Most mangrove trees are protein-rich food for livestock. Therefore, if planted forests on the coast are not carefully protected, they will be destroyed by buffaloes, cows, and goats. Fish sauce is rich in protein and can be pickled. Some fish species like pineapple fish love to eat fish sauce. The sap of the nipa palm tree, taken from the stem of the fruit, is a nutritious and delicious drink that can be exploited to produce sugar, soft drinks, and alcohol. Each hectare of nipa palm can yield 2000 - 3000 kg of sugar, or more than 5000 liters of alcohol, if planted thinly and carefully cared for (Hong, 1997).
Many mangrove species are valuable folk medicinal plants. During the war, people and officers in the war zone used those medicinal plants to treat many diseases. Currently, 20 species of mangrove trees have been investigated in our country that can be used as medicine (Hong, 1999).
Mangroves are the home and birthplace of many aquatic and terrestrial animals.
Mangrove forests provide humus and organic residue (fallen branches, leaves, flowers, and fruits that are decomposed by microorganisms) as food for many intertidal animals, including valuable seafood groups. High economic value such as: Prawns, sea crabs, oysters, snails and many delicious fish species. Some animals are brackish water aquatic animals with stages from post-larvae to nearly adults living in mangrove rivers and canals (tiger prawns, prawns, crabs).
Investigation results of some Vietnamese scientists show that in the mangrove forest there are more than 80 species of Crustaceans (shrimps, crabs, crabs); More than 160 species of molluscs such as cockles, cockles, clams, scallops, etc. live in mud flats and 250 species of fish live all their lives or in stages in mangrove forests. Our country's seafood industry depends heavily on mangrove forests. If forests are lost, seafood will also disappear.
Mangrove forests attract many species of waterfowl and migratory birds. Our country has many bird sanctuaries, during the breeding season there are tens of thousands of birds, including some rare species in the world such as storks, herons, egrets, white-necked cranes, spoon-billed storks... Sorry. There are very few species of high economic value left and are at risk of being destroyed due to over-exploitation and extensive forest destruction, causing loss of their habitat and nesting habitat.
c. Land animals are quite c
Honey is a great source of benefit for mangroves. Every year, mangrove forests in Nam Dinh and Thai Binh produce tens of tons of honey. In addition to the above organisms, there are also some very small organisms that can only be seen through a microscope. These are bacteria and fungi that decompose organic matter as food for many other animals and plants.
1.3.2. Protect coastlines and river banks
Many scientists have likened mangrove strips to green walls protecting estuary coastlines. Since ancient times, our ancestors have known how to keep strips of natural forests and plant more forests on coastal mudflats and estuaries. river to limit the harmful effects of storm winds.
Thanks to the groves planted since 1956, the Phu Luu dyke (Can Loc - Ha Tinh), although weak, was not broken by storms. Strips of forest in the coastal areas of Thai Binh and Nam Dinh, sponsored by the Danish Red Cross, have contributed effectively to resisting strong waves and winds for many years, effectively protecting sea dikes.
Although the Southern region has few storms, the northeast monsoon along with strong tidal currents have caused serious coastal and river bank erosion, especially during the war. After being sprayed with toxic chemicals, many mangrove forests were destroyed. Thanks to reforestation, the phenomenon of river bank erosion in Can Gio district (Ho Chi Minh City) has been greatly reduced in some canals where land has been accreted rapidly.
1.3.3. Expanding land
In newly alluvial coastal beaches and river mouths, there are a number of mangrove species such as white fish sauce and white cork that can grow on loose, salty mud and withstand deep flooding. These are the pioneer plants that invade the swamp. Thanks to the dense respiratory roots on the mud surface, the silt is deposited quickly, the soil is gradually accreted, creating favorable conditions for other tree species such as mangroves, parrots, eagles... to come later and develop into forests.
Many floating beaches in river mouths and coastal areas today have become forests with high economic value such as Con Trong, Con Ngoai at Ca Mau cape, Con Lu, Con Ngan (Nam Dinh), Con Den, Con Vanh (Thailand). Binh) all have mangrove trees growing on alluvial soil and attracting many animals to live.
