1. Roots that multitask
Besides providing support in unstable soil to withstand currents and storms, mangrove roots that take in aboveground air to avoid suffocation in the oxygen poor mud. These roots are called aerial or air-breathing root. Lenticels, which are found on the surface of the roots, are special pores that take in air. These tiny pores only allow air, not water or air, to pass through them. Aerial roots also contain large air spaces called aerenchyma that not only transport water but also act like a reservoir of air during high tide when all aerial roots may be submerged. There are different forms of aerial roots.
To stay upright in soft mud, mangrove tree roots are shallow (no deeper than 2m) forming a wide raft that floats in the muddy unstable ground. Although the roots are shallow, there is often more mass below the surface than above the surface! Additional support is provided by various kinds of roots. These roots also help the plant breathe by increasing the surface area.
Avicennia develop shallow cable roots that spread out form the trunk, with pencil-like roots (pneumatophores) emerging along them. Sonneratia also produce pneumatophores but are in cone-shaped. For Bruguiera, it has knee roots that emerge from the ground then loop back in. Rhisophora have roots sent out form their trunks and branches which arhed done to the ground for extra support and absorption.
Different species have developed different ways of keeping their roots in the air.Rhizophora stylosa is commonly found close to the seaward side of mangroves. It is therefore subjected to high wave energy and has developed a system of stilt or prop roots. These spread far and wide, providing numerous anchors for the tree as well as a large surface area for oxygen-absorbing lenticels. In common with other species, this mangrove also grows aerial roots, extra stilts which arise from the branches or trunk. Studies have shown that these aerial roots alter dramatically in structure when they reach the mud; above it they have about 5% air spaces in their tissues but below this change to 50%.Grey mangrove Avicennia marina grows a series of snorkels or peg/pencil roots, known as pneumatophores. Experiments with a related Avicennia species have shown that those plants growing in coarse coral sand, with a good air supply to the roots, were able to survive after their pneumatophores were removed. However those living in poorly aerated soil died when the pneumatophores were covered. In one situation, where they were covered with oil, the plants responded by growing aerial roots.Orange mangrove Bruguiera gymnorrhiza develops knee roots. These are cable roots which have grown above the surface of the mud and then down into it again.Looking glass mangrove Heritiera littoralis produces buttressed roots which are like flattened, blade-like stilt roots.Cannonball mangrove Xylocarpus granatum is buttressed but the cable roots also appear above the ground in the fashion of knee roots.
Besides providing support in unstable soil to withstand currents and storms, mangrove roots that take in aboveground air to avoid suffocation in the oxygen poor mud. These roots are called aerial or air-breathing root. Lenticels, which are found on the surface of the roots, are special pores that take in air. These tiny pores only allow air, not water or air, to pass through them. Aerial roots also contain large air spaces called aerenchyma that not only transport water but also act like a reservoir of air during high tide when all aerial roots may be submerged. There are different forms of aerial roots.
To stay upright in soft mud, mangrove tree roots are shallow (no deeper than 2m) forming a wide raft that floats in the muddy unstable ground. Although the roots are shallow, there is often more mass below the surface than above the surface! Additional support is provided by various kinds of roots. These roots also help the plant breathe by increasing the surface area.
Avicennia develop shallow cable roots that spread out form the trunk, with pencil-like roots (pneumatophores) emerging along them. Sonneratia also produce pneumatophores but are in cone-shaped. For Bruguiera, it has knee roots that emerge from the ground then loop back in. Rhisophora have roots sent out form their trunks and branches which arhed done to the ground for extra support and absorption.
Different species have developed different ways of keeping their roots in the air.Rhizophora stylosa is commonly found close to the seaward side of mangroves. It is therefore subjected to high wave energy and has developed a system of stilt or prop roots. These spread far and wide, providing numerous anchors for the tree as well as a large surface area for oxygen-absorbing lenticels. In common with other species, this mangrove also grows aerial roots, extra stilts which arise from the branches or trunk. Studies have shown that these aerial roots alter dramatically in structure when they reach the mud; above it they have about 5% air spaces in their tissues but below this change to 50%.Grey mangrove Avicennia marina grows a series of snorkels or peg/pencil roots, known as pneumatophores. Experiments with a related Avicennia species have shown that those plants growing in coarse coral sand, with a good air supply to the roots, were able to survive after their pneumatophores were removed. However those living in poorly aerated soil died when the pneumatophores were covered. In one situation, where they were covered with oil, the plants responded by growing aerial roots.Orange mangrove Bruguiera gymnorrhiza develops knee roots. These are cable roots which have grown above the surface of the mud and then down into it again.Looking glass mangrove Heritiera littoralis produces buttressed roots which are like flattened, blade-like stilt roots.Cannonball mangrove Xylocarpus granatum is buttressed but the cable roots also appear above the ground in the fashion of knee roots.
