INTRODUCTION
Wetlands, as defined by the Ramsar Convention, serve as vital habitats for diverse flora and fauna, offering numerous ecological, economic, and social benefits Xuan Thuy National Park in Nam Dinh province is the first Ramsar site in Southeast Asia and is also designated as one of Vietnam's nine Biosphere Reserves by UNESCO’s Man and Biosphere Program This recognition highlights the park's rich biodiversity and its global significance, particularly its mangrove forest.
The mangrove forest in Xuan Thuy National Park (XTNP) serves as a crucial spawning ground for aquatic species and is a vital habitat for migratory birds, including critically endangered species like the black-faced spoonbill (Platalea minor) and the spoon-billed sandpiper (Calidris pygmaea).
The mangrove forests play a crucial role in safeguarding the sea dyke system that protects the productive Red River Delta and the densely populated rural areas of Nam Dinh and Thai Binh Provinces These forests mitigate the impacts of floods, storms, and high tides by reducing wave energy, limiting erosion, and enhancing the protection of sea dykes Additionally, they support the livelihoods of thousands of people living in the park's buffer zone However, the rapid expansion of aquaculture and agricultural practices has led to significant deforestation, threatening the mangroves and altering the ecological balance of the region This degradation diminishes the vital ecosystem services that the mangroves provide, ultimately affecting human well-being.
Recognizing the important role of mangrove ecosystem at XTNP, recent years, there were several mangrove planting models have been implemented [8, 35,
There has yet to be a thorough assessment of the outcomes from various mangrove planting models This thesis aims to investigate and evaluate the current status of selected mangrove planting models while proposing technical solutions to enhance the conservation and development of mangrove forests in XTNP, Nam Nam province.
LITERATURE REVIEW
Research results on the world
Mangroves, a coastal forest ecosystem frequently flooded by tides, are primarily located between latitudes 25° north and 25° south of the equator, covering around 17 million hectares across 112 countries and territories in Asia, Africa, Australia, and the Americas Southeast Asia boasts the largest mangrove forest area globally, with 4.9 million hectares, representing 35% of the world's total mangrove area Among the 268 plant species identified, 52 are classified as true mangrove species, while the remainder can thrive outside mangrove environments Notably, there are 18 endemic mangrove plant species, including eight true mangrove species.
Mangroves play a crucial role in coastal ecosystems by providing fuel wood and a variety of valuable aquatic and marine products They help stabilize mud and sand, serve as windbreaks, and act as breakwaters, thereby protecting communities, agricultural fields, and cultural sites along the coast Additionally, mangroves significantly contribute to environmental protection in numerous coastal areas (Kathirestan, 2000).
Tomlinson (1986) classified mangrove communities into two distinct groups based on tree species composition: the eastern group, found in the Indo-Pacific region, is characterized by a rich diversity of species, while the western group, located along the tropical coasts of Africa and the Americas, has significantly fewer species—only one-fifth of those in the East (Spalding et al., 1997) Key species in the western group include Rhizophora mangle, A germinans, and Laguncularia racemosa Notably, some tree species in the West can grow larger than their Eastern counterparts, with heights exceeding 50 meters in Brazil and 60 meters in Ecuador.
Mangroves thrive in warm climates with temperatures above 20°C and experience heavy rainfall, averaging over 1,000 mm annually (Larsson et al., 1994) The soils in mangrove ecosystems are nutrient-rich due to tidal waters but have low oxygen levels Daily tidal movements result in soil salinity levels averaging between 1.5% and 2.5% Additionally, the physical and chemical characteristics of mangrove soils are significantly influenced by the sources of alluvium and sediment (Lugo and Snedaker, 1974; Hutchings et al., 1987; Sammut et al., 1996).
The list of mangrove vegetation with a number of species about from 50 to
75 species (Lugo and Snedaker, 1974; Saenger et al., 1983; Blasco, 1984) [93, 103,
The predominant genera of mangroves include Avicennia, Rhizophora, Bruguiera, Ceriops tagal, Excoecaria, and Sonneratia Due to high inundation and salinity, mangrove ecosystems typically exhibit a simplified structure, characterized by a clear dominance of species This often results in a primary tree layer with minimal presence of shrubs and grasses beneath the mangrove canopy.
In tropical regions characterized by hot and humid climates with fertile alluvium, mangroves can grow rapidly, reaching heights of several tens of meters and biomass reserves of hundreds of m³ per hectare Conversely, in sub-tropical areas with poor soils, mangroves typically develop as shrublands, with tree heights limited to a few meters and total biomass not exceeding 50 m³ per hectare The growth rate of mangroves tends to increase during the early years, stabilizing around 10-15 years before gradually declining By the age of 35-40 years, mangroves reach natural maturity, ceasing to grow in size and beginning to age and fall.
Coastal mangroves are among the most threatened ecosystems globally, with their distribution rapidly changing due to factors such as population growth, urbanization, industrial development, land reclamation, and pollution The expansion of shrimp farming areas further exacerbates this issue According to Khan and Ali (2007), the rate at which mangroves are being lost is on the rise.
Kathiresan (2002) studied the functional variation of mangrove ecosystems by comparing ecological variables and physicochemical indices in five rich mangrove areas and 25 degraded zones within the Pichavaram mangrove forest in India The study identified high salinity, low nutrient levels, and poor soil microorganisms as primary causes of natural mangrove degradation Additionally, Onrizal et al (2009) suggested that in cases of degraded mangrove environments, recovery efforts should involve the participation of mangrove species that can effectively replace those that are no longer functional.
2.1.2 Research on mangrove rehabilitation on the world
Havanond (1994) and Aksornkoae (1996) [80, 49], Rhizophora apiculata and
In Thailand, the primary mangrove species are Rhizophora mucronata, which thrive in double mangrove forests where shoots and seedlings exhibit a survival rate exceeding 80% Notably, the survival rate for planted Rhizophora mucronata forests is greater than 94%, as reported by Francis E Putz and H.T Chan (1996) in their study conducted in Malaysia.
1987 to 1992, there were 4,300 hectares of mangroves, with the main crops being:
R apiculata and R mucronata In Indonesia, there are four main cultivated species of mangroves, namely R apiculata, R stylosa, R mucronata and B gymnorrhiza
Koko M (1986) identified a planting method that considers the characteristics and germination abilities of seeds or propagules The author outlines three techniques utilized in various Asian countries: (i) direct planting using propagules, (ii) planting seedlings grown in nurseries, and (iii) utilizing naturally occurring seedlings for planting.
In Indonesia, Soemodihardjo et al (1996) identified two planting techniques: direct planting of seed stalks and the use of container seedlings The recommended planting density is 2,500 trees per hectare, arranged in a 2.0 x 2.0 m configuration, with Rhizophora being the primary crop cultivated.
R apiculata, R mucronata and B gymnorrhiza Planted directly by seed stalks, the survival rate reached 55-70% and indirectly planted with seedlings 3 - 4 months old, the survival rate was higher, reaching 85%
Siddiqi N A and Khan M.A.S (1996) highlight that the appropriate levels of tidal inundation and salinity are crucial for the survival and growth of newly planted seedlings In Bangladesh's new coastal alluvial conditions, S apetala and Avicennia marina were chosen for planting The planting densities varied at 1.2 x 1.2 m, 1.5 x 1.5 m, and 1.7 x 1.7 m Seedlings aged 6-7 months showed a survival rate of 29% to 52% for S apetala, averaging 40%, while A marina achieved a survival rate of 70% After six years, the remaining density of these species was assessed.
