DSpace at VNU: Production Function of Planted Mangroves in Thanh Phu Nature Reserve, Mekong Delta, Vietnam

Production Function of Planted Mangroves in Thanh Phu Nature Reserve, Mekong Delta, Vietnam Author(s): Nguyen Thi Kim Cuc and Erik D de Ruyter van Steveninck Source: Journal of Coastal Research, 31(5):1084-1090 Published By: Coastal Education and Research Foundation DOI: http://dx.doi.org/10.2112/JCOASTRES-D-13-00104.1 URL: http://www.bioone.org/doi/full/10.2112/JCOASTRES-D-13-00104.1 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use Usage of BioOne content is strictly limited to personal, educational, and non-commercial use Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research Journal of Coastal Research 31 1084–1090 Coconut Creek, Florida September 2015 Production Function of Planted Mangroves in Thanh Phu Nature Reserve, Mekong Delta, Vietnam Nguyen Thi Kim Cuc†‡* and Erik D de Ruyter van Steveninck§ † Department of Natural Resources Management Faculty for Water Resources Engineering Water Resources University Dong Da, Hanoi, Vietnam ‡ Mangrove Ecosystem Research Division (MERD) Center for Natural Resources and Environmental Studies (CRES) Vietnam National University (VNU) Ton Duc Thang, Hanoi, Vietnam § Water Science and Engineering Department UNESCO-IHE Institute for Water Education Delft, The Netherlands ABSTRACT Cuc, N.T.K and Ruyter van Stenvenick, E.D de, 2015 Production function of planted mangroves in Thanh Phu Nature Reserve, Mekong Delta, Vietnam Journal of Coastal Research, 31(5), 1084–1090 Coconut Creek (Florida), ISSN 07490208 Through assessment of forest structure, biomass of mangrove plantations in the Thanh Phu Nature Reserve, Mekong Delta, Vietnam was analyzed in correlation with diameter at breast height (DBH, i.e at 1.3 m height) Study plots were set up in 7, 11–22, and 26-year-old planted Rhizophora apiculata Blume plantations There is a significant inverse correlation between DBH and tree density (R2 ¼ 0.73; p , 0.01) To derive an allometric relation to estimate aboveground biomass, 32 trees representing all ages were chosen randomly and harvested at ground level to examine allometric relations We measured the fresh and dry weight of stems (WS), branches (WB), leaves (WL), and aboveground stilt roots (WR) in situ Allometric relationships were satisfied best with DBH as an independent variable (R2 ¼ 0.72, 0.89, 0.87, 0.98, and 0.97 for leaves, branches, stilt roots, stem, and total aboveground biomass, respectively; p , 0.001) The total aboveground biomass was estimated in the plantations to vary between 76 and 320 tons/ha Of this, more than 50% of total aboveground biomass was represented by stems The estimated biomass value of this study is consistent with that of other mangroves in the world Total biomass of R apiculata plantation in Thanh Phu Nature Reserve accounted for about 170,057 tons dry weight or 8056 tons C ADDITIONAL INDEX WORDS: Mangrove plantation, Rhizophora apiculata, aboveground biomass INTRODUCTION Mangroves are among the most important and productive ecosystems in tropical and subtropical regions (Ong, 1993) They provide a variety of ecosystem services, such as sources of food (fish, shellfish, crabs, etc.) timber, fuel wood, and nursery grounds for many commercially important aquatic organisms Mangroves stabilize coastlines and in many cases promote coastal accretion, providing a natural barrier against storms, cyclones, tidal bores, flooding, and other potentially damaging natural forces (Alongi, 2008, 2009; Giesen et al., 2007; Mitsch and Gosselink, 2007; Peter, 1999; Tri, Adger, and Kelly, 1998) However, mangrove forests have declined significantly in SE Asia over the past four decades (1970s–2000s) The main reasons for mangrove loss and degradation have been population pressure resulting in wood extraction and conversion to other land uses such as shrimp ponds, agricultural fields, salt pans, settlements, ports, and industrial estates The resulting environmental impacts have contributed to the decline and degradation of mangrove resources (Hong, 1991; Hong and San, 1993; Macintosh, Ashton, and Havanon, 2002; DOI: 10.2112/JCOASTRES-D-13-00104.1 received May 2013; accepted in revision 21 July 2013; corrected proofs received 