2. Leaves
Many organisms such as leaf-eating crabs feed on the leaves and bark of young Rhizophora trees and seedlings. Mangrove trees have developed many special features to deflect attack by both microorganisms and herbivores, such as leafy tissues that are extremely difficult to be digested. Toxic substances have also been incorporated in some of the leaves. Thick waxy leaves are also an efficient defensive shield against insect attack and fungus infestations, and are also unappetizing to most grazing animals.
Like desert plants, mangroves store freshwater in thick succulent leaves. A waxy coating on the leaves of some mangrove species seals in water and minimizes evaporation. Small hairs on the leaves of other species deflect wind and sunlight, which reduces water loss through the tiny openings where gases enter and exit during photosynthesis. On some mangroves species, these tiny openings are below the leaf's surface, away from the drying wind and sun.
3. Seeds
Living in salt-water condition is tough for adult trees. It will be even tougher for tender seedlings, which are usually dispersed by seawater. Therefore, many mangrove trees have special adaptations to give their offspring the best chance in their harsh habitat by providing them with a good store of food and floatation devices.
Germinating while still on the mother tree is one of the few adaptations. The fruit does not fall away when ripen. The seed germinates. Meanwhile, mother tree channels nutrients to the growing seedling. This is called vivipary.
Another adaptation is that the growing seed does not break through the fruit wall until after the fruit fall off. This is called cryptovivipary, which is the case with Avicennia. Seed coat of its fruit drops away more quickly in water of the right warmth and salinity, usually the best suited spot for an Avicennia seedling.
In others mangrove trees, the growing seedling breaks through the fruit wall to form a stem called hypocotyls, sometimes even roots (Rhizophora, Bruguiera). The whole seedling is then now called a propagule. In the Aegiceras species, seedlings only fall when high tide. After the propagule falls, it floats horizontally and drifts with the tide. After weeks, the tip, which is absorbent, gradually absorbs water and it floats vertically. It will then start to sprout its first leaf from the top and roots from the bottom. When it hits land, it grows more roots to haul itself upright. The long stem is a short cut to sunlight and oxygen as seedlings are often completely submerged at high tide. But the amazing factor is that young seedlings can survive 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.
4. Coping with Salt
A few mangroves have special membranes covering their roots to filter out salt: up to 90% of the salt. Most just draw in seawater then excrete salt in their leaves: you can actually see the salt as a whitish deposit and their leaves taste salty. Others concentrate the salt in bark or leaves which are about to peel or drop off. Others just have tissues which can tolerate high concentrations of salt.
Growing in sea water is almost like growing in a waterless desert and mangroves have developed desert-plant-like features to maintain their internal water balance and minimize water loss: waxy or hairy leaves with sunken breathing holes (stomata) to minimize water loss. They have thick and succulent leaves to store water. Like plants which grow in arid areas, mangroves also grow slowly.
5. A World Traveller
Botanists believe that mangroves originated in Southeast Asia, but ocean currents have since dispersed them to India, Africa, Australia, and the Americas. As Alfredo Quarto, the head of the Mangrove Action Project, puts it, "Over the millions of years since they've been in existence, mangroves have essentially set up shop around the world."
The fruits, seeds, and seedlings of all mangrove plants can float, and they have been known to bob along for more than a year before taking root. In buoyant seawater, a seedling lies flat and floats fast. But when it approaches fresher, brackish water which are ideal conditions for mangroves, the seedling turns vertical so its roots point downward. After lodging in the mud, the seedling quickly sends additional roots into the soil. Within 10 years, as those roots spread and sprout, a single seedling can give rise to an entire thicket. It's not just trees but the land itself that increases. Mud collects around the tangled mangrove roots, and shallow mudflats build up. From the journey of a single seed a rich ecosystem may be born. Do mangrove trees need salt to grow then? No. They continue to grow with freshwater but more slowly. So, outside their specialised niche, they cannot compete with regular plants and are quickly overgrown.
Mangrove trees need regular and large tidal fluctuations in order to flush away the salt. While they can withstand some salt, most cannot survive in water with a constantly high concentration of salt.
Mangroves have different tolerances for these extreme conditions: Some can withstand colder weather and are found in more temperate areas. Others have better breathing roots and thrive in muddier places. Yet others can stand saltier water and are found at the edge of the sea. Some are taller and others are more like large bushes.
These different abilities often result in zones of different species within the mangrove forest.
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