Abstract
Mangrove plays a significant role in climate change mitigation particularly in carbon absorption and minimizing the detrimental impacts of sea level rise, salt-water intrusion and tidal surges. In Bohol Province, Philippines, a small coastal island community known as Banacon is one of the successful cases in mangrove reforestation. Recognizing the site's potential for a carbon sequestration project, a biomass and carbon stock assessment of mangrove trees was done. Using standard sampling techniques and allometric equations, tree carbon storage was measured across stand ages, namely 15-, 20- and 40-year-old plantations. Mature natural stands were also included in the assessment. By estimate, the 40-year-old plantation has the largest carbon density with 370.7 ton ha−1, followed by the 15-year-old plantation with 208.5 ton ha−1, 20-year-old plantation with 149.5 ton ha−1, and lastly by natural stand with 145.6ton ha−1. Overall, Banacon mangroves are in a vigorous condition of storing vast amount of carbon. Local community and government should therefore sustain their commitment in coastal reforestation activities in order to enhance the mangrove carbon stocks. Policies and programs that can help provide incentives and livelihoods to local people who are largely dependent on mangroves should likewise be explored in order to sustainably reap the economic and ecological benefits of conserving mangrove forest.
Introduction
The rehabilitation of mangroves is of considerable importance in many tropical countries such as the Philippines because it plays a crucial role in addressing climate change and climate change-related problems. Mangroves serve as both sources and sinks of atmospheric CO2 (ENFOR 2004; Gevaña et al. 2008). They can be managed to sequester CO2 and store carbon in biomass and soil. Further, mangroves perform protective roles against detrimental climatic impacts. Providing a tidal shield against the abrupt rise in the sea level and salt-water intrusion, and minimizing siltation coming from the eroded soil in the uplands are a few of their foremost ecological services.
Mangrove forests are among the world's most productive ecosystems. They contain a diverse group ofhalophytic species belonging to some 12 genera in eight different families (Lugo and Snedaker 1974). These flora enrich coastal waters, yield commercial forest products, protect coastlines and support coastal fisheries (Kathiresan and Bingham 2001). Growing at the interface between land and sea, they are well adapted to natural stressors (e.g. temperature, salinity, anoxia, ultraviolet rays). However, because they live close to their tolerance limits, they are also vulnerable to disturbances. Mangrove forests are being destroyed because their proximity to population centers that make them favored sites for sewage disposal, human settlements and industry development, and fish pond expansion (Valiela et al. 2001).
Statistics reveal that numerous tracts of mangrove forests are already gone. At the global scale, mangrove deforestation is at an alarming rate of 3500 ha year−1 (Baconguis et al. 1990). In some regions, mangrove forests are in danger of complete collapse. The most recent estimate of the world's mangrove areas is 15.2 million ha (FAO 2007), an estimate that is nearly half of the 1990 level of 24 million ha (Twiley 1992). This trend is likely to persist as human populations continue to push into the mangrove domains.
In the Philippines, mangrove forests had decreased as well. Based on early estimates, it was recorded at around 400,000–500,000 ha (Brown and Fischer 1920; Chapman 1976; Primavera 2000). Recent estimates however suggest that this has trimmed down to 153,577 ha with fairly extensive mangroves left that can be found in Palawan with 41,830 ha (DENR 2005). Consequently, this loss spells a tremendous decrease in forest biomass which could have been instrumental in sequestering atmospheric carbon.
On average, Philippine mangrove has a biomass of around 401.8 ton ha−1 with roughly 176.8 ton ha−1 of carbon stored (Lasco and Pulhin 2000). It is therefore crucial to rehabilitate and protect mangrove forests to sustain their roles in mitigating the impacts of climate change. Moreover, creating a pool of reliable information about their potential for carbon sequestration is relevant for sound forest management interventions.