1.3.4. Environmental protection
Mangrove forests play a huge role in protecting the environment, expanding land and nurturing intertidal animals. But unfortunately, up to now many people still do not know these things, so for immediate benefits they cut down forests indiscriminately.
1.3.5. Climate conditioning
Like inland forests, mangroves have a great effect in regulating climate.
Mangrove forests have absorbed a large amount of carbon dioxide emitted in daily life and industry and released a large amount of oxygen during photosynthesis, making the air fresh, so the people of Ho Chi Minh City compared Can Gio mangrove forest to the " lungs " of the city.
1.3.6. Limit pollution in coastal areas
Mangrove trees have dense roots on the ground that are "nets" that hold solid waste from daily life, healthcare, and industry transferred by the river to the sea. Toxic chemicals such as pesticides and excess fertilizers are also often deposited in mangrove soil. It can be said that mangroves are "giant kidneys" that filter water in coastal areas to protect intertidal creatures.
Thus, mangroves play a very important and irreplaceable role in coastal areas of Vietnam. Forecasting the growth and development of mangroves under the influence of changes in environmental factors is very necessary in the context of the world joining hands to respond to climate change.
1.4. Characteristics of the study area
1.4.1. Natural conditions, resources
a. Geographical location
Xuan Thuy National Park is located in the southeast of Giao Thuy district, Nam Dinh province with the following geographical coordinates:
From 20o 10 ' to 20o 15 ' North latitude
106 o 20' to 106 o 32' East longitude
The Northeast borders the Red River, the Northwest borders the residential area of 5 communes: Giao Thien, Giao An, Giao Lac, Giao Xuan, Giao Hai - Giao Thuy district, Nam Dinh province. The Southeast and Southwest border the East Sea.
* Terrain: Xuan Thuy National Park area has quite flat terrain, sloping from North to South, a type of strong tidal flat. The average height is from 0.5 to 0.9 m, especially in Con Lu, where the height is 1.2 to 1.5 m. The terrain of the intertidal zone is divided by Vop River and Tra River. The terrain of the estuary barriers such as Con Lu, Con Ngan, Con Mo (Con Xanh) has the shape of a small bow-shaped island with its back to the sea. Terrain the
Mangrove tidal flats are low, wide and gentle, distributed between two generations of sand dunes. The main tidal creek system in operation tends to be perpendicular, flowing into the main tidal creek in the Northeast - Southwest direction. The seabed topography is differentiated along the shore, the terrain is steeper the farther out to sea it is (1 - 2 0 ). The National Park's tidal flat terrain can be divided into 3 main types as follows:
- Non-tidal positive terrain with an average height of 1.2 - 1.5 m.
- Regularly flooded terrain has an average height of 0.5 - 0.9 m
- Periodically flooded terrain with an average height of 0.9 - 1.2 m.
b. Climate, weather, hydrology
* Climate - weather:
Xuan Thuy National Park is located in the Gulf of Tonkin region, influenced by the tropical monsoon climate. A year have 4 seasons. Summer is from May to September with hot and humid climate and often influenced by storms or tropical depressions. The coldest winter starts from November of the previous year to March of the following year. At the beginning of winter the air is cold and dry, but at the end of winter the air is cold and moist.
The total amount of radiation is large, from 95 - 105 Kcal/cm 2 /year. Total heat 8,000 0 C -
8,500 0 C/year. The annual average temperature is about 24 0 C, the temperature range during the year (lowest is 6.8 0 C, highest is 40.1 0 C).
The average annual rainfall is 1,175 mm, the total number of rainy days in the year is 133 days (the year with the highest rainfall is 2,754 mm, the lowest is 978 mm).
Main wind direction: Northeast in winter (starting from October to March of the following year). Southeast direction in summer (starting from April to September). The average wind speed is about 4 - 6 m/s. During storms, wind speeds can reach 40 - 45 m/s. In recent times, the number of storms directly hitting this area is very small.
Air humidity is quite high (from 70 - 90%). In October, November, December the humidity is low (<75%), in February, March, April the humidity is high and often accompanied by drizzle. Average evaporation is 86 - 126 mm/month and reaches a maximum in July. Average annual evaporation is 817.4 mm.
The national park is located in the tidal area so it is influenced by the regime
hydrology in the river and tidal regime in the Gulf of Tonkin.
