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Mangrove Forests
A World of their Own
Mangrove forests are one of the most ingenious yet least thought about areas on our planet. Thousands of people a year swim and canoe their way through the watery forests, pulling their way deeper and deeper into the heart of the mangroves. Most of these people donÕt exactly know what the ÒheartÓ of this forest is though; its purpose, and the hundreds of organisms and ecosystems that are connected to it. Without these trees, many young and old organisms in our intertidal zones would be without protection, and the ecosystems at sea would be put at great risk.
There are three main types of mangrove trees that I am going to refer to throughout this paper: red, black and white. Each of these different mangroves is found throughout Florida. They each differ slightly with the color of their bark as well as the process through which they rid the excess salt from their system (Davidson, 1998). The biology of a mangrove tree is similar to that of a normal terrestrial tree, with roots, a trunk, branches, and leaves. ItÕs main different is its adaptation to low oxygen, high salt uptake, gathering of nutrients, and seed dispersal. Since all species of mangroves live in and around water, their soil lacks a sufficient amount of atmospheric oxygen, which is necessary for the plant to photosynthesize. The red mangrove uses large ÒpropÓ roots, which can reach a few feet above the water, to exchange gases and oxygen with the atmosphere through pores on the roots (Lodge, 1998). These mangroves live the furthest from the shore, thus their soil has the highest concentration of salt (Davidson, 1998). Black mangroves on the other hand keep their roots under the soil and send small shoots up about a foot above the soil (Lodge, 1998). These ÒshootsÓ are called pneumatophores, and they serve as the site of gas exchange for the roots of the mangrove. On the exposed areas of each of the pneumatophores are Òair breathingÓ cells, lenticels, which can be open, partially open, or closed based on the state of the environment (Kathirsan & B.L., 2001). The white mangroves form roots in the ground that may form pneumatophores when the oxygen content goes too low. These shoots are much smaller and less noticeable than the extended roots of the black and red mangroves (Lodge, 1998).
High salinity in the marsh soil is a factor that mangroves have learned to adapt to. The red mangrove tree does not allow the salt from entering its roots, while the black mangrove tree allows the salt to filter into the root system, but then excretes it all from the leaves (Lodge, 1998). The white mangroves use neither of these tactics, and instead have two pores on their leaves where they excrete excess salt from their vascular system (Davidson, 1998). Due to the high concentration of salt in the environment, mangrove leaves have many stomata openings and guard cells around these openings controlling the transpiration, water loss, which goes on. The sap from mangrove trees also helps to slow the rate of transpiration, allowing the uptake of water from the soil to be slow, so that salt does not rapidly accumulate in the soil around the roots (Kathirsan & B.L., 2001). Like many intertidal plants, mangroves use floating seeds as their way to reproduce and disperse their offspring. The floating seeds allow for the mangroves to inhabit a wide area of marsh (Lodge, 1998).
While mangrove trees can be classified in the way I just mentioned, they can also be organized in a geographical way. There are three main groups that mangroves can be placed in, Fringe, Basin, and Riverine areas (Ewel et al, 1998). The fringe mangroves inhabit the area closest to the ocean where they encounter high salinity and many tides, basin trees live in the interior of the forest where the soil is less salty and the flooding is more rare, and the riverine trees occupy the area closest to the riverÕs mouth. These trees will have some of the highest productivity, due to the large amount of sediments and nutrients that flow from the river (Lugo, 1980).
While each of the three Florida mangroves occur in different areas, they all grow along the boundary between fresh river water and salty ocean water. Mangroves all over the world live between 25¡N and 30¡S latitude (Valiela, Bowen, & York, 2001). Mangroves thrive in low lying, frequently flooded areas, with loose, fine soil that allows for drainage (Kathirsan & B.L., 2001). Mangrove forests are excellent at trapping sediment that would otherwise wash out to the ocean. Riverine mangrove forests, trees closest to the mouth of the fresh water river, catch most of the sediments due to the heavy load to sediments that rivers carry (Ewel et al, 1998). Their evolutionary ability to excrete salt gives them the tools to grow in this harsh environment. The mangrove swamps are very productive areas for other creatures living there, due to the constant movement of nutrients from the fresh water and ocean tides flowing in and out (Lodge, 1998). The mangrove forests act as a nursery and shelter from predators for small marine animals as well as a feeding ground. Many shrimp live the beginning of their lives in the mangroves due to the constant flow of nutrients from the rivers and also the easy access to the open ocean when they are grown. The dead leaves, limbs, and bark that comes from the trees serves as a food source for many shrimp, crabs, and fish living in either the mouth of the fresh water rivers or the mangrove forests themselves (Kathirsan & B.L., 2001).
