Carbon stock of disturbed and undisturbed mangrove ecosystems in Klang Straits, Malaysia
Introduction
Mangrove forest has a unique setting, occupying the margin between land and sea, in mostly sheltered tropical and subtropical coastlines. It is a crucial coastal ecosystem, providing important coastal and marine resources for economic and social benefits. It also offers environmental services and critical ecological functions (Bosire et al., 2008; Walters et al., 2008) by protecting inland areas from unpredictable disasters such as typhoons and tsunamis. In addition, mangroves act not only as breeding and rearing habitats for many species of fish and shellfish, but also as a source of valuable products including wood, thatch, medicines, dyes and seafood (Kauffman and Donato, 2012). Mangrove forest can also act as a biofilter, including for heavy metal pollution (Usman et al., 2013).
It is very important to note that mangrove forests are carbon-rich ecosystems that are able to mitigate climate change because they sequestrate a substantial amount of organic carbon (OC) (Mcleod et al., 2011; Siikamäki and Sanchirico, 2012). The global mean carbon stock in mangrove forests is higher than any other forest types, such as tropical forest, temperate forest, boreal forest and tropical savannas (Kauffman and Donato, 2012). The organic carbon is mainly stored in living biomass (above-ground and below-ground biomass) and in soil in the mangrove forests.
Mangrove forests can easily lose organic carbon if they are disturbed, for example changed into another landuse (Kauffman and Donato, 2012; Hamilton and Casey, 2016). Annually, mangrove deforestation releases more than 0.02 Pg carbon per year, which is around 2–10% of carbon release from deforestation in tropical areas, even though mangrove forest accounts for only 1% of the tropical forest area globally (Donato et al., 2012). Despite the many recognized ecosystem services of mangrove forests, the mangrove area has declined globally, between 0.16% and 0.39% per year (Hamilton and Casey, 2016). The majority of contemporary mangrove loss occurs in Southeast Asia, at an annual rate between 3.58% and 8.08% (Hamilton and Casey, 2016), largely due to conversion to shrimp and fish aquaculture, rice, oil palm plantations and urban development. More than 114,000 ha of mangrove forests (2.5%) have been converted to aquaculture ponds, rice or oil palm fields from 2000 to 2012 (Richard and Friess, 2016). A recent study estimated that Malaysia now possesses a total of 629,038 ha of mangrove forest, having lost 0.13% per year of mangrove area since 1990 (Omar et al., 2019). Because of the critical roles of mangroves in global carbon sequestration and providing other ecosystem services, and their vulnerability to land use changes, it is important to estimate the carbon stocks in mangrove ecosystems so as to support climate change mitigation strategies and policies (Adame et al., 2015).
Biomass also determines the potential of carbon sequestration depending on the type of forest, maturity age, species distribution and soil conditions (Alongi, 2012). Generally, the carbon sequestration potential increases with plant size and age (Alongi, 2011; Alongi, 2012). Mangroves ecosystems sequester carbon at mean rate of 1110 to 1363 gC m−2 yr−1 globally with 70% of the carbon captured ending up as biomass (Bouillon et al., 2008; Alongi, 2014). Because of this, it is very crucial to determine the carbon stock and its potential sequestration in mangrove ecosystems, in view of the fast changing land use and anthropogenic activities in this region.
Mangrove materials are generally the major source of the carbon that accumulates in the associated soils (Kristensen et al., 2008; Adame and Fry, 2016). However, in mangrove forests located in river- and tide-dominated areas, the relative contribution of allochthonous (terrestrial or marine) organic matter may be higher (Jennerjahn and Ittekkot, 2002). The isotopic signature of soil organic carbon is widely used to detect the sources of the carbon (Stevenson et al., 2005). The more depleted soil δ13C value reflects more mangrove-derived OC accumulated in the mangrove soils (Chen et al., 2018a). The terrestrial nutrient inputs have been found to increase benthic metabolism and nitrogen dynamics, and decrease the burial/storage rate of OC in soils (Molnar et al., 2013; Suarez-Abelenda et al., 2014). These suggested that the anthorpogenic distubances could also regulate the carbon sequestration capacity of a mangrove ecosystem.
Selangor state is the most populated and developed state in Malaysia, with the most economic activities including industries, agriculture, port and residentials. The mangroves of Selangor are found mostly around the mouths of major rivers such as Selangor River, Langat River and Klang River, as well as along beaches and around islands in Klang district. As one of the states in the country with the largest remaining mangroves, the potential for carbon sequestration in Selangor's mangrove ecosystem is of key importance.
The Straits of Malacca is one of the busiest waterways in the world, with many human activities including shipping, transportation, oil and gas, as well as many industries along its coasts. The Klang Islands consist of seven islands, once covered by pure mangrove forest, in the Straits of Malacca. Because of their proximity to the urban and industrial areas of Port Klang, some of these mangrove areas have been heavily affected by anthropogenic activities. For instance, on Telok Gong and Pulau Ketam, part of the mangrove area has been disturbed and reclaimed for industrial area and human settlements. However, some mangrove areas in Klang are under legal protection as undisturbed forest reserves, such as on Pulau Kelang. Thus, this project aims to identify how anthropogenic activities are affecting the remaining areas of the Klang Islands Mangrove Forest in terms of ecosystem carbon storage potential. We hypothesize that human disturbance will reduce the carbon stock potential especially in the soil pool.
Section snippets
Study area
The Klang Straits Islands Mangrove Forest, consisting of seven major islands, is located between two estuaries, the Klang River Estuary and the Langat River Estuary. Both rivers flow into the Straits of Malacca, forming a Klang Delta and Klang Strait. This group of islands is the largest mangrove ecosystem in the state of Selangor. The samplings were conducted within the Klang Islands Mangrove Forest (Fig. 1), which is located in the state of Selangor, on the west coast of Peninsular Malaysia.
Tree carbon stock
Table 2 shows that Teluk Gong had highest total (both above-ground and below-ground combined) tree carbon (89.18 ± 67.46 Mg C ha−1), followed by Pulau Klang (68.79 ± 43.26 Mg C ha−1) and Pulau Ketam (65.68 ± 19.43 Mg C ha−1), even though the number of tree in Telok Gong was reportedly the lowest (n = 174) among sampling (Zakaria et al., 2018). This was due to it having larger size trees compared to the other two sites, with patches of forest reserve within the intensely human impacted area. The
Conclusion
Large mangrove areas like Klang Straits would certainly contribute to large emissions when perturbed, shown in this study where disturbed areas have reduced their carbon stocks. Since the large proportion of carbon is stored in the soil, it is imperative to clearly understand the impact of land use changes as they can contribute to significantly higher carbon emissions and thus contributes to increment of global warming.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The study was supported by University of Malaya, Malaysia (RP019D-16SUS). We thank Forestry Department of Peninsular Malaysia and Selangor State Forestry Department for the permission and assistance during fieldwork campaigns, staff and students of the Institute of Biological Sciences, University of Malaya. We acknowledged Third Institute of Oceanography, Xiamen China for conducting isotope analysis, Institute of Biological Science and Mr. L P Renshaw for English editing service.
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