12 November 2013; published pre-print online 19 December 2013 *Corresponding author: nguyencuc@wru.edu.vn Ó Coastal Education and Research Foundation, Inc 2015 Ong, Gong, and Clough, 1995) Recently, people have begun to appreciate the true value of mangroves, and a growing awareness of the impacts of forest loss has led to renewed efforts to protect and restore mangroves There are also increasing efforts by governments, nongovernmental organizations, and local communities around the world to conserve and rehabilitate mangroves and to manage them in a more sustainable way In the context of climate change, mangroves are known to provide options for both adaptation and mitigation According to Alongi (2008), mangroves function as coastal protection to chronic disturbance events (including climate change) However, the future of mangroves in the face of global change is at risk In order to maintain mangrove functions and services, both quantity and quality of mangrove forest should be preserved Therefore, to support the effort to protect, conserve, and develop mangrove areas, quantitative studies on their functions and services are essential Estimation of total biomass in woody ecosystems is important because of its relevance to nutrient turnover and the potential to store carbon There are several studies on the biomass of mangroves worldwide However, their values are very site specific For example, in low latitudes, primary or mature mangrove forests generally have high aboveground biomass On the other hand, the aboveground biomass is always low in temperate areas and may be related to Mangrove Production in Mekong Delta 1085 Figure Study area in Thanh Phu Natural Reserve, Vietnam different climatic conditions, such as temperature, solar radiation, precipitation, and frequency of storms (Komiyama, Ong, and Poungparn, 2008) This study focuses on a quantitative assessment of aboveground biomass and its partitioning over various tree components with the primary objectives of deriving allometric regression equations for total aboveground biomass and for leaf, branch, stem, and stilt root biomass of Rhizophora apiculata in plantations in Thanh Phu Nature Reserve, Mekong Delta, Vietnam RESEARCH SITE AND METHODS Research Site This study was carried out in 2010 and 2011 in Thanh Phu Nature Reserve, Ben Tre Province, Vietnam (Figure 1) Ben Tre is a coastal province in the Mekong Delta Thanh Phu Nature Reserve covers an area of 4800 and comprises the section of the Mekong Delta coastal zone between the Co Chien and Ham Luong estuaries (two of the mouths of the Mekong River) It is strongly controlled by tides from the sea and the water regime of the Mekong River As is the case with other sites on the eastern coastline of the Mekong Delta, Thanh Phu Nature Reserve is strongly affected by erosion as well as accretion The coastal landscape at Thanh Phu is made up of the following elements: natural mangrove swamp, mangrove plantation, mudflat, sandy beach belts, natural water ways, and shrimp ponds (Pham, 2003) About 60 species of real mangroves were recorded in the reserve The dominant species in the site are R apiculata, Avicennia marina, Avicennia officinalis, Sonneratia spp., and Excoecaria agalloccha More than 80% of the study site is covered by R apiculata plantation Rhizophora apiculata is found in the intermediate estuarine zone in the midintertidal region It is a hardy, fast-growing species that can grow up to 30 m Although it can tolerate a salinity of 65 ppt, for optimal growth a salinity range of 8–15 ppt is required (Robertson and Alongi, 1995) Several other communities consist of natural vegetation, such as Sonneratia alba on mudflats inundated by low tide, Avicennia alba on clay or sandy soils inundated by medium tide, mixed communities of A alba, A officinalis, Rhizophora mucronata, Bruiguiera sexangula on hard clay soil inundated by medium tide, and R mucronata, A alba, A officinalis, B sexangula on hard clay inundated by high tide However, these communities occur as fringes covering only small areas close to channels or river banks These mangroves are an important habitat for a number of aquatic organisms, including some with high economic value The site provides habitats for 60 bird