C in mangroves

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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Forest Ecology and Management 261 (2011) 1325–1335 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Standing biomass and carbon storage of above-ground structures in dominant mangrove trees in the Sundarbans Abhijit Mitra, Kasturi Sengupta, Kakoli Banerjee ∗ Department of Marine Science, University of Calcutta, 35, B.C. Road, Kolkata 700 019, India a r t i c l e i n f o Article history: Received 27 September 2010 Received in revised form 13 January 2011 Accepted 13 January 2011 Available online 4 February 2011 Keywords: Carbon sequestration Mangroves Biomass Carbon a b s t r a c t We evaluated carbon stocks in the above-ground biomass (AGB) of three dominant mangrove species (Sonneratia apetala, Avicennia alba and Excoecaria agallocha) in the Indian Sundarbans. We examined whether these carbon stocks vary with spatial locations (western region vs. central region) and with seasons (pre-monsoon, monsoon and post-monsoon). Among the three studied species, S. apetala showed the maximum above-ground carbon storage (t ha−1 ) followed by A. alba (t ha−1 ) and E. agallocha (t ha−1 ). The above-ground biomass (AGB) varied significantly with spatial locations (p < 0.05) but not with seasons (p < 0.05). The variation may be attributed to different environmental conditions to which these areas are exposed to such as higher siltation and salinity in central region compared to western region. The relatively higher salinity in central region caused subsequent lowering of biomass and stored carbon of the selected species. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Tropical forests are an important component in the global carbon cycle; these forests represent 30–40% of the terrestrial net primary production (Clark et al., 2001). Although the area covered by mangrove forest represent only a small fraction of the tropical forests, their position at the terrestrial–ocean interface and potential exchange of nutrients with coastal water suggests that these forests make a unique contribution to carbon biogeochemistry in coastal oceans (Twilley et al., 1992). Mangrove communities mostly occur along tropical and subtropical coastlines; about 75% of tropical and sub-tropical countries have mangrove forest (William, 2005). These forests play important ecological and socioeconomic roles by acting as a nutrient filter between terrestrial ecosystems and oceans (Robertson and Phillips, 1995), contributing to coastline protection (Vermatt and Thampanya, 2006), providing commercial fisheries resources (Constanza et al., 1997) and nursery grounds for coastal fisheries. Recently, the global warming phenomenon has generated interest in understanding the carbon-storage ability of mangroves species. Although biomass and productivity of mangrove forest have been studied previously in different mangrove forests across the world, these studies were mainly focused on wood production, forest conservation, and ecosystem management (Putz and ∗ Corresponding author. Present address: Subhashish Apartment, Flat No. A-2, 217 Bidhanpally, Garia, Kolkata 700 084, India. Tel.: +91 9433305844. E-mail address: banerjee.kakoli@yahoo.com (K. Banerjee). 0378-1127/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2011.01.012 Chan, 1986; Tamai et al., 1986; Komiyama et al., 1987; Clough and Scott, 1989; McKee, 1995; Ong et al., 1995). The carbon sequestration in this uniquely productive community, however, is a function of biomass production capacity that depends on the interaction between edaphic, climate, and topographic factors. Hence, results obtained at one location may not be equally applicable to another location (Kirui et al., 2006). The Sundarbans mangrove forest, which is the largest single tract of mangrove forest in the world, has no baseline data directly related to carbon storage, although several studies quantified the patterns of biomass production in this forest due to timber production and other commercial activities (Chaudhuri and Choudhury, 1994; Mitra and Banerjee, 2005; Banerjee and Mitra, 2004). The present study aimed to establish a baseline data set of the carbon storage by above-ground structures of three dominant mangrove species (Sonneratia apetala, Avicennia alba and Excoecaria agallocha) in the Indian Sundarbans mangrove. In this study, the carbon content in stem, branch, and leaf biomass was compared in two different physiographic regions having contrasting salinity levels. 