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Spatial Variation in Ecosystem Carbon Stocks and Emissions Resulting from Land-use Conversion: Two Studies in Mangrove Forests of the Dominican Republic
Mangrove forests store more organic carbon across ecosystem carbon pools than most other coastal and forested ecosystems, and are subject to high global rates of deforestation. For these reasons, they are recognized as prime candidates for inclusion in climate change mitigation strategies. However, the ecological drivers of regional and micro-scale variation in mangrove carbon stocks remain poorly understood, and the relative paucity of regionally specific mangrove carbon and land-use conversion emissions values poses a setback to countries seeking to quantify emissions reductions and engage in targeted conservation and restoration. Furthermore, although soil organic carbon (SOC) represents the single largest carbon pool in mangrove ecosystems, contributing an estimated 85 % of ecosystem carbon stocks, and accounting for the majority of greenhouse gas (GHG) emissions arising from mangrove deforestation, regionally specific SOC datasets remain scarce in the Caribbean and Greater Antilles. Those that exist often fail to account for SOC at depths below 1 meter, despite the proven contribution of deep SOC stocks to GHG emissions resulting from land-use conversion. Additionally, few studies have quantified the prevalence of soil inorganic carbon in mangrove forests located in carbonate settings, despite the risk that under-detection of inorganic carbon may result in overestimates of SOC.
To address these gaps, we sampled 20 mangrove forests located in five ecologically distinct coastal regions of the Dominican Republic, quantified total ecosystem carbon stocks (TECS), and provided comprehensive estimates of organic carbon storage and partitioning in biomass and soils to a depth of 3 m (Chapter 2). We also examined relationships among carbon stocks, biomass, and forest structural parameters, and environmental and climate variables (Chapter 2). Finally, we analyzed variation in the edaphic properties of mangrove soils across different regions and depths intervals, and reported inorganic carbon storage and partitioning to a depth of 3 m (Chapter 3).
We found that mangrove forests in the Dominican Republic are highly varied ecosystems that store significant amounts of organic carbon in above- and below-ground pools, with mean total ecosystem carbon stocks of 838 Mg C/ha compared to a global mangrove mean of 856 Mg C/ha. The TECS of individual forest stands varied from 260 ± 59 to 1,311 ± 198 Mg C/ha. We observed significant regional differences in TECS between mangroves in our five study regions, suggesting that regionally specific means can improve the accuracy of nationwide carbon stocks estimates for the Dominican Republic and surrounding nations.
We compared the carbon stocks of intact mangrove forests in Parque Nacional Manglares del Bajo Yuna with those of adjacent, geomorphically similar sites where mangrove forest had been converted to coconut (Cocos nucifera) plantations, and used a stock- change approach to calculate the CO2 equivalent emissions associated with land-use conversion. We found that C. nucifera plantations in Parque Nacional Manglares del Bajo Yuna stored 59 % less organic carbon compared with adjacent, geomorphically similar mangrove stands. We estimate that deforestation and conversion of intact mangrove forests to C. nucifera plantations in this region of the Dominican Republic is likely to result in average emissions of 2160 Mg CO2e/ha over the lifetime of the plantation.
Among mangrove forests, environmental variation was associated with significant differences in tree and downed wood biomass and structural parameters. Interstitial salinity was inversely correlated with forest density in tall (>10 m height) mangrove forests. Also, forests located nearer the marine ecotone stored more carbon in downed wood compared with forests farther inland. However, on the whole, variation in TECS was poorly explained by geomorphic and coastal settings, differences in forest composition and structure, and climate variables. Differences in stand stature drove significant differences in biomass carbon stocks, but not in TECS.
On average, SOC contributed 89 % of total ecosystem carbon, and SOC at depths >1 m contributed 43 %, providing evidence for the importance of sampling soil carbon to depths >1 m in the Dominican Republic. SOC mass at 1m depth, explaining only 8 % of variation in TECS as opposed to 85 % explained by SOC mass at >1 m. While SOC percentages generally decreased with depth, some individual stands and regions subverted this trend and contained rich organic soils with SOC percentages as high as 18 % to depths greater than 1 m.
Our study, the first of its kind in the Greater Antilles region to test 100 % of soil samples for inorganic carbon, offers a more complete picture of the relationship between pH and inorganic carbon stocks in mangroves in the Dominican Republic. Our results indicate that inorganic carbon may account for a substantial (>50%) share of soil carbon in 10 % of mangrove forests in the Dominican Republic, and that environmental variables such as interstitial pH and geomorphology are unreliable predictors of inorganic carbon levels. Therefore, methods that rely on pH and/or field assessments should not take the place of quantitative tests for soil inorganic carbon when determining carbon stocks.
Although the government of the Dominican Republic has demonstrated a commitment to the inclusion of mangroves in Nationally Appropriate Mitigation Action strategies (NAMAS), published inventories of mangrove carbon stocks on the island of Hispaniola and within the Greater Antilles and Caribbean regions as a whole remain scarce. Thus, our regionally specific, scientifically rigorous carbon stocks data contribute significantly to regional and global efforts to address carbon emissions, and underscore the value of mangrove conservation initiatives to climate change mitigation strategies