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https://www.tandfonline.com/doi/full/10.1080/21580103.2013.814593?src=recsys
Abstract
This study was conducted to assess the species diversity and carbon sequestration potential of a natural mangrove stand in Botoc, Pinabacdao, Samar, the Philippines. Using the quadrat sampling technique, 12 plots with a size of 100 m2 were established to facilitate inventory and measurement of trees. Shannon-Wiener index and allometric equations were used to determine species diversity, and biomass and tree carbon storage, respectively. The community's species diversity (H′ = 1.6365) was very low with a total of eight true mangrove species recorded, dominated by Avicennia officinalis with an importance value of 134.80%. Among the plots, a greater percentage of the total biomass was accounted to the above-ground biomass, corresponding to 74% (297.20 t ha−1), while the remaining 26% (103.87 t ha−1) was credited to the root biomass. The total carbon sequestered and stored in the biomass of the natural mangrove stand was 188.50 t C ha−1 equivalent to 691.81 t CO2 ha−1. The biomass and carbon density estimates acquired in this study suggest that natural mangrove forests in Botoc have the potential to sequester and store a huge amount of atmospheric carbon regardless of the very low species diversity.
Introduction
Mangrove forests contribute to the diversity, richness and productivity of coastal ecosystems. As a community easily recognized along the interface between land and sea, mangroves can exist under a wide range of harsh environmental conditions. Apart from the various ecosystem goods and services to coastal inhabitants, mangrove forests provide ecological services such as bioprotection from littoral erosion (Naylor et al. 2002), natural breakwaters, dissipation of the energy of the waves and tsunamis, and protection from cyclonic storms (Alongi 2002). Recently, the role of mangrove forests to sequester substantial amounts of atmospheric carbon dioxide (CO2) and store carbon in its biomass has been underscored (Suwa et al. 2009; Chen et al. 2012). In addition, the economic significance of this coastal ecosystem, referred to as blue carbon sinks, draws significant attention from the global community (Lawrence 2012).
The 2011 Philippine Forestry Statistics (Forest Management Bureau 2011), citing 2003 data, reported a total mangrove area of 247,362 ha. Vast areas of mangroves are located at the southern portion of the archipelago, specifically in Mindanao and Samar, and at the west in Palawan. The Philippines is one of the countries with a high number of true mangrove species, having about 42 species representing 18 families (Samson and Rollon 2011). However, because of their accessibility, these coastal forests have a very high risk of being subjected to numerous pressures related to developmental activities and are often over-exploited. Vast areas of mangroves in this country have been cleared and converted to aquaculture ponds (Walters 2003; Lawrence 2012). Mangrove forests are continuously subjected to unsustainable anthropogenic activities, which, aside from their vulnerability to the impacts of climate change such as sea-level rise, inevitably lead to the degradation of these ecosystems. This loss adds to the remarkable reduction in forest biomass, contributing significantly to the already alarming concentration of CO2 in the atmosphere.
The province of Samar, formerly known as Western Samar, has a relatively long coastline extending over 300 km in the western side of the island. The coastal area, if not limestone cliff or inhabited by humans, is planted with mangroves. The province represents 7% of total mangrove cover of the country (Forest Management Bureau 2011). In the Philippines, Lasco and Pulhin (2004) reported that the estimated mean biomass of mangrove forests is around 409 t ha−1 with a corresponding stored carbon of 184 t C ha−1. Being one of the areas in the country with the largest remaining mangroves, carbon sequestration and storage potential in its biomass is expectedly huge. Moreover, taking into account the density of mangroves in the province of Samar, it was included by the Department of Environment and Natural Resources in the list of conservation priority areas for mangroves (Ong 2002).
Although the Philippines is one of the mangrove-rich countries across the globe, very little is known on the precise amount of biomass and carbon sequestration and storage of this coastal vegetation. Thus, this study aims to assess species diversity and biomass, and estimate carbon sequestration potential of a natural mangrove stand in Botoc, Pinabacdao, Samar, Philippines.
