Study on stand structure of secondary mangrove forest; Sonneratia caseolaris Aegicerus corniculatum stand for introducing silvofishery systems to shrimp culture ponds45241

Study on stand structure of secondary mangrove forest; Sonneratia caseolaris - Aegicerus corniculatum stand for introducing silvofishery systems to shrimp-culture ponds Kazuya Takahash

Study on stand structure of secondary mangrove

forest; Sonneratia caseolaris - Aegicerus corniculatum

stand for introducing silvofishery systems to shrimp-culture ponds

Kazuya Takahashi (1), (*) , Tran Thi Tuyen (1) , Nguyen Trong Linh (1) , Sam Van Tu (1)

(1) School of Agriculture and Natural Resources, Vinh University, Nghe An, Vietnam

* Correspondence: kazu.takahashi.63@gmail.com

Abstract: Secondary mangrove stand; Sonneratia caseolaris - Aegicerus corniculatum in the canal

network adjacent to the shrimp-culture ponds at the Lam River estuary was surveyed Main objective

of this study is to elucidate the relationship between species distribution and flooded duration by brackish water to apply for designing of silvofishery systems to the shrimp-culture ponds Land height from datum line and mean inundated duration range from 1.1 m to 1.9 m and from 9.0 hours day-1 to 19.6 hours day-1, respectively The stand mainly contains three or more herbaceous species

(associate species; Cyperus malaccensis, Acanthus spp and one non-mangrove species; Phragmites

australis) other than two woody species Zones of C malaccensis and Acanthus spp distributing are

overlapped in height and mean inundated duration, ranging from 1.1 m to 1.4 m, from 16.7 hours day-1 to 19.6 hours day-1 and from 1.2 m to 1.4 m, from 16.7 hours day-1 to 18.7 hours day-1, respectively

Since it is reported that Acanthus has more potential to retain nutrients than woody mangrove species, and C malaccensis called mat grass is an economic valued plant, mangrove buffers with herbaceous

species to silvofishery systems in this area are recommended

Keywords: Silvofishery system; mangrove forest; herbaceous species

1 Introduction

Study site named Hung Hoa commune, Vinh city, Nghe An province is located at the estuary of the Lam River, flowing through North-Central Vietnam and shrimp culture

is one of the main industries in this area With rapid expansion of shrimp-culture ponds in the beginning of the 2000s, lagoon with mangrove forests had been decreased, which degraded regional ecosystem services From 1982 to 2000 ca 47,000 ha of salted area had been reduced especially due to changing to shrimp-culture ponds, and ca 1,000 ha of shrimp-culture ponds exist in Nghe An province (as of 2001), accounting for 0.4% of total shrimp-culture ponds in Vietnam (Phuong 2014a) Considering this matter, the central government of Vietnam has launched strategy of Green Economic Development in the Lam River watershed including the estuary (Vietnam socio-economic development strategy 2011

- 2020), however few concrete measures have been conducted

Silvofishery systems in the shrimp-culture ponds, i.e mixture of mangrove silviculture and shrimp- culture, have recently been adopted in many regions e.g Ngoc Hien district, Ca Mau province at Mekong Delta (Clough et al 2002), Semarang city, Indonesia (Hatsui and Budihastuti 2017) and it is evaluated as a community-based shrimp-culture model contributing both for regional economy and mangrove conservation (Syaiful

et al 2016).Moreover, mangrove forests provide several ecological services acting as filters for nutrients and sediments, carbon sink (e.g DOEE, Australian Government 2016, Mitra

2020a), thus silvofishery systems with mangrove buffers are expected to contribute environmental restoration in the regions

For introducing silvofishery systems to the shrimp-culture ponds in Hung Hoa commune, understanding of mangrove stand structure in this region is essential to design the mangrove buffer and to determine water operation Along the Lam River estuary

Sonneratia caseolaris is mainly dominated with Acanthus spp and Cyperus malaccensis at low

layers (Phuong 2014b) Hung Hoa commune has a canal network system flooded by

brackish water adjacent to the shrimp-culture area and secondary mangrove forests of S caseolaris with Aegicerus corniculatum is distributed

With background mentioned above, this study aims to understand stand structure

of secondary mangrove forest; S caseolaris - A corniculatum stand, especially to elucidate

relationship between mangrove/associate species distribution in the stand and inundated duration of brackish water in order to introduce and develop silvofishery systems to shrimp-culture ponds