species, 27 species of reptiles, species of frogs, and 16 species of mammals Five species of the 20 shrimp species are of high commercial value Ninety-eight species of fish use habitats at the site, including 63 saltwater fish, 32 brackish water species, and fresh water species The site is very valuable to local communities for extensive aquaculture (Pham, 2003) MATERIALS AND METHODS Coverage and Stand Structure Current mangrove coverage in Thanh Phu Nature Reserve is the result of long-term succession of the vegetation in the area from a series of human impacts Before the 1960s, 90% of Thanh Phu was covered by natural mangroves (Sub-FIPI II, 1998) During the war, the mangroves were destroyed by sprayed Journal of Coastal Research, Vol 31, No 5, 2015 1086 Cuc and de Ruyter van Steveninck Table Area and stand age of planted mangroves in Thanh Phu Natural Reserve, Vietnam Year Planted 1985 1989 1990 1991 1992 1993 1995 1996 1997 1998 1999 2000 2004 Total Stand Age (y) Area (ha) 26 22 21 20 19 18 16 15 14 13 12 11 24.9 77.7 84.7 91.4 61.8 18.9 145.6 61.5 144.8 42.8 4.0 41.0 4.8 803.9 dioxin Most of the existing mangroves in Thanh Phu Nature Reserve now are planted forest of R apiculata Another small percentage of the area is natural regeneration of Sonneratia caseolaris intermixed with A alba and A officinalis According to the Vo Van Nagan Head of Thanh Phu Nature Reserve Management Board (personal communication, 2011), R apiculata was planted in mud flats with existing A alba at a density of 10,000 trees/ha The forest that was planted before 1985 was totally harvested as production forest in 1995 All the remaining mangroves in the area were planted from 1985 to 2004, with the highest proportion planted in 1995 and 1997 (Table 1) Existing mangroves in the study site range from to 26 years old and cover an area of 803.9 The highest proportion belongs to 14 and 16-year-old forest covering more than 140 Thirty plots each of 10 m 10 m were set in 7, 11–22, and 26year-old planted R apiculata forest Following English, Wilkinson, and Baker (1994) and Clough, Dixon, and Dalhaus (1997), we measured the following variables of all the R apiculata trees present in the plots: (1) (2) (3) (4) density of trees and stilt roots root diameter and height tree diameter at breast height (DBH), a height of 1.3 m height from stratum (bed of sediment/sediment level) to the first branch (5) height from stratum to the first leaf (6) height from stratum to the top of the tree The number of trees of other species was counted Aboveground Biomass Aboveground biomass was measured for 32 trees representing all ages to assess allometric relations These trees were chosen randomly and harvested at ground level Fresh weight of stems (WS), branches (WB), leaves (WL), and aboveground stilt roots (WR) were measured in situ Subsamples of each part were taken for determining the fresh weight to dry weight ratio Dry weights were obtained after oven drying for days at 808C The aboveground dry weight of trees (wAB) was estimated from the dry to fresh weight ratios of the samples (wS ỵ wB ỵ wL ỵ wR) Allometry makes use of the fact that there is proportionality between the relative growths of two different parts of the plant Table Mean SD density, diameter at breast height (DBH), and height of planted Rhizophora apiculata trees in Thanh Phu Natural Reserve, Vietnam Stand Age (y) 26 22 21 20 19 18 16 15 14 13 12 11 No of Plots 3 2 2 2 2 Density (trees/ha) 2080 2867 1850 2500 3900 4500 3533 4000 10,100 4000 4000 7800 13,500 6 6 6 6 6 6 581 321 354 270 310 300 611 294 2095 298 289 2121 1697 DBH (cm) 12.45 11.73 12.67 7.5 8.27 7.35 11.65 10.07 5.42 10.43 10.36 5.81 3.35 6 6 6 6 6 6 1.98 3.3 0.13 2.09 2.04 0.78 2.88 0.63 0.33 1.78 2.13 0.07 Height (m) 13.22 9.77 8.88 8.35 10.02 9.86 11.56 12.40 9.77 7.92 8.29 7.99 8.05 6 6 6 6 6 6 1.27 2.03 4.08 3.35 2.14 1.87 2.31 1.93 1.22 2.17 1.51 1.51 1.79 The relationship between the two variables can be expressed by the generalized allometric equation: y ¼ b xk ð1Þ where x is the independent variable, y the dependent variable, and b and k are the allometric constants Following Boone et al (2011), carbon content of trees was calculated as the product of tree biomass multiplied by wood carbon content