Materials and methods
Study site
This study was carried out in a natural mangrove stand at the coastal village of Botoc (11°36′29″N 124°59′41″E) Pinabacdao, Samar. Samar is located in the Philippine island of Visayas and is 530.15 km from Manila. Pinabacdao, one of the municipalities of Samar, is about 54 km south of the provincial capital, Catbalogan City, (Figure 1). This site was selected based on accessibility and safety in going to and from the natural mangrove stand. The dominant species in these plots is Avicennia officinalis in association with Sonneratia albaand Xylocarpus granatum. Several species belonging to the Rhizophoraceae family were also observed at thesite. The mangrove formation has an estimated area of20 ha.
Figure 1. Map of (a) the Philippines showing the location of the study site in the province of (b) Samar, specifically the coastal village of (c) Botoc, Pinabacdao.
Based on its provincial profile, Samar falls under the Type II and Type IV climate. The former is characterized by having no dry season with a very pronounced maximum rain period which usually occurs in December to January, while the latter has more or less evenly distributed rainfall throughout the year. Pinabacdao, which lies on the western portion of the province, is under the Type IV climate. From 2003 to 2004, the highest amount of rainfall was recorded in the month of November while the month of April had the least rainfall. The mean annual rainfall is 220.82 mm while the mean temperature is 27 °C. Its topography is generally hilly with occasional plain terrain. Approximately 56% of the total land area of Pinabacdao is arable, which is commonly used for agricultural production. Hydrosol and clay loam make up the most of the province's soil cover and it is where most of the agricultural crops are grown.
Data collection
A nondestructive method through the quadrat sampling technique was utilized in this study conducted in June 2011. A total of 12 plots (10 m × 10 m) were established for this research with 20–30 m distance between plots to facilitate the inventory. Within sample plots, a 100% inventory of trees with at least 5 cm diameter was undertaken, where trunk diameters and total height of each tree were measured in addition to species identification. Trunk diameters of Rhizophora species were measured at 30 cm above the highest prop root, whereas the rest were measured at diameter at breast height (DBH). Samples of branch bark and stem (6–8 cm diameter, 10–20 cm long) of each of the major mangrove species in the area were taken to the laboratory for carbon content analysis.
Data analysis
The community's mangrove species diversity was determined using the Shannon–Wiener index. This diversity index is extensively used for comparing diversity between various habitats, including mangrove, as well as to determine species distribution and evenness (Tang et al. 2007; Gevaña and Pampolina 2009; Lumbres et al. 2012). The species diversity has been calculated as follows:
(1)where H′ is the diversity index, pi is the proportion of individuals of species i and ln is natural logarithm.
(1)where H′ is the diversity index, pi is the proportion of individuals of species i and ln is natural logarithm.
Allometric equations for Southeast Asian mangroves developed by Komiyama et al. (2005) were used in the estimation of the above-ground biomass (Wagb) and root biomass (WR), having a coefficient of determination (R2) of 0.98 and 0.95, respectively. The following common allometric equations were used:
(2)
(3)where Wagb is the above-ground biomass (kg); WR is the root biomass (kg); ρ is the wood density of the species; and D is the diameter at breast height. The computed total biomass (above-ground and root) per plot was summed for all plots and averaged to get the mean stand biomass, which was then converted to tons per hectare. The result of biomass carbon fraction analysis was used to convert the biomass value to its carbon equivalent. Carbon stock was determined as the product of the carbon concentration and the corresponding biomass of individual tree.
(2)(3)where Wagb is the above-ground biomass (kg); WR is the root biomass (kg); ρ is the wood density of the species; and D is the diameter at breast height. The computed total biomass (above-ground and root) per plot was summed for all plots and averaged to get the mean stand biomass, which was then converted to tons per hectare. The result of biomass carbon fraction analysis was used to convert the biomass value to its carbon equivalent. Carbon stock was determined as the product of the carbon concentration and the corresponding biomass of individual tree.
