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 shrimpculture 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 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 of salted area had been reduced especially due to changing to shrimp-culture ponds, and ca 1,000 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 shrimpculture 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 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 1,753 mm year-1, minimum precipitation month is March; 48 mm in average, maximum precipitation month is September; 445 mm in average (website; climate Vinh) Nghe An *Middle map: modified cadastral map of Hung Hoa commune published by Hung Hoa commune people’s committee (2018) Figure Study area and location of belt-transects Figure 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, m bars were set up perpendicular to the center line for both sides every m, and all the species occurring and their land coverages by the crowns of trees or plants body of herbs were recorded in the x m rectangular Belttransects surveys were conducted on June 9th and July 12th - 14th, 2019 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 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 ֩◌ 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 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 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 day-1 Land heights of other four species inhabiting range wider than one of P australis, from 1.1 to 1.9 m in land height, inundation duration of 9.0 - 19.6 hours day-1, those species except 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 hours day-1 Herbaceous species; C malaccensis and Acanthus spp tend to intensively distribute on the lower zones than two woody mangrove species, from 1.1 m to 1.4 m in land height, inundation duration of 16.7 - 19.6 hours day-1 and from 1.2 m to 1.4 m in height, inundation duration of 16.7 - 18.7 hours day-1, respectively The bottom line of intensive 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