Research on modeling the effects of flooding (due to climate change) on the mangrove ecosystem in Xuan Thuy National Park, Nam Dinh province

1.1. Mangroves

CHAPTER I OVERVIEW

1.1.1. Concept of mangrove forests and their distribution

According to Phan Nguyen Hong (1999), mangrove forests live in the transition zone between marine and terrestrial environments, the impact of ecological factors affects their existence and distribution. Mangrove trees often appear in various vegetation parallel to the coast and riverbanks. Mangrove trees have a very wide range of adaptation to climate, soil, water, and salinity.

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Mangrove forests are distributed mainly in the equatorial and tropical regions of both hemispheres. However, some species can expand their distribution north to Bermuda (32 0 20' North) and Japan (31 0 22' North) such as Trang, Parachute, Dang, Yellow... (Figure 1.1)

The southern limits of mangroves are New Zealand (38 0 03' S) and southern Australia (38 0 43' S). In these areas, due to the cold winter climate, only fish sauce species usually grows.

Figure 1.1: Map of mangrove forest distribution in the world (Phan Nguyen Hong, 1999)

Calculating the area of mangrove forests accurately is very difficult because they are always changing due to human impact. According to IUCN (1983), the area of mangrove forests in the world is 168,810 km 2 , while Spalding and colleagues (1998) believe that the area of mangrove forests in the world is 198,818 km 2 .

Basically, geological, climatic and biological factors determine the types of development and characteristics of mangrove forests. Geological factors shape soil structure and structure

sediment deposition of the area, meteorological factors causing hydrological processes (tides, rivers, coastal currents...). Biological factors determine the adaptive conditions of plants.

Mangrove species have physiological and reproductive forms that allow them to thrive in harsh environmental conditions (including mangrove environments).

The two countries with the largest mangrove area in the world are Indonesia and Brazil, and the tree size is also very large. In Ecuador there are trees up to 60m tall. In Southeast Asian countries such as Malaysia, the Philippines, Thailand, and South Vietnam, mangrove forests also develop because there are favorable conditions such as abundant rainfall throughout the year, high and low temperature fluctuations, and large swamps. , rich in humus and silt, the tree's height reaches 20 - 30m.

Due to rapid population growth, especially in developing countries, mangrove forests are currently being overexploited or used for other economic purposes. Therefore, the area of mangrove forests in the world is gradually shrinking. There are only very few natural mangrove forests left in many countries. Currently, in some countries, national parks, biosphere zones, animal and plant protection areas, places for research, study, and tourism in mangrove areas have been established.

1.1.2. Origin of mangrove forests in Vietnam

According to Phan Nguyen Hong (1999), the number of mangrove species known is most abundant in the Southern coast (100 species), followed by the Central coast (69 species) and finally the Northern coast (52 species). species). There are differences in the number of species due to differences in geographical, climatic and hydrological characteristics.

Some researchers believe that the center of mangrove formation is Indonesia and Malaysia (Chapman, 1976) from where it spread to other places. According to Phan Nguyen Hong (1991), the transport of seed sources into Vietnam is mainly due to ocean currents and coastal currents.

The southwest monsoon in the summer brings the flow of seeds from the south, but when it reaches latitude 12, the flow changes to the sea, so some species do not spread to the northern coast. That's why many species that are abundant in the South such as: white cork, guava, astragalus, mangrove, cylindrical parrot, split parrot, nipa palm, black fish sauce, white fish sauce... do not appear in the North.

It is also possible that a few of the above species float in the sea for a few months and reach the coast of the Gulf of Tonkin, but because their growth period coincides with winter, they cannot survive (Hong, 1991). Nguyen My Hang and Phan Nguyen Hong

(1996) followed the growth of some species of the Rhizophora family such as mangroves, mangroves, cylindrical parrots, split parrots, lime dwarves, and astragalus, transferred from Can Gio - Ho Chi Minh City to experimental plantings in Thach Ha - Ha Tinh but no results. During hot, humid periods, the tree grows faster than similar tree species in the North, but in the winter of the first year, the tree's top wilts, then branches and continues to grow. By the following winter, parts of the tree die.