In India, two planting methods are utilized for mangrove restoration: direct planting with propagules and using container seedlings measuring 4cm x 10cm This approach involves five species of mangrove plants, including A marina, A officinalis, R apiculata, and S caseolaris, with a recommended planting spacing of 1.5 m x 1.5 m (Untawale, 1996).
Ellison (1999) synthesized 27 global studies on mangrove restoration, highlighting varying rehabilitation goals The analysis revealed that 10 out of the 27 projects aimed to increase forest cover, 6 focused on coastal stabilization, and others targeted climate change mitigation.
The primary objective of mangrove restoration projects is to rehabilitate degraded mangroves, aiming to restore their structural integrity and ecological functions (Lewis, 2005; Gilman and Ellison, 2008) This rehabilitation process enhances coastal stability, promotes alluvium accumulation, and improves coastal protection Additionally, it creates a conducive environment for wildlife, provides resources such as wood and firewood, and enriches the aesthetic value of coastlines, ultimately striving to replicate the benefits of natural mangrove forests.
Research on mangroves in Vietnam
2.2.1 Research on mangrove ecosystems in Vietnam
With its important socio-economic and environmental significance, mangroves in Vietnam have been studied at a very early stage Some typical research projects have been published as follows:
The pioneering research on mangroves in Vietnam was conducted by Vu Van Cuong in his 1964 doctoral thesis titled "Plant Ecosystem and Vegetation in Saigon - Vung Tau, South Vietnam." This work detailed the saltwater and brackish water communities in the Saigon and Vung Tau areas, along with their associated soil characteristics.
Phan Nguyen Hong (1970) conducted a thesis on the ecological characteristics and distribution of flora and vegetation in Vietnam's northern coastal forests, focusing on the growth and biomass of mangrove forests in the Mekong Delta This research is supported by contributions from notable authors, including Barry Clough (1996), Phan Nguyen Hong and Nguyen Hoang Tri (1983), Vien Ngoc Nam (1996), and Dang Trung Tan.
The growth and biomass of mangroves are influenced by several factors, including tidal conditions, soil type, inundation levels, salinity, and organic matter content.
Ngo Dinh Que et al (2003) identified variations in natural geographical conditions to categorize the vegetation of mangrove forests and coastal mangroves in Vietnam They divided the country into three main regions—North, Central, and South—resulting in a classification of 6 regions and 12 sub-regions.
Tran Thi Mai Sen (2005) investigated the impact of salinity, planting time, soil properties, and planting methods on coriander growth in her study titled “Effects of some ecological factors and planting techniques on the survival and growth rate of S caseolaris (L.) Enger in Thai Binh and Nam Dinh provinces.” Conducted in July and August, the research found that coriander thrives best at an average salinity of 5 - 15 To mitigate root system damage and promote healthy plant growth, it is recommended to use potting medium seedlings for planting.
Do Dinh Sam et al (2005) [33] studied an overview of mangrove forests in Vietnam and developed a distribution map of mangrove forests in Vietnam
Tran Triet et al (2007) conducted a study comparing the structure and function of replanted mangrove ecosystems with natural mangrove forests in the Can Gio Biosphere Reserve, Ho Chi Minh City This research highlights the differences between these two types of mangroves and enhances the understanding of tropical mangrove forest ecology The findings have specific applications for the restoration of mangrove ecosystems in Can Gio and other regions.
Nguyen Ngoc Binh, Do Dinh Sam, Ngo Dinh Que, Pham Duc Tuan [4, 5,
33] believe that some natural factors such as topography, coastal seawater salinity, waves, sea breeze, hydrology are Basic factors that directly affect mangrove characteristics
Do Dinh Sam et al (2008) evaluated the production potential of mangrove soils in the Mekong Delta by considering factors such as soil type, soil maturity, organic matter content, and flooding conditions.
In the Mekong Delta's coastal areas, the growth of mangrove plantations is influenced by three key factors: soil type, tidal inundation, and soil maturity Ngo Dinh Que (2003) identified 14 site types in the region, categorizing mangrove soil into 8 types and potential alum mangrove into 6 types Based on the ecological characteristics of tree species and local research, the author recommends specific usage directions for these site types (A, B, C, D).
Hoang Van Thoi (2014) categorized the southern coastal islands, characterized by challenging mangrove environments, gravel, sand, coral debris, and high salinity, into three distinct areas: (1) Coastal islands of the South Central Coast and offshore islands; (2) Islands in the South East Sea; and (3) Islands in the Mekong Delta Each area is further divided into 20 site types based on a combination of three key factors: tidal influence, substrate type, and salinity levels The entire region comprises six groups of cadastral sites: Group A (2 sites), B (4 sites), C (2 sites), D (6 sites), E (2 sites), and F (4 sites) Based on this classification, the author recommended specific mangrove species for each site type group across the southern coastal islands.
Dinh Thanh Giang (2015) categorized mangrove land in the coastal regions of Quang Ninh and Hai Phong based on several criteria, including soil type, salinity, current land use, drying time, inundation depth, soil maturity, and mechanical composition He assigned scores to each site type, classifying the suitability for afforestation and mangrove rehabilitation into four groups: Group I (very convenient), Group II (convenient), Group III (less favorable), and Group IV (limited).
The research findings on site zoning and classification of coastal mangrove land serve as a foundation for establishing standards in planning, selecting tree species, and implementing planting techniques This is crucial for the restoration and development of mangrove forests in our country, particularly in addressing climate change and rising sea levels.
Doan Dinh Tam (2012) introduced innovative techniques for mangrove planting in challenging conditions, significantly enhancing breakwater efforts along Vietnam's northern coast The initial outcomes were promising, with experimental treatments yielding high survival rates and robust plant growth Notably, the various experimental formulas demonstrated effectiveness in difficult environments, such as cohesive sandy sites, rocky soils, deep tidal wetlands, high tide submerged areas, and abandoned shrimp ponds, contributing positively to sedimentation and the improvement of soil and water quality.
Hoang Van Thoi (2011, 2014) conducted experiments on mangrove planting using species such as A marina, R stylosa, and R mucronata across various coastal sites, achieving survival rates exceeding 50% The results indicated that rocky, salty, high-tide, and gravel sites exhibited better survival and growth rates compared to coral and inundated areas The highest survival rate was observed when propagules were planted by digging holes, placing seed pillars, and filling them with surrounding rocks For afforestation in southern coastal islands, an optimal planting density of 6,600 trees per hectare or 5,000 trees per hectare is recommended, with seedlings aged six months showing the best survival and growth Effective crop protection measures include constructing a breakwater parallel to the shoreline and utilizing wooden stakes for plant support.
Ngo Dinh Que et al (2003) [30] conducted a trial planting of 7 ha of breakwater protection mangrove forest with the formula of planting K obovata and
S caseolaris (density of 10,000 plants / hectares) Planting K obovata and mixed with 3 formula K obovata 8,250 plants / hectares, Kourning 800 plants, 1,000 trees, 1,200 plants / hectares at Tan Thanh - Kien Thuy - Hai Phong, on the mud-slurry site (site type MIIIb); medium tidal inundated land
The monitoring and evaluation of the experimental model revealed that the optimal growth of plants occurred with a mixed planting formula of 8250 K obovata plants and 800 additional plants This approach not only enhanced tree growth but also facilitated the development of a two-layer forest structure, thereby improving forest protection effectiveness The study indicated that the forest strip significantly reduced wave height, with waves measuring 1.2 meters outside the mangroves decreasing to only 0.2 to 0.3 meters at the dyke's base Additionally, sediment deposits beneath this forest strip were notably high, averaging approximately 6 cm per year.