The case of Banacon Island in the Province of Bohol is perhaps one of the best when it comes to illustrating the carbon sink potential of mangroves in the Philippines. Mangrove reforestation in Banacon is a community-initiated effort that started in 1957 upon recognizing the dire local needs for fuelwood and construction materials for building boats and houses. Currently, mangrove plantations of Banacon are being managed by the local community with assistance fromthe Department of Environmental and Natural Resources (DENR).
Recognizing therefore the potential of community-managed mangroves of Banacon in carbon storage and sequestration, a study was conducted to assess the actual wood biomass production (both above- and below-ground biomass) from existing natural mangrove forests and mangrove plantations; estimate tree carbon stocks; and recommend policy directions to enhance the role of mangrove forests in climate-change mitigation.
Materials and methods
Site description
The Province of Bohol is located at the central part of the Philippines that is 556.16 nautical miles from Manila and 30 km from Cebu City. Among the 7107 islands, it is the 10th largest island with a total area of394,816.2 ha that is administratively divided into 47municipalities, 1 city and 1109 barangays. It has a population of 1,230,110 individuals with an annual growth rate of 2.92% (PPDO 2008).
Based on the current land-use map, agriculture is the predominant livelihood (Table 1). Agricultural land comprised nearly half of the whole province with 169,678.4 ha. Fishing is the second major source of livelihood particularly of the 30 coastal municipalities. Roughly 33% of the whole population depends on this means, hence local people regarded protection and improvement of marine habitat such as mangroves vital.
Table 1. Land use of Bohol Province.
Among the mangrove areas of Bohol, the Banacon Island in the municipality of Getafe (Figures 1 and 2) is perhaps the most renowned site (Walters 2004). This island lies at the upper northwestern margin of Bohol with the coordinates between 10°03′30′′–10°15′30′′N latitude and 124°03′30′′–124°14′30′′E longitude. It covers around 1775 ha. About two decades ago, it was awarded the “Likas Yaman Award of 1989” of the DENR, and the “Outstanding Tree Farmer Award from the Philippines” of the FAO in 1991 because of its successful development of mangrove plantations. Banacon mangrove is also recognized as one of the largest man-made mangrove forests in Southeast Asia.
Figure 1. Location of the study site.
Figure 2. General view of Banacon Island. a, mangrove plantations mainly composed of bakawans or Rhizophora sp; b, local community.
Mangrove plantations are mainly composed of Rhizophora stylosa, locally known as “bakawan bato.” This species is very adaptive to a high salinity environment of sandy loam to sandy corraline substrates. It is also found to be resilient to frequent tidal surges compared with other Rhizophora species. Various ages of Rhizophora stylosa plantations arecurrently being managed in the island (Figure 3). These plantations are very dense with at least 40,000 trees ha−1 and tree spacing of 0.3 to 0.5 m. Natural mangrove stands are also found fringing these plantations.
Figure 3. General view of R. stylosa plantations based on age. a, 15 years; b, 20 years; c, 40 years. d, natural stand.
Biomass and carbon sampling
A non-destructive method through a quadrat sampling technique was employed to record all the trees within the 10 m × 10 m plots. Site selection was done based on age of the plantations which includes 15, 20 and 40 years old. These were labeled as P-15, P-20 and P-40, respectively. Sampling also covered the mixed natural stand to capture the contribution of other mangrove species in the carbon pool. This site was labeled as P-N. Three plots were established for each stand. The distance between these plots was at least 20 m. Basic information such as species, diameter at breast height above the knee roots (dbh) and height were obtained.
Destructive sampling of 13 trees of R. stylosa at various ages was also done to measure their diameter and estimate the amount of carbon stored. A best-fit regression equation was also generated to predict the incremental growth of carbon stock across age.