For carbon fraction determination, stem and branch bark samples of each major mangrove species in the study area were brought to the laboratory for fresh weight determination. The samples were oven-dried at 105 °C until constant weights were attained (within 48 hours), and an analysis was performed. Samples of 10–20 g each of oven-dried branch bark and stem were brought to the International Rice Research Institute Analytical Services Laboratory for total carbon analysis using the automated carbon analysis–mass spectrometry continuous flow technique. The laboratory analysis resulted in an average carbon fraction of 47%. The acquired carbon concentration is also the carbon default value recommended in the Intergovernmental Panel on Climate Change IPCC (2006).
The total C-stock was estimated by obtaining the sum of the above-ground and root C-stocks. The ratio of molecular weight of CO2 to carbon was used in the conversion of biomass C-stocks to their CO2equivalent.
Results and discussion
Species diversity
The species composition of the natural mangrove stand in Botoc, Pinabacdao, Samar, and trunk diameter and total height of each species measured are shown in Table 1. The total number of stems in the 12 10 m × 10 m plots is136, of which Avicennia officinalis was observed in almost all of the plots. The two species Sonneratia alba and Xylocarpus granatum coexisted in the stand along with another five Rhizophoraceae species, specifically Aegiceras corniculatum, Rhizophora stylosa, Rhizophora apiculata, Rhizophora mucronata, and Ceriops decandra. The intertidal vegetation is a natural old-growth stand with trunk diameters ranging from 5.0 cm to 70.7 cm, total height varying between 2.0 m and 14.1 m, and averages of 12.9 cm and 5.6 m, respectively. Avicennia officinalis was recorded with the largest girth with 70.7 cm; the same species also registered the tallest at 14.1 m. Plot 7 registered the most number of species with 22 individuals.
Table 1. Species composition and community structure of the natural mangrove stand in Botoc, Pinabacdao, Samar, Philippines, using non-destructive quadrat sampling. SD = standard deviation.
The analyses of the importance value of species and species diversity index are summarized in Table 2. Of the eight mangrove species subjected for analysis, Avicennia officinalis belonging to the Acanthaceae family turned out to have the highest relative frequency of 27.03%, relative dominance of 71.74%, relative density of 36.03%, and therefore had the highest importance value of 134.80%. As shown in Table 2, a total of 49 individual Avicennia officinalis species was recorded in 10 plots. This was followed by Sonneratia alba from the Lythraceae family and Xylocarpus granatum from the Meliaceae family, with 54.66% and 46.03% importance values, respectively. The summed importance values of species belonging to the family Rhizophoraceae was 50.50%, of which a little more than half was accounted for by Rhizophora apiculata. The average density of mangroves in the study site was 1133 trees ha−1. The importance value is derived from the sum of the relative frequency, relative dominance and relative density of a species in a community or forest. It mainly provides a better index than density alone, taking into account the significance or function of a species in its habitat (Rotaquio et al. 2007).
Table 2. Analyses of importance value and Shannon–Wiener diversity index of species belonging to five families in the natural mangrove stand in Botoc, Pinabacdao, Samar, Philippines, using non-destructive quadrat sampling.
With regards to species diversity as presented in Table 2, the calculated diversity index using Shannon–Wiener index was H′ = 1.6365. The overall diversity index was considered very low based on the diversity scale used by Gevaña and Pampolina (2009). This is primarily due to the lack of species variation in the mangrove stands. A number of studies coincidentally concluded that the mangroves had very low diversity indices due to their unique stand formation compared to other tropical forest ecosystems (Lu et al. 1998; ENFOR 2004; Gevaña and Pampolina 2009; Gevaña et al. 2009; Kovacs et al. 2011).
