
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We became familiar
with various Mangrove swamps during our travels in Australia. We
first came across them in the complicated system of creeks and inlets
in Sydney. They become much more extensive on the Queensland coast,
the North coast and down the west coast, becoming more sporadic and
with fewer species, as the water cools further south, and are said to
occur on one fifth of the Australian coast ( there are none in
Tasmania) They are the third greatest area of mangrove forest in the
world.
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Peg Roots or pneumatophores are about 6 inches tall. We came across these in a mangrove swamp in a Sydney inlet a couple of days after we first arrived. We were still jet-lagged and the sight of parrots, common British plants like Sow
Thistle growing as garden weeds, and walking through these aerial roots, growing in glutinous mud, all added to the profound culture shock. |
Fascinating habitat
though it is, I have a love/hate feeling towards mangroves, as not only is
it home to a myriad of creatures from the canopy to within the mud
under one’s feet, most intrusive being the voracious Salt -water
Mosquito which attack within minutes of you entering the swamp, day
or night. They bite ferociously and will penetrate at least one layer
of shirt, seem undeterred by the most powerful repellent, and their
itchy bites are fearsome and persistent. Prone as I am, to being
bitten by just about every insect with mouthparts, these mozzies
really had a field day with me.
All the senses are
on the alert in this unique habitat, so strange to us, being used to
the mixed deciduous woodlands native in temperate Britain and Europe.
It’s unlike any other type of woodland, although the oval,
evergreen leaves don’t seem unusual, it is the aerial roots of
various kinds that arrest the attention.
Even the sense of
taste is used if you try the white crystalline specks on the leaves
of some mangrove species. They are salty and are a strategy for
excreting the salt from the saline habitat the trees are growing in.
The fine glutinous
mud which is always accumulating as the trees’ roots trap the
sediments is unpleasant stuff to walk in. My mother’s term was
‘clarty’ meaning sticky and clinging….it could have been
invented for mangrove mud rather than the word for sticky clay etc
used by Nottinghamshire farming folk when she was a girl.
The most affected
senses are those of hearing and sight – the incessant whining of
the mozzies and other insects, the calls of elusive birds, the ‘snap,
crackle and pop’ of holes opening up in the mud as the tide goes
out and great tower shells begin to open up and move, mud crabs
emerge from their holes and signal to each other with enlarged red or
yellow claws; mud skippers come to life and run at each other across
the mud, using their front fins as legs and signalling aggressively
by raising and lowering their dorsal fins like luminous flags.
Red Mangrove Crab.
Yellow Signal Crabs wave their large claws at the first sign of movement.
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| Mud Skippers. One is displaying by erecting its dorsal fin. As the mud is exposed, they can walk on it by using their pectoral fins as front legs. |
The tangle of roots
make any sort of headway though these mangrove forests very difficult
and further complicated by a network of creeks and gullies so the
best means of access is using the boardwalks constructed in various
reserves up and down the coast. Then you can stroll and pause to
watch the birds and take in the surroundings. Bird-watching isn’t
easy; either small and elusive honey-eaters, robins, flycatchers and Warblers tend to keep to the canopy, feeding on insects and pollen
and nectar when the mangroves are flowering. The predatory birds like
mangrove herons and kingfishers are well camouflaged and sit
motionless and difficult to spot.
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| Impenetrable root tangle. |
From our log,
partway up the central Queensland coast:
“We made our way
to the head of the bay and came to a small creek among the mangroves.
We watched a host of little fish swimming into it as the tide came in and saw a
sting-ray hiding under the mangrove roots. We started back to camp,
but Kim called us back for a bird. It was a Bush Thick-knees, ad we
stalked it through the mangroves and had a marvellous ten minutes or
so, watching it. They are the birds which puzzled us in the night
with their loud wild wailing cries.”
Beach Thick-knees. These birds are related to our Stone Curlews.
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| Young mangroves. Many species produce a prolific crop of seeds which will float in water but won't germinate until they wash up at the high water line. |
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| These venerable mangrove trees were growing in fissures in a rocky reef partway up the Western Australian coast. They had huge stout and gnarled trunks bowing to the wind. |
From log, again
partway up the central Queensland coast:
“We were heading
for the Edmund Kennedy National Park, a mangrove and swamp area south
of Cairns. On the way we passed alongside Hinchinbrook Island.