2. Methods 2.1. Study site and sampling description The River Ganga emerges from the Gangotri glacier, about 7010 m above mean sea level in the Himalayas, flows down to the Bay of Bengal and spreads over Bangladesh (which comprises 62% of the total Sundarbans) and India (38% of the total Sundarbans) Author's personal copy 1326 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 covering a distance of 2525 km. The Indian part is known as Indian Sundarbans that covers a Biosphere Reserve area of 9630 km2 and houses 102 islands. The unique biological productivity, taxonomic diversity and aesthetic beauty of the Indian Sundarbans have been recognized with the crowns of World Heritage Site and Biosphere Reserve in 1987 and 1989 respectively by UNESCO (Mitra and Banerjee, 2005). From the Indian Sundarbans, two sites with contrasting aquatic salinity were considered in this study. One site was from the western region (low saline), locally known as Sagar South (88◦ 01′ 47.28′′ N Latitude and 21◦ 31′ 4.68′′ E Longitude), and the other site was from the central region (high saline) – located at Canning (88◦ 40’36.84′′ N Latitude and 22◦ 18′ 37.44′′ E Longitude) adjacent to tide fed Matla River (Fig. 1). The station in the western region is at the confluence of the River Hooghly (continuation of Ganga–Bhagirathi system) and the Bay of Bengal. The western region of the deltaic lobe receives the snowmelt water of Himalayan glaciers after being regulated through several dams on the way. The central region on the other hand, is fully deprived from such supply due to heavy siltation and clogging of the Bidyadhari channel in the late 15th century (Chaudhuri and Choudhury, 1994). The substrate of the Indian Sundarbans is mostly Fig. 1. Location of sampling stations in the western and central regions of Indian Sundarbans. Central region Stem Biomass (Kg) Stem Biomass (Kg) Western region y = 146.55x + 54.699 R2 = 0.8703 100 80 60 40 20 60 y = 99.851x + 26.339 R2 = 0.9655 50 40 30 20 10 0 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0 0.05 0.1 0.15 DBH (m) Branch Biomass (Kg) Branch biomass (Kg) 10 5 0 25 20 y = -16.167x + 20.927 R2 = 0.1444 15 10 5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0 0.05 0.1 DBH (m) Leaf Biomass (Kg) y = -6.2599x + 18.875 R2 = 0.0144 15 0.2 0.25 0.2 0.25 0.3 Central region 25 20 0.15 DBH (m) Western region Leaf Biomass (Kg) 0.3 Central region y = -12.198x + 34.852 R2 = 0.0374 30 25 20 15 0.25 DBH (m) Western region 40 35 0.2 10 5 0 14 12 y = -8.6863x + 11.275 R2 = 0.0756 10 8 6 4 2 0 0 0.05 0.1 0.15 DBH (m) 0.2 0.25 0.3 0 0.05 0.1 0.15 0.3 DBH (m) Fig. 2. The relationship between DBH and stem, branch and leaf biomass of Sonneratia apetala in the western and central regions of Indian Sundarbans. Author's personal copy 1327 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 Table 1 Relative abundance of mangrove species (mean of 15 plots/region) in the study area. No./100 m2 Species Relative abundance (%) Western region 13 9 7 6 6 3 2 1 1 Sonneratia apetala Excoecaria agallocha Avicennia alba Avicennia marina Avicennia officinalis Acanthus ilicifolius Aegiceros corniculatum Bruguiera gymnorrhiza Xylocarpus molluscensis Central region Western region 5 9 8 4 4 4 ab ab ab 27.08 18.75 14.58 12.5 12.5 6.25 4.17 2.08 2.08 Central region 14.70 26.47 23.53 11.76 11.76 11.76 – – – ‘ab’ means absence of the species in the selected plots. silt and clay, but in some places of the western region, there is a pure silt substrate. Both the regions exhibit productive mangrove vegetation, but high salinity in the central region likely reduces their growth. Freshwater mangrove species (Heritiera fomes, Nypa fruticans and S. apetala) are extremely rare in the central region. In both regions, selected forest patches were ∼12 years old. In each region, 15 sample plots (10 m × 10 m) were established (in the river bank) through random sampling in the various qualitatively classified biomass levels for each region (n = 30). Seasonal sampling