TDA-IME Project Final Report June, 2013 evaporation rates increase salinities at the landward margin, while salinity concentrations along the seaward edge are moderated by tidal inundation. The most salt-resistant species are thus found in the landward zone, often alongside extensive areas of bare sand with hypersaline soil conditions (Blasco, 1984). Mangroves are found in both humid and arid climates. However, mangrove growth and species diversity are greatest in humid tropical areas where rainfall is plentiful and quite evenly distributed throughout the year, as in most of Southeast Asia. The critical factor for mangrove productivity is that the ratio of rainfall to evapotranspiration should be balanced, or positive. Rainfall does not appear to limit the global distribution of mangroves, but it does have a significant influence on the distribution and zonation of species. Rainfall serves as a primary controlling factor on salinity in mangrove environments; rainwater also leaches excess salts from mangrove soils, and thereby helps to keep soil salinity within tolerable limits, although this also leads to nutrient loss from mangroves (Boto, 1982). Rainfall periodicity is yet another critical factor affecting mangrove species distribution. In climates that are humid throughout the year, salts are leached continuously from soils by heavy, but evenly distributed rainfall. Salinity levels are generally constant and stable throughout the year. The greatest diversity of species exists in such regions, including Indochina. In arid climates, or monsoonal areas with strongly seasonal rainfall distribution, the low rainfall or drought periods lead to high evaporation rates and, consequently, increased soil salinities. During the rainy season, this situation reverses, and soil salinity drops considerably. Mangrove diversity in arid and monsoonal regions is low because only a few species are able to tolerate the prevailing large salinity fluctuations and high extremes of salinity. This situation is well illustrated in Hong Kong, where cool, dry winters and hot, wet summers combine with marked seasonal salinity fluctuations. Consequently, despite being technically in the tropics, Hong Kong has only nine mangrove species (Hodgkiss, 1986; Maxwell, 1993). Sea Level Variability Coastal mangrove-covered landscapes and landforms exist at their present locations as a result of eustatic changes in sea level and global and local land adjustments (Kjerfve, 1990). During glaciation periods, a considerable fraction of the world’s oceans was frozen into continental glaciers. At such times, global sea level reached a glacial low stand, with coastlines located near the outer edges of the continental shelves. Most recently, a glacial maximum occurred at the beginning of the Holocene period 16,000 years ago, when eustatic sea level stood 130 m below the present mean sea level. Then, following the onset of the most recent interglacial stage, the rising sea level invaded and flooded outer continental shelves, river valleys and low-lying coastal plains, and the present-day coastal plain estuaries and coastal lagoons were formed. Eustatic sea level reached within a few meters of its present elevation approximately 5,000 years ago (Fairbridge, 1980). Presently, eustatic sea level along tectonically stable coasts has been rising at a mean rate of 0.15 cm/century; at the same time, the global temperature has increased as a consequence of a warming climate, the greenhouse effect, and the melting of glaciers, sea ice, and the fringes of major ice caps. The present rate of eustatic sea level rise of 0.15 m/century is actually quite small compared to the 1.4 m/century rise in the early Holocene after the most recent glaciation maximum. However, eustatic sea level rise is only half of the story. The most important factor controlling sea level is relative sea level change: the sum of eustatic sea level rise and net changes in land elevation. Most mangrove coasts are experiencing relative sea level rise at a rate 2-10 times that of eustatic sea level rise. This represents the combined effect of eustatic sea level rise plus changes in land elevation resulting from sediment compaction, withdrawal of fluids (potable water, oil and gas), and regional tectonics. 27
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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