Analysis Of The Pattern And Driving Force Of Net Primary Productivity Of Vegetation In The Regions Of China’s Grain For Green Program

The Grain for Green Program implemented by the Chinese government is one of the largest ecological restoration projects in the world,which has a huge carbon sink potential and contributes significantly to the goal of carbon neutrality.However,the characteristics of the vegetation dynamics of retired farmlands and the driving forces of carbon sinks in retired farmlands are not yet clear.Therefore,it is important to understand the spatial and temporal characteristics of vegetation in retired farmlands,and to analyze the impact of climate change and human activities on vegetation dynamics and the mechanism of action to promote the sustainable development of Grain for Green Program.In this paper,we determined the extent of retired farmlands during 2001-2020 based on three phases of Globe Land30 land cover data,and determined the net primary productivity（NPP）of retired farmlands during 2011-2020 by CASA（Carnegie-Ames-Stanford approach）model simulation.The spatial and temporal characteristics of vegetation NPP during 2011-2020 were analyzed by combining meteorological,vegetation type and rural population density data,using the Sen’s trend test method,correlation analysis,constraint line method and residual trend analysis,and the influence of climate change and human activities on the changes of vegetation NPP and their relative contribution rates were explored.The main conclusions are as follows:（1）The area of Grain for Green Program during 2001-2020 reached 261,523.08 km²,including 131,297.76 km² from 2001-2010 and 130,225.32 km² from 2011-2020.The retired farmlands was concentrated in northeast,southwest and middle and lower reaches of Yangtze River in China,among which The forest and shrubland was concentrated in the mountainous and hilly areas of the South Central China and Southwest Chian,and the grassland was concentrated in the North China,Northwest China and Southwest China.（2）The spatial distribution of vegetation NPP on retired farmlands during 2011-2020 was high in the southeast and low in the northwest,with obvious spatial heterogeneity,the annual average value ranged from 477.77 to 976.22 g C·m^-2·year^-1,and the ten-year total value reached 2014.41 Tg C.From 2011 to 2020,the NPP of Grain for Green Program showed an overall increasing trend,with significant increases in the Changbai Mountains,Loess Plateau and Yunnan-Guizhou Plateau,and significant decreases in the North China Plain,Huaihe River Basin and southern Yunnan Province.（3）The NPP is influenced by the annual average values of temperature and precipitation during 2011-2020,and the temperature and precipitation show a spatial distribution pattern decreasing from southeast to northwest,with an overall increasing trend in temperature and insignificant changes in precipitation.2011-2020 showed an overall highly significant decreasing trend of rural population density and an overall highly significant increasing trend of animal husbandry output value in pastoral areas.The correlation between vegetation NPP of retired farmlands and temperature and precipitation was mainly positive,and the correlation with rural population density and animal husbandry output value was not high.The constraint relationship between rural population density,animal husbandry output value and NPP of retired farmlands was well fitted,and the constraint lines were all single-peaked quadratic curves.（4）The relative anthropogenic contribution to the vegetation NPP in most of Grain for Green Program during 2011-2020 was 0,indicating that most of the project implementation areas were climate-driven vegetation changes.The percentage of municipalities with positive anthropogenic contribution to vegetation NPP was higher than the percentage of municipalities with negative anthropogenic contribution to vegetation NPP.The cities with positive contribution rates were located in northern China,the Hengduan Mountains and the Wuyi Mountains,and those with negative contribution rates were located in the southern Tibetan Plateau and the middle and lower reached of the Yangtze River.This study analyzed and compared the spatial and temporal patterns of vegetation NPP changes and drivers in the Grain for Green Program,and maked corresponding recommendations for the project.The results of the paper show that the vegetation restoration in the project area has been effective since the implementation of Grain for Green Program,and there is a significant relationship between vegetation growth and climate and human activities.The study provided theoretical support for further implementation of the Grain for Green Program and other ecological restoration projects to improve ecosystem service functions and sustainable development.