in both regions was carried out in the low tide period. S. apetala, A. alba and E. agallocha dominated the mangrove forests at the seaward end of the Sundarban estuary where all mangrove sampling took place. In the western region, S. apetala (27% relative abundance) and in the central region E. agallocha (26.5% Central region 25 Stem Biomass (Kg) Stem Biomass (kg) Western region y = 131.84x - 4.5154 R2 = 0.7928 20 15 10 5 0 0.15 0.2 0.25 y = 97.256x - 6.118 R2 = 0.9561 0 0.05 0.1 0.15 DBH (m) DBH (m) Western region Central region y = -10.24x + 8.0971 R2 = 0.0311 0.2 0.25 6.00 5.00 4.00 y = -2.9877x + 4.1774 R2 = 0.0058 3.00 2.00 1.00 0.00 0 Leaf Biomass (Kg) 0.1 Branch Biomass (Kg) 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.05 0.05 0.1 0.15 0.2 0.25 0 0.05 0.1 0.15 DBH (m) DBH (m) Western region Central region 6.00 0.2 0.25 0.2 0.25 3.50 Leaf Biomass (Kg) Branch Biomass (Kg) 0 16 14 12 10 8 6 4 2 0 5.00 4.00 y = -5.754x + 4.4 R2 = 0.0305 3.00 2.00 1.00 0.00 3.00 2.50 2.00 y = -2.3154x + 2.3671 R2 = 0.0111 1.50 1.00 0.50 0.00 0 0.05 0.1 0.15 DBH (m) 0.2 0.25 0 0.05 0.1 0.15 DBH (m) Fig. 3. The relationship between DBH and stem, branch and leaf biomass of Excoecaria agallocha in the western and central regions of Indian Sundarbans. Author's personal copy 1328 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 Central region y = 112.67x + 19.119 R2 = 0.9091 40 30 20 10 y = 44.96x + 12.654 R2 = 0.6131 25 20 15 10 5 0 0 0 Branch Biomass (Kg) 30 Stem Biomass (Kg) 50 0.05 0.1 0.15 0.2 0 0.1 0.15 0.2 12.00 0.25 0.3 y = -1.8586x + 8.5765 R2 = 0.0067 10.00 8.00 6.00 4.00 2.00 0.00 0.25 0 0.05 0.1 0.15 DBH (m) 0.2 0.25 0.3 DBH (m) Central region Western region Leaf Biomass (Kg) 12.00 Leaf Biomass (Kg) 0.2 Central region 5 10.00 y = -8.3261x + 9.9478 R2 = 0.0648 8.00 0.15 Western region 10 0.05 0.1 DBH(m) y = 14.42x + 13.078 R2 = 0.0811 0 0.05 DBH (m) 20 15 0 0.25 Branch Biomass (Kg) Stem Biomass (Kg) Western region 6.00 4.00 2.00 6.00 5.00 4.00 3.00 y = -2.2604x + 4.8986 R2 = 0.0222 2.00 1.00 0.00 0.00 0 0.05 0.1 0.15 0.2 0 0.25 0.05 0.1 0.15 0.2 0.25 0.3 DBH (m) DBH (m) Fig. 4. The relationship between DBH and stem, branch and leaf biomass of Avicennia alba in the western and central regions of Indian Sundarbans. Table 2 Seasonal variation in AGB (t ha−1 ) of three dominant mangrove species in Indian Sundarbans (mean ± SD of 15 sampling plots in each region is presented). Mangrove vegetative part Monsoon (September, 2009) Stem Branch Leaf AGB Postmonsoon (December, 2009) Stem Branch Leaf AGB Premonsoon (March, 2010) Stem Branch Leaf AGB Sonneratia apetala Excoecaria agallocha Avicennia alba Western region Central region Western region Central region Western region Central region 104.09 42.64 22.88 169.61 ± ± ± ± 0.20 0.34 0.38 0.92 21.68 9.03 4.33 35.04 ± ± ± ± 0.45 0.12 0.39 0.96 14.09 6.30 3.22 23.61 ± ± ± ± 0.01 0.38 0.38 0.77 9.27 3.81 1.85 14.93 ± ± ± ± 0.28 0.2 0.15 0.63 27.20 12.42 7.07 46.69 ± ± ± ± 0.29 0.35 0.52 1.16 15.56 6.30 2.96 24.82 ± ± ± ± 0.44 0.36 0.57 1.37 113.35 43.07 23.97 180.39 ± ± ± ± 0.39 0.21 0.44 1.04 22.18 9.09 4.41 35.68 ± ± ± ± 0.56 0.54 0.46 1.56 15.25 6.48 3.25 24.98 ± ± ± ± 0.08 0.50 0.44 1.02 9.98 3.90 1.87 15.75 ± ± ± ± 0.10 0.48 0.44 1.02 29.18 12.98 7.10 49.26 ± ± ± ± 0.43 0.61 0.38 1.42 16.65 6.33 3.05 26.03 ± ± ± ± 0.72 0.47 0.23 1.42 116.74 43.43 22.85 183.02 ± ± ± ± 0.33 0.46 0.69 1.48 22.45 7.77 4.15 34.37 ± ± ± ± 0.51 0.01 0.32 0.84 15.55 6.03 3.21 24.79 ± ± ± ± 0.49 0.56 0.42 1.47 10.12 3.60 1.80 15.52 ± ± ± ± 0.33 0.34 0.20 0.87 30.34 11.91 6.90 49.15 ± ± ± ± 0.58 0.33 0.63 1.54 17.14 5.79 2.99 25.92 ± ± ± ± 0.68 0.80 0.61 2.09 Author's personal copy 1329 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 western region central region 145 y = 0.821x + 65.042 R2 = 0.6291 135 AGB (Kg) AGB (Kg) 140 130 125 120 0 20 40 60 80 100 125 120 10 15 20 25 30 35 90 80 70 60 50 40 30 20 10 0 0 40 5 10 western region AGB (Kg) AGB (Kg) 130 125 120 10 15 15 20 25 central region y = 1.16x + 113.05 R2 = 0.1389 5 60 Branch Biomass (Kg) 145 0 50 y = 0.6759x + 61.009 R2 = 0.0807 Branch Biomass (Kg) 135 40 central region 130 140 30 western region AGB (Kg) AGB (Kg) 20 Stem Biomass (Kg) y = 0.7581x + 108.92 R2 = 0.0864 5 10 Stem Biomass (Kg) 140 0 y = 0.7659x + 38.082 R2 = 0.5912 