TDA-IME Project Final Report June, 2013 25 SECTION 2 Description of the Indochina Mangrove Ecosystems Physical and Hydrological Features Introduction Mangrove wetland forests are the dominant natural ecosystem separating land and sea in a narrow fringe along sedimentary coasts in most of the world’s tropical and subtropical regions. Mangroves are most prevalent and biologically diverse along open coastlines and within estuaries and deltas in Southeast Asia; this is considered by Ellison et al. (1999) to be due to a higher in situ evolution of mangrove taxa here compared to other regions. Globally, mangroves occur in 124 countries (FAO, 2007). Mangrove ecosystems are a vegetated landform complex that changes continually as a result of dynamic biological and physical processes occurring over a spectrum of time scales (Thom, 1982), Hydrological processes are fundamental to the extent, distribution, structure and productivity of mangrove ecosystems (Kjerfve, 1990). Mangrove forests are inundated by ocean tides on at least an intermittent basis; they are also flooded regularly by rivers and runoff from the land (Kjerfve, 1990). The extent, physiography, structure, and productivity of mangrove wetland forests are a function of the degree and regularity of tidal inundation and terrestrial drainage, which are two processes serving to deliver sedimentation from land and marine sources. In fact, hydrological differences from site to site may be the dominant factor controlling the structure and productivity of coastal mangrove systems (e.g. Tomlinson, 1986; Saenger, 2002). Certainly, the regular tidal flooding by saline coastal waters limits the competition from non-mangrove species, except in the areas dominated by mangrove associates near the inland boundary, or “back-mangrove” zone. Mangrove Setting The most extensive mangroves are found on large sedimentary plains within river deltas and adjacent to river mouths with high freshwater discharge and regular inundation by meso- to macro -tides (tidal range greater than 1 m and 4 m, respectively), in wet humid tropical or subtropical settings. These are also the areas which, on a global scale, are most affected by relative sea level variability as a result of changes in land elevation due to human interventions (physical infrastructure and extraction of water, oil, and gas) and coupled with regional tectonic processes. Some of the most extensive mangrove systems in the world are located in deltaic regions that exhibit a moderate tidal range of 2-4 m. In this situation, freshwater may actually be a more critical factor in controlling mangrove productivity than tidal range. Although mangrove species vary in their salinity tolerance, salinity is not necessarily a major influence over mangrove zonation. Most mangrove species are facultative halophytes and thus they do not require saline conditions for survival. Rather, they prefer brackish to saline waters during at least part of the year, possibly because salinity eliminates competition from non-salt tolerant plant species, or glycophytes (Thom, 1967). Mangroves become stunted when the salinity exceeds 40 ppt, or if drought conditions are prevalent. The geomorphological structure of mangrove ecosystems includes natural mud banks (levees), extensive mud flats, plus numerous creeks and smaller waterways which intersect the forested wetland. Mud sediments rich in organic materials dominate, but silt- and sanddominated sediments become more common further inland.
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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