Climate and the mangrove ecosystem

Professor Phan Nguyen Hong describes the potential impact of climate change on Vietnam's mangrove ecosystem.

The author is Director of the Mangrove Ecosystem Research Division at Vietnam National University in Hanoi and co-author of Mangroves of Vietnam, published by the World Conservation Union, Bangkok.

Vietnam has over 3000km of coastline along which mangrove forests not only cover a considerable area — 400,000ha before the war — but also play a significant role in maintaining ecological health and the social and economic well-being of the many coastal inhabitants.

In recent decades, the mangrove forests of Vietnam have been affected by many detrimental changes in extent, composition and actual forest quality. These changes are due to various causes but the principal ones are those resulting from human activities and climate-related impacts.

There are four climate-related factors that have direct impacts on the mangrove ecosystem:

• air temperature;
• frost;
• rainfall; and
• monsoons and storms.

Mangrove species are diverse in equatorial, subequatorial and humid tropical regions where annual temperature is high and temperature amplitude is small. Low temperature reduces the tree size, leaf area index and species composition of the flora as well as the complexity of communities.

These characteristics can be clearly seen when comparing the mangrove forests at Tien Yen and Quang Ninh in the north of Vietnam and Vung Tau in the south. At Vung Tau, the numbers of communities and of the species in each community are much larger and the succession is also more complicated than in the north.

Frost caused by low temperature damages the mangroves in the north of Vietnam, especially on days with a low tide. On January 17th and 18th in 1961, for example, it was recorded in Quang Ninh that a number of mangrove leaves became dry and died when the temperature fell to below 2°C.

Rainfall, as well as temperature, has a significant influence on the distribution and zonation of mangrove species. Regions with low rainfall, such as along the estuaries of the Luy River in Phan Ri, the Cai river in Phan Thiet and the Ba Ngoi in Kkanh Hoa, have flora systems that are poor and scattered with stunted trees.

The influence of rainfall can also be seen in two areas of southern Vietnam. The mean annual temperatures at Vung Tau and at Ca Mau are not very different (within 1°C) but the average annual rainfall is 1,357mm in Vung Tau with 124 rainy days and is 2,360mm in Ca Mau with 165 rainy days. The substrate sediment at Ca Mau is thicker, though, so there are five more species than at Vung Tau and the tree size is also larger.

The significant influence of rainfall on distribution and species composition is because rainfall regulates salt concentrations in both soil and plants as well as providing a source of freshwater for the mangroves. This is an important factor when propagules begin to take root and also in their season of blooming and fruiting. If, however, high rainfall occurs over a short period and other months of the year are prone to drought, the conditions can be considered unfavourable for the growth and distribution of mangroves.

The north-east monsoon has the worst effect on mangrove communities. During the winter months, it brings cold air to the north and to north central Vietnam, thereby causing a sudden decrease in air and water temperatures. These, in turn, seriously affect the growth and composition of mangroves as well as many other tidal creatures. In south Ha Tinh, Quang Binh and Quang Tri, sand carried inland by high-speed north-east monsoons fills up creeks and salt and brackish swamps damaging the mangroves inside the estuaries.

In the south of Vietnam, north-east monsoons cause large waves to erode the east coast, destroying many mangrove areas and felling thousands of trees. Many benthos die after being brought on to land together with the mud and sand by the waves and some zooplankton species have to move away.

Remote sensing data shows that in just two years, from 1973 to 1975, the Con Loi-Ben Tre area lost 460ha and the Hau river mouth lost 350ha. The Ca Mau peninsula alone lost 9,630ha of coast, accounting for 81% of the extent of Vietnamese coastal erosion.

According to the remote sensing data of SOYUZ, between the 28th of April 1992 to the 17th of June 1993, 600ha of mangroves from the Bo De river mouth to past the Rach Goc estuary fell down due to winds.

The eroded area has now spread to the tip of the Ca Mau cape. The monsoons also carry sand from the sea, forming a thick layer on the mangrove forest ground near the coast. When this occurred the pneumatophore of Bruquiera and Avicennia could not work so a lot of trees died standing in large patches.

The dry, hot south-west monsoon also causes a lot of devastation to the mangrove forests along the brackish water river mouths in north central Vietnam, especially when the tide is low in June to August every year. At these times, the salinity in the unflooded mangrove soil rises to very high levels (40 to 45%). This leads to the death or migration of some species of brackish mollusca and polychaete.

Tropical depressions and storms constrain the distribution of mangroves. In the coastal plains of the north, the soil is very rich in alluvia but mangroves only form narrow stretches inside the river mouths because they cannot grow at spots with strong waves and winds.

Typhoons occurring in the Yen Hung district — Quang Ninh, Hai Phong, Thai Binh, Ha Nam and Ninh Binh — have broken the sea dykes, destroying the naturally growing mangrove forests and those planted by people to protect the dyke. This has also resulted in the devastation of the sheltered nurseries and spawning grounds of many species of sea animals as well as water birds. A storm accompanied by heavy rain can break mangrove branches, cause flowers and fruit to fall off and carries a lot of seedlings into the sea. Storm No. 6 on the 17th of August 1991 washed away more than 70% of the newly-planted seedlings at some communes of Thach Ha district in Ha Tinh province.

