mangroves

mangroves ‘Mangrove’ is the general term given to a variety of halophytic (salt-tolerant) trees and shrubs found in low-energy intertidal areas. While some of these species can tolerate relatively cool temperatures, the major mangrove forests are found in the tropics and subtropics, where the mean winter water temperature is over 20 °C. Like the salt marshes found in higher latitudes, mangroves grow best in low-energy, sheltered environments such as estuaries, river deltas, and less-exposed open coast areas. The extent of an individual mangrove forest is largely dependent on coastal relief: the largest expanses of mangrove forest are found along gently sloping shorelines such as those found on major river deltas. The Sundarbans, for instance, which is the largest continuous area of mangrove forest in the world, has developed on the extensive delta of the Ganges, Brahmaputra, and Meghna rivers. Tidal range is also important, since a high tidal range will flood a greater area of coast and allow a correspond-ingly bigger (and often more diverse) mangrove forest to develop.

The fine sediment that is characteristic of the low-energy environments in which mangroves grow is frequently oxygen-poor (or even oxygen-free). This is a result of the small grain size of clay and silt particles, which maximises water retention at low tide and so limits the penetration of oxygen into the sediment, and to the high quantities of organic material in the sediment, the breakdown of which depletes the sediment of oxygen. Owing to the oxygen-poor nature of the sediments (and to provide anchorage in the muddy substrate) mangrove trees and shrubs have extensive root systems that may be aerial (in the form of prop roots) or have numerous breathing roots (pneumatophores) which extend up above the mud surface. Once established, mangrove vegetation promotes rapid vertical build-up of sediments by the trapping of suspended sediments by the extensive above-ground root network.

Longer-term mangrove forest development is intimately linked to changes in relative sea-level. Under a stable, or falling, sea-level, mangroves will tend to prograde or expand seawards, as long as the supply of sediment is sufficient to allow the growth of new mangrove areas. Mangroves in deltaic areas, however, are often subject to a rising relative sea level, owing to land subsidence caused by sediment compaction (a process in which delta sediments compact under the weight of overlying sediment). Provided that the sediment supply is sufficient, the mangrove substrate can still build up rapidly, offsetting this rise in sea level. Where sea-level rise is more rapid or sediment supply is lower, mangroves will undergo erosion and landward migration. Over shorter timescales, mangrove development can be dramatically influenced by tropical cyclone activity. Because of their dominantly tropical and subtropical location, mangrove forests are often subject to damage from severe tropical storms. Trees may suffer direct breakage and leaf-loss, and extensive erosion of the muddy mangrove substrate may occur. In areas prone to regular tropical cyclones, the frequency of these storms may limit the extent to which the mangrove ecosystem can develop.

Andrew B. Cundy