themselves in suitable areas and grow into trees.
Introduction: cology of the mangrove, that is, the plant
community of woody evergreen shrubs
"The word mangrove is used both located within the tidal zone of the Hong
for an ecological group of species
concerned with the distribution of mangrove
species, their taxonomy, their morphological
and anatomical adaptations, succession of the
community, the planting of mangrove species
to cut down wave action, and their environ .
mental requirements . In this last-named
category he mentions : wet hot climates in the
tropics and subtropics, abundance of humus,
water not too deep, mixed salt and fresh
water, a bay or river mouth where tide and
wind action are not too strong .
Along the coast of Mainland China
work has also been done. Ho (1957) speaks
of the community and mentions the following
environmental influences : contour of the
coast, kind of parent material, ocean currents,
the tide, temperature, and salinity.
On the muddy beaches and particularly round Deep
Bay there is a dwarf mangrove formation
by Rhizophoraceae, Aegiceras and various
Cyperaceae" (1960, p. 126) . The 1 :25,000
maps of Hong Kong and the New Territories
(1957) also indicate the location of man.
groves . Tregear (1958), in his classification
of land, writes of marshlands and mangrove.
He says : "It (the mangrove) seldom reaches
more than four feet in height and generally
is only two to three feet" (1958, p. 16) .
My project began from these statements,
and its object was to acquire data on the
mangrove community of Hong Kong.
Aside from the effects of wars and human activities, mangrove forests are susceptible to chan in climatic, edaphic and hydrological conditions. The effects of these enviroc
Although Vietnam is situated in a tropical and monsoon region, there are differences in dim which arise due to latitudinal differences. Climate conditions therefore differ between north south. Among climatic factors, temperature, rainfall and wind have the most significant influence on the composition and quality of mangrove vegetation (Hong, 1991).
Temperature
The mean annual temperature at sea level is about 270C in the south and decreases northwards to about 21 0C in the extreme north.
The northeast monsoon brings sudden cold air to the north, thereby affecting the growth composition of flora in the region. Lugo and Zucca (1977) noted that the typical responses mangrove communities to decreasing temperatures and increasing daily and annual thermal amplitudes, are reductions in the number of species and in the height and size of trees. This can be observed along the coast of northern Vietnam. In addition, frost often occurs at night on days in winter and can be a constraint to the growth of mangroves. It has been recorded that on January 17-18, 1961, a number of leaves of Bruguiera gymnorrhiza and Clerodendron inerme in Tien Yen district, Quang Ninh province, became dry and died when the temperatures went down to 00C.
High temperatures, or sudden fluctuations in temperature, can also have an adverse effect on mangroves. On April 18, 1990, at Duyen Hai district (Ho Chi Minh city) when the air temperature went up to 400C and the soil temperature rose to 420C, physiological activities of Rhizophora apiculata, such as transpiration and photosynthetic efficiency, were minimised (Hong, 1991). Similarly, Millan (1971) found that a number of Avicennia propagules died 48 hours after exposure to a temperature of 39-400C.
Rainfall
Most areas in the country receive about 2,000 mm of rainfall annually. Mangroves require certain amount of fresh water for optimum growth, even though they are salt tolerant spec (halophytes). Rain regulates salt concentration in soil and plants and provides an extra source of freshwater, in addition to river water, for mangroves and this favours their physiological processes. Southwest monsoons from the Indian Ocean bring heavy rains to Southeast Asia during the summer months. Consequently, the most dense mangrove forests are found in this region. For instance, mangroves flourish at Ca Mau cape, where the rainfall is 2000-2200 mm annually with 120-150 rainy days per year. On the other hand, mangroves are sparse along the small estuaries of Khanh Hoa coast where they receive less than 1,000 mm/year of rainfall. Similarly, in the central part of Vietnam, where high rainfall occurs over a short period of time and other months of the year are prone to drought, conditions are unfavourable for the growth and distribution of mangroves. Moreover, while the southwest monsoons bring heavy rains from April to August, over the western range of Truong Son mountains (southern Laos), north-central Vietnam is affected by a long dry spell. At this time, soil salinity and evaporation rates are very high. This, together with high evapo-transpiration and insufficient water absorption, constrains the growth of mangroves in this region.
Winds
Winds directly or indirectly affect the establishment of mangrove forests in many ways. Local winds increase the rate of evaporation and decrease air and water temperatures. The southern coast is influenced mainly by the northeast monsoons.
