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Science of the Total Environment 706 (2020) 135416 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv The impacts of degradation, deforestation and restoration on mangrove ecosystem carbon stocks across Cambodia ⇑ Sahadev Sharma a,1, Richard A. MacKenzie b, , Thida Tieng c, Kim Soben d, Natcha Tulyasuwan e, Amomwan Resanond e, Geoffrey Blate f, Creighton M. Litton a a Department of Natural Resources and Environmental Management, University of Hawaii at Manoa, 1910 East-West Rd., Honolulu, HI, USA b USDA Forest Service, Institute of Pacific Islands Forestry, 60 Nowelo St., Hilo, HI, USA c Asian Institute of Technology, Klong Luang, Pathumthani 12120, Bangkok, Thailand d Royal University of Agriculture, Khan Dangkor, Phnom Penh, Cambodia e USAID LEAD RDMA Program, 87 Wireless Road, Bangkok, Thailand f USAID, 1300 Pennsylvania Ave. NW, Washington, DC, USA highlights graphical abstract Deforestation reduced total Our results concluded, that deforestation and degradation results in significant losses of TEC stocks from ecosystem carbon (TEC) stocks of mangroves. While the prevention of deforestation and degradation is the most effective strategy for cli- intact mangroves by 60%. mate change mitigation and adaptation, it appears that restoration results in mangroves that can con- TEC stocks from degraded mangroves tinue to combat climate change after 25–30 years. forests did not differ from intact forests. TEC stocks from 25-year-old restored mangroves were similar to intact mangroves. A gridded sampling approach effectively captured TEC variability across an entire country. article info abstract Article history: Mangrove forest conservation can help reduce global C emissions. Despite this benefit to climate change Received 1 September 2019 mitigation and adaptation, mangrove forests are being deforested or degraded at an alarming rate, Received in revised form 30 October 2019 though restoration efforts may offset these losses. The impacts of deforestation to C stocks are relatively Accepted 5 November 2019 intuitive and result in significant decreases in C stocks. It remains unclear how degradation from selective Available online 23 November 2019 harvesting of trees affects C stocks or how effective restoration efforts are at restoring C stocks. Editor: Jose Julio Ortega-Calvo Furthermore, total ecosystem C (TEC) stocks of pristine mangroves can significantly vary spatially. To address these issues, we conducted an intensive, national assessment of mangrove forests across Cambodia using a grid approach to: 1) examine how land use land cover (i.e., pristine, deforested, Keywords: Blue carbon degraded, and restored forests) impacts TEC stocks, and 2) how TEC stocks vary spatially across the coun- Carbon inventory try. TEC stocks from deforested mangroves were always lower than pristine forests, resulting in an overall À1 Rhizophora plantation loss of 60% C (480 Mg C ha ). However, TEC stocks from degraded and 25-year-old restored mangroves À Climate change mitigation forests did not differ from pristine forests. Mean TEC in mangroves was 784.7 ± 30.1 Mg C ha 1, ⇑ Corresponding author. E-mail addresses: [email protected] (S. Sharma), [email protected] (R.A. MacKenzie), [email protected] (T. Tieng), [email protected] (K. Soben), [email protected] (N. Tulyasuwan), [email protected] (A. Resanond), [email protected] (G. Blate), [email protected] (C.M. Litton). 1 Present address: Institute of Ocean and Earth Sciences, University of Malaya, C308 Institute of Postgraduate Studies Building, Kuala Lumpur, Malaysia. https://doi.org/10.1016/j.scitotenv.2019.135416 0048-9697/Ó 2019 Published by Elsevier B.V. 2 S. Sharma et al. / Science of the Total Environment 706 (2020) 135416 Gridded sampling decreasing from 957.2 ± 32.8 Mg C haÀ1 in the northern region to 628.9 ± 33.1 Mg C haÀ1 in the central region to 386.2 ± 19.1 Mg C haÀ1 in the southern region of Cambodia. Intensive sampling in mangroves across Cambodia verified impacts of deforestation reported elsewhere, revealed the lack of degradation impacts on TEC stocks, and demonstrated the effectiveness of restoration on TEC stocks after only 25 years. Our gridded sampling approach was able to capture spatial variability across Cambodia and provide a more realistic TEC stock information that can be used for national reporting or participation in C markets. Ó 2019 Published by Elsevier B.V. 1. Introduction As the number of TEC stock assessments have increased over the past decade, it has become clear that the C storage can vary The large carbon (C) stocks in mangroves forests resulting from substantially across global regions and within countries. For exam- the removal and storage of C from atmospheric CO2 have been well ple, across Southeast Asian mangroves, TEC varied from 442 to documented (i.e., Donato et al., 2011; Murdiyarso et al., 2015). 