0 145 135 90 80 70 60 50 40 30 20 10 0 20 90 80 70 60 50 40 30 20 10 0 25 y = 1.1156x + 62.336 R2 = 0.1212 0 2 4 6 8 10 12 14 Leaf Biomass (Kg) Leaf Biomass (Kg) Fig. 5. The relationship between AGB and stem, branch and leaf biomass of Sonneratia apetala in the western and central regions of Indian Sundarbans. Table 3 Seasonal variation in AGCS (t ha−1 ) of three dominant mangrove species in Indian Sundarbans (mean ± SD of 15 sampling plots in each region is presented). Mangrove vegetative part Monsoon (September, 2009) Stem Branch Leaf AGCS Postmonsoon (December, 2009) Stem Branch Leaf AGCS Premonsoon (March, 2010) Stem Branch Leaf AGCS Sonneratia apetala Excoecaria agallocha Western region Central region Western region 44.65 17.90 10.59 73.14 ± ± ± ± 0.40 0.63 0.56 1.59 9.69 3.76 1.94 15.39 ± ± ± ± 0.45 0.64 0.64 1.73 5.88 2.58 1.36 9.82 ± ± ± ± 51.34 18.47 11.48 81.29 ± ± ± ± 0.61 0.36 0.46 1.43 9.98 3.82 1.99 15.79 ± ± ± ± 0.40 0.65 0.57 1.62 6.60 2.71 1.42 10.73 53.93 19.76 11.10 84.79 ± ± ± ± 0.37 0.53 0.32 1.22 10.34 3.40 1.93 15.67 ± ± ± ± 0.39 0.48 0.71 1.58 6.98 2.55 1.49 11.02 Avicennia alba Central region Western region Central region 0.52 0.48 0.35 1.35 3.77 1.53 0.75 6.05 ± ± ± ± 0.41 0.33 0.45 1.19 12.26 5.46 3.40 21.12 ± ± ± ± 0.67 0.32 0.56 1.55 6.98 2.64 1.33 10.95 ± ± ± ± 0.55 0.56 0.18 1.29 ± ± ± ± 0.35 0.48 0.32 1.15 4.16 1.59 0.79 6.54 ± ± ± ± 0.46 0.44 0.53 1.432 13.68 5.87 3.44 22.99 ± ± ± ± 0.37 0.33 0.34 1.04 7.65 2.79 1.44 11.88 ± ± ± ± 0.46 0.54 0.32 1.32 ± ± ± ± 0.57 0.40 0.45 1.42 4.33 1.50 0.82 6.65 ± ± ± ± 0.38 0.42 0.51 1.31 14.44 5.49 3.39 23.32 ± ± ± ± 0.27 0.39 0.53 1.19 8.02 2.61 1.42 12.05 ± ± ± ± 0.12 0.49 0.39 1.00 Author's personal copy 1330 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 western region 35.00 25.00 y = 0.8446x + 12.514 R2 = 0.6761 30.00 25.00 AGB (Kg) AGB (Kg) central region 20.00 15.00 10.00 10.00 15.00 20.00 25.00 0 6 AGB (Kg) 20.00 y = 0.8807x + 20.83 R2 = 0.1131 2.00 4.00 6.00 8.00 16 15.00 10.00 0.00 0.00 10.00 1.00 2.00 3.00 4.00 5.00 6.00 Branch Biomass (Kg) western region central region 35.00 25.00 30.00 25.00 AGB (Kg) AGB (Kg) 14 y = 1.6631x + 8.9866 R2 = 0.3011 Branch Biomass (Kg) 20.00 y = 1.5126x + 21.255 R2 = 0.1075 20.00 y = 2.8266x + 9.4954 R2 = 0.2738 15.00 10.00 5.00 5.00 0.00 0.00 12 5.00 5.00 10.00 10 central region 25.00 15.00 8 western region 20.00 10.00 4 Stem biomass (Kg) 25.00 15.00 2 Stem Biomass (Kg) 30.00 AGB (Kg) 10.00 0.00 5.00 35.00 0.00 0.00 15.00 5.00 5.00 0.00 0.00 y = 1.0283x + 5.4284 R2 = 0.7426 20.00 1.00 2.00 3.00 4.00 5.00 6.00 0.00 0.00 0.50 Leaf Biomass (Kg) 1.00 1.50 2.00 2.50 3.00 3.50 Leaf Biomass (Kg) Fig. 6. The relationship between AGB and stem, branch and leaf biomass of Excoecaria agallocha in the western and central regions of Indian Sundarbans. Table 4 Seasonal and spatial variations of stem, branch and leaf (A) biomass and (B) carbon of three mangrove species from Indian Sundarbans. Variable Species F cal Stem F crit (p < 0.05) Branch Leaf (A) Biomass Between season Sonneratia apetala Excoecaria agallocha Avicennia alba 1.27 13.76 9.37 0.19 15.31 7.08 1.86 7.0 1.61 19.0 19.0 19.0 Between region Sonneratia apetala Excoecaria agallocha Avicennia alba 630.51 719.51 760.09 2977.06 3289.47 1270.32 3621.79 13312.0 4610.28 18.51 18.51 18.51 Between season Sonneratia apetala Excoecaria agallocha Avicennia alba 1.30 9.65 7.95 0.42 10.43 5.64 1.23 10.71 2.14 Between region Sonneratia apetala Excoecaria agallocha Avicennia alba 238.82 233.20 312.14 502.56 2116.0 1386.48 1383.23 1302.89 4617.92 (B) Carbon 19.0 19.0 19.0 18.51 18.51 18.51 Author's personal copy 1331 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 relative abundance) were the dominant mangrove species at the sampling sites. Above-ground biomass (AGB) in these species refers to the sum of total stem, branch and leaf biomass that are exposed above the soil. AGB of individual trees of three dominant species in each plot was estimated and the average values of 15 plots from each region were finally converted into biomass (t ha−1 ) in the study area. 2.2. Above-ground stem biomass estimation The stem volume of five individuals from each species in each of the 15 plots per region (n = 5 × 15 = 75) was estimated using the Newton’s formula (Husch et al., 1982). V= h 6(Ab + 4Am + At ) where V is the volume (in m3 ), h the height measured with laser beam (BOSCH DLE 70 Professional model), and Ab , Am , and At are the areas at base, middle and top respectively. Specific gravity (G) of the wood was estimated taking the stem cores, by boring 7.5 cm deep with mechanized corer. This was converted into stem biomass (BS ) as per the expression BS = GV. The stem biomass of individual tree was finally multiplied by the number of individuals of each species in 15 selected plots in both western and central Indian Sundarbans. 