The northeast monsoons from the East Sea normally occur between December and April and cause heavy erosion along the east coast from Vung Tau to Ca Mau cape (figure 1.3). During the winter months, these mons6ons bring cold air to northern Vietnam, thereby causing a sudden decrease in air and water temperatures which, in turn, seriously affects the growth and composition of mangroves.
In the central part, along Ha Tinh, Quang Binh and Quang Tri coasts, sand carried far inland by these monsoon winds fills a number of creeks, salt and brackish swamps and damages the mangroves inside the estuaries. Strong winds also change the surface salinity, resulting in a change in the vertical salinity distribution. Furthermore, if the wind blows in the same direction as the spring tide, the combined effect brings saline water further inland. This can be observed in the mangroves at the Tien estuary at Thoi Thuan, Thua Duc and B'inh Dai districts in Ben Tre province, where the northeast monsoons cause increases in the salinity of the rivers. This has made some halophytes such as Avicennia a/ba, Xylocarpus granatum, A. officinalis, grow further inland on the mudflats of canals and creeks and some brackish water species such as Sonneratia caseolaris and Acanthus ilicifolius have been found far inland (Hong, 1984).
Tropical depressions, storms and typhoons often occur during the wet season in the central and northern coasts of Vietnam. Generally speaking, mangroves do not naturally occur in areas directly affected by storms. Strong winds often cause very strong waves which induce surges and the sea water rises to its highest level when spring tides combine with typhoons. For example, during the typhoons of August 1955 and July 1992, the sea level rose by 5-6 m and some sea dykes of Hai Phong and Quang Ninh collapsed. In August 1991. more than 100 ha of Rhizophora stylosa and Bruguiera Gymnorhiza seedlings planted outside the sea dykes of Thach Ha district (Ha Tinh province) were washed away by a storm. On the other hand, strong waves may have a beneficial effect for mangroves, resulting in the establishment of some mangrove species, such as Aegiceras corniculatum and Rhizophora stylosa, on the mainland flooded by sea water.
Hydrological Factors
In addition to climatic factors, hydrological changes in tides and ocean currents may also affect the development of mangroves in coastal regions.
Tides
Tides affect vegetation, the structure and salinity of soil and rates of evaporation. In turn, tides are influenced by winds, rainfall and stream flows.
Thuy (1984) classified 8 tidal zones along the coast, which can be further generalized into 4 zones, namely, the central coast, the north coast, the south-east coast and the western Ca Mau coast. The central coast is exposed to an irregular diurnal and semi-diurnal tide. The north coast is exposed to a regular diurnal tide while the south-east coast is subjected to a semi-diurnal tide. The western coast at Ca Mau receives mixed diurnal and semi-diurnal tides, although the diurnal tide is prevalent.
Compared to semi-diurnal tides, diurnal tides are less favourable for the growth of mangrove species, especially those which have no pneumatophores, due to the following reasons:
- In areas of diurnal tides, the period of inundation is doubled, requiring a high storage of oxygen in the roots.
- The period of exposure to sunlight is longer in areas of diurnal tide. Consequently, the high temperatures cause increased evapotranspiration in soil and trees, thereby affecting physiological processes of mangroves (Hong, 1991).
Hence, the mangrove forests of southern Vietnam, which are affected by semi-diurnal tides, develop much better than those in the north. The Avicennia alba in the northwest of Ca Mau cape are smaller than those in the east because the northwest is poor in alluvium and is affected by diurnal tides.
Tidal amplitude also affects mangrove development. Along the coast of Quang Ninh and northern Hai Phong, the tidal amplitude is large (3-4 m) and tide currents are strong. Their maximum velocity is more than 2 knots during low tide and 3 knots during neap tide (Thuy, 1984). Large amplitudes and high velocities of these tides widen channels and cause erosion in mangrove areas, thereby minimizing mangrove seed and propagule settlement.
In estuaries with small tidal amplitudes, such as those in the central part of Vietnam and the northwestern Ca Mau coastline where tidal amplitude is 0.5-1 m, there is little transportation of seedlings and sediment. In these areas, mangroves are distributed along a narrow track. In areas where the tidal amplitude is only 2-3 m and the relief is not as steep as in the Mekong estuary, mangroves grow well and can be found far inland.
Ocean Currents
Mangrove fruits, seeds and propagules are transported into Vietnam mainly by currents under the influence of winds. Due to the effect of the southwest monsoons, currents move from the Indian Ocean to Indonesia, Malaysia and up to the coast of southern Vietnam, so that mangrove species here are quite similar to those of other Southeast Asian countries.