(10 times higher than when there is no forest)
RESEARCH OBJECTIVES AND METHODS
Research goal, ofjectives
To assess the current status of mangrove planting models and propose several solutions to improve the efficiency of conservation and development of mangrove forests at XTNP
- To research the current status of mangrove planting models
- To assess the growth characteristics of mangrove planting models
- To investigate the quality of mangrove planting forests
- To research some technical solutions to enhance and restoration of mangroves in XTNP.
Methods
In the part the data were collected by searching the documents and from the published papers which is related to mangrove forest, mangrove planting models, nature condition of XTNP
The current forest inventory utilized established study routes and sample plots, following the methodology outlined by Aksornkoae and Kongsangchai in 1980.
Sample plots were set up according to the method of Fujiwara, K (1987)
[74], English et al (1997) [74, 68]: Size of each sample plot is 400 m 2 (20m x 20m) The total number of sample plots to be established at the study site are 9 plots for 3 mangrove planting models
Scientific names of mangrove species were identified according to Hong, P.N.,
(1999), FAO (1994), Chapman, V.J., (1976) [18, 70, 57] Scientific names are determined based on comparison characteristics of reproductive and vegetative parts
A survey of tall trees in the plot was conducted, focusing on key growth indicators such as the diameter of the root (D00) and the height of the tops (Hvn) for each individual tree.
The diameter (D00) was indirectly measured using the original circumference (C00) with a measuring tape For the Sonneratia caseolaris and Avicennia marina species, measurements were taken 50 cm from the root In the case of Bruguiera gymnorrhiza, measurements were conducted at the upper part of the rhizome, as the root bud, which develops from the bud, features numerous bark holes and cracks that facilitate air intake, functioning as the tree's respiratory root (Phan Nguyen Hong, 1999) [18].
The height of a forest tree, referred to as Hvn, is measured using a graduated scale with an accuracy of 10 cm This measurement is taken from the base of the tree to the top of its growth.
To assess the quality of trees, we categorize them based on specific indicators: Good trees (G) are characterized by their straight growth, absence of pests, and a well-balanced canopy In contrast, Bad trees (B) exhibit crooked growth, are afflicted by diseases, and have significant pest infestations Trees that fall between these two classifications are identified as medium quality (M), representing an intermediate form of growth and health.
Planting model: ……… Address: ……… Plot number:………Area (m 2 ): ………… Distance to the bank (m): ………… Survey route:……… Coordinate X: ……… Date:……… Coordinate Y: ……… Investor: ………
STT Species Doo (mm) Hvn (cm) Quality
Data collection for regeneration: Evaluating the regeneration capacity of the mangrove planting model by indicators such as composition, density, vitality of regenerated plants
In each panel plot (5mx5m), the author conducted an investigation of the following criteria:
- Name of regeneration species was identified by botany experts
- The diameter was determined with a glass caliper with an accuracy of up to mm
- The height of the regeneration tree was measured with a tape measure and a marking pole with an accuracy of up to mm
- The quality of regeneration tree was classified into 3 levels (Good - G, Medium - M, Bad - B) In which:
Good quality trees are those with good growth, straight stems, evenly developed foliage, no dead ends, no pests
Bad quality trees are crooked, topless, scattered trees, affected by pests and diseases
Medium quality trees are those in intermediate form in the above two types
- Determine soure of regeneration trees: Regeneration of seeds or regeneration of shoots
- Investigate the number of regeneration trees
The collected results were recorded in table of regeneration investigation
Distance to the bank (m): Survey route:
Determining the current status and the change in the structure of mangrove communities through the status of individual trees
Specialized software such as MapInfo or ArcGIS was used to identify and delineate maps [88, 115]
Field survey: Conduct field surveys by routes and regions to collect data
Mapping: The main contents of the process are:
- Organize information by files, analysing and entering data;
- Organize information by audience classes;
- Create information layers according to thematic maps;
- Edit, design and present for map layout
Data collected from surveys and sample plots were summarized and analyzed using SPSS V.23 and Microsoft Excel, following the guidelines of Tuat, H.N et al (2006) This analysis focused on the growth characteristics of mangrove planting models.
N: is the number of individuals of the species or the total number of individuals in a plot;
* Survival rate of mangroves planting models:
The survival rate (%) of planting models was determined by the number of trees divided by the number of trees according to the design density of the model multiplied with 100
* Percentage of tree quality of each investigated species according to the formula:
(G, M, B) %= (Ni/N) x 100 Where: G, M, B: is the quality of the tree Good, Medium and Bad;
Ni: is the number of quality trees i;
N: is the total number of survey trees in the plot b) Regeneration trees characteristics of the mangroves planting models:
* Formular of regenerative tree in plot:
- Determination and composition of each species is calculated according to the formula:
Ki : Coefficient of the species i ni: is the number of individual trees for species i in th e plot m: is the total number of seedlings in plot
In the species composition formula, which has a large nesting coefficient, it is written first The number part represents the composition coefficient, the letter part stands for the species name
Regeneration density based on total number of regeneration trees on area unit (usually using hectares), which is calculated by the formula:
N: is the total number of seedlings in the plot
S: is the area of plot for regeneration investigation (m 2 )
* Determining the quality of regeneration trees:
When studying tree species regeneration, it is essential to assess the quality of the regenerated trees, categorizing them into good, medium, and bad quality The classification of trees is based on a calculated percentage of each quality type using a specific formula.
Where: N% is the percentage of the regeneration tree (good, medium and bad); n: is total number of regeneration trees (good, medium and bad);
N: is total number of regeneration trees
* Distribution of regeneration trees by height class
Conducting statistics on the number of regeneration trees according to four height levels: Level I: tree height < 0.5m; level II: tree height 0.5 - 1m; level III: tree height 1-1.5m and level IV: tree height > 1.5m
GEOGRAPHIC CONDITIONS AT XUAN THUY NATIONAL
Location
XTNP is located in Giao Thuy district, about 40 km from Nam Dinh city and
130 km from Hanoi, located in the geographical coordinates:
The Northeast borders by the Red River
The Southeast and Southwest regions are adjacent to the South China Sea, encompassing a core zone that includes parts of Con Ngan, Con Lu, and Con Xanh (Con Mo), covering a total area of 7,100 hectares.
The buffer zone includes: The rest of Con Ngan and 5 communes of Giao Thien, Giao An, Giao Lac, Giao Xuan and Giao Hai of Giao Thuy district [45]
Figure 4.2 Map of study area
Topography
The study area focuses on the tidal flat located outside the sea dyke, which encompasses dunes, river beds, and tidal creeks These tidal flats are formed from sediments sourced from the Red River and the East Sea, consisting of sand, mud, and clay The spatial and temporal accretion of alluvial sediments is influenced by factors such as the volume of alluvium, river flow dynamics, tidal movements, and human activities like dyke construction, afforestation, and shrimp farming, which have shaped the current geomorphology Overall, the terrain exhibits a low elevation gradient from North to South and East to West, with average heights ranging from 0.5 m to 0.9 m, while Con Lu features a notable sand strip reaching heights of 1.2 m to 1.5 m.
The division of the National Park tidal flats by sea dykes, Tra rivers, tidal creeks and downstream Vop river into: Con Ngan, Con Lu and Con Xanh
Con Ngan islet has been partially separated from the surrounding area due to the construction of a sea dyke, positioning it between the dyke and the Tra River The northern section of Con Ngan has largely been converted into aquaculture plots, while the remaining land near the Tra estuary consists of wetlands and bare land The vegetation along the islet features Aegiceras corniculatum and K obovata mangroves, extending from the bog to the southern tip of Con Ngan.