Bacteria living in the mangrove forests are critical for the mangroves survival as well as the ecosystems in the open ocean. There are many nitrogen fixating bacteria that help to keep nitrogen cycling through the environment. There are also other bacteria that help to break down and process wastes that pass through the mangroves from the industries upriver, which help to keep these wastes from reaching the ocean organisms (Kathirsan & B.L., 2001). As well as keeping excess nitrogen from flowing into other water sources, mangrove trees absorb carbon dioxide for photosynthesis and keep the carbon in the soil around them (Learn About Mangroves).
Crabs have also found a special place among the mangrove forests, using the area above and below the elevated roots as protection from predators and deadly temperature changes. Some mangrove crabs feed high up in the mangrove trees, which actually speeds up the recycling of the nutrients found in the mangrove detritus. Crabs also help to aerate the soil around the roots of the mangrove trees, allowing water and nutrients to travel more freely throughout the soil. Mangroves do no grow well in stagnant water, so this aeration helps to keep the mangroves healthy (Kathirsan & B.L., 2001).
While mangrove forests are essential to the intertidal ecosystem for protection against waves and hurricanes, they are also extremely delicate (Davidson, 1998). Damage to mangroves can come from natural disasters such as high waves, winds, or hurricanes; or human destruction. Destroying the mangroves has unseen consequences, such as a loss of vegetation and animals living within the mangrove forests, which eventually causes changes in these separate ecosystems (Kathirsan & B.L., 2001). The sea grass and coral reefs living around the mangroves depend on the filtering system of the mangrove trees to keep unwanted nutrients and sediment from depositing in these areas (Learn About Mangroves). Excess nutrients in these areas can cause algae to grow on coral beds, eventually killing them, and an overload of sediment in the sea grass beds can block out the sun, inhibiting their ability to photosynthesize (Davidson, 1998). It can also result in erosion of the soil and a loss of essential nutrients the mangroves were holding close to land. With the sediment not being caught by the forests, it will eventually deposit off shore or on other mangrove pneumatophores, blocking their ability to transfer oxygen (Ewel et al, 1998). While mangroves are excellent filters for organic pollution, such as nitrogen and phosphorus, an overload of pollution in these forests can cause disease and death among the trees (Kathirsan & B.L., 2001). Destroying the mangrove trees can also affect the flooding of the area when strong rains come. Mangrove marshes naturally flood on a normal basis, so if they are cut down and the land is commercialized, this land will flood very often. The forestÕs soil, mangrove trees, and other vegetation also help to slow the velocity that the water is coming in at, so when it does reach buildings it will be at a much slower pace (Ewel et al, 1998).
Many mangrove forests have been destroyed from human interaction. In some areas, fresh water supplies from rivers are being used up, thus killing the mangroves from the lack of fresh water. Mangrove trees are also being destroyed to use the land for aquaculture as well as using the wood for fuel and lumber. The aquaculture shrimping industry has taken over large parts of mangrove forests, eventually leaving them barren and unable to support mangrove growth (Kathirsan & B.L., 2001). Some shrimping companies will leave the mangrove forests in tact, and use the ecosystem to raise the shrimp, trapping them in the forest until they are ready to harvest. Not all shrimping tactics are this kind to mangrove forests. Other companies will tear down full forests of mangroves, and dig ponds to raise the shrimp. The pond viability does not last long, and eventually more ponds must be dug to continue the raising of shrimp (Ewel et al, 1998). Mangrove forests are an extremely important part of our land and oceanÕs ecosystem. Humans, animals, and other plants rely strongly on these mangrove forests to stay healthy and bountiful. Without our dedication to these forests, they will soon be no more, leaving our costal cities, sea grass beds, and coral reefs at danger.
Works Cited:
Davidson, O (1998). The Enchanted Braid. New York, New York: John Wiley & Sons, Inc..
Ewel, K, Twilley, R, & Ong, J (1998). Different Kinds of Mangrove Forests Provide Different Goods and Services. Global Ecology and Biogeography. 7, 83-94.
Kathiresan, K, & Bingham, B.L. (2001). Biology of Mangroves and Mangrove Ecosystems. Advances in Marine Biology. 40, 81-251.
Learn About Mangroves. Retrieved June 6, 2008, from Mangrove Action Project Web site: http://www.mangroveactionproject.org/mangroves
Lodge, T (1998). The Everglades Handbook. Boca Raton, FL: CRC PressLLC.
Lugo, A (June 1980). 1. Mangrove Ecosystems: Successional or Steady State? Mangrove Ecosystems: Successional or Steady State?. Biotropica. 12, 65-72.
Valiela, I, Bowen, J, & York, J Mangrove Forests: One of the World. 51, Retrieved 06/2/2008,
from http://links.jstor.org/sici?sici=0006-3568%28200110%2951%3A10%3C807%3AMFOOTW%3E2.0.CO%3B2-L .
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