2 Methodology

2.1 Study area

Secondary mangrove forest in the study site is dominated by S caseolaris and A corniculatum in the canal network adjacent to the shrimp-culture ponds at the Lam River

estuary in Hung Hoa commune, Nghe An province, Vietnam (Fig.1) It is regularly flooded

by brackish water; estimated mean highest spring tide and mean lowest spring tide in 2019 are 2.7 m and 0.5 m, respectively (Center for Oceanography 2018) This canal network has four gates to control water input and output from/to the Lam River for irrigation and flood prevention The Lam River is an international river, originated in Laos flowing mainly through Nghe An province, Vietnam into the Gulf of Tonkin (website; background of key river basins, World Bank document) Climate of Vinh city, Nghe An province is affected by North East monsoon, annual average temperature is 24.6 ℃, the warmest month is June; 30.1

℃ in average, the coldest month is January; 18.3 ℃ in average and annual average rainfall is

precipitation month is September; 445 mm in average (website; climate Vinh)

Nghe An

Figure 1 Study area and location of belt-transects

Figure 2 Tidal change at the Lam River estuary (Cua Hoi; 18° 48′ N, 105° 46′ E): June - July 2019,

cited from Center for Oceanography 2018

2.2 Belt-transects

To understand vegetation gradients corresponding to the land height from datum line and inundation duration, three belts; B1 - B3 (B1: 18° 41′ 27′′, N 105° 45′ 32′′ E - 18° 41′

26′′ N, 105° 45′ 31′′ E; B2: 18° 41′ 25′′ N, 105° 45′ 33′′ E - 18° 41′ 23′′ N, 105° 45′ 32′′ E; B3: 18°

41′ 23′′ N, 105° 45′ 34′′ E - 18° 41′ 21′′ N, 105° 45′ 32′′ E) were selected in the mangrove stands (Fig 1) At each belt center line was set up horizontally, height from the center line to the land surface was measured basically every 2.5 m, if water body was existed water depth was also recorded After measuring height, it was converted to the land height based on the relationship between real water depth measured and tide height (Center for Oceanography 2018) For understanding of vegetation gradients, 2 m bars were set up perpendicular to the center line for both sides every 5 m, and all the species occurring and their land coverages

by the crowns of trees or plants body of herbs were recorded in the 4 x 5 m rectangular

2.3 Inundated duration

Since the vegetation occurs raging approximately from 1.0 m to 2.0 m in land height, average inundated duration per day of this range in unit of 0.1 m was estimated by

*Middle map: modified cadastral map of Hung Hoa commune published by Hung Hoa commune people’s committee (2018)

comparing the land height with change in tide level at Cua Hoi; 18° 48′ N, 105° 46′ E (Center for Oceanography 2018) i.e cumulative inundated time for each land height in year 2019 divided by 365 days ֩◌

3 Results

3.1 Vegetation gradients on the belt-transects; B1 - B3

Fig.3 displays vegetation gradients on the three belts with cross sections of the lands B1 is located at the most upstream among three belts (Fig.3a), vegetation length from the left side to the right side is 31.3 m It includes 0.1 - 0.2 m higher spots, but mostly flat at 1.4 - 1.5 m in land height A corniculatum and P australis are dominant through the belt, except for C malaccensis being dominant at around 1.2 m in land height on the right side

B2 is at middle of three belts (Fig 3b), vegetation length from the left side to the right side is 45.0 m It has creek like shallow water course having less vegetation It is 0.2 - 0.3 m higher than B1, it is almost flat, but the left side land is 0.1 - 0.2 m lower than the right side

On the left side P australis is dominant, while on the right side, A corniculatum is dominant S caseolaris is scattered through the belt

B3 is located at the most downstream among three belts (Fig.3c), vegetation length from the left side to the right side is 64.1 m including no vegetation zone at creek like shallow water course Through the belt, land height is 1.1 - 1.4 m, lower than upstream belts; B1 and B2 On the left side A corniculatum with C malaccensis is dominant, while on the right side

S caseolaris with Acanthus spp is dominant In the middle part there is a slightly higher spot where Acrostichum aureum is inhabiting