1.1.3. Necessary ecological factors for mangrove forests to grow and develop

Mangroves grow and develop well in places with the following factors (Chapman, 1976):

1.1.3.1. Soil quality

Alluvial marshes containing many nutrients brought in by tides are the best conditions for mangrove plants to grow and develop. In coastal areas with a lot of sand, little silt, or places with a lot of gravel, some species of mangrove trees can still survive but are short and stunted. For example, mangrove trees in Ca Mau cape (Ca Mau province) have a growth rate of 0.8 - 1m/year in height, and 0.6 - 0.8 cm/year in diameter (Hong et al., 1999 ). Meanwhile, the growth rate of this species in Cam Ranh Bay (Khanh Hoa) where there is a lot of sand is only 0.4 - 0.6m/year; low tree with many branches.

1.1.3.2. Topographic

Coastal areas and estuaries with many shielding islands, gentle coastlines, less affected by storms, are favorable for mangrove trees to grow and widely distribute.

1.1.3.3. Salinity of soil and water

Many mangrove species grow well in areas with an average salinity of 1.5 - 2.5%. However, there are some trees adapted to brackish water with low salinity (0.5 - 1%) along the banks. estuaries such as Sour Soap , Nipa Coconut, and Acanthus. In general, when salinity is too high or too low, many plant species do not grow normally.

1.1.3.4. Temperature

Mangrove species only grow well in areas with annual average temperatures above 20 o C, with little variation. In the North of our country, there are cold winters, so mangrove forests have few large trees, few species, and slow growth. In the South, the average annual temperature is 24 - 27 o C, with little fluctuation, so the forest has many large, fast-growing trees and species composition is also richer. With strong temperature fluctuations in winter over the years

Recently (2008-2011) many areas of mangrove forests, especially the distribution area of

Sonneratia caseolaris species have declined in health or died.

1.1.3.5. Tide

Estuarine and coastal areas have daily tides that provide nutrients and moisture to the swamps, making them very suitable for the growth of mangrove trees. In deeply flooded or rarely flooded beaches, plants grow poorly or will even die if continuously flooded for many days, such as tiger trees, parrots, snakeheads, etc. in shrimp ponds.

1.1.3.6. Fresh water stream

The flow of fresh water from the river provides silt and other nutrients to the plants. Fresh water dilutes the salinity of sea water, creating favorable conditions for plants to grow.

1.1.3.7. Amount of rain

Like other plants, mangrove trees need rainwater, especially during the period of flowering, fruiting, and propagule formation. When germinating, seedlings need fresh water to reduce salinity in the soil. Where there is a lot of rain, trees thrive, where there is little rain, the trees are barren. For example, in Inisophai (Australia) with a rainfall of 3,500 - 4,000 mm/year, mangrove trees grow very well, with up to 92 species of trees in the forest. And in Iran due to low rainfall and heat

At high altitude, mangrove forests only have Avicennia marina . The tallest tree is 3 - 5 m.

1.1.4. Some biological characteristics of mangrove species

1.1.4.1. Root system above ground

Living in an environment periodically flooded with tidal water, mangrove trees have a number of adaptive characteristics: First of all, the root system. Some typical mangrove trees such as mangroves, parrots, ornamentals, cork, fish sauce... often have developed root systems. In addition to underground roots, these plants have additional above-ground roots, responsible for the function of respiration and helping the plant stand steady in muddy, unstable conditions.

determined.

In mangrove trees, the roots grow from the trunk, the branches grow longer, branch out and stick into the ground like upturned teeth; fish sauce and cork have respiratory roots growing upside down from the roots

Figure 1.2 : Diagram of above ground roots

1. Roots supporting trees and mangrove trees

2. Respiratory roots in fish sauce 3. Knee-shaped roots in parrots

lying horizontally on the ground, looking like spikes to get air; Parrots have respiratory roots that rise from horizontal roots near the ground like knees (Figure 1.2). These roots

with underground roots that keep the tree standing firmly on soft, turbulent mud.

