| In this paper the spatial distribution pattern of carbon sequestration and inter-annual changes of carbon preliminary were studied for three tree species of Pinus massoniana, Cunninghamia lanceolata, Eucalyptus plantations under different afforestation measures which are main afforestation species on Guangxi. The cost present value and benefit net present value of stand carbon sequestration were compared by using economics principle. And based on a case study of four CDM carbon sinks reforestation projects of southwestern China, cost dynamic changes of carbon sequestration under the CDM projects and the different afforestation models in tCER and lCER were explored. This study is helpful to choice more reasonable afforestation measures of main plantation species, and to provide scientific basis on reducing cost-benefit uncertainty of carbon sequestration, and meanwhile to provide reference for implement feasibility and periodic monitoring, verification and certification of CDM project. The main research results were as follows:1. Carbon spatial distribution and cost-benefit changes of Cunninghamia lanceolata plantations under different afforestation measuresAs the stand age increased of Cunninghamia lanceolata under afforestation measures, Arbor layer carbon and net present value of benefit increased, but present value of cost decreased. The sequences of carbon cost present value of different afforestation measures were: no fertilization < fertilization, thinning < no thinning, and full reclamation < no reclamation, and net present value of benefit was contrast with the cost present value.Total carbon sequestration was respectively 157.86 tC·hm-2 and 143.74 tC·hm-2 of 15-year-old Cunninghamia lanceolata under fertilization and no fertilization, of which arbor layer accounted for 41.26% and 36.67%, understory vegetation layer accounted for 3.50% and 2.43 %, litter layer accounted for 1.67% and 1.16%, soil layer accounted for 56.14% and 59.73%. The annual net carbon sequestration was separately 4.62 tC·hm-2·a-1 and 3.86 tC·hm-2·a-1. The present value of carbon sequestration cost was in turn 220.1$·(t C)-1 and 210.0$·(t C)-1at 15-year-old, and the net present value of benefit was respectively 370.9$·(t C)-1 and 371.9$·(t C)-1.Total carbon sequestration was respectively 150.41 tC·hm-2 and 146.92 tC·hm-2 of Cunninghamia lanceolata under thinning (16-year-old) and no thinning (15-year-old), of which arbor layer accounted for 44.44% and 27.31%, understory vegetation layer accounted for 2.77% and 0.42%, litter layer accounted for 3.08% and 5.99%, soil layer accounted for 49.71% and 66.28%. The annual net carbon sequestration was separately 4.73 tC·hm-2·a-1 and 3.30 tC·hm-2·a-1. The present value of carbon sequestration cost was in turn 236.9$·(t C)-1 and 319.0$·(t C)-1 at 15-year-old, and the net present value of benefit was respectively 302.7$·(t C)-1 and 266.8$·(t C)-1.Total carbon sequestration was respectively 135.78 tC·hm-2 and 129.49 tC·hm-2 of 6-year-old Cunninghamia lanceolata under full reclamation and no reclamation, of which arbor layer accounted for 15.02% and 12.67%, understory vegetation layer accounted for 10.80% and 8.65% , soil layer accounted for 74.19% and 78.69%. The present value of carbon cost was in turn 403.9$·(t C)-1 and 474.7$·(t C)-1 at 6-year-old, and the net present value of benefit was respectively 223.6$·(t C)-1 and 152.8$·(t C)-1.2. Carbon spatial distribution and cost-benefit changes of Pinus massoniana plantations under different afforestation measuresWhile the stand age increased of Pinus massoniana under afforestation measures, Arbor layer carbon and net present value of benefit increased, but present value of cost decreased and net present value of benefit under different thinning intensity decreased at certain stand age. The sequences of carbon cost present value of different afforestation measures were: no fertilization < fertilization, controlled burning < no burning, and control < different thinning, and net present value of benefit was contrast with the cost present value.Total carbon sequestration was respectively 141.31 tC·hm-2 and 121.39 tC·hm-2 of 9-year-old Pinus massoniana under fertilization and no fertilization, of which arbor layer accounted for 45.71% and 40.15%, understory vegetation layer accounted for 1.27% and 0.75%, litter layer accounted for 1.73% and 2.74%, soil layer accounted for 51.30% and 56.36%. The annual net carbon sequestration was separately 7.65 tC·hm-2·a-1 and 5.89 tC·hm-2·a-1. The present value of carbon sequestration cost was in turn 188.3$·(t C)-1 and 177.8$·(t C)-1 at 9-year-old, and the net present value of benefit was respectively 139.9$·(t C)-1 and 170.6$·(t C)-1.Total carbon sequestration was respectively 116.19 tC·hm-2 and 149.43 tC·hm-2 of Pinus massoniana under controlled burning (13-year-old) and no burning (8-year-old), of which arbor layer accounted for 41.78% and 9.76%, understory vegetation layer accounted for 2.96% and 5.72%, litter layer accounted for 2.51% and 0.60%, soil layer accounted for 52.75% and 83.92%. The present value of carbon sequestration cost was in turn 393.4$·(t C)-1 and 511.6$·(t C)-1 at 8-year-old, and the net present value of benefit was respectively -52.7$·(t C)-1 and -165.0$·(t C)-1.Total carbon sequestration was in sequence of 235.76 tC·hm-2, 239.37 tC·hm-2, 237.97 tC·hm-2 and 229.74 tC·hm-2 of 25-year-old Pinus massoniana under intensity, intermediate, mild thinning and control, of which arbor