- Con Lu islet: located between Tra river and tidal creek separating from Con Xanh Con Lu is located near to Con Ngan parallel To the East and Southeast, Con
Lu features a prominent sand strip that remains unaffected by tidal flooding, with some sections covered by casuarina trees At the end of Con Lu, there is a unique blend of sand and alluvial ground, while the surrounding region consists of mangrove areas dominated by Kandelia obovata.
Con Xanh Islet, located adjacent to Con Lu, has an average altitude ranging from 0.5m to 0.9m and is primarily composed of sea sand, which continues to accrete, enhancing its area and sand dunes The islet is consistently flooded during high tide, while at low tide, it reveals two distinct sand strips situated in the East and Southeast, showcasing the natural formation of dunes that contribute to land expansion.
River beds and tidal creeks are characterized by high humidity and frequent flooding, leading to the accumulation of alluvial deposits such as mud, clay, and sand This process raises the soil level and narrows the water flow Additionally, these areas are partially filled with muddy sediments, covering approximately 4,000 hectares, which presents significant potential for future land expansion.
Climate
The study area exhibits the climatic characteristics typical of a coastal plain, featuring a humid tropical climate (K = 1.5 ÷ 2.0) It experiences a cold winter wind with two months where the average temperature drops below 18°C In contrast, summer months see average temperatures exceeding 25°C The rainy season occurs during the summer and autumn, spanning from May to October, while the dry season lasts for two months without any drought Additionally, spring is characterized by prolonged, light drizzle, and the coastal plain showcases a unique seasonal rhythm, with four distinct seasons throughout the year.
The annual radiation levels range from 95 KCal to 105 KCal/cm², with a total annual heat volume between 8000°C and 8500°C The average annual temperature is 24°C, while monthly temperatures vary from 16.3°C to 20.9°C January experiences the lowest temperature at 6.8°C, whereas summer peaks at a high of 40.1°C.
The rainy season spans from May to October, with annual average rainfall varying between 1500 mm and 1715 mm The highest recorded total rainfall in a year reached 2754 mm, while the lowest was 978 mm.
- Humidity: Average air humidity is 84% Average annual evaporation is 817.4 mm Average monthly evaporation is from 86 ÷ 126 mm/month The highest evaporation is in July
- Wind: In the winter, the prevailing wind direction is from the North, in the summer it is from the East and Southeast Average wind speed is 3 m / s ÷ 4 m / s
The annual weather patterns are characterized by distinct seasonal changes: winter brings cold and dry conditions, spring features cool, drizzly, and humid weather, summer is marked by hot temperatures accompanied by thunderstorms and showers, while autumn experiences cool weather with heavy rain and storms.
Hydrology
XTNP is supplied with water and sediment from the Red River At Ba Lat estuary, there are 2 main rivers: Tra River and Vop River, there are also drainage creeks
The Tra River, stretching approximately 10 km from the Ba Lat estuary to the sea, serves as the boundary between Con Ngan and Con Lu In its downstream section, the river is characterized by alluvial flats, often reducing to creeks during low tide.
The Vop River begins at Ba Lat and flows into the sea, with its downstream section marking the boundary that separates the Zone from the surrounding areas to the north and northwest.
The average alluvium concentration at the Ba Lat estuary is approximately 1.8 g/l, which serves as the primary source for sedimentation and the territorial expansion of the Zone.
The Vop River features a short tidal creek that divides Con Lu and Con Xanh, flowing from the Ba Lat estuary to the sea.
Tidal
The Hon Dau tidal system significantly influences the planting area, characterized by a diurnal tide cycle of approximately 25 hours and two semi-diurnal tide periods lasting 5-7 days each month The tidal range varies from 3 to 4 meters, with a moderate daily amplitude averaging between 150 and 180 cm The highest tide recorded is 3.3 meters, while the lowest is 0.25 meters This diurnal tidal pattern has contributed to the healthy maintenance and development of the mangrove forest.
Tidal variations typically feature one high tide and one low tide approximately every half month, with instances of three low tides and two high tides occurring within a month The tidal amplitude is notably higher during the dry season, particularly from December of the previous year to February of the following year.
Salinity
Seawater salinity in the study area fluctuates significantly between 9% and 26% throughout the year, influenced by seasonal changes and the hydrological conditions of the Red River Survey data indicates that the peak salinity occurs in January, while the lowest monthly average is recorded in August at 9%.
Figure 4.3 Diagram of average salinity of each month in XTNP, 2020
Mangrove forests status
Mangroves thrive in flat, gently sloping marshlands and coastal estuaries, where they are protected by numerous islands from storm impacts Each species of mangrove tree is uniquely adapted to varying elevations and terrain Key distributions of these flora communities are evident in these regions.
The community comprising A corniculatum, S caseolaris, A marina, and Acanthus ilicifolius exhibits natural regeneration, predominantly in the upper layer with A corniculatum The lower layer is characterized by Acanthus ilicifolius and Cyperus malaccensis Additionally, certain areas within this community show development of A corniculatum and S caseolaris.
The commune of K obovata and A corniculatum features a mixed structure divided into three distinct layers The uppermost layer consists of cork trees, which rise 8-10 meters high and are scattered along the forest's edge The tree layer serves as the primary ecological dominant, comprising K obovata and A corniculatum A marina grows sporadically between these two species, while in highland areas, Derris trifoliate climbs to the canopy's top, leaning against other trees.
On some sandy land of Con Lu islet towards the sea, there are populations of
A marina that grow naturally and branch early There are also a very small number of individuals R stylosa, B gymnorrhiza, S caseolaris [20, 21, 22]
* General discussions related to natural conditions:
The study area experiences significant variations in tidal volume, which positively impacts mangrove growth To enhance the effectiveness of afforestation efforts, it is essential to analyze the tidal regime and implement efficient construction methods.
The study area in the Red River estuary experiences lower seawater salinity compared to neighboring regions due to the influx of fresh water from the Red River This reduced salinity creates a favorable environment for the growth of brackish mangrove species, such as Soneratia caseolaris and Nypa fruticans, particularly in the central and southern regions However, during the winter months, from December to March, seawater salinity tends to rise as fresh water flow decreases, which can hinder the establishment of newly planted mangroves by increasing the risk of barnacle attachment, ultimately affecting their survival rates.
RESULTS AND DISCUSSION
Mangrove forest status in Xuan Thuy National park
5.1.1 Mangrove area in Xuan Thuy National park
According to the inventory results, the total area of mangrove forests in XTNP, Nam Dinh Province is 1,641.52 ha in 2020, including:
- The area of special-use forest is 1,056.68 ha, of which the forested land area is 1,017.48 ha and the vacant land area is 39.20 ha
The mangrove forest buffer zone spans an area of 584.84 hectares across four communes: Giao Thien, Giao Lac, Giao Xuan, and Giao An Within this zone, the forested land covers 582.72 hectares, while the unforested land accounts for 2.12 hectares.
Table 5.1 Current status of mangrove forest area in XTNP
No Content Total Non-forest cover area (ha)
5.1.2 Distribution characteristics of mangrove forest in XTNP
The results of the actual investigation show that mangrove forests are mainly distributed in the core zone of XTNP These include the main communities as follow:
+ The community A corniculata - S caseolaris - K obovata - A marina
This community is distributed in the north of the National Park and grows alternatingly and divided into 3 distinct layers The overstory layer is S caseolaris
The plant species, which grows to a height of 1.8 to 7 meters, is spreading along the borderline and emerging from the forest canopy It is expected to become the dominant species in the National Park's ecosystem in the future This species represents the main ecological layer, highlighting its significance in the park's biodiversity.