However, the content in different species and structures is different For example, in Micronesia in the western Pacific Ocean, carbon content in Bruguiera gymnorrhiza was 46.3%, in R apiculata 45.9%, and in S alba 47.1%, with an average for all species of 46.4% (Boone et al., 2011) This carbon content of 45.9% for R apiculata has been used to calculate the partitions and aboveground carbon of mangrove trees in the present study RESULTS Coverage and Stand Structure In the study area, R apiculata accounted for more than 90% of the number of trees The two species, S alba and Avicennia spp coexisted in the vegetation The stem density of R apiculata in the area varied with stand age and ranged from 1850 to 14,700 trees/ha (Table 2) Correlation between density and DBH of the mangrove trees in the study area is significant at the 0.01 level with R2 ¼ 0.73 (Figure 2) The height of the trees in the study area ranged from 7.92 m to 13.22 m Aboveground Biomass Aboveground biomass correlated positively with stem DBH for leaves, branches, stilt roots, stems, and total aboveground biomass (R2 ¼ 0.72–0.97; p , 0.001; Figure 3) Combining these equations with tree densities in the study plots, total aboveground mangrove biomass ranged from 76 to 320 tons/ ha, depending on the age of the plots (Table 3) More than 50% of aboveground biomass was accounted for by stems Stilt root and branches have almost the same proportion of 18% The smallest contribution came from leaves Total biomass of the planted mangroves in Thanh Phu Nature Reserve equals 170,057 tons dry weight, with an estimated 8056 tons of carbon (Table 3) Journal of Coastal Research, Vol 31, No 5, 2015 Mangrove Production in Mekong Delta 1087 Figure Correlation (p , 0.01) between diameter at breast height (DBH at 1.3 m in cm) and tree density of the mangrove trees in Thanh Phu Natural Reserve, Vietnam DISCUSSION In newly planted plots, the density of the vegetation is mainly dependent on mortality rates Results from previous research in the area show that density of planted mangroves was 8500– 8800 trees/ha in 2–5-year-old plantations (Sub-FIPI II, 2003) Subsequently, density relies upon self-thinning and natural regeneration processes Besides natural processes, the density of vegetation in the study area is affected by human activities Mangrove species are long-lived perennials with long reproductive lives According to Hutchings and Saenger (1987), flower primordials develop on plants when they are to years old Kandelia candel in Longhai, Fujian, China, started flowering and producing propagules at about years old, and the number and density of flowers varied among plants of different ages (Chen, 2000) In our study area, R apiculata begins to flower and produce propagules after about years (personal observation) Natural regeneration led to an increase of tree density with stand age and to increased variation between the smallest and the largest tree diameters, heights, and basal areas This early start of flowering resulted in an increase of tree density in older plantations As Clough and Scott (1989) have pointed out, in dense mangrove stands, height is not a parameter that can be estimated rapidly for each tree over relatively large mangrove areas, but since the simple form of the relationship (using only DBH) provides an accurate estimate, there is no need for additional input variables The coefficient of determination of the relationships between DBH and weight of tree organs was the highest in stem weight, and coefficients exceeded 0.8 for all components except weight of leaves (R2 ¼ 0.72) These results agree well with those of Clough (1992) that allometric relationships between stem diameter and weight of leaves and propagules are generally less robust than those for stem weight or total weight, because leaves and propagules are more easily broken off the tree by strong winds and waves Moreover, weights of leaves and propagules may have phenological variations even within trees of the same age Selected allometric equations for various mangrove species for aboveground biomass in relation to DBH (in centimeters) are provided in Table The allometric relations were stronger when only DBH was used as the independent variable Several studies