Forest ecosystems act as a carbon sink and contribute to climate change mitigation. Research on the spatiotemporal patterns of C storage and density in forest ecosystems is essential to understand the role of forest ecosystems in the C sink and is helpful to select the efficient forest management practice to maximize the C sequestration potential. We quantified the C storage in forest ecosystems in Hunan Province in southern China over two decades by combining forest inventory data with field survey. The C storage in forest ecosystems in Hunan Province increased from 820.2 Tg to 1277.8 Tg over the two decades, with 457.6 Tg (134.4 Tg in vegetation and 323.2 Tg in soil) of C sequestration in the forest ecosystems. Forest C storage increased sharply from 1996 to 2007, but slowly from 2007 to 2015. The mean annual C sequestration was 25.8 Tg yr(-1), with 19.0 Tg yr(-1) in soil and 6.8 Tg yr(-1) in vegetation. The C density in forest ecosystems increased from 110.3 Mg ha(-1) in 1996 to 130.8 Mg ha(-1) in 2013. C densities varied in forest types, with the highest value in evergreen broadleaved forest ecosystems. The uneven spatial distributions of forest C storage, density, and sequestration in Hunan Province exhibited similar pattern with the highest in the western Hunan and the lowest in the central Hunan. The forest ecosystems in Hunan Province present a significant C sequestration potential (1321.5 Tg, including 1029.2 Tg in biomass C and 292.3 Tg in soil C), given the proportion of the area of young and middle-aged forests (71.3%). To maximize the C sequestration potential of forest ecosystems in Hunan Province, future forest management should focus on the conversion of forest type, the selection of tree species in reforestation, and the prevention of adverse human disturbances in young and middle-aged forests.