The Response Of Forest Productivity And Carbon Storage To Landscape Pattern Change In Three Gorges Reservoir Area

The Three Gorges Reservoir Area?TGRA?is an ecological barrier in the middle and lower reaches of the Yangtze River.Affected by the Three Gorges Reservoir Project,land use/cover in this area has dramatically changed during 1992²012.Under this background,this paper analyse the land use/cover change before and after the project implementation,characterize the landscape pattern grain size effect and its dynamic response to LUCC,then discover the forest landscape pattern evolution and fragmentation process based on the land use map in 1992,2002,2006 and 2012?represented important stages of the Project construction?.Meanwhile,applying the local parameterized Biome-BGC model to simulate net primary production?NPP?and vegetation,litter,soil carbon density of main forest types,also reveal the dynamic responses of forest ecosystem NPP and carbon storage to LUCC in TGRA.Main conclusions are as follows:?1?During 1992²012,the farmland area in the Three Gorges Reservoir area decreased by 42.5%,the forest land area increased by 65%,and the construction land and water area increased by 6 times and 2 times respectively.Forest land is mainly distributed at the east top,northeast and southwest of the center part,and the west end of the reservoir.Farmland is mainly distributed at the north of the center part while some scattered forest land.The construction land is concentrated in Chongqing "one hour economic circle" and its surrounding area.?2?The behavior of the metrics in response to altering grain size can be grouped into five types: decreased or increased monotonically,increased stair-step,no obvious response and first increased then rapidly declined.The optimal grain size for landscape pattern analysis is 30⁶0m.Grain size effect of shape metrics showed obvious response to land use change process,while the responses of aggregation metrics,area-edge metrics and diversity index were not affected by the grain size.Landscape pattern of the TGRA became more fragmentary and heterogeneous in 1992²012.?3?i)During 1992²012,forest area and forest coverage rate increased gradually.The structure of forest landscape was relatively stable.ii)Forest was unevenly distributed in the space with most concentrated in the east and center part.During the past 20 years,forest area increased rapidly in the central part,while kept steady in the east and west part.iii)The distribution of patch area shows obvious polarization,which means small fragmented patches were in large quantity while the large patches with areas above 100 hectares were in small quantity.But the polarization was obviously relieved,scattered forest patches were significantly reduced,continuous patches were both increased in quantity and area.?4?i)During 1992²012,forest area?FA?and mean patch size?MPS?in TGRA were significantly positive affected by the average altitude and annual precipitation.ii)In human factors,the urbanization rate had a significant negative impact both on FA and MPS,which means that the influence of urbanization on forestry resources is not only in quantity but also in structure.iii)The forestry investment amount played a significant positive role both in FA and MPS,the elasticity coefficients were 2.269 and 0.158 respectively.This result indicates that increasing the investment amount of ecological construction and protection can directly increase the forest area,then alleviate the fragmentaion of forest landscape pattern.iv)The total papulation,buildings under construction,total forestry output value had significant negative impacts on FA,and highway mileage,rural per capita net income had significant negative impacts on MPS.?5?i)During 1992²014,the NPP of different forest types in TGRA had obvious interannual variation and similar change trend: a curve of continuous alternation of fluctuation over time.ii)The average value of NPP on different forest types from high to low was evergreen needle leaf forest?ENF?550.29 gC m-2 a^-1,needle leaf and broad leaf mixed forest?MIX?504.49 gC m-2 a^-1,deciduous broad leaf forest?DBF?458.69 gC m-2 a^-1,evergreen broad leaf forest?EBF?456.40 gC m-2 a^-1,Shrub274.99 gC m-2 a^-1.iii)The carbon density of different forest types from high to low was ENF 104.76 t hm-2?MIX100.87 t hm-2?DBF 96.98 t hm-2?EBF 93.65 t hm-2,Shrub 80.25 t hm-2.iv)The soil carbon density of different forest types accounts for 62.5%⁸3.9% while the proportion of vegetation and litter carbon density was 28.5%³3.4%,2.87%⁴.27%.?6?i)The results of Biome-BGC model validation show: the simulation value of ENF NPP and MIX NPP was slightly larger than the actual value by plot investigation,the total carbon density was close to the measured value.ii)The simulation values of NPP and carbon density of DBF,EBF and Shrub was lower than actual value.iii)Litter decomposition,biomass distribution,and initial setting of soil environment lead to the uncertainty of soil carbon density.iv)Generally speaking,the simulation result of forest ecosystem in the Three Gorges Reservoir Area by Biome-BGC model reflects the actual situation.?7?i)During 1992²012,the total NPP and unit NPP of forest ecosystem and NPP of different fforest types in TGRA all had obvious interannual variation,affected by land use change and extreme weather.The total NPP and average NPP of forest ecosystem in 1992,1996,2002,2006,2012 were 7.827 Tg C,11.338 Tg C,13.346 Tg,7.171 Tg C,14.999 Tg C and 3.833 tC hm-2 a^-1,5.437 tC hm-2 a^-1,5.235 tC hm-2 a^-1,2.415 tC hm-2 a^-1,4.465 tC hm-2 a^-1.ii)Excluding extreme hot weather in 2006,the NPP of forest ecosystem and main forest types maintained a continuous growth trend,but the growth slowed.iii)In 2006,the total NPP and average NPP of forest ecosystem decreased by 46.3% and 53.9% respectively compared with the same period in the year of 2002 due to the anomaly high temperature and low precipitation.iv)Therefore,the average production of forest ecosystem is mainly influenced by ecophysiological characteristics of plants and the environmental factors such as solar radiation,precipitation and soil,while the total production of forest ecosystem is largely affected by the human factors.?8?During 1992²012,the vegetation,litter,soil and total carbon storage increased continuously but the growth slowed after a rapid rise.The total carbon storage of forest was 200.845 Tg C?205.128 Tg C?250.084 Tg C?291.699 Tg C and 328.421 Tg C in 1992,1996,2002,2006 and 2012 with increase of 2.13%,21.92%,16.64% and 12.59%.In a word,the accumulation of new afforestation,the increase of forest stock and the improvement of forest landscape pattern result in a obvious upward trend of forest carbon storage.