Primary succession of mangrove forest in con ong trang, mui Ca Mau biosphere reserve

s. The mangrove at the South Coast has richer species composition than other zones. 1.2.3. The impact of ecological factors to distribution of mangrove plants The mangrove plant distribution depended on various ecological factors including rainfall, temperature, tide, salinity and soil, The number of individuals, number of species and tree size were affected by temperature and precipitation (Phan Nguyen Hong and Hoang Thi San, 1993). Tidal inundation was the main factor affecting the mangrove plant distribution (Giesen et al., 2007). Mangrove forests might occur on variety of sediments, mud, coral reefs; however, they grew best on muddy soil. The appropriate salinity for mangrove plants was about 5 – 30‰ (Lacerda et al., 2001). 1.2.4. Types of mangrove plant distribution Mangrove plants had two main form of distribution depending on structure of mangrove community. They are random distribution and aggregate distribution. 1.3. Carbon accumulation of mangrove forest Mangroves were one of the most carbon-rich forest types in the tropics. Carbon accumulation of mangrove forest depended on specie composition, density, age and height classes, topography. In general, above ground carbon pools accounted for about 40 – 45% of total carbon stock and in the root was 30 -75% -5- 1.4. Mangrove succession 1.4.1. Ecological succession 1.4.1.1. Concept: The view of succession began with Clements’ concept (1961). According to Clements, succession of plants was an indeterminate chain of change. 1.4.1.2. The causes of succession Ecological succession was a process that caused by many reason in which there were some main causes: Changes of natural condition impact on community in ecosystem; Living activities of community; Interation among species in community 1.4.1.3. Classification of succession Succession was classified into primary succession, secondary succession and decomposition succession. 1.4.2. Mangrove succession 1.4.2.1. Theory of mangrove succession The mangrove succession was a unique example of the primary succession of tropical coastal vegetation, because this ecosystem changed rapidly over time and in the space of material cycle. 1.4.2.2. Studies in the world Chen and Twilley (1998) suggested the model of effects of salinity and soil nutrient content on distribution of three mangrove plant species. Ball (1980) studied secondary succession of mangrove forest in the Southern Florida. 1.4.2.3. Studies in Vietnam Phan Nguyen Hong and Hoang Thi San (1993) built succession diagram and identified typical species for each stage in various region. Based on the soil conditions, Phung Trung Ngan and Chau Quang Hien (1987) stated the primary succession types of mangrove forests -6- in Mui Chua (Quang Ninh province), CuaDai, Cua Ham Luong (Ben Tre province), the coast of Ho Chi Minh city and Ca mau province (Minh Hai). According to Thai van Trung (1998), the mangrove succession had three stages with various species composition. The recent studies showed the replacement of communities following the changes of environmental and soil conditions in Ca Mau province. Many authors described the succession mainly based on monitoring the current distribution status of mangroves and environment status. However, the relationship between species composition and environmental factors have not quantitative analyzed. Therefore, it is essential to have quantitative analysis of succession over the time and space in the relationship with environmental condition in mangrove forests. 1.5. Research results on mangrove forest in Con Ong Trang Con Ong Trang was a region formed according to natural process without human effects. Therefore, the mangrove ecosystem on the dunes of Ong Trang estuary was selected for many studies in recent times. There were several authors have published their works relating to the investigating species composition of Con Trong or Con Ngoai (Ong Trang estuary, Ca Mau province), studying factors impacting the vegetation, learning the principle of forest structure in this area. These studies were carried out in the recent times from 2007 to 2014 and study area was only Con Trong or Con Ngoai instead of for all dunes of Ong Trang estuary (belonging to Mui Ca Mau Biosphere Reserve). There was also not have any research on primary succession in Con Ong Trang. -7- CHAPTER 2. STUDY AREA AND METHODS 2.1. Study area The study area included three sand dunes namely Con Trong, Con Ngoai and Con Moi with various forming time. These dunes were located at Ong Trang estuary, Ngoc Hien district, Ca Mau province). In the thesis, this area was jointly referred to Con Ong Trang. 2.3. Methods 2.3.1. Methodology: The methodology of this thesis based on: The view of Thai Van Trung (1998) and methodology of space-for-time substitution (Walker and Moral, 2003). 