The article discusses two tree species, Aegiceras corniculata and Kandelia obovata, with A marina also present, thriving in the canopy due to its competitive nature and tidal dispersal Additionally, Derris trifoliata grows on other trees, often overshadowing them in the canopy layer.
The A corniculata - K obovata - S caseolaris community is found in the Con Lu area, located north of XTNP, with an average plant height ranging from 2 to 7 meters A corniculata and K obovata dominate through natural regeneration, while S caseolaris occupies a limited area Additionally, scattered B gymnorhiza trees are present within this community.
Figure 5.1 The community Aegiceras corniculata - K obovata – S caseolaris at
(Source: Tran Van Sang, 2022) + The community K obovata - R stylosa - S caseolaris This community is distributed in Con Lu islet, north of XTNP The average height of the plants is 2.18
- 7.09m The K obovata dominates mainly, while R stylosa and S caseolaris occupy a small area
Figure 5.2 The community of K obovata - R stylosa - S caseolaris XTNP
Despite the limited species diversity in tidal mudflat communities, which primarily include A corniculata, K obovata, S caseolaris, A marina, R stylosa, and B gymnorhiza, these pioneer species are crucial for the protection and stabilization of their ecosystems Consequently, it is essential to focus on the conservation and expansion of mangrove species like A marina and R stylosa to ensure the sustainability of these vital habitats.
Figure 5.3 The community of Aegiceras corniculata - S caseolaris in XTNP
Current status of some mangrove planting models in XTNP
5.2.1 The models of K obovate monoculture
The research focuses on the K obovata forest model, initiated by the Danish Red Cross (MRC) in collaboration with the Vietnam Red Cross in 1997, covering a total area of 2,403 hectares, which includes 1,091 hectares of K obovata, 332 hectares of S caseolaris, and 980 hectares of R stylosa Current surveys indicate that only approximately 678 hectares of mature mangroves remain (IFRC, 2010).
The survey results indicate that the project implementation relied on the local Commune Red Cross Society, utilizing straightforward planting methods Afforestation was conducted using sprouts collected from mother trees, which were then planted in alluvial fields in Giao An commune at a density of 10,000 trees per hectare (1m x 1m) The afforestation techniques employed were basic, lacking technical design, protection, and post-planting care (IFRC, 2010; IFRC, 2011) [19, 23, 24].
Figure 5.4 Techniques for applying K obovate monoculture afforestation model in 1998 at XTNP
(Source: IFRC, 2010) 5.2.2 Planting model of K obovata and S caseolaris
The K obovata and S caseolaris forest restoration model was initiated as part of the "Protection and Development of Coastal Protection Forests in Nam Dinh Province (2015-2020)" project, which is aligned with the National Target Program for Climate Change Response (SP-RCC) across Nam Dinh province This initiative focused on a mangrove restoration area of 77.27 hectares located in the XTNP area, specifically within the Giao Thien, Giao An, and Giao Lac communes The forest restoration project commenced in 2016 and included management and protection measures that were implemented over a five-year period, from 2016 to 2020.
The forest restoration project in Giao Thuy district, Nam Dinh province, focuses on native plant species, specifically S caseolari and K obovata, which thrive in estuaries and coastal alluvial regions.
For afforestation, seedlings of S caseolari and K obovata aged 18 to 24 months were utilized, with specific seedling standards outlined in Table 5.2 The planting density was set at 3,333 plants per hectare, employing a mixed species ratio of 1:1 (1.5m x 2m) The project enhanced local planting holes by using collected surface alluvium for filling After planting, the mangrove plants received care and protection for a duration of five years.
Table 5.2 Standards for seedlings of the K obovata and S caseolaris model
No Species Height (m) Root diameter
The post-planting management mechanism involves the Project Management Board of the Department of Agriculture and Rural Development of Nam Dinh province overseeing the project for its initial four years Upon completion, the board will transfer the forest to the forest owner, XTNP, for ongoing management and utilization.
Figure 5.5 Techniques for applying Planting model of K obovata and S caseolaris in 2016 at XTNP
5.2.3 Plantation models of R stylosa and B gymnorhiza
Scale, area and implementation time: The model of planting R stylosa and B gymnorhiza in XTNP is a research project funded by Nam Dinh Department of
The project, covering an area of 3.0 hectares, was developed in the Con Lu region within the core zone of XTNP by the Institute of Ecology and Works Protection (WIP) between 2014 and 2016 It focused on the cultivation of R stylosa and B gymnorhiza species.
2015 and monitored, tended and protected for 1 year after planting
The project focuses on the selection of indigenous mangrove species that thrive in the specific conditions of XTNP, prioritizing two key species for forest restoration: R stylosa and B gymnorrhiza.
The planting techniques involved using 12-month-old seedlings of R stylosa and B gymnorrhiza, nurtured in 20cm x 20cm polyethylene bags A mixed-species afforestation model was implemented at a 1:1 ratio, with a density of 3,300 plants per hectare, maintaining a distance of 1.5 meters between plants and caves The project enhanced local planting holes by cultivating alluvium at the surface and filling it around the plant stump, securing the mangrove trunk with three stakes post-planting Monitoring, tending, and protection of the model were conducted during the first year after planting.
Table 5.3 Standards for seedlings of the R stylosa and B gymnorhiza model
No Species Height (m) Root diameter
Figure 5.6 Techniques for applying Planting model of R stylosa and B gymnorhiza in 2015 at XTNP
After the project's completion in 2016, the implementing unit transferred technology and forest restoration models to XTNP for ongoing management Additionally, training courses were conducted to enhance the understanding of mangrove importance and afforestation techniques among Red Cross members in the buffer zone communes of XTNP.
Table 5.4 Planting technical for planting models at XTNP between 1998 and 2016
No Model Project name Time
K obovata MRC 1998 2,331 25 - Using only propugules
- Seedlings snowed in the polyethylene bag, 12 – 18 months old and using three stakes to keep stable seedlings
Mixed species: R stylosa and B gymnorrhiza
- Seedlings snowed in the polyethylene bag, 12 months old and using one stakes to keep stable seedlings
Between 1998 and 2005, K obovata monoculture afforestation was carried out using straightforward techniques that involved high-density planting of propagules, without any maintenance or replanting efforts.
SP-RCC and WIP projects have successfully carried out forest restoration initiatives through afforestation, utilizing high-quality seedlings nurtured in polyethylene plastic bags To ensure stability against strong winds, they employed 1-3 stakes for each tree trunk The projects included replanting efforts within three years of initial planting and ongoing care for 1-4 years post-planting.
Figure 5.7 Map of the current mangroves planting models in XTNP,
Nam Dinh province between 1998 and 2016
Effectiveness evaluation of mangroves planting models
5.3.1 Survival rate of mangrove planting models
The survival rate indicates how well mangrove species adapt to their environment and assesses the effectiveness of the planting techniques used for their growth.
In addition to environmental factors like temperature and salinity, the survival rate of newly planted mangroves is influenced by external elements such as storms that can damage trees, as well as pests and diseases Furthermore, fishing activities conducted beneath the mangrove canopy and the movement of boats can negatively impact these trees, further diminishing their survival rate.