There are also indirect relationships between climate change and the mangrove ecosystem through changes in sea level.

Inundation is only one of the effects. As sea level rises, coastal erosion and the severity of coastal flooding will increase and coastlines will recede unless they are stabilized by dykes or through sand nourishment. Salt water intrusion into ground water, rivers, bays and estuaries will increase. Changes in rainfall patterns and in temperature will modify salinity gradients in estuaries and alter rates of river delta sedimentation. Coastal currents and upwelling patterns are likely to shift geographically and change in intensity. All of these "sea changes" will affect the biodiversity in coastal zones.

There are a number of causes for sea level rise in the Vietnamese coastal and riverine areas. Some are active in the present-day while others threaten the future.They include:

• the north-east monsoon;
• increased riverflow;
• local heavy rains;
• alluvium accumulation;
• human activities; and
• the greenhouse effect.

North-east monsoons have contributed significantly to sea level rise in Vietnam. The monsoon occurs in the dry season from November to the following April when the tide level is at its highest in the year (October to December). This results in salt water intrusion far inland in many areas, particularly so in the Mekong river delta. According to the documents of the Mekong River Committee, when the wind speed went up to 5ms-1, the water level increased by 10cm. When the wind speed went up to 10ms-1, the water level rise was 20cm. When there was no wind, the water level increase only accounted for 4cm.

Meteorological factors also affect sea level. Increased riverflow is a major cause but this usually only occurs in the rainy season and is a short-term effect. Again, on the timescale of days, sea level rises highest in the days with spring tide and storms. Heavy rains may cause localized rises in sea level. If any of these factors change in frequency or severity, a longer-term change in sea level may result.

Alluvia, the result of erosion borne from inland by rivers, accumulates partly on the river bed and along the river basin and partly near the river mouth forming small islets.

Dyke and embankment construction in mangrove forests and on accretions for shrimp ponds or agricultural production, for example, constrains water distribution. The building of dams for reservoirs and hydrological plants reduces the flow of river water resulting in salt water invasion and subsequent salt intrusion far inland. In Minh Hai, for example, the use of ground water without proper planning has reduced the water amount rapidly. This can lead to landslides in the mangrove areas favouring an even higher rise in sea water.

Human activity is likely to prove one of the major long-term influences on sea level, as is the global environmental problem of climate change caused by greenhouse gas emissions.

Preliminary studies from the General Department of Meteorology and Hydrology show that the sea level may rise by up to 1.5mm a year. The annual tide level data in the years from 1982 to 1992 at Ca Mau and Genh Hao stations show that the average level rise over that period has been 1.5cm.

Sea level rise, together with monsoons and storms, accelerates the speed of mangrove coastal erosion. This erosion results in the destruction of many rich and various mangrove forests as they fall into the sea, as has occurred all along the east coast of the Ca Mau cape. Erosion also destroys the shelter of a great many tidal and forest animals as well as the spawning grounds of some fish and shrimp species.

Sea level rise facilitates the invasion of mangroves into the mainland, killing other cultivated plants. For example, the land in some coastal areas has been used for one-crop rice or subsidiary crop cultivation in the rainy season but, in the past few years, a large extent has had to have been left fallow due to salt intrusion. This has occurred in Can Gio district, in Ho Chi Minh City and in the Long Phu district.

In some other districts, such as Thach Ha, Cam Xuyen of Ha Tinh province and Yen Hung of Quang Ninh province, salt intrusion has destroyed the living places of some field creatures and facilitated the invasion of mangroves into the land. This has changed the properties, the distribution and the succession of some biological communities.

Sea level rise has prevented soil accumulation so tidal flats become deeply flooded. This is likely to hinder the development of pioneer mangrove communities like Avicennia alba and Sonneratia alba in the river mouths and accretions because their pneumatophores lying under deep water cannot get the air necessary for the trees.

Humans, too, have experienced the impact of sea level rise. In recent years, the level of spring tide in November and December has risen. This has flooded the floors of some low stilt houses in the Ngoc Hien district of Minh Hai province as well as flooding several village paths. In Ba Tau hamlet of the Vien An Dong commune in Ngoc Hien, salt water has submerged and flowed even into the high fields of marrows and beans.

Coastal ecosystems, especially mangroves, have rich biodiversity resources but are easily destroyed by both natural and human impacts.

There are many environmental factors that affect these ecosystems as a whole, but climate change plays an important role as it not only influences the biodiversity directly but also has indirect impacts through factors such as the environmental hydrology and edaphon.

High population growth and unplanned economic activities have significantly damaged the biodiversity resource, thus affecting climate change and sea level rise. This issue has not been given sufficient concern in Vietnam. Studies should be supported and promoted to gain comprehensive data. This will enable us to find solutions for the possible disasters that may result from climate change.

Further information

Professor Phan Nguyen Hong, Mangrove Ecosystem Research Division, Vietnam National University, 91 Nguyen Khuyen St, Van Mieu, Hanoi, Vietnam. Fax: 84-42-35990. Email: merd@netnam.org.vn.