2.3. Above-ground branch biomass estimation The total number of branches irrespective of size was counted on each of the sample trees. These branches were categorized on the basis of basal diameter into three groups, viz. <6 cm, 6–10 cm and >10 cm. The leaves on the branches were removed by hand. The branches were oven-dried at 70 ◦ C overnight in hot air oven in order to remove moisture content if any present in the branches. Dry weight of two branches from each size group was recorded separately using the equation of Chidumaya (1990) as per the expression: Bdb = n1 bw1 + n2 bw2 + n3 bw3 = central region 10 20 AGB (Kg) AGB (Kg) y = 1.0938x + 20.388 R2 = 0.8513 0 30 40 50 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 y = 1.014x + 12.399 R2 = 0.7908 0 5 Stem Biomass (Kg) y = 1.7154x + 37.479 R2 = 0.3844 AGB (Kg) AGB (Kg) 70 40 30 20 10 0 0 5 10 15 20 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 0.00 2.00 y = -0.0432x + 64.95 R2 = 0.0001 AGB (Kg) AGB (Kg) 70 40 30 20 10 2.00 4.00 6.00 8.00 Leaf Biomass (Kg) 30 4.00 6.00 8.00 10.00 12.00 5.00 6.00 central region western region 0 0.00 25 Branch Biomass (Kg) 80 50 20 y = 1.269x + 23.428 R2 = 0.195 Branch Biomass (Kg) 60 15 central region western region 50 10 Stem Biomass (Kg) 80 60 ni bwi where Bdb is the dry branch biomass per tree, ni the number of branches in the ith branch group, bwi the average weight of branches in the ith group and i = 1, 2, 3, . . ., n are the branch groups. western region 80 70 60 50 40 30 20 10 0  10.00 12.00 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 0.00 y = 1.2355x + 28.342 R2 = 0.0821 1.00 2.00 3.00 4.00 Leaf Biomass (Kg) Fig. 7. The relationship between AGB and stem, branch and leaf biomass of Avicennia alba in the western and central regions of Indian Sundarbans. Author's personal copy 1332 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 The branch biomass of individual tree was finally multiplied with the number of trees of the species in all the 15 plots for each station. 2.4. Above-ground leaf biomass estimation One tree of each species per plot was randomly considered for estimation. All leaves from nine branches (three of each size group) of individual trees of each species were removed and oven dried at 70 ◦ C and dry weight (species wise) was estimated. The leaf biomass of each tree was then calculated by multiplying the average biomass of the leaves per branch with the number of branches in that tree. Finally, the dry leaf biomass of the selected mangrove species (for each region) was recorded as per the expression: Ldb = n1 Lw1 N1 + n2 Lw2 N2 + · · · + ni Lwi Ni where Ldb is the dry leaf biomass of selected mangrove species per plot, n1 , . . ., ni are the number of branches of each tree of three dominant species, Lw1 , . . . , Lwi are the average dry weight of leaves removed from the branches and N1 , . . ., Ni are the number of trees per species in the plots. grinding and random mixing the oven-dried stems (n = 23 for each species per region), branches (n = 60 for each species per region) and leaves (n = 615 for each species per region). 2.6. Statistical analysis To explore the relationships between AGB, DBH, stem, branch and leaf biomass along with stored carbon in these above-ground biomass, scatterplots, allometric equations and correlations were computed (n = 225). To assess whether biomass and carbon content varied significantly among sites and among seasons, analysis of variance (ANOVA) was performed. A logarithmic transformation was applied on the response variable; above-ground biomass (AGB) and above-ground carbon stock (AGCS) to improve normality. All statistical calculations were performed with SPSS 9.0 for Windows. 