For mangrove forests, competition for nutrient space also reduces mangrove plants’ survival
Table 5.5 Survival rate of planting models at XTNP Model Plot Species Age
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
RB: The plantation model of R stylosa and B gymnorhiza with ratio 1:1 Sc: The abbreviation of S caseolaris
Ko: The abbreviation of K obovata
Rs: The abbreviation of R stylosa
Bg: The abbreviation of B gymnorhiza
Based on data on survival rates of 3 mangrove planting models in table 5.5, it shows that:
In a mixed-species planting model of K obovata and S caseolaris with a 1:1 ratio, monitoring revealed that after four years, S caseolaris had a survival rate of 74.75%, significantly higher than K obovata's 44.44% The lower survival rate of K obovata can be attributed to the rapid growth of S caseolaris, which competed for nutrients and light, leading to K obovata becoming completely shaded under the canopy of S caseolaris Consequently, some K obovata individuals were unable to adapt to the reduced light conditions and gradually perished.
The survival rate of K obovata at 20 years old was only 50.17%, primarily due to outdated afforestation techniques used in 1998, which involved planting sprouts on alluvial ground This method made the sprouts vulnerable to waves and tides, resulting in significant washout and contributing to the low survival rates.
The 5-year survival rate of the plantation model for R stylosa and B gymnorhiza was the highest among the three models studied, with survival rates of 89.39% for R stylosa and B gymnorhiza.
The survival rates of R stylosa and B gymnorhiza in this afforestation model were comparable, both reaching 94.70% This high survival rate can be attributed to the use of high-quality seedlings in the model.
Seedlings nursed in polyethylene bags for over 12 months exhibit resilience against waves and tides The study area’s conditions are well-suited for the two plant species, Rhizophora stylosa and B gymnorhiza For future afforestation projects, selecting these species is recommended to enhance survival rates.
Figure 5.8 Survival rate of mangrove plants in plantation models
(In which: Sc – S caseolaris, Ko1 - K obovata in the plantation model of K obovata and S caseolaris, Ko2 - K obovata in the plantation model of K obovata,
Rs - R stylosa and Bg - B gymnorhiza)
The results from Figure 5.8 indicate that among the four mangrove species planted across three plantation models, R stylosa and B gymnorhiza exhibited the highest survival rates, while S caseolaris and K obovata had the lowest This finding suggests that when considering afforestation efforts in XTNP, the selection of K obovata should be approached with caution, as previous afforestation models have typically recorded survival rates of only around 50%.
5.3.2 The growth situation of the plantation model evaluation
The growth of forest plants refers to the increase in size and weight of the plants, which includes the development of new organs, cells, and structural elements (Tran Thi Mai Sen, 2005 and 2007) Additionally, the overall growth of a forest community is determined by the cumulative growth of the individual plants within that ecosystem.
In the study area, the growth characteristics of some mangrove species in mangrove plantation models are shown in Table 5.6:
Table 5.6 Growth characteristics of mangrove species in mangrove plantation models
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
RB: The plantation model of R stylosa and B gymnorhiza with ratio 1:1 Sc: The abbreviation of S caseolaris
Ko: The abbreviation of K obovata
Rs: The abbreviation of R stylosa
Bg: The abbreviation of B gymnorhiza
For each model, the growth of species in planted forest models is different, which is reflected in the criteria of base diameter and plant’s height
In the S caseolaris and K obovata community, the average growth metrics for both species across plots 01, 02, and 03 were recorded, with a base diameter of 97.02±18.21 mm and an average height of 732.19±63.46 cm Notably, K obovata exhibited significant variation in growth parameters across the three plots, with a root diameter of 34.67±7.0 mm and an average height that reflects this disparity.
The height of S caseolaris and K obovata in the study area averages 120.83 ± 23.53 cm This difference can be attributed to the plantation model, which employs a 1:1 ratio that does not align with the natural mangrove succession, where S caseolaris and K obovata typically exist in a 1:9 ratio (Chung, 2014) S caseolaris is a fast-growing species with significant biomass, while K obovata grows slowly and has a considerably smaller biomass Consequently, in a 1:1 afforestation design, K obovata struggles to compete for nutrients against the more dominant S caseolaris.
At the time of field study, the research team observed that most of K obovata were under the canopy of S caseolaris and showed signs of stunting and poor growth
In the plantation model of K obovata, the average growth parameters indicate a base diameter of 76.59 mm and a peak height of 185.12 cm At 22 years old, the root diameter is notably small, suggesting that dense planting has led to increased competition for nutrients, resulting in greater height growth but limited root diameter expansion.
In the plantation model of R stylosa and B gymnorhiza, the average growth patterns of these two species exhibited contrasting trends in base diameter and height across the plots Additionally, data on the root diameters of R stylosa and B gymnorhiza were analyzed.
27.46±4.56 mm and 71.56±9.79 mm, respectively However, the data on the peak height of the two species was 106.86±13.02 cm and 93.00±12.08 cm, respectively
The study revealed that despite varying ecological characteristics and research durations among the species, the plantation model of S caseolaris exhibited the largest root diameter and maximum plant height In contrast, the R stylosa and B gymnorhiza plantation model demonstrated greater uniformity in growth parameters, such as base diameter and peak height, compared to the other models Additionally, the K obovata plantation model showed the most imbalance in growth indicators relative to the other two models.
Figure 5.9 Average growth chart of plantation models
Research findings indicated that at 4 years old, the height growth of plant species in the S caseolaris and K obovata plantation model was the highest, while the R stylosa and B gymnorhiza model exhibited the lowest growth However, the differences in root diameters among these plantation models were not significant, ranging from 49.51 mm to 76.59 mm.
5.3.3 Growth quality of mangroves in plantation models
Characteristics of natural regeneration in forest restoration models
In mangrove ecosystems, different species exhibit unique regeneration methods S caseolaris reproduces through seeds, similar to terrestrial forest plants, while K obovata, R stylosa, and B gymnorrhiza regenerate via propagules These propagules, which are seedlings attached to the fruit, germinate immediately after ripening without a resting phase on the parent plant Notably, K obovata, R stylosa, and B gymnorrhiza typically regenerate beneath the canopy of the mother tree, with R stylosa and B gymnorrhiza capable of being planted directly into the ground upon falling if the tide has receded However, no regenerated S caseolaris trees have been observed in the studied plantation models.
Table 5.8 Composition and density of regeneration trees in the plantation models
Model Plot Age No of regeneration tree (tree/ha)
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
RB: The plantation model of R stylosa and B gymnorhiza with ratio 1:1 Sc: The abbreviation of S caseolaris
Ko: The abbreviation of K obovata
Rs: The abbreviation of R stylosa
Bg: The abbreviation of B gymnorhiza
The survey results indicate that in the S caseolaris - K obovata model and the pure-species plantation model of K obovata, only K obovata exhibits natural regeneration at exceptionally high densities, ranging from 2,300 to 12,850 trees per hectare Notably, standard plots located near the sea with soft mud beds show a higher density of regenerated trees compared to those near the shore with solid mud beds This high density of K obovata regeneration is attributed to the soft mud bed, which provides an ideal substrate for propagules from neighboring areas to drift with tidal currents, allowing them to cling and grow into mature trees.