1.1.4.2. Fruit and sprouts

The fruits and seeds of mangrove trees are also very special. In mangroves, parrots, ornamentals, and dachshunds of the Rhizophoraceae family, the seeds germinate as soon as the fruit is still on the mother tree into a long part attached to the fruit called a propagule that has all the parts of a tree. future seedlings. It is called the phenomenon of "giving birth on the mother tree" ( vivipary ) (Figure 1.3). When the sprout is ripe, it separates from the fruit and falls down, sticking into the mud and growing into a seedling. There are seeds that also germinate on the mother tree, but the sprouts only lie in the fruit. After falling into the mud, they grow again, like in tiger plants, fish sauce, and nipa palms. That is the phenomenon of "half-birth " ( cryptovivipary ).

Figure 1.3 : Fruit and propagules of mangrove

1. Red mangrove fruit; 2. Fruits and propagules are still very young; 3. Old fruits and propagules; 4. Pillar

The sprout separates from the fruit and falls to the ground

Finally, for trees with common fruits and seeds such as bean sprouts, holly, cork ... the ripe seeds fall to the ground and germinate immediately into seedlings.

1.1.4.3. Groups of mangrove trees

Mangrove plants adapt to salt water regimes in different ways.

Can be divided into the following 2 groups:

 Salt- excreting group : Plants absorb salt water into their bodies and then excrete the salt through special glands, called salt-excreting glands , on the leaves. For example, fish sauce, tiger prune, and ho ho .

Salt accumulating group (salt accumulating group) Trees can also absorb salt water into their bodies and then filter it, while harmful salts accumulate in old leaves . When they fall, the salts are excreted out of the body like in trees. price, twilight foul, dong, duoc, trang, parrot parachute (Hong, 1991).

1.1.5. Ecological succession of mangrove forests in the study area

According to the research results of Phan Nguyen Hong (1991) on the coastal vegetation of Northern Vietnam, it can be seen that in Xuan Thuy National Park there are 2 ecological successions corresponding to the typical communities in this area: : Primary succession of mangrove communities with the pioneering role of amphora species and primary succession

Born as a pioneer of the Sour Sourdough tree.

1. Primary succession of mangrove communities with the pioneering role of amphora species

This ecological succession can be divided into 4 stages including:

- Pioneer stage of the fish sauce plant: First, on the beaches that emerge from the water at low tide, the land is still in the form of liquid mud, lots of sand, seeds of the fish sauce species from other places are brought and kept here. . Thanks to the special structure of the sprouts, they are curved and covered with dense hairs that act as anchors, creating conditions for the seeds to firmly stick in the mud and grow into plants. When the fish sauce plant grows, it will create many horizontal roots in a circular direction around the stem. From these roots, two types of roots will arise: nutritional roots and respiratory roots. It is thanks to this root system that humus and seedlings of other species are retained, creating conditions for the emergence of new tree species.

- Mixed phase: The pioneer community plays an important role in holding the soil, causing the mudflat to rise, the period of periodic tidal flooding during the day shortens, and the mud gradually compacts. Seedlings of other species such as tigers, parrots, etc., transferred to the area, will be kept and will grow and develop quickly in the fish sauce community. Gradually these species break through the canopy, competing for light and nutrients with the pioneer species. Most of the individuals of the pioneer species cannot compete so they are eliminated, only those individuals that rise to the top survive. Due to their good ability to tolerate shade, their numbers are quite large. On the other hand, due to their high salt tolerance, most of them occupy coastal locations, especially creeks. The parakeets live in relatively low places, while the parakeets are distributed high up near the shore.

- Parachute parrot dominant stage: When the mudflats are raised and stabilized in the ground, only flooded at high tide, the soil composition has changed, the mud is compact with clay, the growth of salt-tolerant species such as tiger, replanting is slow, although parrots have high shade tolerance, they have an advantage in competing for light and nutrients, so their growth rate is faster than other species, they surpass the canopy and become the dominant tree species.

- Final successional stage: In swamps that are raised to the point where high tides only occasionally rise, only a few parrot trees survive, other species gradually die because the mud hardens and the pyrite-rich soil is polluted. oxidized to acid sulfate form. A community of trees and shrubs is no longer flooded or invaded.