suggested Figure Allometric relations (p , 0.001) between diameter at breast height (DBH at 1.3 m in cm) and leaves, branches, stilt roots, stem, and aboveground biomass in Thanh Phu Natural Reserve, Vietnam that estimation of biomass on the basis of a combination of tree height and stem diameter is less robust than when either are measured alone (Clough, Dixon, and Dalhaus, 1997; Komiyama et al., 1988, 2000; Tam et al., 1995) Moreover, the height of individual trees is difficult to measure accurately in an extensive closed canopy These factors make diameter easier to obtain in the field than height, and thus DBH is a useful variable for estimating mangrove biomass, in addition to the accuracy of the allometric relation Diameter at 1.3 m height was also used to estimate biomass components of mangroves in Biscayne National Park, Florida (Michael et al., 2001); in Satun Province, southern Thailand (Komiyama et al., 2000); and in the north of Vietnam (Cuc and Ninomiya, 2007) Several studies that have used regressions to investigate biomass adopt the DBH or the perimeter at breast height as the independent variable (Soares, 1997) For mangrove communities the following studies have adopted these independent variables: Amarasinghe and Balasubramaniam (1992); Cintron and Schaeffer-Novelli (1984); Clough and Scott (1989); Fromard et al (1998); Gong and Ong (1995); Imbert and Rollet (1989); Ong, Gong, and Wong (1980, 2004); Ong et al (1984); Putz and Chan (1986); Silva (1988); Slim and Gwada (1993); Steinke, Ward, and Raijh (1995); Sukardjo and Yamada (1992); Tam et al (1995) However, some studies used equations based on height and DBH for the estimation of aboveground biomass of mangrove species (Cintron and Schaeffer-Novelli, 1984; Imbert and Rollet, 1989; Lee, 1990; Suzuki and Tagawa, 1983) Mackey (1993) calculated the biomass of individuals using predictive regression of biomass on height or girth In the same way, Sherman, Fahey, and Martinez (2003) found high significant allometric relationships between tree parabolic Journal of Coastal Research, Vol 31, No 5, 2015 1088 Cuc and de Ruyter van Steveninck Table Biomass of leaves, branches, stilt roots, stems, and total aboveground biomass and carbon content of planted Rhizophora apiculata in Thanh Phu Natural Reserve, Vietnam Stand Age (y) 26 22 21 20 19 18 16 15 14 13 12 11 Total Leaves (tons/ha) Branches (tons/ha) Stilt Roots (tons/ha) Stems (tons/ha) Total Aboveground biomass (tons/ha) Total Carbon (tons/ha) Total Biomass of Thanh Phu (tons) Total Carbon of Thanh Phu (tons) 22.92 27.96 21.13 9.75 18.58 16.84 33.97 28.53 20.25 30.66 30.24 18.03 10.10 37.16 44.65 34.42 13.88 27.12 23.84 54.16 43.81 26.50 47.51 46.77 24.02 11.68 38.62 46.80 35.68 15.50 29.88 26.71 56.82 46.93 31.01 50.64 49.90 27.83 14.63 119.39 143.88 110.47 45.69 88.90 78.57 174.58 142.25 88.52 153.99 151.67 80.01 39.74 218 263 202 85 164 146 320 262 166 283 278.59 149.89 76.15 100 121 93 39 75 67 147 120 76 130 128 69 35 5431 20,456 17,083 7752 10,165 2759 46,514 16,094 24,073 12,104 1114 6150 362 170,057 2493 9390 7841 3558 4666 1266 21,350 7387 1050 5556 511 2823 166 78,056 volume (which is based on height and DBH) and aboveground biomass components (total, leaf, trunk, branch, and prop roots) Ross et al (2001) studying Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle mangroves in America used both simple and multiple regression models for the estimation of aboveground biomass They developed models for stem, branch, leaf, stilt root, and total biomass estimation, based on diameter at 30 cm above ground, height, and crown volume Fromard et al (1998) also estimated the biomass of A germinans, L racemosa, and Rhizophora spp through the use of DBH as an independent variable Komiyama et al (2002) explain that the allometric relationship for stem weight is usually expressed as a function of stem diameter and height, such as DBH2H, which differs between tree species, forcing the determination of a series of allometric equations for all tree species The species-specific allometric relationships were analyzed based on the specific gravity of stems, with the aim of establishing a common equation for predicting the stem weight of mangroves The total aboveground biomass of the