5.4.2 Distribution of regenerated trees according to height
The results of the study on the height distribution of regenerated trees are presented in Table 5.9:
Table 5.9 Height distribution of regenerated trees
Model Plot Age Regeneration level (%)
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
RB: The plantation model of R stylosa and B gymnorhiza with ratio 1:1 Sc: The abbreviation of S caseolaris
Ko: The abbreviation of K obovata
Rs: The abbreviation of R stylosa
Bg: The abbreviation of B gymnorhiza
Table 5.9 highlights a significant disparity in height levels among regenerated trees, with the majority found in height levels I (< 0.5m) and II (0.5-1m), while levels III and IV show almost no presence This trend can be attributed to the rapid regeneration and growth of K obovata and B gymnorrhiza shortly after fruit ripening However, the harsh conditions of cold winters, characterized by low temperatures and tidal flooding during the day followed by drying at night, lead to the demise of these regenerated plants Consequently, field investigations reveal a near absence of regeneration trees at higher height levels.
The regeneration capacity indicates how favorable external environmental conditions are for flowering, fruiting, germination, and the growth and development of seedlings This capacity is assessed using criteria such as density and tree quality Forest conditions significantly influence the formation stage of regeneration trees Consequently, the results obtained can inform measures to enhance regeneration efforts.
For each plantation model with good, medium and bad quality of regenerated trees investigated in plots, there are always changes which is shown in Table 5.10:
Table 5.10 Quality of regenerated tree layers in plantation models
Model Plot Age Regeneration quality (%)
Table 5.10 clearly illustrates the growth conditions of regenerated trees across various plantation models, categorizing them into good, medium, and poor growth quality.
- In the S caseolaris - K obovata plantation model, regenerated trees have 84.13% of the trees with very high growth, 14.7% of average growth and poor quality at a very low rate of 1.17%
- In the pure-species plantation model of K obovata with 100% of the regenerated trees being K obovata, the average rate of good quality is 80.67%, medium quality is 14.96%, and bad quality is 4.37%
In the R stylosa and B gymnorrhiza model, all regenerated trees exhibit high quality; however, the survey revealed a limited number of regenerated plants in the plot, predominantly at a uniform height Notably, only B gymnorrhiza demonstrates natural regeneration beneath the mother tree.
The research indicates that the percentage of high-quality regenerated trees across the three models ranges from 80.67% to 100%, while the proportion of medium and low-quality trees is minimal, between 1.17% and 14.96% The survey revealed that most regenerated trees are found in the S caseolaris - K obovata plantation and the pure-species K obovata model, with K obovata being the only species that naturally regenerates However, the number of regenerated plants capable of maturing into mother plants is very limited, highlighting a low potential for these trees to develop into adult specimens despite the high density of regeneration under the mother tree.
Characteristics of natural regeneration in mangrove planting models
5.5.1 Density of natural regenerated trees in mangrove planting models
Mangrove species exhibit various regeneration methods in wetland environments The S caseolaris species reproduces through seeds, similar to certain plants found in mountain forest ecosystems In contrast, R stylosa, K obovata, and B gymnorrhiza regenerate via propagules, which germinate immediately after ripening without a resting period on the parent plant, resulting in seedlings that remain attached to the fruit.
In a study by Nguyen Hong (1999), it was observed that K obovata and B gymnorrhiza are successfully regenerating beneath the mother tree However, S caseolaris and R stylosa have not yet been identified as regenerated trees in the examined planting models.
Table 5.11 Composition and density of regenerated tree layers in planted forest models
Model Plot Age No of regeneration tree
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
RB: The plantation model of R stylosa and B gymnorhiza with ratio 1:1 The survey results in Table 5.11 showed that in the mixed species model of
K obovata - S caseolaris and in the K obovate monoculture model, only K obovata trees regenerated naturally, the density of regenerated trees is very high at range from 2,300 trees/ha to 12,850 trees/ha The standard plots close to the sea can have a soft mudflat with a higher density of regenerated trees than the standard plots near the shoreline, where a solid mud background is possible The reason why K obovata trees regenerated at such a high density is because the soft mudflat easily forms a substrate to support the K obovata species from nearby areas drifting with the tidal currents to cling to and grow into regeneration trees While the densities of regenerated plants in the mixed species model of R stylosa and B gymnorhiza ranged from 700 to 900 plants/ha The survey results showed that most of the regenerated plants in the model were B gymnorhiza species
Figure 5.11 Naturally regeneration trees in mangroves planting models
(a- mixed species model of K obovata - S caseolaris; b – monoculture model of
K obovate; c – mixed species model of R stylosa - B gymnorhiza)
Source: Tran Van Sang, 2022 5.5.2 Distribution of regeneration trees according to height class
The study results on the height distribution of regeneration trees at planting models are presented in Table 5.12:
Table 5.12 Height distribution of regeneration trees
Model Plot Age Regeneration level (%)
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
The plantation model of R stylosa and B gymnorhiza in a 1:1 ratio reveals significant differences in tree height levels, as indicated in Table 5.12 Most of the plantation models are concentrated in height levels I (< 0.5 m) and II (0.5-1 m), while levels III and IV are nearly absent This phenomenon occurs because B gymnorhiza can regenerate and grow rapidly in the early stages of K obovate species after propagule ripening However, low winter temperatures and tidal flooding during the day, followed by drying at night, lead to the death of all regeneration trees Consequently, field investigations show a lack of regeneration trees at height levels III and IV.
5.5.3 The quality of the regenerative trees
The regenerative capacity of a forest is influenced by external environmental conditions that promote flowering, fruiting, germination, and seedling growth This capacity is assessed through criteria such as tree density and quality The conditions within the forest significantly affect the development of the regeneration tree layer Consequently, the results obtained can inform measures to enhance the regeneration of trees.
For each mangroves planting model, regeneration trees with good, medium and bad quality investigated in plots always dramatically changed in Table 5.13:
Table 5.13 Quality of regeneration trees in mangroves planting models
Model Plot Age Regeneration quality (%)
SK: The plantation model of S caseolaris and K obovata with ratio 1:1 KO: The plantation model of K obovata
The plantation model of R stylosa and B gymnorhiza in a 1:1 ratio demonstrates distinct growth patterns in regeneration trees, categorized into good, medium, and bad quality Table 5.13 clearly illustrates these growth situations, allowing for a comprehensive analysis of the results.
In the mixed species model of S caseolaris and K obovata, the regeneration of trees demonstrated impressive growth rates, with an average of 84.13% classified as very high growth Additionally, 14.7% of the trees exhibited medium growth, while only 1.17% were identified as having poor quality regeneration.
- In the monoculture model of K obovate, with 100% of the regeneration trees was K obovate species, the average rate of good quality was 80.67%, medium quality trees were 14.96%, bad quality trees about 4.37%
In a mixed species model of R stylosa and B gymnorhiza, 100% of the regeneration trees are of good quality However, the survey revealed a limited number of regeneration trees in the plots, predominantly at the same height level (level I) Notably, only B gymnorhiza exhibits naturally regenerating trees beneath the mother tree.
The regeneration trees in the three mangrove planting models exhibited a high percentage of good quality, ranging from 80.67% to 100% In contrast, the percentages of medium and poor-quality regeneration trees were notably low, between 1.17% and 14.96% The survey revealed that the S caseolaris - K obovata plantation model and the K obovata monoculture model primarily featured natural regeneration of K obovata However, the potential for these regeneration trees to develop into mother trees was minimal This indicates that, despite the high density of regeneration trees beneath the mother tree canopy, their ability to mature into adult trees is significantly limited.
Proposing some solutions to improve the efficiency of mangrove restoration
The assessment of vegetation in XTNP has revealed several native mangrove communities that are well adapted to the area's conditions Suitable mangrove species are those that have naturally existed and thrived in the park for an extended period, contributing to the unique mangrove environment of XTNP Careful consideration is necessary when introducing new species, as they can potentially overwhelm native flora, leading to degradation and disruption of the mangrove structure Therefore, thorough research is essential to select appropriate introduced species for ecosystem restoration efforts.