Figure 3 Vegetation gradients on the belts; B1 - B3 with cross sections

3.2 Relationship between species distribution and land height, inundated duration

Land height data on A corniculatum, S caseolaris, C malaccensis, Acanthus spp and P australis distributing were plotted in Fig.4 and distribution pattern of each species with

inundated duration (Fig.5) was summarized in table 1

Distribution range in land height of these five species are not significantly different,

but P australis, which is neither mangrove nor associate species (Phuong 2014 b), tends to

distribute intensively at 1.4 - 1.7 m in land height, inundation duration of 12.7 - 16.7 hours

for A corniculatum, intensively distribute rather lower zones, especially S caseolaris

intensively distributes from 1.3 m to 1.4 m in land height, inundation duration of 16.7 - 17.7

distribute on the lower zones than two woody mangrove species, from 1.1 m to 1.4 m in

distribution zone of Acanthus spp is shifted 0.1m higher than one of C malaccensis

In the zone of 1.1 - 1.4 m in land height equivalent to B3 land height, land cover by

C malaccensis is correlated with land height; r=0.91, p<0.01, but one by Acanthus spp is not

correlated with land height; p=0.09 (Fig.6)

Figure 4 Vertical distribution pattern of

five species

Figure 5 Inundated duration at each land height

Table 1 Summary of distribution pattern of five species

C malaccensis r= 0.91 (P<0.01) Figure 6 Land height vs coverage

4 Conclusions and discussion

4.1 Stand structure

S caseolaris - A corniculatum stand in the study site is tide dominated type of

mangrove forest (Woodroffe 1992), regularly flooded by brackish water except for the gate being operated Its land height ranges from 1.1 m to 1.9 m, mean inundated duration is 9.0

non-mangrove/associate species; P australis being distributed intensively at MWL; 1.7 m S caseolaris is typical mangrove species at river estuaries (Khoon et al 2004) in South East Asia often with A corniculatum (Phuong 2014 b) in North Central Vietnam

The stand includes two or more herbaceous mangrove/associate species; C malaccensis; associate species and Acanthus spp.; mangrove spp (Phuong 2014b) Their

intensive distribution zones are overlapped Mean inundation duration and mean water

study from Can Gio, Vietnam reported that Acanthus ebracteatus forms community with C malaccensis and other species at the zone flooded by tides 1.5 - 2.0 m in height (Hong 2000),

spp spp

corresponding to the observation in the study site In the intensive distribution zone of C malaccensis; 1.1 - 1.4 m in land height, expansion of the habitat area along with increasing of

land height was observed; r=0.91, p<0.01

4.1 Toward introduction of silvofishery systems to shrimp-culture ponds

Mangrove buffers in silvofishery systems are expected to provide ecological functioning of filter for nutrients and sediments In fact, nitrogen and phosphorus retention effect by mangrove species was reported from silvofishery systems in Indonesia (Hatsui and Budihastuti 2017)

4.1.1 Filter for nutrients

Focusing on nutrient retention ability of each species, few studies have been conducted, but Mitra (2020b) reported concentrations of nutrients in senesced leaves of

Acanthus spp are higher than A corniculatum and S caseolaris in both of nitrogen and phosphorus concentration, indicating that Acanthus spp have higher potential of nutrient

retention Nutrient retention abilities depend on various environmental factors, but mixture

of woody mangrove species and Acanthus spp could have more advantage in terms of

nutrients retention

4.1.2 Filter for sediments

In contrary to nutrient retention, plenty of studies on sediment retention effect of mangrove forests have been reported not only for elucidating filtering effect but also evaluating erosion prevention effect Previous study from Ba Lat estuary of the Red River,

Vietnam reported sediments trapped by A corniculatum with Acanthus ilicifolius in dry

2017) Regarding relationship between mangrove/associate species inhabiting in the study site and sedimentation rate, few studies were reported, but it was reported that herbaceous species have rather higher potential of sediment trapping (Willemsen et al 2016)

4.1.3 Economic aspects

In order to operate mangrove buffers sustainably, it would be better for buffers

themselves should produce some economic valued goods In this area C malaccensis called

mat grass has traditionally been cultivated as a weaving materials, even in year 2018, 17 households still cultivate it and cultivation area of 1.6 ha provides income of 20 - 30 USD

per person month (hearing from Hung Hoa commune people’s committee 2019) Thus, C malaccensis is appropriate species for mangrove buffer component considering regional

economy It is expected to process it as eco-friendly commodities to circulate in markets