Supplied with water from the Red River, the Red River has a basin area of 143,700 km 2 with a main stream length of 1,130 km. The average total water volume is 114.10 9 m 3 /year and the sediment flow is 115 million tons/year. This sand and mud flow contributes to accreting the Red River Delta with an average speed of 80 - 100 meters/year. In the flood season, the flow accounts for 75 - 90% of the total water volume of the year and carries up to 90% of the sand and mud, causing flooding of the delta, filling up river channels and making the estuary area dry. sweetened. On the contrary, in the dry season, the estuary area is narrowed, the tide rises, bringing salt water deep into the continent along the rivers.
The study area has two main rivers: Vop river and Tra river, in addition to a number of small natural water supply and drainage creeks.
The tidal regime in the Xuan Thuy National Park area has a fairly homogeneous diurnal tidal regime, the tide has an average cycle of 24h45', the time of rising and falling tides is different (TD = 11h11', TR = 13h43' ). Maximum fluctuation amplitude is 3.0 - 3.5 m, average is 1.7 - 1.9 m and minimum is 0.3 - 0.5 m. The maximum tide level is about 4.0 m and the lowest is about 0.08 m. Every month, on average, there are 2 periods of high water, each period lasting from 11 to 13 days with a day-night amplitude of 1.5 - 3.0 m, and between them are periods of low water, each period lasting 2 - 3 days. , with a small fluctuation amplitude of 0.5 - 0.8 m.
The tides of the Gulf of Tonkin have a special influence on the Ba Lat estuary and Xuan Thuy National Park is located within it. One of its impacts is saltwater intrusion. Tides propagate into the river in the form of moving wedges; The top of the saline wedge acts as a spillway for fresh water carrying suspended sediment particles to overflow, while larger particles moving on the bottom surface are stopped and cause sedimentation. In the flood season, sea water salinity drops to low, on average 9 - 17‰ and in the dry season months it increases from 23 - 3 2‰. In the estuary from December to May, the average salinity increases and reaches the highest value in January.
Water surges in the study area occur mainly under the influence of wind during storms. Rising water causes flooding and destroys residential and economic structures, especially seafood farming lagoons. In addition, during the winter, under the influence of the Northeast monsoon, which has high speed and stable blowing off the coast of the Gulf of Tonkin, the region
This area often experiences rising water, however the rising water value due to the Northeast monsoon is not high, on average about 25 - 30 cm.
In the coastal area of Giao Thuy district, waves have a strong impact on the redistribution of sand and mud in the river, forming coastal mudflats at estuaries such as Thu dunes, Ngan dunes... During monsoon days. Prolonged strong winds in the Northeast and active storms, large waves and rising water always threaten coastal dikes, especially in recent years mangrove forests - an active agent protecting the coast and coastal plains. When the sea is seriously cleared to build aquaculture ponds, the level of wave damage increases.
Recently, due to unreasonable human interventions and some unfavorable natural factors, the water regime in the Ba Lat River estuary mudflats has become abnormal. The area adjacent to the Red River estuary has been freshwater due to the Vop dam blocking the Vop and Tra rivers being filled in the middle, preventing the normal circulation of the two water sources; The fresh water source of the Red River and the salt water source of the Giao Hai Sea. Sweet-loving plant species have had strong growth conditions (such as Sour and Reed, Sedge) in the Red River estuary, while salt-loving plant species have died (such as parrots). On the contrary, the land far from the river mouth is salinized, and Ha (a mollusk that lives on mangrove trees) grows very strongly, causing mangrove trees to be damaged in many places and mangrove forests to be underdeveloped. even died en masse.
The change in hydrological regime leads to changes in biodiversity of the estuary. Biodiversity is sharply reduced, economically valuable aquatic species cannot develop, giving way to less valuable species of freshwater ecosystems. Similarly, when salinity is quite high all year round, it is difficult to obtain economically valuable aquatic species. The decline in quantity and quality of forest resources and aquatic flora and fauna is an inevitable consequence leading to the shrinking of feeding and residence habitats for migratory birds and other wildlife.