1267 Mg C haÀ1 (Murdiyarso et al., 2015) and from 154 to While this highlights how mangrove conservation can promote cli- 1484 Mg C haÀ1 in West-Central Africa (Kauffman and Bhomia, mate change mitigation and adaptation, deforestation and degra- 2017). Within Indonesia, mangrove TEC stocks varied from 593 dation of mangroves continues to be a major threat to their to 1397 Mg C haÀ1 (Murdiyarso et al., 2015). The wide range of existence. Deforestation, a land use and land cover (LULC) defined TEC stocks reported across global regions is due in part to esti- here as the complete removal of a mangrove forest or stand of mates from most countries being restricted to a very small sample mangrove trees where the land is thereafter converted to a non- size consisting of only a few study plots or transects. Using an ade- forest use, has occurred globally at a rate of 1 to 2% per year. Loss quate number of plots to accurately estimate TEC stocks coupled of mangrove forests has largely been due to conversion of man- with a random and intensive grid approach would cover the grove forests to shrimp and fish aquaculture, rice, oil palm planta- entirety of existing mangrove ecosystems and effectively capture tions, and urban development. In Southeast Asia, where 60% of the spatial variability of TEC stocks at a country level. A grid approach world’s mangroves are found (FAO, 1994; Spalding, 2010), more for national assessments would also allow the development of than 114,000 ha of mangrove forests (2.5%) have been converted higher tier level emission factors that would increase the accuracy to aquaculture ponds, rice or oil palm from 2000 to 2012 and credibility of national reporting and the capacity of participa- (Richards and Friess, 2016). Conversion of mangroves to shrimp tion in voluntary C markets. ponds in Indonesia, Central America, and the Caribbean has The primary goal of this study was to conduct an intensive resulted in C stocks 2–8 times lower than adjacent pristine man- country-wide assessment of TEC stocks in mangrove forests across grove forests (i.e., C loss of 554 ± 230 Mg C haÀ1 assuming that Cambodia to address the following objectives: (1) compare TEC baseline conditions of deforested sites were similar to pristine ones stocks across various mangrove LULC types (pristine, deforested, (Kauffman et al., 2017a)). Similar impacts are expected to occur degraded, and restored); and (2) examine regional patterns in across SE Asia with mangrove deforestation, yet few studies have mangrove TEC stocks across Cambodia. We hypothesized that: quantified this despite the large areas of mangroves there, the high (H1) TEC stocks would be highest in pristine mangroves because rates of deforestation, and the potential loss of large amounts of of C loss from timber extraction and soil degradation in deforested terrestrial C. and degraded plots; (H2) TEC stocks would be greater in degraded Forest degradation is a LULC characterized by a change in the plots than deforested plots due to the forest structure that still original forest condition through disturbances such as illegal cut- remains in degraded plots after the harvest of a few select trees ting for fuelwood, burning, or human structures that compact soils versus complete clear cutting of deforested plots; and (H3) or alter hydrology. Forest degradation can also be caused by natu- restored mangrove plots would have lower TEC than pristine man- ral phenomena such as cyclones, diseases, die back, tsunami etc. grove plots due to the young age and development of the above This leads to a reduction in capacity of the forest to provide goods and belowground forest biomass. or services but does not result in a land cover change (i.e., conversion to non-forest land) (Bahamondez and Thompson, 2016; Krauss et al., 2010; Simula, 2009). As a result, degraded sites, 2. Materials and methods unlike deforested sites, still support some forest structure and thus land cover that can be difficult to quantify from remote sensing. 2.1. Study area This suggests that while degraded sites may have lower TEC stocks compared to pristine forests, they will likely be greater than defor- Cambodia lies within the tropics between 10° and 15°N latitude ested mangroves. Furthermore, if degradation is not included as a and 102° and 108°E longitude, with 435 km of coast (Fig. 1)(Rizvi LULC, field or remote sensing assessments may overestimate TEC and Singer, 2011). While mangrove forests are found along the stocks of mangrove forests. entire coastline of Cambodia, we focused our study on the Koh To offset losses from deforestation as well as to reduce C emis- Kong and Preah Sihanouk provinces as more than 90% of Cambo- sions, several countries and non-governmental agencies have dia’s mangroves are found there (FAO, 1994). Within Koh Kong invested heavily in projects to restore mangroves by planting province, we sampled mangroves from Peam Krasoap Wildlife monospecific stands of Rhizophora sp. (Wylie et al., 2016).

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