3. Results 3.1. Relative abundance 2.5. Carbon estimation Nine and six mangrove species were recorded respectively in the selected plots in the western and central regions. The mean order of abundance of these species was S. apetala (27.08%) > E. agallocha Central region Western region 64.00 63.00 62.00 61.00 60.00 59.00 58.00 57.00 56.00 55.00 54.00 AGB Carbon Content (Kg AGB Carbon Content (Kg Direct estimation of percent carbon in the above-ground biomass was done by Vario MACRO elementar CHN analyzer, after y = 0.3606x + 28.878 R 2 = 0.6197 0 20 40 60 80 40.00 35.00 30.00 25.00 y = 0.336x + 16.911 20.00 R2 = 0.5822 15.00 10.00 5.00 0.00 100 0 10 20 30 y = 0.3382x + 47.976 R2 = 0.0878 0 5 10 15 20 25 30 35 35.00 30.00 25.00 y = 0.3035x + 26.845 20.00 R2 = 0.0832 15.00 10.00 5.00 0.00 40 0 5 10 AGB Carbon Content (Kg) AGB Carbon Content (Kg) 2 R = 0.1499 5 10 15 Leaf Biomass (Kg) 20 25 Central region y = 0.5333x + 49.545 0 15 Branch Biomass (Kg) Western region 59.00 58.00 57.00 56.00 55.00 54.00 60 40.00 Branch Biomass (Kg) 64.00 63.00 62.00 61.00 60.00 50 Central region AGB Carbon Content (Kg) AGB Carbon Content (Kg Western region 64.00 63.00 62.00 61.00 60.00 59.00 58.00 57.00 56.00 55.00 54.00 40 Stem Biomass (Kg) Stem Biomass (Kg) 20 25 40.00 35.00 30.00 y = 0.5111x + 27.343 25.00 2 R = 0.1302 20.00 15.00 10.00 5.00 0.00 0 2 4 6 8 10 12 Leaf Biomass (Kg) Fig. 8. The relationship between AGB carbon content and stem, branch and leaf biomass of Sonneratia apetala in the western and central regions of Indian Sundarbans. 14 Author's personal copy 1333 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 Central region 16.00 AGB Carbon Content (Kg) AGB Carbon Content (Kg) Western region 14.00 y = 0.3709x + 5.5574 12.00 2 R = 0.671 10.00 8.00 6.00 4.00 2.00 0.00 10.00 y = 0.4525x + 2.4113 8.00 2 R = 0.7389 6.00 4.00 2.00 0.00 5 10 15 20 25 6 y = 0.3933x + 9.1672 2 R = 0.1161 6.00 4.00 2.00 10.00 8.00 14 16 y = 0.7376x + 3.9555 2 R = 0.3044 6.00 4.00 2.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 0.00 10.00 1.00 2.00 3.00 4.00 5.00 6.00 Branch Biomass (Kg) Central region AGB Carbon Content (Kg) Western region AGB Carbon Content (Kg) 12 12.00 Branch Biomass (Kg) 16.00 14.00 12.00 y = 0.6791x + 9.3446 2 R = 0.1115 8.00 6.00 4.00 2.00 12.00 10.00 y = 1.2569x + 4.1744 8.00 2 R = 0.2783 6.00 4.00 2.00 0.00 0.00 0.00 10 0.00 0.00 10.00 8 Central region 8.00 0.00 4 Western region 14.00 10.00 2 Stem Biomass (Kg) 16.00 12.00 0 Stem Biomass (Kg) AGB Carbon Content (Kg) 0 AGB Carbon Content (Kg) 12.00 1.00 2.00 3.00 4.00 5.00 6.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 Leaf Biomass (Kg) Leaf Biomass (Kg) Fig. 9. The relationship between AGB carbon content and stem, branch and leaf biomass of Excoecaria agallocha in the western and central regions of Indian Sundarbans. (18.75%) > A. alba (14.58%) > Avicennia marina (12.5%) = Avicennia officinalis (12.5%) > Acanthus ilicifolius (6.25%) > Aegiceros corniculatum (4.17%) > Bruguiera gymnorrhiza (2.08%) = Xylocarpous molluscensis (2.08%) in the western region and E. agallocha (26.47%) > A. alba (23.53%) > S. apetala (14.70%) > A. marina (11.76%) = A. officinalis (11.76%) = A. ilicifolius (11.76%) in the central region (Table 1). On the basis of relative abundance, three dominant species, namely S. apetala, A. alba and E. agallocha were, considered for carbon stock estimation in their respective above-ground structures. These species constituted 60.4% and 64.7% of AGB of the mangroves in the western and central regions, respectively. 3.2. Stem biomass The stem biomass contributed significantly to AGB of the selected species in both the regions (Figs. 5–7). The stem biomass differed significantly between the western and central regions (p < 0.05) (Table 4A). For all three species and in both regions, the stem biomass (Y) showed significantly positive correlations with DBH (X) (Figs. 2–4). In both the regions, the stem biomass temporally varied as per the order March, 2010 > December, 2009 > September, 2009 (Table 2). The seasonal difference, however, was not statistically significant (Table 4A). 