In computing for the tree biomass (kg), allometric equations developed by Komiyama et al. (2005) for the above-ground biomass (Wagb) and roots (Wr) were employed. These equations have a coefficient of determination (R2) of 0.98 and 0.95, respectively that are comparably reliable with earlier equations for mangrove tree biomass (e.g. Putz and Chan 1986; Clough and Scott 1989; Sukristijono and Yamada 1992; Slim et al. 1996). Aboveground biomass (Wagb) comprised the biomass of stem, branches and leaves while root biomass (Wr) included the biomass of stilt roots and pneumatophores. All biomass values in each plot were summed to get the total biomass (expressed in tons). Biomass was converted to the equivalent amount of carbon by multiplying the biomass by 0.45, an average carbon content value for tropical trees based on Lasco and Pulhin (2003).
The study employed the following general allometric equations for mangroves:
For Rhizophoras, root biomass was computed using the following formula:
Peat or sediment carbon was not included in the sampling because the study aimed to highlight the specific contribution of trees to site's carbon stocks. However, further work to account this carbon pool is regarded essential fully measure the site's carbon stock. By estimate, mangrove sediment contributes to around 22 kg m−2 to 130 kg m−2 carbon, which values largely depend on vegetation type, topography and burial rate (Fujimoto 2000).
Results and discussion
Species composition
Table 2 shows the list of species together with their corresponding frequencies, diameter and height values. Rhizophora stylosa was found common in all sites while three other species were identified in the natural stand. The plots within the 15 Year Old Plantation registered the most number of trees with 180 to 324 stems per plot. This was followed by the 40 Year Old Plantation with 111 to 229 stems per plot. Distribution of frequency followed a trend: P15 > P40 > P20 > PN.
Table 2. Species composition across the stands.
In terms of diameter, Avicennia marina in the natural stand registered the largest value with 68.2 cm. In general, trees recorded at this stand have larger dbhvalues compared with other trees observed in the plantations. A similar finding was also observed for height where Rhizophora stylosa and Avicennia marinaat the natural stand registered the tallest height among others.
Biomass and carbon storage
Biomass and carbon stocks were largely influenced by tree diameter and density. The summary of biomass and carbon stocks of the plantations and natural stands is presented in Figure 4. Among the stands, the 40-year-old plantation has the largest total biomass and carbon storage with 823.7 ton ha−1and 370.7 ton ha−1, respectively. These values are reflective of the large diameter trees recorded (3.5–15.6 cm) and the dense condition of the stand that accounts to 111–229 stems per plot. Among the plots, Plot 2 in P-40 has the largest estimates with 948.0 ton ha−1 of biomass and 426.6 ton ha−1 of carbon storage (Table 3).
Figure 4. Summary of biomass and carbon density values across different mangrove stands.
Table 3. Biomass and carbon stocks of R. stylosa plantations and natural stands in Banacon, Bohol.
Surprisingly, values in 15-year-old plantation were larger than the 20-year-old plantation despite the differences in the tree diameter in favor of P-20 stand. This finding is also attributable to the large density observed in P-15 with 180–324 stem per plot vis-à-vis 60–149 stem/plot in P-20. On the average, the totalbiomass and carbon stock in P-15 stand were 463.4 ton ha−1 and 208.5 ton ha−1, respectively. Biomass and carbon stock in P-20 were 332.2 ton ha−1 and 145.9 ton ha−1, respectively.
In the natural stand, mean biomass and carbon stock were 323.6 ton ha−1 and 145.6 ton ha−1, respectively. Among the plots, Plot 2 registered the largest estimates where Avicennia marina (4.6–68.2 cm) was noted as the major contributor to total biomass. In general, the biomass and carbon storage of this stand followed a trend: Plot 2 > Plot 3 > Plot 1.