Between the shore and the island were extensive mangroves and a
convoluted system of creeks.
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Looking across to Hinchinbrook Island.
Cassowary |
As we looked across the passage to Hinchinbrook, where our road passed through an
area of vine-hung rainforest, what should stride out across the
road right in front of us but a Cassowary! Good job we were going
slowly. It had a great ‘helmet’ and a red ‘scarf’ of blue and red skinny wattles round its neck and a flouncing and quivering silken cape of
black plumes. It immediately disappeared into the thick scrub at the
side of the road.
We found our way
along sand tracks to the camp-site in the Edmund Kennedy National Park. It’s
obviously new (this was in 1989) with posh ablutions, no other
visitors and an unused barbie.
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| Smoky fire at Edmund Kennedy to deter the mozzies. When I picked up a handful of dry gum leaves to make more smoke, a small black scorpion scuttled away. |
A terrific dawn
chorus after a restful night, but the mozzies are still about. After
breakfast we walked through the wide belt of mangroves till we
reached the coast. We hadn't reckoned with the distance, the heat,
and the mozzies. The place was heaving with birds but as soon as we
stopped we were attacked and eaten alive by the mozzies which bit us
through our shirts. Quiet green-brown flooded creeks wound among the
mangrove roots. Notices in the camp-site warned of Estuarine
Crocodiles’(the notorious ‘salties’) but we were out of luck
even though we were creeping round very quietly, hoping to see one.
But all we saw were mud skippers, crabs and garfish.
Tony climbing among the stilt roots, to give an idea of their size.
These mangroves produce 'propagules' which when mature, with a small leafy sprout already growing, drop into the water or mud below like spears, and immediately send down roots and start to grow.
Mangrove Kingfishers hide in the foliage above the creeks.
And very good
views of a Mangrove heron before it flew into a mangrove near us and
completely disappeared, so good was his camouflage.”
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| Showing the stilt roots. |
From log, Aug.’93 (going north up the Western Australian coast)
"After the Eighty
Mile beach, we re-joined the highway and crossed a vast flat
sandplain with a sparse scatter of small trees, before turning down
another dirt track heading west towards the coast. 23 kilometres
down nasty corrugations, we came to Port Smith, our destination
picked from the map as it looked an interesting night stop.
“ Pulled up in the
shade of a big tree at the Campsite ($10 for the night ; ie about £4
sterling) and the very helpful lady suggested various walks to see
plenty of birds.
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| The Land Rover belonging to this place was still in use, at least it was in 1993, so the manufacturers would be gratified. No MoT needed in the outback? |
After pitching up, we went for a walk along a coralline rocky ridge behind the mangroves lining the creek. Lots of birds including a few new for the trip.
Port SmithNext morning...up before 6 and walked down to the seaward side of the mangroves where we walked along over wet, very fine sand looking out over various creeky inlets in a wide cove with the sea visible a long way off. Lovely views of several Sacred Kingfishers and Mangrove crabs with brilliant red claws.”
Reef Heron feeding among young mangroves as the tide came in.
Sunbird in the mangroves. Beautiful iridescent plumage on its back.
From log, mid
August’93. Now staying at the Broome Bird Observatory.
“After breakfast
we drove down to One-Tree at the end of the track and walked up
behind the mangroves to Crab Creek. We were amazed at the heavy crop
of Mangrove beans on the trees. But we were especially looking for
‘little jobs’ in the trees. We had some success, seeing
Broad-billed flycatcher, Dusky Warbler and M. had a quick glimpse of
a Mangrove Golden Whistler. There were Stilts on the mud at the
edge of the mangroves as the tide came in. It was 9am by now and
getting hot. The threatening cloud and mist of earlier was clearing.
Mangrove Golden Whistler.
White-throated Whistler.
While staying at the Bird Observatory outside Broome, we enrolled in a Ringing Course and during sessions of mist-netting in the mangrove creeks, we saw several specialist birds of this habitat, including these two Whistlers.
Mangrove habitat is, like so many others, under threat from various human pressures: taking firewood, clearing for fish farming, and widespread destruction for resort development along tropical and sub-tropical coasts.
These unique ecosystems cannot be replaced.