* Land resources
According to the investigation results, in the area of Xuan Thuy National Park there are 3 types of soil including: deep gley salty soil, typical marine sandy soil, deep gley salty mangrove soil, details are as follows:
Table 1. 4: Area of different types of land in the study area
Saline soil with many deep gleys
Typical, saturated, marine sandy soil
Salty soil, mangrove parrot, deep glay
Land not investigated (land of rivers, streams, water surface
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Source: Land map of Nam Dinh province
The land in Xuan Thuy National Park is formed from alluvial deposits of the Red River system. Sedimentary material includes two main types of alluvial mud (gradually consolidated to become a layer of loam) and deposited sand (deposited and moved due to external forces to become sand dunes). The different levels of consolidation of the soil types and the level of sand dune elevation have contributed to the detailed differences in soil strata types and soil distribution. The alluvial layer is transported and deposited by the current to form the coastal estuarine soil layer, which is determined by the delta coastal soil layer with the following types:
- Light soil, mixed sand and light loam, small portion of pure sand.
- Average soil, average meat.
- Heavy soils range from heavy loam to clay.
Below are some physical and mechanical properties and granularity of surface sediments of the soil layer in the study area:
+ Clay in plastic state: Common in sandy areas, has black brown, pink brown or gray brown color. Particle content of 0.25 - 0.1 mm accounts for 10 - 25%, particle content of 0.1 - 0.05 mm accounts for 21 - 36%, particle content of 0.05 - 0.01 mm accounts for 10 - 13%, Particle levels smaller than 0.005 mm account for 16 - 25%. Natural density varies between 1.84 - 1.95 tons/m 3 , density 2.71 - 2.76 tons/m 3 . According to mechanical properties, this class is relatively weak
has high natural moisture, mechanical and physical properties characterized by low durability and deformation under large construction loads. It can then be used as a foundation for an earth embankment, but the stability period and level of foundation settlement are similar. relatively large or used as dike material when its moisture content decreases.
+ Clay in a soft plastic state: This type of soil is often found at a depth of less than 5 m in a relatively compact soft plastic state, the clay is relatively uniform in color from gray brown to light brown, and has a particle content of 0.25 - 0 1 mm accounts for 22 - 39%, particle class 0.1 - 0.05mm accounts for 10 - 26%, particle class 0.05 - 0.01 mm accounts for 18 - 30% particle class 0.01 - 0.005 mm accounts for 4 - 17% of the particle size is smaller than 0.005 mm, accounting for 18 - 27%. According to the physical and mechanical properties, this soil layer has greater physical and mechanical strength indicators than the flexible clay layer above. This type of soil is relatively well consolidated, has average natural moisture, has low deformation under the load of the above works, and is suitable for making the foundations of works with not too large a load above. used as dike building material.
+ Fine-grained sand: Usually found right on the dune surface, the sand is opaque gray, white gray mixed with mica scales. The particle composition is shown as follows: particle size larger than 0.25 mm accounts for 12 - 52%, particle class 0.25 - 0.1mm accounts for 39 - 62%, particle class 0.1 - 0.05 mm accounts for 2
- 35% grain size 0.05 - 0.01 mm accounts for 1 - 2%, diameter M d10 is in the range of 0.07 - 0.12 mm, M d60 is in the range of 0.15 - 0.36 mm, degree The density of sand varies from 0.33 - 0.51. In general, this is a relatively homogeneous sand layer with relatively good density, but when flooded under the influence of currents and the load of the construction, it easily turns into flowing sand, causing erosion and landslides.
Unstable soil groups are also strongly influenced by diurnal tides, waves, floods and coastal currents, are not yet consolidated and are in liquid mud form. The deep lower layer has gradually stabilized and formed layer B, the upper layer is no more than 20 cm thick. The group of trees belonging to the mangrove forest type plays an active role in fixing the soil layer, gradually raising the coastal elevation. The amount of silt at Ba Lat estuary averages 1.8 grams per liter of water, forming the basis for the formation of alluvial dunes extending in the southwest direction. The pH of the soil layer is quite stable (meat - meat weighs from 7.2 - 7.6) and the level of salinity with NH density fluctuates from 17.2 - 20 milligrams in 100 grams of dry soil sampled. Loose mud or fixed soil is rich in nutrients, suitable for many species of mangrove plants. Very clearly shows the close relationship and beneficial interaction between soil and flooded forest populations,
* Water Resources