Carbon density, storage and spatial distribution in young stands within five afforestation patterns following the ‘Grain for Green’ program (GGP, i.e., conversion of farmland to florets) were investigated in Huitong County, Hunan Province. The five afforestation patterns included Pinus massoniana pure forests, Cinnamomum camphora pure forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, and Elaeocarpus sylvestris and Cinnamomum camphora mixed forests. The results showed that the mean carbon densities of all organs were 05296, 05188, 05178, 05376, and 05355 gC/g for the tree species Pinus massoniana, Cinnamomum camphora, Elaeocarpus sylvestris, Michelia chapensis, and Manglietia insignis, respectively. The average carbon densities in different organs of the five tree species ranked in the order as: trunk>root>leaf >branch>bark. Carbon densities ranged from 04380 to 05380gC/g, 05060 to05200gC/g and 000786 to 001485 gC/g in living-understory layer, litterfall layer and soil layer (0-60 cm depth), respectively within the five afforestation patterns. Three years later after the conversion of farmland to forests, soil organic carbon density was increased 53.6%, 39.2%, 38.6%, and 248% in Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, Elaeocarpus sylvestris and Cinnamomum camphora mixed forests and Cinnamomum camphora forests, but decreased 18.7% in Pinus massoniana forest when compared with pre-converted farmland. Carbon storage in different organs was positively related to the biomass of these respective organs. It was found that Elaeocarpus sylvestris and Cinnamomum camphora mixed forest had the highest carbon stocks in the tree biomass pools (1.22 tC/hm2), while Elaeocarpus sylvestris and Michelia chapensis mixed forest had the lowest value (020 tC/hm2). Trunk and root stored more carbon than other organs, and the two organs accounted for 57.5% of the total carbon storage in all tree organs. At the early stage of the five forested ecosystems, the soil layer (0-60 cm) had the largest carbon storage, ranging from 74.52 to119.31 tC/hm2, and accounted for more than 96.2% of the total carbon storage in the ecosystems. Carbon storage in the plant layer ranged from 063 to 2.96 tC/hm2, which accounted for 064 to 3.8% of the total carbon storage of the ecosystems. The litterfall layer had the smallest proportion of the carbon storage within the ecosystems. After conversion of farmland to forestland, the carbon storage was increased 19.48, 27.72, 41.64, 26.82 tC/hm2 in Cinnamomum camphora pure forests, Cinnamomum camphora and Elaeocarpus sylvestris mixed forests, Michelia chapensis and Manglietia insignis mixed forests, Elaeocarpus sylvestris and Michelia chapensis mixed forests, respectively, but decreased 1.68 tC/hm2 in Pinus massoniana pure forests.. Our results indicated that ‘Grain for Green’ program, especially in the patterns of farmland to mixed forests, had great potential to enhance carbon sequestration in terrestrial ecosystems of southern China.