Mangrove ecosystem services (ES) support the global carbon (C) cycle. This study aimed to assess factors affecting the loss or gain of C stocks in mangrove forests in Thailand. Two fundamental considerations were taken into account, including ES supplied by mangroves from the perspective of C stocks, and the potential for C loss resulting from human activities conducted in mangrove forests. Three different land-use types in mangrove forests were studied: an area encroached upon by the local population (L1), a conservation area (L2), (both of which were dominated by the mangrove species Avicennia alba), and a seaside area. Based on their average height and diameter at breast height (DBH), most of the mangrove trees were determined to be young. The highest importance value index (IVI) was seen for A. alba, at 224.73 (L1) and 213.79 (L2). Above- and below-ground C levels were 189.97 t-Cha-1, 77.11 t-Cha-1 in L1 and 81.73 t-Cha-1, 32.54 t-Cha-1 in L2. Soil C stocks were 60.95 t-Cha-1 (L1) and 43.71 t-Cha-1 (L2). Statistical analysis indicated that nitrogen was the crucial factor influencing soil C in both L1 and L2. Overall, the total mangrove C stocks in L1 were estimated to be 328.64 t-Cha-1, which surprisingly was higher than in L2, at 290.34 t-Cha-1. The potential change in C stocks was then assessed. This showed that demand for mangrove resources resulted in the permanent loss of C stocks, particularly within plant communities, as the major fraction of C was from above-ground C stores. The loss of 1 ha of mangrove vegetation was estimated to result in the loss of 77.71-189.97 t-C/ha-1 and 32.54-81.73 t-Cha-1 in L1 and L2, respectively. Different approaches to mangrove management based on the differing supply and demand for ES are recommended.