Impact on VietnamImpact on VietnamThe climate of Vietnam is most favourable for mangrove forest development. Already damaged by war and human encroachment, global warming presents a further threat to this valuable ecosystem.
The mangrove species found in Vietnam are diverse. Due to the fact that the country stretches over many latitudes, climate conditions are very different between north and south and species and distribution differ greatly. Mangrove forests in the south are considerably richer in species than those in the north.
As part of a naturally existing ecosystem, mangroves play an important role in the preservation and protection of coastal and estuarine regions. Moreover, with wise management, the rich resources that the mangrove ecosystem offers could be of great assistance to the development of livelihoods for local communities.
To assess the future threat of sea level rise, it is necessary to begin by considering the present-day vulnerability of the mangrove ecosystem.
Prior to the 1960s, the estimated area of mangrove forest in Vietnam was 400,000 hectares. Between 1962 and 1971, chemical toxicants released by the United States destroyed 104,123 hectares. Since then, further reductions have occurred as a result of agricultural demands and overexploitation for fuelwood and charcoal.
The conversion of large areas of mangrove to aquaculture and agriculture with their interlacing dykes has restricted the surface of tide water and is contributing to current sea level rise.
The construction of dykes, dams and embankments, as well as hydroelectric stations, has lead to decreased discharge downstream in the rainy seasons, also increasing the intrusion of salt water inland.
These problems have often been compounded by rash pumping of groundwater. This can lead to delta subsidence, as seen in the Ca Mau peninsula of South Vietnam. The cutting of forests in river drainage basins exacerbates the situation.
The mangrove ecosystem is particularly vulnerable to human activity and to environmental change because its development only occurs under very specific conditions.
Mangrove forests develop in shallow water and rather calm areas, such as in gulfs, estuaries sheltered by capes, narrow straits or along coasts protected by archipelagos.
Salinity is one of the most important determinants of mangrove forest growth and distribution.
Mangroves develop well in places where the salt concentration is between 20 and 35%. Too high a salt concentration (40-80%) diminishes the number of species and their size. In places where salt concentration reaches 90%, only a few species can exist and even then they grow slowly. No natural mangroves can be found where the salinity is too low.
Mangroves do need a certain amount of freshwater during their growth. Freshwater from rivers, channels and rain dilutes the salinity of sea water, creating brackish water suited to many species during specific stages of their growth.
As sea level rises and river flow alters, the distribution of salinity and freshwater in mangrove areas will change. The mangrove ecosystem will respond by changing in productivity, areal extent or species diversity or by migrating.
Study of the impact of recent changes in sea level suggests that three particular factors have been important in determining the response of mangrove ecosystems: sedimentation rates; erosion rates; and the ability to migrate inland to agricultural areas.
Sea level rise induced by global warming could alter substantially the status of Vietnam's mangrove forests, with serious consequences for coastal protection and resource management.
Based on material provided by
Professor Phan Nguyen Hong, Director, Mangrove Ecosystem Research Centre, Hanoi National Pedagogic University
"Policy Responses: The challenge facing Vietnam"
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