| The western China is rich in natural resources,with about 70%of the China’s land area and 28%of the China’s population,and is an important supplier of energy and raw materials in China,playing an important role in maintaining the country’s ecological and environmental security.However,the natural ecological environment in western China is extremely fragile and faces a series of ecological damage and degradation problems.Since the end of last century,China has implemented a series of major ecological projects,such as the project of returning farmland to forest and grass,the project of returning pasture to grass and the project of protecting natural forests,etc.,and the ecological environment in western China has undergone significant improvement.Some research has been carried out on the environmental effects of ecological engineering,but systematic studies of the spatial and temporal patterns of vegetation activity and its drivers in the west are lacking.Therefore,a comprehensive assessment of the dynamic changes of vegetation structure and function in western China since the implementation of ecological projects and quantification of the relative contributions of climate change and human activities to vegetation dynamics can not only provide a scientific basis for improving the rational use of regional land resources,but also provide decision support for rational adjustment of ecological projects and ecosystem management.Firstly,this study used multi-source land use data,includingCLCD,ESA CCI-LC,GLASS-GLC,CAS-LULC and Globe Land30,and analyzed the land use patterns and dynamic changes of major land use types such as forest lands,grasslands and croplands in western China during 2000-2020;Second,based on the Normalized Difference Vegetation Index（NDVI）,Gross Primary Production（GPP）,and Vegetation Optical Depth（VOD）were used to assess the spatial and temporal dynamics of ecosystem functions,and quantitatively assess the relative contribution of climate change and human activities to vegetation recovery during 2000-2018 using residual analysis;Finally,the spatial and temporal dynamic characteristics of Net Primary Production（NPP）in western China from the early 2000s to 2018 and its drivers were studied using the CASA（Carnegie-Ames-Stanford Approach）model and factor analysis methods.The main findings and conclusions are as follows:（1）In the past 20 years,the area of grasslands and bare land/desert in western China has generally shown a decreasing trend at a rate of-0.41±0.73 million km2·a-1 and-0.02±0.51million km2·a-1 respectively,while the overall increasing trend of forests area was the most significant,followed by urban(0.25±0.17 million km2·a-1),water bodies and croplands.The area of forests in both southwestern and northwestern China has been increasing,while the area of croplands and grasslands both showed a trend of"decreasing in the southwest and increasing in the northwest",among which the trend of increasing forest lands and grasslands in northwestern China was not monitored in CAS-LULC.The increase of forest lands in southwestern China was mainly due to the transfer of croplands and grasslands,while the increase of forest lands and croplands in northwestern China was mainly from bare land/desert.The largest area of spatial land use change was based on CAS-LULC,while the smallest area of change was based on CLCD.All datasets show that the area of forest and grasslands was expanding in the Loess Plateau,Sichuan,Chongqing,Yunnan,Guizhou and Guangxi from2010 to 2020.（2）During 2000-2018,Ku-VOD,C-VOD,NDVI,BEPS-GPP,and MODIS-GPP in western China continued to increase,with growth rates of 7.7×10–4 a-1、1.2×10–3 a-1、1.7×10–3 a-1、8.1 gC·m-2·a-1and 5.5 gC·m-2·a-1.Except for BEPS-GPP,all indicators showed higher growth rates in northwestern China than in southwestern China.Except for MODIS-GPP,all vegetation indicators showed that the vegetation growth rate in western China after2010 was higher than that before 2010.NDVI had the highest positive correlation with temperature（30.51%of the total western area）,GPP had the highest positive correlation with precipitation（BEPS-GPP:50.14%,MODIS-GPP:28.63%）,and VOD had a smaller and sporadic distribution with climate.Human activities dominated the increase of VOD,NDVI and GPP in the western China,with contributions ranging from 59%～100%,and the highest contribution to VOD（100%）,especially in the Loess Plateau.Human activities inhibited the growth of vegetation VOD in Inner Mongolia,Qinghai-Tibet Plateau,Guizhou and Yunnan,but promoted the increase of NDVI and GPP.Compared with the post-2010 period,human activities before 2010 contributed more to the Loess Plateau and central Inner Mongolia,and the eastern part of the southwestern China.The climate change suppressed the pre-2010vegetation in the eastern part of the southwestern China and promoted the post-2010vegetation growth.（3）Between the early 2000s and 2018,NPP increased significantly at a rate of 1.90 gC·m-2·yr-1 in western China,with a higher rate of increase in the southwest than in the northwest.The rate of NPP increase after 2010 was higher than before 2010,with a doubling of the rate of increase in the southwest.Compared with NPP under constant climate factors and projected NDVI simulations,the contributions of human activities,radiation,precipitation,and temperature to the annual average NPP increase were 139%,44%,-56%,and-27%for 2001-2018,respectively.Between 2000-2009 and 2010-2018,the contribution of human activities to the annual average NPP was higher after 2010,especially in the Loess Plateau and the Yunnan-Kweichow Plateau,where its positive contribution to vegetation in the western China turned negative due to weakening solar radiation,with the negative contribution mainly concentrated in Shaanxi and the negative contribution was mainly concentrated in Shaanxi and northern Inner Mongolia.The increase in precipitation promoted the vegetation growth in the Sichuan basin and northern Inner Mongolia.The increase in temperature continued to make a positive contribution to Yunnan and Guangxi.Main conclusions:The implementation of ecological projects had contributed to the increase of forest and grassland areas in the Loess Plateau and southwest China.The results of NDVI,VOD,GPP and NPP also indicated that human activities have effectively contributed to the recovery of vegetation in the western part of China,and that ecological projects were important for the improvement of vegetation structure and function.Future eco-engineering focuses on areas where vegetation function has declined or where the contribution of human activities remains low,thus achieving high-quality sustainable development of the ecological environment.As the results of different data sources and methods vary to a certain extent,the assessment of ecological benefits should be adapted to the time and place,and should combine multiple data sources and ground-truthing data to improve the accuracy and scientificity of the assessment. |
