Mangrove Root Function

Mangrove root system has developed uniquely due to its exposure to tides and its waterlogged substrates condition, therefore it has multiple function in absorption and plant positioning.

Mangroves grow in the tropical and subtropical water areas. Fringing along the coastline and river mouths, they present several layers of zonation and shelter the land effectively. The unique structure of the rooting system helps trapping sediment carried in the incoming currents and high tides. In the extended process, the vegetation regenerates the fine deposit into peaty soil, which work much protecting the land from abrasion.

The environment where the plants thrive, is a wet, waterlogged swamp, which is inundated periodically for once or two times a day, or depends on the characteristics of local tidal periods.

The swamp has the anaerobic condition limiting the oxygen intake from the root. For this reason, the plants have physically developed their roots exposed to the air. Modified differently from other terrestrial vegetation, mangroves developed the aerating component. The above-ground part of the root system will enable the plants to allow the gas exchange process.

Mangrove seedlings establish soon after ripe fruits drop and drift away into a suitable environment, the closest fine sediment substrate they may get. The embryo often has developed since it is still attached to the parent plant. Sometimes even when tide doesn't occur, the fruit just plops down into the soft mud under its parent plant and erects its first anchoring roots.

The first soft and white epicotyls still function similarly to other plant root to absorb nutrients into the seedling body. Soon after they grow more into some light brownish shoots it gets hardened and unbendable. When the plant grows fully developed, roots erect from its branches. Some grow downward, forming more anchoring branches and penetrate the substrate, and function for absorption and support of the tree. Some others develop fully to be aerial roots that may never touch the ground at all.

Tomlinson (1994) in The Botany of Mangroves, explains about several unique types of aerial roots in mangroves, such as:

1. Stilt-roots

The species of Rhizopora, Bruguiera, and Ceriops form branched aerial roots, from the lower branches of the tree. They grow harder, and become shallow stands to support the position of the adult tree.

2. Pneumatophores

Avicennia and Sonneratia have their buried root system growing outward from the center. From the lines here, they grow vertical roots in various heights. Avicennia has ones of less than 30 cm while Sonneratia with thicker enlargement of the structure may erect the root for as high as 3 m.

3. Root knees

From the horizontal roots of Bruguiera and Ceriops the root knees can be seen as loops growing upwards, before continuing their horizontal form.

4. Plank roots

This kind of root is found in Xylocarpus growing outward of the horizontal root up because of the cambium activity.

From a transversal section of a pneumatophore, it can be observed that the cortex tissue is mainly composed of intercellular spaces and they are supported by bands of wall thickening. There are lenticels as the vital component for aeration. The structure of the parenchyma develops into bigger cell forms which allow air circulation and gas exchange to run well.

Clayton and King (1990) in Biology of Marine Plants stated that the component plays an important role, since root growth in anaerobic soil is encouraged by the air spaces system (aerenchyma). The system either lets parts of the roots to respire normally above water or contains enough oxygen for the plants to undergo respiration during tidal flooding especially for example, in Avicennia marina, a species which is the most frequently exposed to the tides.

There are also a number of toxic compounds which are naturally present in anaerobic soil such as ferrous and manganeous ions. A great deal of bacterial activity in the anaerobic condition produce sulphide compounds that are toxic to plants. Gas exchange and oxidation at the root surface may avoid contacts with the toxins. In mangroves it is made possible by the presence of oxygen flux at the aerenchyma.

The underground root system has the original function of a root. The massive roots grow rapidly penetrating the peat substrates and form spreading anchor into it. Some older plants will likely have built big and strong descending prop roots, sometimes exceeds 3 meters high, as buttresses to support the plants. Through root hair and their capillary passages, the absorption of water and nutrients occur, even in anaerobic soil condition. The movement of nutrients through the vessels and from the substrates is controlled by the osmotic system. Even though the system has a high selectivity for nutrient uptake, as it is a high saline environment, the nutrient uptake may contain higher salt than required. The excess salt will then transported to excretive glands such as those located in the leaves.

Both underground and aerial root built a cohesive system to the tree. According to Tomlinson, the root system plays double parts as the anchoring and absorbing system. There is a cable system that connects the lines of anchoring, absorbing, and gas exchange. This system supplies the transport system in such a way so that the water and nutrients can move from the absorption sites to the trunk and the processing body of photosynthesis in the leaves. On the other hand, digested assimilates can also be transported from the trunk to the root system. The cable system lies along the root cables buried close to the surface of the substrate or inside the lower aerial roots above the ground.

With such unique combination from each part of their root system, mangrove plants have been able to survive through its environment for years.

Besides forming the very important green belt of the island, mangroves also play the significant role as the nourishing ground and the roots has something to do with it. The intertwining root construction above the ground often keeps wave current at minimal which provides still and safe environment for larvae and juveniles of crustacean and other associated organisms to grow their early stages. Some bottom dwelling organisms such as mud crabs and worms burrow tunnels and enable oxygen circulation throughout the soil. Other bacterial organisms get advantage from rich nutrient as the consequence of decayed litters from the trees or entrapped sediment from the surrounding waters.

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