2.3.2. The area of regions formed in different time periods in mangrove forest in Con Ong Trang: Con Ong Trang were divided into areas forming in different periods of time (Figure 2.4; Table 2.3). 2.3.3. Method of investigating quadrats: In each quadrat, growth indicators of tree were determined including the scientific name and morphological characteristics of species; the number of individuals of each species, diameter at breast height and tree height. Table 2.3. Number of quadrats in areas of Con Ong Trang Formation periods Con Trong Con Ngoai Con Moi Areas No. quadrats Areas No. quadrats Areas No. quadrats Before 1962 V_CT 4 1962-1979 IV_CT 12 1979-1992 III_CT 11 III_CN 20 1992-2004 II_CT 5 II_CN 5 2004-2016 I_CT 11 I_CN 6 I_CM 18 Total 43 31 18 2.3.4. Method of species identification: Indentifying scientific name of species according to morphological comparison method based on main documents including Cay co Viet Nam (Pham Hoang Ho, 1999- 2003), Rung ngap man Viet Nam (Phan Nguyen Hong et al., 1993), -8- Mangroves of the Kien Giang biosphere reserve Vietnam (Norm Duke, 2012) 2.3.5. Methods of environmental parameters survey, soil collection and soil characteristics analyse a. Classification of soil according to Decision 5365/QD-BNN- TCLN b. Determination of the tidal regime c. Soil sampling: Collecting soil samples in two soil layers: 0- 20 cm and 20-60 cm. d. Measurement of salinity and pH e. Soil samples analysis with several parameters including content (%) of nitrogen, phosphorus, potassium, content of carbon and texture in both soil layers. 2.3.6. Data analysis 2.3.6.1. Softwares used in data analysis included Microsoft Excel 2016, Statgraphics Centurion XVI., Biodiversity Pro 2.0, Primer 6.1.6 2.3.6.2. The formula for calculation a. Importance Value Index (IVI): IVI = (RN + RF + RBA)/3 b. Biodiversity indices included the species abundance, the evenness index, the diversity Shannon Wiener’s index and the Simpson’s dominance index were calculated using PRIMER 6.1.6 software. c. Biomass and forest stock, carbon storage Biomass: AGB = 0,251 x ρ x Dbh2,46 (kg); BGB = 0,199 x ρ0,899 x Dbh2,22 (kg) Carbon in aboveground biomass: CAGB = 0,47 x AGB Carbon in belowground biomass:CBGB = 0,39 x BGB -9- CHAPTER 3. RESULTS AND DISCUSSION 3.1. The formation of dunes at Ong Trang estuary over time According to the results of analysis and processing of maps and satellite images, the area of Con Ong Trang at the time of the survey (2016) was determined to be 284.21 ha, of which Con Trong was 165.15 ha, Con Ngoai was 84.9 ha and Con Moi was 34.16 ha. The area of these dunes in periods were calculated, the results were shown in Table 3.1. In general, the accretion was mainly in the Northwest direction. In different time periods, the area of these dunes had increased trend, but the rate of increase varried due to the accretion (or erosion) in the region. 3.2. Primary succession in Con Trong 3.2.1. Tidal and soil characteristics in Con Trong 3.2.1.1. Tidal inundation in Con Trong: 20% of the area was frequently flooded with tides, 40% inundated by low tides while 40% inundated by medium tides. 3.2.1.2. Soil characteristics in Con Trong: soil was classified according to texture as well as value of pH, salinity, content of N, P, K of two soil layers were determined. 3.2.2. Plant diversity and forest structure in Con Trong 3.2.2.1. Mangrove plant species composition in Con Trong The study identified 10 true mangrove species belonging to 4 families in Con Trong mangrove. In which several species occurred outside the quadrats including A. ilicifolius, X. moluccensis and R. mucronata. In addition, a number of regenerated individuals of C. zippeliana were also found in quadrats. -10- 3.2.2.2. The importance value indices of mangrove plants in Con Trong The two dominant species in Con Trong were R. apiculata and A. alba. The IVI of R. apiculata was the highest (IVI = 54.98%); followed by A. alba (IVI = 29.88%). 3.2.3. Mangrove plant distribution in Con Trong Aggregation was the main distribution types of mangrove plants in Con Trong (Table 3.8). At the similarity of 40%, A. alba and R. apiculata were belong to one group. At the 20% level, three species including A. alba, R. apiculata and B. parviflora were in a group. 3.2.4. Effects of tides and soil charactersistics on mangrove plants distribution in Con Trong 3.2.4.1. Effects of tides on the mangrove plant distribution in Con Trong: the results of MDS analysis identified the distribution of dominant species according to various types of tidal inundation Fig. 3.5. 3.2.4.2. Effects of soil types on the mangrove plant distribution in Con Trong: The distribution of A. alba and R. apiculata were shown in Fig. 3.6. 