Looking at the 4 stages of primary succession above, it can be seen that in the study area the ecological succession is in a mixed stage with the rapid development of species of mandarins and black tigers, while the fish sauce tree is gradually being destroyed. thinned naturally and replaced by other tree species.

2. Primary succession of the mangrove community with the pioneering role of the sorrel species

This succession occurs in the Tra River estuary, including the following stages:

- The pioneering stage of the Sour Soap species: On the alluvial ground just inside the river mouth, the soil is still soft, flooded at low tide, there are no trees or a few scattered sedges. The inside of the beach, where the land is higher, is inundated with average tides, and sour crabs settle. Sour soil has a horizontal root system that is distributed radially. Nutrient roots penetrate the soil, have many branches, and respiratory roots grow upward. In addition to the respiratory function, it also helps retain organic humus and residue. as seeds and seedlings of other species are brought by sea water. It is the adaptation to nutrient-rich swampy soil that has caused the Sora Sora to grow quite strongly, forming a dominant population.

- Mixed stage of Sour Soap - Tiger - Trang - Umbrella Parrot - Acanthus: After the bog is gradually raised, thanks to the respiratory root system of Sour Soup, seedlings of species such as Tiger, Acanthus or Each sedge clump is formed. But in places where there are high mounds, trumpet gourds appear and they often climb up trees. This vegetation has a good effect of retaining silt, so the sedimentation rate is quite fast. When the system of small plants has developed, which has the effect of retaining seeds and seedlings of species such as Trang, tiger and parrot... the seedlings of these species will grow and develop into vegetation. under the canopy of the Sour cork species.

- Degeneration stage: When the foundation has been raised, rarely flooded with tides, tree species such as Sour Soap, Pistachio, Trang... that are no longer suitable for the new environment will be eliminated and replaced by populations. Tree and shrub communes adapt to living conditions that are not flooded by tides.

According to the stages of primary succession above, in Xuan Thuy National Park, in the estuaries, a cluster of sorrel trees grows on the mudflats just inside the river, the soil is still loose, flooded at low tide, under the canopy of sorrel trees is Tree species such as tiger, cup trumpet, ornamental plant, grass... Thus, this complex is in the mixed stage but the advantage still belongs to the Sour cork tree.

1.2. Climate change and mangrove ecosystem

1.2.1. Climate change and sea level rise around the world

Assessments by the world's leading experts on the greenhouse effect and global climate change, especially ocean fluctuations and changes, show that the world's ocean has warmed significantly since the end of the 1990s. 1950. There are two main causes of sea level rise: thermal expansion of the ocean (water expands and takes up more space as it warms) and ice melting. From calculations to control global emissions that cause the greenhouse effect, experts have made elaborate calculations about ocean warming leading to a rise in water levels due to thermal expansion and melting of rivers. ice, ice sheets and ice sheets in Greenland and the Five Poles. In addition to general results based on documentary sources from 1961 to 2003, the assessment also focuses on examining changes over each decade,

However, sea level changes are not uniform throughout the world's oceans: In some regions, the rise rate may be several times higher than the global average speed, while in other regions it may decrease. Over the past decade, sea levels have risen fastest in the Western Pacific and Eastern Indian Oceans.

Some processes for estimating global sea level rise do not have enough scientific basis, even for estimating how much the earth will warm by the end of this century. Many scientists point out that the melting ice sheets separating from Greenland and Antarctica will move to warmer waters so the melting rate will be faster and the melting rate could double in the last 5 to 10 years. (Tran Thuc, Duong Hong Son, 2012).

Current measurements of sea level are based on two methods: measurements at coastal oceanographic stations and measurements by satellite. The oceanographic station shows changes in water level compared to the station's elevation mark. To be able to know water level changes due to water volume and other physical factors, oceanographic station data need to eliminate factors due to geological movements of the ground. Among the changes due to glacial isostatic adjustment (GIA), tectonics, subsidence, and sedimentation, the change due to GIA is calculated in a global geodynamic model. Estimating the effects of geological movements in general will not be possible if there are not enough measurement locations or geological data. However

Research on modeling the effects of flooding (due to climate change) on the mangrove ecosystem in Xuan Thuy National Park, Nam Dinh province - 2

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