| Forest resource is the largest carbon pool in terrestrial ecosystem, which contributes carbon to the global carbon cycle incomparably. The Three Gorges Reservoir Area locates in the upstream to midstream of Yangtze River under a special ecological status, where is the key area of ecological and environmental protection in China. The forest ecosystem in this reservoir plays a important role in soil and water conservation, and carbon sink increase, etc. Simulating the carbon storage and productivity of forest ecosystem using model method, on one hand, can make for a more objective assessment of forest ecological functions of Three Gorges Reservoir Area; on the other hand, is conducive to explore more precise regional carbon accounting theory and technical methods through the localization improvement of model parameters.This study considers the Masson Pine, Chinese Fir and other major forest types in Three Gorges reservoir area as the research objects. Firstly, the Age-Volume yield curves are created by comparing sevaral growth functions based on forest resource inventory of continuous sample survey data for each forest type, as the driving force for model to simulate forest carbon stocks and productivity; Secondly, volume-to-biomass conversion parameters and forest biomass component (stem, bark, branch, foliage) proportion parameters of every main forest type were estimated using the power function model and multiple logit model and plots by collecting a large number plots from literature data in various forest types.Ultimately, the model simulation of forest ecosystem carbon stocks and productivity in Three Gorges Reservoir finished, using the estimated yield curve, volume-to-biomass conversion parameters and forest biomass component proportion parameters parameters, biomass turnover and litter decomposition parameters searched from literature,adding the base information as age, area, etc. in Three Gorges Forest Inventory data.Model parameters estimation results are as follows: (1) The Age-Volume growth models of main forest types in Three Gorges Reservoir Area were estimated using the mathematical statistics method of nonlinear regression, based on the optimal model of Richards, Logistic and Korf growth function. Finally the most suitable models for every forest type were chosen through comparing the results of model fit statistics and sampling accuracy test. It shows that the Richards growth function appears its strong adaptability, the Logistic and Korf growth function. All the fitting results of best curves for every forest type are good, their determine coefficient R2are all above0.5, and the RMSE varied from10.99m3·hm-2to21.07m3·hm-2, the results of accuracy test are between44%-80%. The residuals between predicted and observed volume which show normal distribution, increased with the age rising. Coniferous mixed forest has the greatest potential to growth, its accumulation growth limit is352.356m3·hm-2, and the cypress forest is poorest, with the growth extreme value less than80m3·hm-2. All the growth models can provide reference and basis to stand growth prediction and forest management for this region.(2) The model fitting results of volume-to-biomass conversion for every forest type show good relations except evergreen broad-leaved forest, and the predicting residuals increased with the increment of the independent variable value. The root mean square errors of them are all controlled within6.520t·hm-2-23.123t·hm-2. The results of accuracy test are between31.14%-91.79%, all the forest types of it except evergreen broad-leaved forest are above70%. On the other hand, The results of proportion models for biomass components show significant relationship in addition to evergreen broad-leaved forest, and the predicting residuals decreased with the independent variable value increased. The root mean square error of stem, bark, branch and leaf biomass proportion of total tree biomass were all less than0.1, as0.031～0.085,0.005～0.041,0.029～0.103and0.016～0.083respectively. Their results of accuracy test reach91.04%～96.14%,62.71%～94.48%,63.70%-94.47%and-8.86%～83.92%. The models obtained (except the models for evergreen broad-leaved forest) in this paper are reasonable, and the model parameters also can offer reference for the study of the subtropical forest biomass parameters. Model simulation results are as follows:(1) The carbon storage of dead organic matter (DOM) pool of forest ecosystems reaches103.636×106t in Three Gorges Reservoir Area, and the carbon storage of underground DOM is3times more than the aboveground. For different forest types, Masson Pine forest has the highest carbon stocks in DOM (accounts for26.1%to total DOM carbon stocks), follow in descending order are deciduous broad-leaved (22.7%), coniferous and deciduous mixed (20.2%), coniferous mixed (15.2%), Cypress(5.5%), evergreen broadleaved (5.0%), Chinese Fir (3.4%) and temperate pine forest (1.9%). The carbon stocks in DOM carbon density of different types of forest varied greatly, shows that the coniferous forest is significantly less than broadleaved and mixed forests, especially the Cypress forest types which has the lowest DOM carbon intensity.(2) The carbon storage of forest ecosystem reaches151.018×106t in Three Gorges Reservoir Area, where soil organic carbon stock is up to80.163×106t, accounts for53.1%to the total forest ecosystem carbon stocks, and with the biomass carbon stock of47.382×106t (31.4%), litter carbon reserves of23.472×106t (15.5%). The forest ecosystem carbon stocks of different forest are quite different from3.144x106t to40.706×106t. The Masson Pine forest which has the largest distribution area gets the highest ecosystem carbon stocks, while the Cypress forst with lower productivity reserves a minimum. The average forest ecosystems carbon density (vegetation, litters and soil) of107.353t·hm-2of Three Gorges Reservoir Area is below the national average level of258.83t·hm-2, which is far below the world average level of275t·hm-2(vegitation and soil). The average soil organic carbon density is56.984t·hm-2, with the vegetation of33.682t·hm-2, litter of16.685t·hm-2. For different forest types, the ecosystem carbon density of coniferous forest is less than broadleaved and mixed forests, in which the Cypress forest owns the minimal carbon density, the conifer-deciduous mixed forest gets the maximum carbon density.(3) The simulation results of Net Primary Productivity (NPP) and Net Ecosystem Productivity (NEP) for forest ecosystem in Three Gorges Reservoir show that:the average vegetation NPP is3.92t C·hm-2·a-1, of which more than two thirds of the carbon is assigned to litters, results in the emissions of2.71t C·hm-2·a-1. The average NEP is1.21t C·hm-2·a-1. The vegetation fixes5.513×106t atmospheric C every year, in which1.694×106t C sequestrate in ecosystems, and release3.819x106t C returned to the atmosphere because of litter decomposition.. The average annual NPP of every forest types varies from2.20-5.08t C·hm-2·a-1. The carbon fixation capacity of forest in descending order as follows:coniferous and deciduous mixed> coniferous mixed> evergreen broad-leaved> deciduous broad-leaved> Masson Pine> Chinese Fir> temperate pine> Cypress forest. The carbon sequestration capacity are not consistent with its fixation ability fixed carbon, with average annual NEP at0.63t C·hm-2·a-1～2.14t C·hm-2·a-1. The carbon sequestration capacity of forest in descending order as follows:coniferous mixed> Masson Pine> Chinese Fir> evergreen broad-leaved> deciduous broad-leaved> coniferous and deciduous mixed> temperate pine> Cypress forest.By the comparison with estimated results used measured data, CBM-CFS3model simulation results of vegetation carbon density are reasonable, but the litter and soil organic carbon density express the overestimation and underestimation respectively, the dead organic matter including the deadwood and the DOM initialization methods may be the main reasons. NPP estimation results are basically consistent with the measured projection data. Overall, the average forest ecosystems carbon density of Three Gorges Reservoir Area derived from model is lower than the national, and far below the world level. But benefit from its young-middle age structure, the carbon sequestration potential of forest in this area will appear quickly in the future. |
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