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| Mangrove
Trees 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 unstable and poor in oxygen (anaerobic). They also have to deal with swollen rivers carrying silt during the wet season, as well as violent storms that hit the coasts. Salt solution: To deal with salt, all mangrove trees exclude some salt at the root level, and all can tolerate more salt in their tissues than "normal" plants, often in quantities that would kill other 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. These include Bruguiera, Sonneratia and Rhizophora. A few can tolerate high levels of salt in their tissues and their sap can be up to one-tenth as salty as sea water. They then secrete the excess salt through special cells on their leaves. |
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Why
don't mangroves grow in freshwater swamps?
One reason could be that they are unable to grow as fast as other freshwater plants and are soon overwhelmed. They may also be unable to cope with the bacteria and fungi found in freshwater. |
Avicennia does this best and is often the only tree to survive
is hot salty regions. Some other mangrove associates also do this:
Sea Holly (Acanthus spp.). Although mangrove trees are adapted to grow in salt water, they require regular flushing with freshwater. They will die if immersed in saltwater all the time. |
Root
of the matter: Mangrove roots not only provide support in unstable
soils and to withstand currents and storms, but also breathe air. To avoid
suffocation in the oxygen poor mud, mangrove trees snorkel for air. They
develop aerial or air-breathing roots. These take in aboveground air. 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 underwater. The function of aerial roots are
to absorb air or/and to provide structural support in the soft mud. 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 (left) called pneumatophores (meaning "air carrier" in Greek). 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.Rhizophora send out roots from their trunk and branches which arch down to the ground (right) for extra support and air absorption.
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.
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 floatation devices.
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 (right) 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 stem
(called a hypocotyl), sometimes even roots (Rhizophora, Bruguiera).
The whole seedling is then called a propagule (potential plant). In some
trees, the seedlings only fall at high tide! (Aegiceras).When the propagule finally falls, at first it floats horizontally, and drifts with the tide. It can survive for long periods at sea. The tip is water absorbent while the top end is water repellent. After some weeks, 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 aerial 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 water conserving features of 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). They may also store water in succulent leaves. Mangroves also protect their hard won parts with spiny leaves (e.g., Sea Holly) or waxy leaves; and high levels of tannins and other toxins (e.g., Blind Your Eye). Mangrove plants are thus a precious resource of chemicals that have myriad potential uses for humans.
Role in the habitat:
| 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 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. |
How
did the term
"mangrove" arise? We don't really know. It might be derived from some of the ancient names given to some mangrove trees: the Portuguese called them mangue, the Spanish mangle, the Malays manggi-manggi or mangin. |
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. There are even tiny moth larvae that feed on pneumatophores.
Natural water filter: Underwater, a huge number of filter-feeders are fastened on 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 . As the mud builds up and soil conditions improve, other plants can take root. Mangrove trees also reduce the damage from violent storms.
LINKS
- Mangroves: more than mud and mozzies! by the Queensland Environmental Protection Agency: fact sheet on mangrove adaptations and role in the habitat.
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buy these references & others, visit Nature's Niche |
- Peter K L Ng and N Sivasothi, "A Guide to the Mangroves of Singapore I: The Ecosystem and Plant Diversity", Singapore Science Centre, 1999 (p. 77-87: adaptations of mangrove plants, photos).
- Colin Field, "Journey among Mangroves", International Society for Mangrove Ecosystems, 1995 (p. 40-48: adaptations of mangrove trees to their environment, photos).
- Michael Mastaller, "Mangroves: The Forgotten Forest Between Land and Sea", Tropical Press, 1997 (p.7: why mangroves don't grow in freshwater; p. 15-53: adaptations of mangrove trees, photos).