| Vegetation net primary productivity(NPP)is the basis of the ecosystem material and energy cycle,and it is also an important part of the carbon cycle.Land use/cover change(LUCC)is one of the main drivers of NPP.At global or regional scales,studying the impact of LUCC on NPP caused by human activities is not only beneficial to understand the global carbon cycle and its regulatory mechanisms,but also is of great significance for a profound understanding of its response to and feedback to global climate change.Therefore,this paper analyzed the characteristics of spatio-temporal changes of land use/cover from 1990 to 2015 in the Three Gorges reservoir area(TGRA),revealed the long-term evolution process of land use/cover in the TGRA,estimated the NPP in the TGRA from 2000 to 2015 based on the CASA model,and analyzed of temporal and spatial dynamics of NPP in the TGRA,further explored the effect of land use change on NPP in the TGRA over the past 15 years.The main findings are as follows:(1)Woodland and farmland are dominant areas in the TGRA,which together account for more than 93.82% of the total land area.From 1990 to 2015,the land use/coverage in the TGRA has undergone significant changes.The main features are: the area of woodland has increased by 12,842.82 km2(71.02%);the largest area reduction was observed in farmland with a total decrease of 14,948.81 km2(39.05%);the smallest area reduction was observed in grassland with a total decrease of 254.80 km2;the water area increased by 86.40%,and the construction land area increased by 10.06 times.The woodland are contiguously distributed in the four counties of Hubei,and Wushan,Wuxi,Fengjie in the northeast of the top,Shizhu,Fengdu and Wulong in the southwest of the center part,and the southern part of Jiangjin in the end of reservoir.Most of the farmland is distributed in the north of the center part and the end of reservoir.Water area is concentrated in the Yangtze River main channel and the main tributary basin.Construction land is mainly distributed in Chongqing “one-hour economic circle”.(2)The land use transfer matrix analysis of the TGRA from 1990 to 2015 shows that the transfer of woodland,farmland,and construction land is more frequent.The reduced farmland is mainly converted to woodland and construction land,of which the area of farmland converted to woodland is as high as 12974.52 km2,at the same time,there is a small amount of forest land reclamation as farmland;the farmland area occupied by construction land is 1383.34 km2;in addition,there is a small part of farmland converted to water areas and unused land;the area of grassland has been reduced,which mainly converted to woodland.During the process of land-use change,the area of farmland in the reservoir area has decreased steadily,and the number of the population has increased steadily,the per capita farmland has decreased,and the contradiction between people and land has become increasingly severe.(3)The woodland and farmland landscapes in the six landscape types in the TGRA always dominate the landscape.From 1990 to 2015,the largest patch index of woodland increased,the number of patches and density of patch increased,the shape index decreased,the degree of influence of woodland on the reservoir area became larger,the degree of fragmentation of woodland decreased,and the shape tended to be simplified;The number and density of patches increased,the degree of aggregation decreased,the density of edges increased,the farmland became more fragmented,and the degree of isolation between patches increased;the number of patches and patch density of grassland decreased,and the degree of aggregation increased,indicating the degree of grassland fragmentation.The number of patches and patches density in water areas,construction land and unused land increased,and the degree of patch fragmentation was strengthened.The overall landscape heterogeneity of the TGRA is enhanced,landscape dominance and landscape connectivity are reduced,showing a trend of fragmentation,diversification,homogenization and irregularity.(4)The average NPP in the TGRA in 2000,2005,2010 and 2015 was 527.99?562.32?554.28 and 601.60 g C?m-2?a-1.The change process of the NPP total value is the same as the NPP average value.From 2000 to 2015,the total amount of NPP showed a fluctuating growth trend.The total NPP increased from 30.69 Tg C in 2000 to 34.96 Tg C in 2015,with an increase of 4.27 Tg C(13.91%).The monthly change of vegetation NPP in the reservoir area showed a unimodal curve,the lagest NPP was observed in July,and the smallest NPP was observed in December,accounting for only 2.14% of the average annual NPP.The period from March to September is the main accumulation stage of NPP,and the total NPP of these seven months reached more than 85% of the annual total NPP.(5)The spatial distribution of NPP in the TGRA is uneven and there is obvious spatial heterogeneity.The county with the highest average NPP in the reservoir area is Wuxi County,which has reached 643.98 g C?m-2?a-1,and the smallest NPP was observed in the main urban area of Chongqing.Throughout the 15 years,only the average NPP of the main urban area of Chongqing in the TGRA has decreased(by 6.69 g C?m-2?a-1).The average NPP of the other 19 districts and counties has increased,the largest increase was observed in Shizhu County(114.44 g C?m-2?a-1).With the increase of altitude,the average NPP and total NPP in the reservoir area both increased first and then decreased,reaching the maximum at 1800-2100 m and 300-600 m altitude respectively,and they were 700.20 g C?m-2?a-1 and 9.36 Tg C.The middle and low altitude areas are the main distribution areas of vegetation NPP in the reservoir area.The total amount of vegetation NPP in the area below 1500 m accounts for 90.23% of the total NPP area in the reservoir area.(6)The average NPP of the woodland in the TGRA was the highest,which was 567.70-643.91 g C?m-2?a-1,followed by the farmland 520.02-613.20 g C?m-2?a-1.The non-vegetation type NPP is relatively low,the average NPP of construction land is lowest,which is 149.90-184.27 g C?m-2?a-1.In terms of annual average total amount,woodland and farmland are the main contributors to the total NPP of vegetation in the TGRA.The average contribution rates for each year are 49.01% and 49.39% respectively;the distribution range of other land types is small,and the total amount of NPP is not high.The lowest contribution to regional total NPP is unutilized land,with an average contribution rate of 0.25%.From 2000 to 2015,the total amount of NPP of different land use types was significantly different in each research period,and its change trend was similar to that of land use type's area.(7)NPP loss caused by conversion of farmland into construction land is significantly greater than other land-use conversion methods,and its NPP loss is 0.1644 Tg C,accounting for 50.94% of the total NPP losses during the same period.The total NPP loss for conversion to construction land is the largest,and the total NPP revenue for conversion to woodland is the largest.In the recent 15 years,urbanization has led to a significant increase in the area of construction land.However,during the entire study period,the total NPP in the TGRA has not decreased,but has shown an increasing trend,indicating that the restoration of farmland to forests and the protection of natural forests under the guidance of major national macroeconomic policies have played a catalytic role in restoring the ecological environment of the reservoir area.At the same time,landscape pattern characteristics and changes have a significant impact on NPP in the reservoir area.The increase in marginal density,patch density,and landscape diversity led to an increase in NPP in the reservoir area.With the increase of patch aggregation and maximum patch index,the reservoir area NPP showed a decreasing trend.Therefore,it is an important way to increase regional carbon sequestration capacity by appropriately returning farmland to forests,afforesting,renewing mature forests,enhancing heterogeneity and abundance of region vegetation when planning and constructing cities. |
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