The surface sea currents which carry seeds and propagules northward to the Vietnamese coast, during the southeast and east monsoons (June to November), change their direction at latitude 12. This prevents some mangrove species from reaching the north of Vietnam and they settle instead on Hainam island. This phenomenon is one of the main reasons for the difference in species composition of the mangroves in the south and north of Vietnam.
Fresh Water Currents
Fresh water from rivers and channels brings necessary nutrients and alluvium to mangrove forests. It also dilutes sea water, creating brackish water suitable for the growth of many species. Experiments on the growth of Kandelia candel, Rhizophora apiculata and R. stylosa conducted in different coastal areas of Vietnam show that it is unusual for mangroves to grow in places lacking fresh water (Hong, 1991).
Salinity
Surface water salinity in mangrove areas is greatly affected by seasonal rainfall and evaporation. Heavy rainfall in the wet season is said to be the main factor causing low salinities of 0.5-10 ppt in the Red River estuary and 1 - 10 ppt in the Mekong estuary.
Mangroves develop well in places such as Ca Mau Cape where the mean salt concentration is 22-26 ppt. High salt concentrations of 33-36 ppt diminishes the size and number of species, as is evident on Quang Ninh coast and Cam Ranh p~ninsula. On the other hand, few mangrove species grow in areas where the salinity is very low. Mangrove species such as Avicennia alba, A. marina, Rhizophora mucronata and R. stylosa can adjust' to different salinity levels and can tolerate high soil salinity during the dry season and low salinity during the rainy season. However, Nypa fruticans and Cryptocoryne ciliata (grass) can grow only in a brackish environment with a salinity of 5-15 ppt and will die when planted in very saline coastal areas.
Coastal zone erosion
A rise in sea level accelerates the speed of coastal erosion. Strong waves deposit sand and sediment in mangrove areas, covering pneumatophores and killing trees (figure 4). Analysis of remote sensing data from Landsat shows the extent of erosion in certain mangrove areas. For example, Con Loi, Ben Tre province, has lost 460 ha, Hau Giang estuary, 350 ha and the eas~ern coast of Ca Mau peninsula, 630 ha. In recent years erosion has increased, especially at Ca Mau cape. According to remote sensing data from 28 April 1982 to 17 June 1983, more than 600 ha were eroded from the Bo De estuary to Rach Goc estuary, the width of the eroded areas reaching up to 200 m at some locations (Luu, 1990). The main cause of this erosion is believed to be the impact of the strong waves due to the northeast monsoons.
In coastal zones with brackish water, local people often transplant rice in the wet season and leave the land fallow in the dry season when fresh water supply is low. High lands, rarely flooded with sea water were used for coconut or pineapple plantations. With increased erosion, however, brackish water has invaded these areas and brought with it Nypa fruticans, Sonneratia caseolaris and Acanthus ilicifolius.
Soils
Research on mangrove soils in Vietnam has been limited to a few localities such as Yen Hung district, Quang Ninh province and Ca Mau cape, Mjnh Hai province. Mangroves can survive on different waterlogged and anaerobic substrate such as sediments, coarse sand, silt, peat soil or coral reef. However, they grow best on silt-clay soils. This kind of soil is prevalent along the coastline, estuaries and closed gulfs of Vietnam.
Soils in mangrove areas are formed by alluvium from rivers and by sediments from the sea. Mangrove soils are rich in nutrients, such as magnesium, sodium, phosphorous and potassium. The physical and chemical characteristics of the soil depend on the source of alluvium and sediments, which in turn influence mangrove distribution.
In the northern part of Vietnam, mangroves grow on mudflats where the alluvium originates from lateritic hill soils, so the trees remain small. Along the coast of limestone islands such as those in Bai Tu Long Bay or Ha Long Bay, mangroves are stunted and of low diversity because the mudflats derive detritus and sand from limestone and clay-limestone.
Topography
Mangroves develop in shallow waters and calm areas such as gulfs, estuaries behind capes, narrow straits or along coasts protected by archipelagoes such as Ha Long Bay and Bai Tu Long Bay. Changes in Vietnam's coastal topography have influenced mangrove distribution. Along the coast of the central part, there are almost no mangroves because the coastline is uneven and unprotected from violent wave action, which makes it difficult for young plants to establish themselves.
CONCLUSION
The extent of Vietnam's forest depends heavily on conducive environmental conditions which help in the transportation of propagules and in the physiological growth of different mangrove species. In addition to the factors described above, the differences created by geographical conditions are also important.