layer accounted for 51.29%, 50.66%, 52.20% and 47.22%, understory vegetation layer accounted for 1.65%, 1.64%, 2.05% and 2.13%, litter layer accounted for 1.74%, 1.98%, 1.49% and 2.12%, soil layer accounted for 45.33%, 45.72%, 44.26% and 48.53%. The annual net carbon sequestration was in turn 5.16 tC·hm-2·a-1, 5.20 tC·hm-2·a-1, 5.31 tC·hm-2·a-1 and 4.73 tC·hm-2·a-1. The present value of carbon sequestration cost was in sequence of 111.0$·(t C)-1, 103.4$·(t C)-1, 96.2$·(t C)-1 and 106.1$·(t C)-1 at 25-year-old, and the net present value of benefit was respectively 195.8$·(t C)-1, 211.1$·(t C)-1, 213.5$·(t C)-1 and 244.4$·(t C)-1.3. Carbon spatial distribution and cost-benefit changes of Eucalyptus plantations under different afforestation measuresWhile the stand age increased of Eucalyptus under afforestation measures, Arbor layer carbon and net present value of benefit increased, but present value of cost decreased and net present value of benefit under different levels of fertilization decreased at certain stand age. The sequences of carbon cost present value of different afforestation measures were: fertilization < no fertilization, fertilization III < other fertilization levels, and net present value of benefit was contrast with the cost present value.Total carbon sequestration was respectively 83.44 tC·hm-2 and 74.53 tC·hm-2 of Eucalyptus under fertilization (3-year-old) and no fertilization (6-year-old), of which arbor layer accounted for 35.58% and 15.82%, understory vegetation layer accounted for zero and 9.12%, litter layer accounted for 2.03% and zero, soil layer accounted for 62.39% and 75.06%. The present value of carbon sequestration cost was in turn 361.9$·(t C)-1 and 2549.3$·(t C)-1 at 3-year-old, and the net present value of benefit was respectively 515.5$·(t C)-1 and -1676.3$·(t C)-1.Total carbon sequestration was in sequence of 227.84 tC·hm-2, 232.19 tC·hm-2 and 216.56 tC·hm-2 of 16-year-old Eucalyptus under different fertilization levels (I, II, III), of which arbor layer accounted for 72.32%, 65.75% and 68.25%, understory vegetation layer accounted for 0.17%, 0.95%, and 0.79%, litter layer accounted for 1.09%, 1.32%, and 1.79%, soil layer accounted for 26.42%, 31.97%, and 29.17%. The annual net carbon sequestration was in turn 10.48 tC·hm-2·a-1, 9.87 tC·hm-2·a-1 and 9.59 tC·hm-2·a-1. The present value of carbon sequestration cost was in sequence of 183.2$·(t C)-1, 181.5$·(t C)-1 and 164.3$·(t C)-1 at 16-year-old, and the net present value of benefit was respectively 416.7$·(t C)-1, 426.0$·(t C)-1 and 429.8$·(t C)-1.4. The effects of clear cutting and burning on stand carbon sequestration of Cunninghamia lanceolata and Pinus massonianaThe effect of clear cutting on biomass carbon of Cunninghamia lanceolata is more obvious than Pinus massoniana; and two tree species had separately a loss of 63.97% and 46.45% of biomass carbon. Controlled burning had greater influence on biomass carbon of Pinus massoniana, which had a loss of 27.42%, but Cunninghamia lanceolata had a loss of 9.49% of biomass carbon. Meanwhile, two stand types were reduced of humus layer carbon sequestration by controlled burning, and it was not obvious to the soil part under the humus layer.5. Carbon cost changes of four CDM afforestation and reforestation projectsThe tCER costs of four CDM projects were decreased gradually, and lCER costs of the projects were increased after reducing under the entire project period. The order of tCER costs of four projects: Multiple-purposes reforestation on degraded lands in Longyang, Yunnan > Afforestation and reforestation on degraded lands in Northwest Sichuan > Reforestation on degraded lands in Northwest Guangxi > Facilitating reforestation for Guangxi watershed management in Pearl River Basin. The lCER costs of multiple-purposes reforestation on degraded lands in Longyang and reforestation on degraded lands in Northwest Guangxi were higher than other two CDM projects. The tCER costs were equal to lCER costs of four projects at the end of the first commitment period, and tCER costs were less than lCER cost at the end of other commitment periods.The tCER and lCER costs of different afforestation models on four CDM carbon projects at the end of the commitment periods were as follows: The costs of Liquidambar formosana + Cunninghamia lanceolata and Liquidambar formosana + Pinus massoniana were higher than Pinus massoniana + Schima superba and Pinus massoniana + Quercus sp., and Eucalyptus was lowest in facilitating reforestation for Guangxi watershed management in Pearl River Basin; The cost of Cunninghamia lanceolata was higher than others, and Eucalyptus was lowest in Reforestation on degraded lands in Northwest Guangxi; The cost of Picea asperata was higher than others, and Poplus szechuanica was lowest in afforestation and reforestation on degraded lands in Northwest Sichuan; Pinus armandi was higher carbon cost than others, and Pinus yunnanensis + Pinus khasya was lowest in multiple-purposes reforestation on degraded lands in Longyang.The sensitivity of carbon cost-benefit was analyzed through single-factor analysis. Carbon sequestration per unit area was a greater impact on the carbon cost present value and timber selling price was more sensitivity to the benefit net present value, when it was fluctuated separately±30% of carbon sequestration per unit area, the cost of afforestation, land rent and selling price of wood.
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