mangrove stands ranged from 76 to 320 tons dry weight (DW)/ha (Table 3) The differences in biomass were due mainly to the differences in stand age The ratio of partitioning biomass to total aboveground biomass ranged from 10% to 55% for leaves, branches, stilt roots, and stems The proportion of stem biomass is highest This result is normal for woody plants that add secondary growth A similar trend of high biomass accumulation in nonphotosynthetic organs in mature forest was reported by Komiyama et al (1988) The biomasses in this study are comparable with those of some other primary forests, such as those in Halmahera, Indonesia, and Andaman Island, India (Komiyama et al., 1988); 15-year-old R apiculata in Phuket, Thailand (Christensen, 1978); and 28-year-old forest in Matang, Malaysia (Ong, Gong, and Wong, 1982) of 357, 159, and 212 tons/ha, respectively, but smaller than those estimated in some natural mangroves or very mature forests (Komiyama et al., 1988; Putz and Chan, 1986) of 270 and 299 tons/ha, respectively The variation in net primary productivity of mangrove species may be related to the geographical location (Clough, 1992), species, stand density, and growing season (Aksornkoae, 1993), as well as stand age (Ong, Gong, and Wong, 1985) Apart from the geographical location and forest structural attributes, the net primary productivity depends on abiotic factors such as hypoxic conditions, tidal height, frequency of tidal inundation, availability of nutrients, salinity, and climatic factors (Aksornkoae, 1993; Hutchings and Saenger, 1987) Table Allometric relations between diameter at breast height (DBH at 1.3 m in cm) and aboveground tree weight (Wtop in kg) for various mangrove species Data compiled from Komiyama, Ong, and Poungparn (2008) Species Avicennia germinans A marina Laguncularia racemosa Rhizophora apiculata Rhizophora mangle Rhizophora spp Bruguiera gymnorrhiza Bruguiera parviflora Ceriops australis Xylocarpus granatum Common equation Allometric Equation Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop Wtop ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ 2.40 0.140 DBH 0.0942 DBH2.54 0.308 DBH2.11 0.102 DBH2.50 0.209 DBH2.24 0.235 DBH2.42 0.178 DBH2.47 0.128 DBH2.60 0.105 DBH2.68 0.186 DBH2.31 0.168 DBH2.42 0.189 DBH2.34 0.0823 DBH2.59 0.251 pD2.46 0.168 pDBH2.47 R2 N DBH max (cm) Sources (in Komiyama et al., 2008) 0.97 0.99 0.97 0.97 0.99 0.98 0.98 0.92 0.99 0.99 0.99 0.99 0.99 0.98 0.99 45 21 22 70 17 57 17 23 17 25 26 15 104 84 unknown 35 10 unknown 28 unknown 32 25 25 16 20 25 49 50 Fromard et al (1998) Imbert and Rollet (1989) Comley and McGuinness (2005) Fromard et al (1998) Imbert and Rollet (1989) Ong, Gong, and Wong (2004) Imbert and Rollet (1989) Fromard et al (1998) Clough and Scott (1989) Clough and Scott (1989) Clough and Scott (1989) Clough and Scott (1989) Clough and Scott (1989) Komiyama, Poungparn, and Kato (2005) Chave et al (2005) Journal of Coastal Research, Vol 31, No 5, 2015 Mangrove Production in Mekong Delta More and more mangroves around the world are affected by human activities, and all may be influenced by global changes in climate or sea level Because mangrove coverage is being reduced, we hope that future exploitation of mangroves will be preceded by environmental impact assessments that will include estimates of biomass ACKNOWLEDGMENTS We thank Department of Agriculture and Rural Development of Ben Tre Province and Thanh Phu Nature Reserve’s Management Board for their unstinting support for data collection and field survey The work reported here was undertaken as part of the research programme ‘‘PRoACC— Post-doctoral Research Programme on Climate Change Adaptation in the Mekong River Basin.’’ The project is funded by the Netherlands Ministry of Development Cooperation (DGIS) through the UNESCO-IHE Partnership Research Fund This research project is a joint initiative of UNESCO-IHE Institute for Water Education and many partner institutions in the Lower Mekong countries and China 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Education Delft, The Netherlands ABSTRACT Cuc, N.T.K and Ruyter van Stenvenick, E.D de, 2015 Production function of planted mangroves in Thanh Phu Nature Reserve, Mekong Delta, Vietnam Journal of