3.2.4.3. Effects of soil characteristics on the mangrove plant distribution in Con Trong The results of PCA analysis indicated that community in Con Trong was divided into three groups. With the increasing of salinity but decreasing of pH and potassium content, the number of R. apiculata increased. While mixed community (R. apiculata - A. alba) was effected by of phosphorus content as well as salinity. 3.2.5. The alteration of plant species composition in Con Trong The IVI of A. alba in newly form areas (2004 – 2016) was the highest. After that the value declined in following stage of mangrove -11- growth. In contrast, IVI of R. apiculata made an increase with forest age, from 27,23% (2004-2016) up to 79,35% (1962-1979). 3.2.6. The succession of forest structure in Con Trong The density of mangrove trees in Con Trong gradually dropped from beginning to the stable mature stage; and the rate of decrease differed in various stages. Meanwhile, average of tree diameter and height rised and there were statistically significant differences among areas. 3.2.7. Carbon accumulation in stages in Con Trong 3.2.7.1. Biomass and carbon stock in Con Trong The average biomass at the time of survey was 423.43 ± 157.76 t ha-1. The average carbon in aboveground biomass was 140.60 ± 53.78 t ha-1; in belowground biomass was 48.47 ± 16.92 t ha-1. 3.2.7.2. Carbon accumulation in stages in Con Trong The carbon accumulation increased with forest age from beginning until the age of 37-54 but went down in following stage. This was due to the decrease of individual density in community. 3.2.8. Primary succession diagram in Con Trong Primary succession in Con Trong had three stages (Figure 3.9). Stage 1 was early stage of succession taking place during first 12 years with several typical environmental characteristics. Most area was frequently flooded tides, soil was mainly fluid-mud, pH was neutral (6.12 ± 0.75), salinity was about 31.68 ± 5.5‰. A. alba was the pioneer species that can spread and developed. Subsequently, R. apiculata grew mixing with A. alba. Nevertheless, A. alba was the absolute dominant species in this stage. In the stage 2 (from 12 to 54 years), the topographic elevation gradually raised, along with the changes in tidal inundation and soil -12- characteristic created favaroble condition for the growth of R. apiculata. Therefore, R. apiculata formed a mixed comminity with A. alba and gradually dominates at the end of this period. Environmental features of area in stage 3 were medium tidal inundation and soft clay ground. In this stage, R. apiculata together with B. parviflora formed a mixed community; there were also many individuals of A. officinalis. 3.3. Primary succession in Con Ngoai 3.3.1. Tidal and soil characteristics in Con Ngoai 3.3.1.1. Tidal inundation in Con Ngoai: 45% of total quadrats was frequently flooded with tides. 3.3.1.2. Soil characteristics in Con Ngoai: Soil in Con Ngoai had 4 ground types and in term of texture, 96% of area was clay. 3.3.2. Plant diversity and forest structure in Con Ngoai 3.3.2.1. Mangrove plant species composition in Con Ngoai There were 8 true mangrove species belonging to 3 families in Con Ngoai mangrove forest. In which several individuals of N. fruticans appeared outside the quadrats. 3.3.2.2. The importance value indices of mangrove plants in Con Ngoai The IVI value of R. apiculata and A. alba did not significantly differ, respectively 48.14% and 40.19%; there was no completely dominant species. 3.3.3. Mangrove plant distribution in Con Ngoai Mangrove plants in Con Ngoai had two distribution types including aggregated and random distribution (Table 3.20). The topographic elevation and tidal inundation were relatively different -13- among transects. Therefore, many groups of plants formed in the corresponding environmental conditions. 3.3.4. Effects of tides and soil charactersistics on mangrove plants distribution in Con Trong 3.3.4.1. Effects of tides on the mangrove plant distribution in Con Ngoai: MDS analysis showed the distribution of A. alba, R. apiculata, B. parviflora in various types of tidal inundation. 3.3.4.2. Effects of soil types on the mangrove plant distribution in Con Ngoai: A. alba mainly appeared in fluid-mud and R. apiculata distributed in mud and soft clay. 3.3.4.3. Effects of soil characteristics on the mangrove plant distribution in Con Ngoai Mangrove plant communities in Con Ngoai was divided into 3 groups. In which mixed community (R. apiculata-A. alba) grew in high soil pH areas. The number individual of A. alba was direct ratio to potassium content. 3.3.5. The alteration of plant species composition in Con Ngoai There was an alteration of species composition in Con Ngoai mangrove. While S. alba was only present in the early stage of 1992- 2016, A. officinalis and C. zippeliana appeared later, until the period 1979-1992; B. parviflora and B. cylindrica are also found in 1992- 2004 period. Althouh A. alba and R. apiculata are both present in growing stages of the forest, IVI value of these two species had various increase or decrease. 