TDA-IME Project Final Report June, 2013 Freshwater runoff provides favourable conditions for mangrove colonization because it supplies nutrients and leaches the soil, thus keeping soil salinity within a tolerable range. The frequency and range of tidal inundation are critical factors in arid regions with low annual (or only seasonal) freshwater flow and high rates of evapotranspiration. Under these climatic conditions, bare hypersaline flats are found frequently just landward of the high tide level. These flats rarely support vegetation and are only inundated by an occasional extreme spring tide; this decreases evaporation and temporarily alleviates the hypersaline soil conditions. Progradation can, however, be episodic as seasonal storms can add a dynamic, even cyclic dimension to the process (Maxwell, 2000). Deltas are typically prograding because of the continuous supply of alluvial deposits from river runoff, which favours the colonization of mangroves. Mangroves contribute to land-building because they colonize a suitable substratum first and then add to progradation later, once they have become established. The substratum also acts as a major geomorphological influence on mangrove development and zonation (Thom, 1967). Fine-grained alluvial mud, silt, and sand deposited into deltas are ideal substrata for mangrove colonization. Although some mangrove species do colonize on gravel or rocky substrata, their growth is stunted to varying degrees because of the sheer lack of soil, as well as the abrasiveness of the substratum to their roots (Maxwell, 2000). Exposed beaches are also ill-suited for mangrove growth because wave- and wind-deposited marine sands, and constant sediment movement, can cover and suffocate mangrove pneumatophores. Mangrove establishment is optimal on sheltered coasts with muddy alluvial sediments containing high content of silt. Important examples in Indochina where these ideal natural conditions for mangroves occur include the Gulf of Thailand, the Melaka Straits and the Mekong Delta. Climate and mangroves Climate plays a fundamental role in the growth and zonation of mangroves (Kjerfve, 1990; Mangrove Task Team, 1994), with climatic conditions regulating mangrove growth from macro- to micro-spatial scales. Temperature is believed to have the greatest overall influence on the latitudinal distribution of mangroves (Tomlinson, 1986). Although mangroves are generally associated with tropical and subtropical regions, species vary considerably in their thermal tolerance, and several species survive into the temperate zone, including southern China and Okinawa Island, Japan. Mangrove distribution extends to higher latitudes along coasts with warm western boundary currents, but frost is a limiting factor for mangrove growth. Cold temperatures result in decreased tree height, a reduced leaf area index, and reduced species richness and forest complexity. Mangroves which employ C3 photosynthesis have the ability to adapt to cold temperatures, and transpiration demands on mangroves decrease in colder climates (Clough et al., 1982). Temperature stress has a major influence on mangrove physiology, while cold temperatures and frost are clearly limiting factors for mangrove development. Overall, mangroves species are hardiest and most diversified in regions where the coldest monthly mean temperature is greater than 20°C and the annual temperature range is less than 10°C (Chapman, 1977). Low temperature is a powerful limiting factor on mangrove plant diversity, as witnessed by the presence of just one species at the southern biogeographic limit of mangroves in New Zealand, where only cold-tolerant ecotypes of Avicennia marina survive (Maxwell, 2001). Species zonation differs between mangrove ecosystems in humid climates as opposed to arid climates (Blasco, 1984). In humid climates, rainfall leaches the soil more effectively in the landward zone, whereas at the seaward fringe, salinities are elevated by frequent periods of tidal inundation. The less salt-resistant species thus exist in the landward zone. But in arid climates subject to dry seasons, or long periods of drought and little river flow, high 26
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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