Mangroves
updated
Aug 09
What
are mangroves? The word 'mangrove' is used to refer to
both the plants as well as the entire community of plants. According
to Tomlinson, mangroves may be defined as tropical trees that grow
only in the intertidal and adjacent communities.
Mangroves
at the Chek Jawa Boardwalk, Aug 09
Mangrove trees
grow where no tree has grown before. They are able to survive inundation
by salt water twice a day, and in 'soil' which is soft, unstable
and poor in oxygen (anaerobic). They also have to deal with rivers
carrying silt during the wet season, as well as violent storms that
hit the coasts.
Root of the matter: Mangrove
roots not only provide support in unstable soils and to withstand
currents and storms, but also breathe in oxygen. To avoid suffocation
in the oxygen-poor mud, mangrove trees snorkel for air. They develop
aerial or air-breathing roots that grow upwards from the ground.
All aerial tree roots have on their surface, special tiny pores
to take in air (lenticels). Only air can get through the lenticels,
not water or salts.
All aerial roots also contain large air spaces (aerenchyma). These
not only transport air, but also provide a reservoir of air during
high tide when all the aerial roots may be submerged. Roots for
absorbing nutrients are tiny and emerge near the muddy surface.
Aerial roots can take on different forms. Avicennia
develop shallow cable roots which spread out from the trunk.
Along these cable roots emerge short pencil-like roots called pneumatophores
(meaning "air carrier" in Greek). According to Hogarth, a 3-metre
tall Avicennia can have 10,000 pneumatophores! Sonneratia
also produce pneumatophores, but these are cone-shaped instead.
Bruguiera sends out
knee roots, that emerge from the ground then loop back in, often
with a knobbly bump at the highest point of the loop that resembles
a knee. Rhizophora
send out stilt roots from their trunk which arch down to the ground
for extra support and air absorption. They also produce prop roots
from their branches which, well, prop up the these branches. Xylocarpus
produces flat, sinuous plank roots that act as buttresses to the
trunk.
Most mangrove trees lack a heartwood and instead have narrow vessels
that are densely and evenly distributed throughout the wood. Thus,
they are able to withstand damage to the bark and outer trunk.
Salt solution: All mangrove
trees exclude most of the salt in seawater at the root level, and
all can tolerate more salt in their tissues than "normal" plants.
But some have more effective ultrafiltration at the root level to
exclude more salt. Any salt that gets through are believed to be
stored in old leaves which are later shed.
A few can tolerate high levels of salt in their tissues. They then
secrete the excess salt through special cells on their leaves. Avicennia
does this best and is often the only tree to survive is hot salty
regions. Some other mangrove plants like Sea
Holly (Acanthus spp.) also do this.
Tough toddlers: If it's hard for adult trees to cope
with their environment, it's even harder for tender seedlings which
are usually dispersed by seawater. Thus many mangrove trees have
special adaptations to give their offspring the best chance in their
harsh habitat. Many provide their seedlings with a good store of
food and flotation devices to travel to new places.
The fruit is the portion that first forms before germination. In
some, the fruit does not fall away when it ripens. Instead, the
seed within the fruit starts to germinate while it is still on the
mother tree, and the mother tree channels nutrients to the growing
seedling (vivipary).
In some plants, the growing seed does not break through the fruit
wall while the seed is on the mother plant but only after the fruit
falls off (cryptovivipary). This is the case with Avicennia
and the seed coat of its fruits drops away more quickly in water
of the right warmth and salinity, usually in a spot best suited
for an Avicennia seedling.
In others, the growing seedling breaks through the fruit wall to
form a long green finger-like stem (called a hypocotyl), sometimes
even roots (Rhizophora, Bruguiera). At this point, the 'fruit'
is now called the calyx. The entire assembly of hypocotyl and calyx
is called the propagule (which means 'potential plant').
When the propagule finally falls, at first it floats horizontally,
and drifts with the tide. Some can survive for long periods at sea.
The tip is water absorbent while the top end is water repellent.
After some time, the tip gradually absorbs water and the seedling
floats vertically and starts to sprout its first leaf from the top,
and roots from the bottom. When it hits land, it hauls itself upright
by growing more roots, then sprouts more leaves. The long stem is
a short-cut to sunlight, and oxygen as seedlings are often completely
submerged at high tide. Amazingly, young seedlings can survive being
completely underwater until they are big enough to grow air-breathing
roots, at about 1-2 years. Meanwhile, they depend on stores of air
in air spaces (aerenchyma) in their stems.
Water water everywhere, not a drop to drink:
Freshwater is as precious to a mangrove tree as to a desert plant.