3.3.6. The succession of forest structure in Con Ngoai The density in areas of Con Ngoai were not statistically significant different. During the period 1979-1992, the mangrove tree had a significant increase in diameter and height. -14- 3.3.7. Carbon accumulation in stages in Con Ngoai 3.3.7.1. Biomass and carbon stock in Con Ngoai The average carbon in aboveground biomass was 114.57 ± 44.23 t ha-1; in belowground biomass was 40.34 ± 14.50 t ha-1. 3.3.7.2. Carbon accumulation in stages in Con Ngoai The carbon accumulation was not statistically significant difference between the first stage and the second stage; carbon stock made a strong increase in the third stage (180.45 ± 52.64 t ha-1). 3.3.8. Primary succession diagram in Con Ngoai Primary succession in Con Ngoai was presented as following: Stage 1 took place during early 24 years. In this stage, area was frequently flooded tides with fluid-mud, soil pH was quite low and average salinity was 35.59 ± 3.83 ‰. A. alba was dominant species and there were also individuals of S. alba scattered in the population of A. alba. In the end of the stage, R. apiculata together with A. alba formed A. alba – R. apiculata community. In the second stage, forest age was over 24 years old, environmental parameters and soil features clearly changed. Corresponding to the changes of environment, R. apiculata became dominant species instead of A. alba; these species formed mixed community. 3.4. Primary succession in Con Moi 3.4.1. Tidal and soil characteristics in Con Moi The fluid-mud type was not found in Con Moi proving that soil was rather tight. The proportion of soft clay in Con Moi was 50%. -15- 3.4.2. Plant diversity and forest structure in Con Moi 3.4.2.1. Mangrove plant species composition in Con Moi There were 6 true mangrove species belonging to 2 families in Con Moi mangrove forest. In which several individuals of N A. ilicifolius appeared outside the quadrats. 3.4.2.2. Plant diversity in Con Moi The IVI of A. alba was the highest with 75,68% while other species had very low IVI indicating that A. alba was absolutely dominant in Con Moi. 3.4.2.3. Mangrove forest structure in Con Moi The average density was 3228 ± 696 tree ha-1, the average diameter and height were respectively 7.6 ± 4.1 cm and 7.4 ± 2.7 m. 3.4.3. Mangrove plant distribution in Con Moi The distribution of species namely A. alba, R. apiculata and B. parviflora were mainly cluster. At the similarity of 40%, R. apiculata and B. parviflora always distributed together while the rest do not form any group. 3.4.4. Effects of tides and soil charactersistics on mangrove plants distribution in Con Moi 3.4.4.1. Effects of tides on the mangrove plant distribution in Con Moi: The distribution of dominant species according to tidal inundation were analyzed and presented in Figure 3.21. 3.4.4.2. Effects of soil types on the mangrove plant distribution in Con Moi: While A. alba could distribute in variety types of soil, R. apiculata mainly presented in clay soil types. 3.4.4.3. Effects of soil characteristics on the mangrove plant distribution in Con Moi -16- The results of PCA analysis indicated that A. alba adapted to high salinity and low pH. In contrast, B. parviflora and B. cylindrica distributed in areas with low salinity and high pH value. 3.4.5. Carbon accumulation in stages in Con Moi The average carbon in aboveground biomass was 60.33 ± 19.76 t ha-1, accounting for a large proportion of 73%. The average carbon in belowground biomass was 90.19 ± 19.03 t ha-1. 3.5. Primary succession in Con Ong Trang The three dunes formed in Ong Trang estuary including Con Trong, Con Ngoai and Con Moi were named as Con Ong Trang. 3.5.1. Tidal and soil characteristics in Con Ong Trang 3.5.1.1. Topographic elevation of Con Ong Trang Topographic elevation fluctuated from 0.6 to 1.1 m. 3.5.1.2. Tidal inundation in Con Ong Trang The results showed that 34% of the area was frequently flooded with tides, 35% inundated by low tides. 3.5.1.3. Soil characteristics in Con Ong Trang a. Soil types in Con Ong Trang The solid mud and soft clay had a large proportion in the study area and the ratio was different among the dunes. b. Soil texture in Con Ong Trang The percentage of clay in soil was 39.6 – 59.3%, higher than ratio of silt or sand. c. Soil characteristics in Con Ong Trang Soil pH value measured in this area was in the range of neutral. The average salinity was 34.85 ± 6.42‰. The average content of nitrogen, phosphorus and potassium between the two soil layers were not statistically significant different. -17- 3.5.2. Plant diversity and forest structure in Con Ong Trang 3.5.2.1. Mangrove plant species composition in Con Ong Trang Mangrove forest in Con Ong Trang had 11 true mangrove species belonging to 5 familias. Of which, there were 4 species found outside the quadrats. The study indicated that a large number of individuals (52.04% of total individuals) belonged to Rhizophoraceae, followed by Acanthaceae (46.82%). 