3.3. Branch biomass The branch biomass of the three studied species exhibited no significant relationship with their respective DBH and neither contributed to AGB of the species (Figs. 2–7). ANOVA results confirm significant spatial variation in branch biomass between the trees in the western and central regions for all the three species (p < 0.05), but no significant seasonal variation was observed (Table 4A). 3.4. Leaf biomass The leaf biomass of the three studied species exhibited no relationship with their respective DBH and did not contribute to AGB (Figs. 2–7) ANOVA results confirm significant spatial variation in leaf biomass between the trees in the western and central regions Author's personal copy 1334 A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 Central region AGB Carbon Content (Kg) AGB Carbon Content (Kg) Western region 35.00 30.00 25.00 y = 0.478x + 9.1968 20.00 2 R = 0.8483 15.00 10.00 5.00 0.00 0 5 10 15 20 25 30 35 40 45 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 50 y = 0.4458x + 5.5154 R2 = 0.7877 0 5 10 Stem Biomass (Kg) y = 0.7413x + 16.797 2 R = 0.3746 15.00 10.00 5.00 0.00 0 AGB Carbon Content (Kg) AGB Carbon Content (Kg) 30.00 20.00 2 4 6 8 10 12 14 16 18 0.00 2 2.00 4.00 6.00 8.00 Central region y = 0.0138x + 28.397 2 R = 5E-05 10.00 5.00 2.00 4.00 6.00 8.00 30 R = 0.1905 Western region 15.00 0.00 0.00 25 y = 0.5527x + 10.407 Branch Biomass (Kg) 30.00 20.00 20 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Branch Biomass (Kg) 35.00 25.00 20 Central region 10.00 12.00 AGB Carbon Content (Kg) AGB Carbon Content (Kg) Western region 35.00 25.00 15 Stem Biomass (Kg) 20.00 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0.00 Leaf Biomass (Kg) 10.00 12.00 5.00 6.00 y = 0.5704x + 12.402 R2 = 0.0902 1.00 2.00 3.00 4.00 Leaf Biomass (Kg) Fig. 10. The relationship between AGB carbon content and stem, branch and leaf biomass of Avicennia alba in the western and central regions of Indian Sundarbans. for all the three species (p < 0.05), but no significant seasonal variation was observed (Table 4A). 3.5. Carbon content Table 3 shows the seasonal variation of carbon stock in the above-ground biomass of the selected species. The order is S. apetala > A. alba > E. agallocha in both the western and central regions. The stored carbon in the above-ground structures exhibits the trend Stem > Branch > Leaf. For stem biomass, the stored carbon ranged from 3.77 ± 0.41 t ha−1 (in E. agallocha of central region during September, 2009) to 53.93 ± 0.37 t ha−1 (in S. apetala of western region during March, 2010). For branch biomass, the value ranged from 1.50 ± 0.42 t ha−1 (in E. agallocha of central region during March, 2010) to 19.76 ± 0.53 t ha−1 (in S. apetala of western region during March, 2010). For leaf biomass, the values ranged from 0.75 ± 0.45 t ha−1 (in E. agallocha of central region during September, 2009) to 11.48 ± 0.46 t ha−1 (in S. apetala of western region during December, 2009). ANOVA results indicated significant differences in carbon content in AGB for all species between the sites (p < 0.05) but not between the seasons (Table 4B). It is interesting to note that stored carbon in AGB shows significant positive correlation with stem biomass, but not with branch and leaf biomass (Figs. 8–10). 4. Discussion Our observed mangrove stem biomass in the western region (22.10 t ha−1 for E. agallocha to 111.39 t ha−1 for S. apetala; averaged over the three seasons) is around values of a R. mangle stand (12.5 t ha−1 ) in Florida, USA (Coronado-Molina et al., 2004) and of Komiyama et al. (2000) in a secondary mangrove (Ceriops tagal) forest in Southern Thailand (92.2 t ha−1 ). The relatively higher stem biomass of similar aged trees in the western region compared to the central region (22.10 t ha−1 for S. apetala, 9.79 t ha−1 for E. agallocha and 16.45 t ha−1 for A. alba) may be attributed to better hydrological and soil characteristics contributed by the River Ganga–Bhagirathi system. Mangroves, in general, grow better in brackish water and, in extreme saline conditions, growth is stunted (Mitra et al., 2004). The western region of Indian Sundarbans provides ideal growing conditions for mangroves due to fresh water input from the Himalayan Glaciers after being regulated by the Farakka dam. Five-year surveys (1999–2003) on water discharge from Farakka dam revealed an average discharge of (3.4 ± 1.2) × 103 m3 s−1 . Higher discharge values were