They have to expend energy to get rid of the salt in every drop
of water they use. Thus mangroves have many of the water conserving
features found in desert plants. To minimise water loss through
evaporation they may have thick waxy leaves, hairy leaves (to trap
an insulating layer of air near the leaf and thus reduce water loss
through evaporation). They may also store water in succulent leaves.
Mangroves also protect their hard won water from leaf-eating animals
with spiny or waxy leaves; and high levels of tannins and other
toxins. Mangrove plants are thus a precious resource of novel chemicals
that have myriad potential uses for humans.
The Fart of Life: Mangrove mud
is usually black and stinky. But this is an indication of life and
not necessarily of death and decaying rotten things. Mangrove mud
is very fine and well compacted. Thus there is little space for
oxygen. But mangrove mud is rich in tiny bits of decaying plants
and animals (detritus). Special bacteria thrive on this detritus.
These bacteria are special because they can breathe both with and
without oxygen. At first, they use up all the oxygen. Then they
start to use sulphur, releasing hydrogen sulphide as a by-product.
Hydrogen sulphide turns things black and smells of rotten eggs -
hence black and stinky mud! These bacteria are in turn eaten by
tiny animals which in turn are eaten by larger ones. Thus these
bacteria support food chains in the mangroves and beyond.
Role in the habitat: Mangrove
trees provide many services for other plants and animals, including
humans. Humans use mangrove trees and plants for timber, to make
charcoal and for making other things such as baskets, ropes
and other useful items. Parts of mangrove plants are eaten by people
are food and for medicinal purposes, fed to their livestock, and
used to catch fish. But the most valuable service is probably as
a natural filter and in stabilising and protecting coasts (see below).
Refuge: Tree climbing crabs
and sea snails climb up their aerial roots at high tide to avoid
aquatic predators. The roots provide a surface for all kinds of
creatures to settle on, from algae to shellfish. And the tangle
of roots provide hiding places for young fishes and shrimps from
larger predators. Their branches provide shelter for creatures from
Proboscis Monkeys and nesting sites for large herons, to crevices
for insects and other tiny creatures. The
mudlobster plays a key role in further enriching the mangrove
habitat and allowing a wide variety of plants and animals to thrive
in the back mangroves.
Food: While on the tree, leaves
are eaten by all kinds of creatures. Monkey snack on the shoots
and leaves, small insects nibble on them. Fallen leaves are an important
source of nutrients both within the mangrove habitat and when it
is flushed out to the coral reefs. The leaves are rapidly broken
up by crabs and other small creatures, and further broken down by
micro-organisms into useful minerals. Bees and other insects visit
mangrove flowers that produce nectar. Bats rely on the nectar of
the large pom-pom flowers of
Sonneratia and it is these bats that also pollinate our
durians! There are even tiny moth larvae that feed on pneumatophores.
Natural water filter: Underwater,
a huge number of filter-feeders fasten onto the tangle of roots:
barnacles, sponges, shellfish. These filter feeders clean the water
of nutrients and silt. As a result, clear water washes out into
the sea, allowing the coral reef ecosystem to flourish.
Stabilise the coast and river banks: Their roots prevents
mud and sand from being washed away with the tide and river currents.
Mangrove trees also slowly regenerate the soil by penetrating and
aerating it (other creatures such as crabs and mud lobsters also
help in this effort). As the mud builds up and soil conditions improve,
other plants can take root. Mangrove trees also reduce the damage
from violent storms.
Sadly, despite the important role of mangroves, they are often looked
upon as smelly places which can be put to 'better' use. In Singapore,
we have lost a large proportion of our original mangroves to development.
We should appreciate and protect our remaining mangroves.
|
Pneumatophores
of Sonneratia.
Chek Jawa, Jul 07
Prop
and stilt roots of Rhizophora.
Pulau Semakau, Jan 09
Knee
roots of Ceriops tagal.
Pulau Semakau, Jan 09
Buttress
and knee roots
of Bruguiera
gymnorrhiza.
St. John's Island, Aug 09
Cable
and pencil roots
of Avicennia
rumphiana
Chek Jawa, Aug 09
Propagule
of Bruguiera gymnorrhiza
with
red calyx and long green hypcotyl.
Tuas, Nov 03
Salt
crystals deposited on
an Avicennia officinalis
leaf.
Tuas, Nov 03
Floating Avicennia
seedlings.
Chek Jawa, Nov 03
Avicennia
seedling taking root.
Tuas, Nov 03
Mudlobsters
and their mounds
add to richness of life in the mangroves.
Tuas, Nov 03
|
Oxford University Press. 273 pp. 
Cambridge University Press. USA. 419 pp.
.
Second edition. Prentice Hall. 271 pp.