On the other hand, all of the species recorded in the natural mangrove forest of Botoc are among the 42 true mangrove species thriving in the Philippine archipelago (FAO 2005; Samson and Rollon 2011). According to FAO (2007), true mangrove species grow only in a mangrove environment and do not extend into terrestrial plant communities and are morphologically, physiologically and reproductively adapted to saline, waterlogged and anaerobic conditions. Following the International Union for Conservation of Nature Red List of Threatened Species, only Ceriops decandra is identified as nearly threatened, while others belong to the least-concern category (Polidoro et al. 2010). This species also represents the smallest number of individuals observed in this study.
Biomass and C-stock
The allometric method of estimating the biomass of a forest community is the most common as it is non-destructive and less tedious than other methods (Kridiborworn et al. 2012). In particular, the allometric equations (Komiyama et al. 2005) used in this study only employ stem diameter and wood density, which are easily quantifiable. These two variables essentially influenced the biomass and carbon stocks of each individual species.
Table 3 shows the biomass pools, C-stocks and equivalent CO2 in different species diameter ranges in the natural mangrove forest in Botoc. From accumulation of above-ground and root biomass of individual trees of the Botoc mangrove forest, the total biomass was estimated tobe from 59.73 to 1091.53 t ha−1with an average of 401.07 t ha−1. With a 47% carbon content of the biomassbased on laboratory analysis, its C-stock was 188.50 t C ha−1, which was equivalent to 691.81 t CO2 ha−1. In terms of the equivalent C-stock of the biomass, the value ranged from 28.07 to as high as 513.02 t C ha−1. The biomass C-stock was equivalent to 103.03 to 1882.78 t CO2 ha−1, which was held and stored in the biomass, both above-ground and roots. Among the established sample plots, Plot 8 had the largest total biomass and carbon storage with 1091.53 t ha−1 and 513.02 t C ha−1, respectively, having an equivalent 1882.78 t CO2 ha−1. This was closely followed by Plot 2, with total biomass and carbon storage of 1015.74 t ha−1 and 477.40 t C ha−1, respectively, with an equivalent 1752.05 t CO2 ha−1. This could be attributed to the large girths of the trees recorded (5.10–70.70 cm) within the plots. Although Plot 7 comprised the most number of individuals among the plots, it only ranked fifth on biomass and carbon stock contribution. Furthermore, the above-ground biomass ranged from 41.43 to 828.11 t ha−1, with a mean of 297.20 t ha−1, while the root biomass varied from 18.31 to 263.42 t ha−1, with an average of 103.87 t ha−1.
Table 3. Biomass pools, C-stocks, equivalent CO2 and above-ground-to-root biomass (T/R) ratio in different species diameter ranges in the natural mangrove stand in Botoc, Pinabacdao, Samar, Philippines. SE = standard error.
In this study, the greater percentage of the total biomass was attributed to the above-ground corresponding to 74%, while the remaining 26% was attributed to the roots. The summary of the above-ground and root biomass and the corresponding C-stocks of the natural mangrove stand in Botoc, Pinabacdao, Samar, is presented in Figure 2. The result of the biomass estimations obtained in this present study is within the range (7.9 t ha−1 in Florida, USA, to 460 t ha−1 in Malaysia) reported by Komiyama et al. (2008). Globally, the biomass estimates of the Botoc's mangrove was relatively higher than that of Rhizophora apiculata in Sarawak Mangrove Forest in Malaysia (Chandra et al. 2011), Ceriops tagal in Satun, Thailand (Komiyama et al. 2000), and Kandelia obovata in Okinawa, Japan (Khan et al. 2009). Furthermore, the biomass estimates of the natural mangrove stand of Botoc, Pinabacdao, Samar, was comparable with the values obtained by Camacho et al. (2011) on a community-managed mangrove forest and natural stand in Bohol, Philippines, dominated by R. stylosa. Camacho et al. (2011), who also used the common allometric equations of Komiyama et al. (2005), reported a total biomass ranging from 173.9 to 948.0 t ha−1 compared to the 59.73 to 1091.53 t ha−1 of the present study. According to Komiyama et al. (2008) in the review of biomass of mangrove forests in various countries undertaken over several years, the variation in the biomass estimates depends not only on species but also on ecological conditions and geographical location.