Selecting fast-growing mangrove species with high biomass is crucial for effective afforestation, aiming for a survival rate of at least 50% after four years (MARD, 2016) [1] These species play a vital role in creating carbon sinks, helping to reduce greenhouse gas emissions while addressing the challenges posed by climate change and rising sea levels Additionally, they enhance coastal protection, minimizing damage from storms, winds, and waves.
Mangrove species selected for afforestation must be validated through effective models demonstrating their growth At XTNP, several mangrove species, including S caseolaris, K obovata, R stylosa, and B gymnorhiza, have been utilized for afforestation efforts However, research indicates that K obovata, a native tree species predominantly found in the national park, exhibits poor growth and is susceptible to degradation, pests, and diseases compared to other tree species.
To ensure the success of afforestation projects, it is crucial to select species that are easy to breed and provide healthy seedlings The quality of seedlings directly impacts the effectiveness of mangrove restoration activities; unsuitable or diseased species can lead to project failure Therefore, seedlings must be chosen based on their potential for robust growth, including factors such as stem diameter, height, and disease resistance.
- Mangroves species should be listed in Decision No 1205/QD-BNN- TCLN, 5365/QD-BNN-TCLN and 4147/QD-BNN-TCLN of MARD [1, 2, 3]
- The species should be common native mangroves species good growing or being planted in XTNP and Red River Delta: S caseolaris, K obovata, A corniculatum, R stylosa, A marina and B gymnorrhiza
- Results of selecting suitable species for mangroves restoration in table 5.14 below:
Table 5.14 Table of criteria for selecting species for mangrove restoration project at XTNP
Growth very fast, high biomass, good protecting for coastal area
Implemented successfull at the field, growing and developing well
It is an easy species for breeding at nursery
Listed in the decision of the Ministry of Agriculture and Rural
The number 1 is lowest suitable level
The number 2 is medium suitable level
The number 3 is highest suitable level
Table 5.14 identifies three mangrove species as the most suitable for afforestation: S caseolaris, R stylosa, and B gymnorrhiza S caseolaris thrives in unstable mudflats, while R stylosa is best suited for stable mudflats with medium tidal flooding.
B gymnorrhiza species is suitable in areas with stable alluvial substrates and high tidal flooding
5.6.2 Solutions for restoring mangroves at erosion areas, high wave energy
Planting mangroves requires careful consideration of wave conditions, as waves exceeding 0.4m can adversely affect seedlings by damaging their young roots and leading to mortality To ensure the success of mangrove planting projects, it is essential to construct a soft barrier to mitigate wave energy during the first two years, a critical period when the trees are vulnerable to high waves (Trinh Van Hanh, 2014) [16].
A study by GIZ Vietnam indicates that the optimal wave-reducing structure is a permeable dam design, effective for waves less than 0.4 m and with a lifespan of approximately 2 years This structure can be constructed using local materials, such as bamboo and melaleuca, as noted by Chu Van Cuong and Saron Brown (2012) and GIZ (2014).
The coastline of XTNP, approximately 7.2 km long, is currently experiencing significant erosion due to high waves and strong currents at Con Lu islet (Huong P.T.T., 2018; Loi T.T et al., 2019; ChuqiLong et al., 2021) To address this issue, implementing a soft-wall solution is essential to mitigate wave energy, promote sediment accretion, and support the successful restoration of mangroves (Chu Van Cuong et al., 2015).
In regions where wave heights range from 0.4 m to 0.8 m, constructing wave-reducing fences can effectively mitigate negative impacts on mangrove seedlings Utilizing local materials such as bamboo and meleuca, these soft walls not only diminish wave energy but also prevent coastal erosion and promote alluvial formation The design of the soft walls—whether in single or double rows, or even taller configurations—should be tailored to the geological characteristics and wave regime of the project area, in accordance with TCVN 10405: 2020.
Figure 5.12 The permeable dam model used bamboo material for reducing high wave and preventing erosion the Red River Delta
Source: Tran Van Sang, 2022 5.6.3 Technical solutions to improve the effective of mangrove restoration in XTNP
The study on the current status of mangroves at XTNP revealed the classification of planting conditions in the area The research team proposed solutions for selecting species and silviculture techniques tailored to three distinct site conditions: highly suitable, suitable, and difficult natural conditions.
Table 5.15 Proposing technical solutions for mangrove restoration
The ideal planting conditions are found in the stabilized mudflat areas of the estuary and alluvial flats within mangrove ecosystems These mangroves typically have an elevation ranging from 0.0m to +0.2m, with sand content below 50% Additionally, the drying time of the mudflats varies between 5 to 8 hours per day.
2 Seedling type Seedlings breeding in polyethylene bag
5 Planting time From June to September
6 Planting method Mixed species R stylosa - B gymnorrhiza or S caseolaris monoculture
8 Technical support Pluging one stake to keep the stability of the seedling
Suitable planting condition group: The mangrove belt area has a width of about
30m -100 m, the sand percentage from 50%-70%, the elevation from - 0.2 m to 0.0 m, the time drying mudflat time 8h to 10h/day
2 Seedling type Seedlings breeding in polyethylene bag
Seedling old: ≥ 12 months Doo ≥ 0.8 cm Hvn ≥ 0.6 m
Seedling old: ≥ 12 months Doo ≥ 1.0 cm Hvn ≥ 0.8 m
5 Planting time From June to September
8 Technical support Pluging one stake to keep the stability of the seedling
The difficult planting conditions are characterized by areas experiencing significant coastal erosion, where surface mudflat erosion is prevalent In these regions, the mangrove belt remains sparse, with sand composition exceeding 70% Additionally, the mudflat elevation is below -0.2 meters, and the drying time is limited to less than 4 hours per day.
2 Seedling type Seedlings breeding in polyethylene bag
Seedling old: ≥ 12 months Doo ≥ 1.5 cm Hvn ≥ 1.2 m
5 Planting time From June to September
8 Technical support Pluging three stakes to keep the stability of the seedling
For this mudflat type, it is necessary to apply the solution of soft wall to reduce high waves, and prevent erosion, reduce flow to keep sedimentation and stabilize alluvial
5.6.4 Solutions for planning the development of mangroves forest in XTNP
The afforestation sites have been categorized into three groups based on their natural conditions: high suitability, suitable, and difficult Utilizing field investigation results and mapping software, we have compiled the area of these site groups, as detailed in Table 5.16 below.
Table 5.16 Area of nautural condition sites for mangrove restoration
1 High suitable natural conditions (ha) 59,8 263,5 120,9 444,2
According to Tran Van Sang (2022), the XTNP region has a total mudflat area of approximately 553.0 hectares, indicating significant potential for mangrove restoration This area includes 108.8 hectares within the core zone and 444.2 hectares in the surrounding buffer zone Field investigations have categorized the mudflat area based on various natural conditions.
- The area of mangroves development planning normally using techniques (distributed at buffer zone communes: Giao An, Giao Lac and Giao Xuan commune) is 444.2 ha;
The mangrove restoration planning area in Giao Thien commune spans approximately 108.8 hectares and will implement support solutions like permeable dams and bamboo fences This region is characterized by eroded mudflats, high sandy conditions, low elevation, and exposure to strong waves To ensure the success of mangrove planting activities, it is essential to utilize these solutions to reduce wave impact, enhance alluvial conditions, and maintain high seedling standards.
Figure 5.13 The map of mangroves planning restoration solution at XTNP