With reasons mentioned above, buffers of S caseolaris - A, corniculatum with

herbaceous species are recommended to set up to the shrimp-culture ponds in Hung Hoa commune

Acknowledgments: Special thanks to Huong Hoa Commune People’s Committee, Nghe An

province for providing valuable information and kind support

References

Center for Oceanography (2018) Cua Hoi In Tide tables 2019 vol.1; ed Quang D.N.; Publishing house of natural science and technology: 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam; 93-117

Climate Vinh en.climate-data.org (accessed on 29 August 2019)

Clough, B., Johnston D., Xuan, T.T., Phillips, M., Pednekar, S.S., Thien, N.H., Dan T.H., Thong P.L (2002) Silvofishery Farming Systems in Ca Mau Province, Vietnam In Report prepared under the World Bank, NACA, WWF and FAO Consortium Program on Shrimp Farming and the Environment Work in Progress for Public Discussion; Published

by the Consortium; 73 pp

Department of Environment and Energy, Australia (2016) Coastal wetlands -mangroves and saltmarshes environment.gov.au (accessed on 19 June 2019)

Hastuti, E.D., Budihastuti, R (2017) Nutrient accumulation in the sediment of silvofishery ponds in Semarang MakaraJournal of Science, 21(4): 195-201

Hong, P.N (2000) Effects of mangrove restoration and conservation on the biodiversity and environment in Can Gio district, Ho Chi Minh City Phan Nguyen Hong, International Workshop Asia-Pacific Cooperation on Research for Conservation of Mangroves, 26-30 March, 2000 -Okinawa, Japan; 14 pp

Khoon, G.W., Aksornkoae, S., Tri, N.H., Vongwattana, K., Fan, H., Santoso, N., Barangan, F., Havanond, S., Sam, D.D., Pernetta, J (2004) Inundation regime In mangrove

in the South China Sea; UNEP/GEF Project Co-ordinating Unit, United Nations Environment Programme: UN Building, 9th Floor Block A, Rajdamnern Avenue, Bangkok

10200, Thailand; 6

Mitra, A (2020a) Ecosystem services of mangroves: an overview In Mangrove forests in India; Springer Nature Switzerland AG: Gewerbestrasse 11, 6330 Cham, Switzerland; 1-30

Mitra, A (2020b) Mangroves: nutrient retention box In Mangrove forests in India; Springer Nature Switzerland AG: Gewerbestrasse 11, 6330 Cham, Switzerland; 87-114

Phuong, V T (2014a) Geographical distribution of mangrove forest in Vietnam In national report on mangroves in South China Sea Vietnam; Research Centre for Forest Ecology and Environment: Dong Ngac, Tu Liem, Hanoi, Vietnam; 1-4

Phuong, V T (2014 b) Species distribution and formation In national report on mangroves in South China Sea Vietnam; Research Centre for Forest Ecology and Environment: Dong Ngac, Tu Liem, Hanoi, Vietnam; 5-14

Santen, P.V., Augustinus, P.G.E.F., Janssen-Stelder, B.M., Quartel, S., Tri, N.H (2017) Sedimentation in an estuarine mangrove system Journal of Asian Earth Science, 29: 566-575

Syaiful, E., Mohammad, R.R., Iskhaq I., Andy, M (2016) Community-based mangrove forests conservation for sustainable fisheries Journal Silvikultur Tropika, 7(3): 42-47

Willemsen, P.W.J.M., Horstman, E.M., Borsje, B.W., Friess, D.A., Dohmen-Janssen, C.M (2016) Sensitivity of the sediment trapping capacity of an estuarine mangrove forest Geomorphology, 273: 189-201

Woodroffe, C (1992) Mangrove sediments and geomorphology In Tropical mangrove ecosystems; eds Robertson, A.I and Alongi, D.M.; American Geophysical Union: Washington DC, USA; 7-41

World Bank document: Background of key river basins, Vietnam natural hazards project

(http://documents.worldbank.org/curated/en/323821468127184247/pdf/E29930REVISED00r

onmental0Background.pdf (accessed on 15 October 2019)