Figure 2. Summary of the above-ground and root biomass and the corresponding C-stocks of the natural mangrove stand in Botoc, Pinabacdao, Samar, Philippines. AGB = above-ground biomass, AGB C-stock = above-ground biomass carbon stock, RB = root biomass, RB C-stock = root biomass carbon stock.
The above-ground biomass to root biomass (T/R) ratio ranged from 2.22 to 3.21 or an average of 2.60 (Table 3). The result was consistent with the value given in Komiyama et al. (2008), which was from 1.1 to 4.4. In comparison to upland forests, mangrove forests’ T/R ratio was significantly lower because a large amount of biomass tended to be allocated in the root system in order to maintain a bottom-heavy tree form to stand upright in wet and soft mud (Komiyama et al. 2008).
Regarding the total natural mangrove cover of the province of Samar, which is equal to 16,167 ha (Forest Management Bureau 2011), it has potential to store a substantial quantity of 3.05million t C and an estimated amount of 11.18million t CO2. The recent economic significance of this intertidal ecosystem as blue carbon sinks must be explored in an attempt to balance conservation and the need for sustainable livelihoods. Blue carbon sinks refer to the coastal ecosystems, mangroves in particular, and their ability to transfer and store carbon in their sediments and within plant parts at rates far greater than those of terrestrial forests (Lawrence 2012). A blue carbon fund is the ocean equivalent of reducing emissions from deforestation and forest degradation (REDD), for carbon sequestration in coastal areas. This opportunity for carbon trading, in addition to the renowned array of ecosystem services and ecological functions that we may benefit from these mangrove forests, can lead to notable strategies for climate change mitigation. It is in relation to climate change mitigation that the undervalued ecosystem services of mangroves are now gradually being overcome and gaining much attention worldwide. Specifically, the payment for ecosystem services (PES), which is defined by Wunder et al. (2008) as a voluntary transaction where a well-defined environmental service is bought by a service buyer from a service provider, coupled with a carbon credit system may pave the way for convergent opportunities as adaptive management tools to attain the dual goals of uplifting the lives of coastal inhabitants and protecting the global marine carbon sinks (Warren-Rhodes et al. 2011).
Conclusion
The biomass and carbon density estimates acquired in this study suggest that the natural mangrove forest in Botoc, Pinabacdao, Samar, has the potential to sequester and store as high as 188.50 t C ha−1despite the lean species diversity. The estimated value is comparable to the mean carbon storage of Philippine mangroves derived by Lasco and Pulhin (2004). However, its vulnerability to unsustainable anthropogenic activities and impacts of climate change such as sea-level rise threatens the existence of mangrove forests. It is imperative therefore to explore sustainable funding and tangible incentive systems to balance mangrove conservation with sustainable livelihoods for coastal inhabitants. Among the emerging options with promising opportunities to assist ecological conservation and economic development in the coastal zone are PES and the recently established blue carbon fund, the ocean equivalent of REDD, for carbon sequestration on coastal areas. Moreover, further study on developing a biomass equation for different Philippine mangroves species applicable at wide DBH classes is also necessary. The availability of site- and species-specific allometric equations will refine present biomass estimates for mangroves, which are highly important in the light of carbon trading.
Acknowledgements
The authors would like to acknowledge the local government unit as well as the coastal communities of Botoc, Pinabacdao, Samar, Philippines, for their assistance during the data gathering. This study was carried out with the support of the Forest Science and Technology Projects [Project No. S211013L010410] provided by the Korea Forest Service.
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References
https://www.tandfonline.com/doi/full/10.1080/21580103.2013.814593?src=recsys