Overall, greater biomass and carbon stock are stored in the plantations than in natural stand. Apparently, the very dense spacing of trees and the presence of silvicultural management to improve timber stock were regarded instrumental for hastening the accumulation of tree biomass. In a similar study conducted by Gevaña and Pampolina (2009) on a mature Rhizophora-dominated stand in San Juan, Batangas, Philippines, smaller carbon stock was however observed. By comparison, carbon stock in that site was estimated to only around 115 ton ha−1, a value which is reflective of its lower density i.e. 17–49 stems per 200 m2. Similarly, estimates for natural stands were also larger compared with other findings abroad. For instance, Lugo et al. (1990) measured about 141.6 ton ha−1 in the mixed mangroves situated within 10° and 35° latitude. Other studies also showed smaller estimates such as in Phuket, Thailand with 159.0 ton ha−1 (Christensen 1978), Boca Chica in Mexico with 135.0 ton ha−1 (Day et al. 1987) and Hainan in China with 248 ton ha−1 (Lin et al. 1990). The comparably highest per-site biomass can be observed in Malaysia with 257.4 ton ha−1 to 286.8 ton ha−1 (Ong et al. 1979), which wasattributed to its dense and protected condition aswell.
Looking at the carbon storage of individual trees sampled, estimates generally increases correspondingly with age (Figure 5). Plotted in a linear regression with acorrelation coefficient of r = 0.89, trend in carbon density can be described as slowly increasing in the first ten years, and dramatically increasing afterwards. At the age of 50 years, individual tree carbon density of aR. stylosa could possibly reach as much as 60 kg. Alikely reason for this trend is the dense condition of the stands that enhances tree growth as trees compete intensely for sunlight. Furthermore, the varying topography, hydrologic regime, erosion, and exposure to current may also hold as significant factors for faster growth and survival (Kaly and Jones 1998; Samson and Rollon 2008). Such assumptions however need further assessment in order to specifically identify the environmental factors that affect biomass and carbon stocks accumulation.
Figure 5. Carbon stock of Rhizophora stylosa across age.
In general, mangrove forest of Bohol, which covers 10,597.62 h has the potential to store as much as 2,010,474 tons of carbon. Roughly, carbon storage canalso reach as high as carbon 37,621.52 tons within 40years if half of the coastal area (101.5 ha) of the province will be developed or rehabilitated into mangrove plantation like that of Banacon. These estimates somehow suggest the good carbon storage and sequestration potential of Bohol.
Conclusion
Tree biomass and carbon stock of mangrove plantations and natural stands in Banacon, Bohol were assessed. Overall, the high estimates observed depict the vigorous condition of the site. More carbon is stored in the plantations than in the natural stand due to high density planting. Observations at tree level also showed that carbon storage capacity of R. stylosa trees generally increases correspondingly with age. Carbon stock of this tree is slowly increasing in the initial plantation stage and dramatically increases at 11years and onwards. At the age of 50 years, its carbon storage capacity could possibly reach 60 kg. Considering therefore that mangrove plantations can be established using high density spacing, carbon sequestration potential can be further enhanced.
Imperative to effective carbon sequestration are the active roles of local communities in protecting and managing the plantations and the sustained financial and technical support that the local government provides. Given that Bohol showcases a huge potential for carbon sequestration projects in the future, further studies to include other sites is essential to accurately estimate the total carbon budget of the province.
On policy level, more mangrove plantations can be established by the local communities if proper incentives are given to them by the government. One incentive is to allow community members to sustainably harvest part of plantations that they themselves established by amending laws such as RA 7161 (1991; An Act Incorporating Certain Sections of the National Internal Revenue Code Of 1977) that prohibit such important livelihood and fuelwood source. This policy action together with a very clear tenure is expected to spur massive mangrove plantations among local communities. Moreover, adopting incentive-based conservation programs such as payment for environmental services (PES) and Reducing Emissions from Deforestation and Forest Degradation projects (REDD) should also be explored in order to stimulate protection and enhance biodiversity, carbon stocks, water, aesthetics and local livelihoods.
Acknowledgement
This research was supported by the ASEAN-Korea Environmental Cooperation Project (AKECOP). It is also partially supported by the Korea Research Foundation as well as the Korea Forest Service.
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References
https://www.tandfonline.com/doi/full/10.1080/21580103.2011.621377
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