3.5.2.2. The importance value indices of mangrove plants in Con Ong Trang The two dominant species in Con Ong Trang were A. alba and R. apiculata. The IVI value of these species were 44.07% and 42.73, respectively. 3.5.2.3. Diversity indices in Con Ong Trang The Shannon-Wiener diversity indices varied from 0.38 to 0.6 indicating that the mangrove forests in the study area have low diversity. The Simpson’s dominance indices ranged from 0.64 to 0.78 proving that there were many dominant species in the communities. 3.5.2.4. Mangrove forest structure in Con Ong Trang The average density of individuals in Con Ong Trang was 2746 ± 887 tree ha-1; however, the tree densities were different among the dunes. The curve of N-Dbh distribution had left deviation and number of individuals was generally in inverse ratio to diameter classes. The frequency distribution of height was also various in the study area. 3.5.3. Mangrove plant distribution in Con Ong Trang 3.5.3.1. Types of distribution of plant species in Con Ong Trang Species in Con Ong Trang had two distribution types including random and aggregated distribution. -18- 3.5.3.2. Distribution of mangrove species in communities of Con Ong Trang The result of similarity coefficient analysis showed that this coefficient of A. alba and R. apiculata was the highest. At the similarity of 40%, there was only a group that included R. apiculata and A. alba; At the similarity of 20%, the species of the mangrove forest in Con Ong Trang were divided into 3 groups, only S. alba did not belong to any group. 3.5.4. Effects of tides and soil charactersistics on mangrove plants distribution in Con Ong Trang 3.5.4.1. Effects of tides on the mangrove plant distribution in Con Ong Trang The distribution of species regarding to tidal inundation was presented as following: A. alba mainly mostly distributed in areas with frequent flooding and low tide flooding; R. apiculata adapted to areas with low and medium tidal inundation; B. parviflora mainly distributed in areas flooded by medium tides. 3.5.4.2. Effects of soil types on the mangrove plant distribution in Con Ong Trang A. alba adapted to mud soil; R. apiculata mainly distributed where ground types were solid mud and soft clay. Soft clay was also appropriate to the growth of B. parviflora. 3.5.4.3. Effects of soil characteristics on the mangrove plant distribution in Con Ong Trang The results of PCA analysis indicated that mixed community of A. alba and R. apiculata was dominant where soil have high salinity, low pH value and low content of nitrogen. In contrast, with lower -19- salinity as well as higher pH value and %N, mixed community of R. apiculata and A. alba developed favourably. 3.5.5. The alteration of plant species composition in Con Ong Trang There was an alteration of species composition in Con Ngoai mangrove over time. The pioneer species, A. alba, was gradually replaced by R. apiculata and B. parviflora. Many of mangrove species also appeared and disappeared in the proccess of succession of the mangrove forest. From data of three dunes, the research identified IVI value of species in 5 stages with various time periods (Table 3.44). 3.5.6. The succession of forest structure in Con Ong Trang The density of individuals tended to decline from the first stage to later stages; Meanwhile, the average diameter and height made an increase over time. The frequency distribution of diameter and height classes in each stage were also calculated. 3.5.7. Carbon accumulation in stages in Con Ong Trang 3.5.7.1. Biomass and carbon stock in Con Ong Trang a. Biomass of mangrove forest in Con Ong Trang The total biomas in the study area ranged from 186.99 t ha-1 to 423.43 t ha -1. Belowground biomass was less than aboveground biomass. In addition, aboveground biomass in the dunes was different. b. Carbon in biomass of mangrove plants: Carbon in aboveground biomass was 73 – 74% of total carbon in biomass. The amount of carbon in biomass also differed among study areas. c. Carbon accumulation in soil until 60 cm depth Soil carbon was 41.292 ± 12.14 t ha-1 in soil layer 0 – 20 cm and 80.978 ± 26.31 t ha-1 in soil layer 20 – 60 cm. -20- d. Carbon stock in carbon pools Carbon stock in carbon pools was high in areas that formed earlier; the trend of increase and decrease was uneven and different depending on various pools (Figure 3.42). Carbon stock in biomass rose faster than soil carbon. 3.5.7.2. Car