observed during the monsoon with an average of (3.2 ± 1.2) × 103 m3 s−1 , and the maximum of the order 4200 m3 s−1 during freshet (September). Considerably lower discharge values were recorded during pre-monsoon with an average of (1.2 ± 0.09) × 103 m3 s−1 , Author's personal copy A. Mitra et al. / Forest Ecology and Management 261 (2011) 1325–1335 and the minimum of the order 860 m3 s−1 during May. During post-monsoon discharge, values were moderate with an average of (2.1 ± 0.98) × 103 m3 s−1 . The lower Gangetic deltaic lobe also experiences considerable rainfall (1400 mm average rainfall) and surface runoff from the 60,000 km2 catchment areas of Ganga–Bhagirathi–Hooghly system and their tributaries. All these factors (dam discharge + precipitation + runoff) increase the dilution factor of the Hooghly estuary in the western part of Indian Sundarbans – a condition for better growth and increased mangrove biomass. The central region, on contrary, does not receive the freshwater input on account of siltation of the Bidyadhari River which may be attributed to low stem biomass of the selected mangrove species inhabiting the zone. The branch biomass in the western region of Indian Sundarbans is almost similar to the values in a secondary mangrove (C. tagal) forest in southern Thailand as documented by Komiyama et al. (2000). Mangrove branches with stunted growth in the central region are related to high salinity (Mitra et al., 2009) in and around the Matla River. The leaf biomass of dominant mangrove trees in Sundarbans (1.80–23.97 t ha−1 ) is comparable to those studied elsewhere, e.g., 2.1–15.0 t ha−1 in Avicennia forests of Australia (Briggs, 1977), 6.2–20.2 t ha−1 in Rhizophora apiculata young plantations of Thailand (Aksomkoae, 1975), 13.3 t ha−1 in Rhizophora patch in Matabungkay Beach Batangas Province (de la Cruz and Banaag, 1967) and 8.1 t ha−1 in a mature Rhizophora forest of southern Thailand (Tamai et al., 1986). Significant lower leaf biomass of selected species in the western region compared to the central region is again a reflection of unfavourable salinity in the central region that promotes early leaf fall (Mitra et al., 2004). It is noteworthy that AGB of selected mangroves is accounted solely due to stem biomass (which is a direct function of DBH) and is therefore a permanent indicator of biomass production, unlike branches and leaves that contribute substantially to litter fall and less to permanent biomass (Figs. 2–4). Mangroves are unique storehouse for carbon. The global storage of carbon in mangrove biomass is estimated to be 4.03 pg, 70% of which occurs in coastal margins from 0◦ to 10◦ latitude (Twilley et al., 1992). In the present study we observed significantly higher stored carbon in the above-ground structures of the species thriving in the western region (73.14–84.79 t ha−1 in S. apetala, 9.82–11.02 t ha−1 in E. agallocha and 21.12–23.32 t ha−1 in A. alba) compared to the central region. The hypersalinity of the central part of Indian Sundarbans may be considered as one of the important reasons for stunted growth and relatively lower stored carbon in the AGB of the selected species (15.39–15.79 t ha−1 in S. apetala, 6.05–6.65 t ha−1 in E. agallocha and 10.95–12.05 t ha−1 in A. alba). Records show that surface water salinity has increased by 40.46% in central region, and decreased by 46.21% in western region of Indian Sundarbans over a period of 27 years (1980–2007) due to blockage of fresh water flow from western side of Indian Sundarbans to central region (Mitra et al., 2009) on account of massive siltation (Chaudhuri and Choudhury, 1994). In conclusion, this study demonstrates that the biomass and carbon storage capacity of mangrove species vary with spatial locations due to varying salinity, perhaps moderated by soil and water management. Effective soil management, tidal interactions (through artificial canalization) and sufficient flow of freshwater 1335 into a mangrove system are important mediators of biomass production of mangrove species. Acknowledgments The financial assistance from the Ministry of Earth Sciences, Govt. of India (Project sanction No. MoES/11-MRDF/1/34/P/08, dated 18.03.2009), is gratefully acknowledged. 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