| Society and economy as well as the ecological environment we depend on have been critically influenced by global warming in recent decades.As one of the most important parts in the terrestrial ecosystem,vegetation provides a wide range of ecological funct ions such as storm prevention,sand immobilization,soil and water conservation,and climate regulation.Vegetation cover change is very complicated in China due to rapid growth of national economy and population,increasing impact of human activities on the vegetation ecosystem and the regional difference of climate change.It is necessary to understand the spatio-temporal variation of vegetation cover and scientifically analyze the interaction between vegetation,climate,and human activities,which can contribute to the formulation and implementation of the national strategy for ecologically sustainable development.However,the current studies differ greatly in the research scale,region,and period,and the attribution analysis of the national vegetation cover change needs further investigation.Therefore,based on the long-term data of climate,land use,and Normalized Difference Vegetation Index?NDVI?datasets and various methods of Kriging interpolation,trend analysis,coefficient of variation,correlation analysis,multiple regression and residual analysis,this study explored the changes of climate,land use,and vegetation cover in China from 1982 to 2015 at varied spatiotemporal scales,and then evaluated the drivers of vegetation cover change and their contributions.Finally,we analyzed the impacts of land surface processes dominated by vegetation cover change on surface air temperature in the Loess Plateau where vegetation cover has been undergone significant restoration with a widely-used OMR method?Observation Minus Reanalysis?worldwide.The main conclusions of this study are as follows:?1?Climate warming in China is significant during 1982-2015 and the average increasing trend of annual mean temperature is about 0.35?/10a?P<0.01?.The change rates of annual mean temperature varies in each region,and it has a clear decline in the eight vegetation sub-regions as follows:Qinghai-Tibet Plateau alpine meadow region?V8?,Temperate desert region?V7?,Temperate grassland region?V6?,Evergreen broadleaf forest region?V4?,Deciduous broadleaf forest region?V3?,Rainforest region?V5?,Broadleaf–coniferous mixed forest region?V2?and Coniferous forest region?V1?.While the annual total precipitation showed an unobvious trend in China,it increased the fastest in V5 and decreased the fastest in V1.Moreover,the seasonal temperature and precipitation generally showed certain interdecadal variations in China,whereas their overall change trends showed great spatial heterogeneity.On average,seasonal mean temperature increased rapidly before 1998,and then increased slowly,and even decreased in winter;precipitation in autumn decreased obviously before 1998,and then increased obviously,which was contrary to the change of precipitation in summer.In recent 34 years,the temperature in spring showed the fastest increase,while the temperature in winter showed the slowest increase;there was no significant change in precipitation in each season.At the provincial scale,while the annual mean temperature showed the fastest increase in Shanxi,it showed the slowest increase in Hainan;while the annual total precipitation showed the fastest increase in Hainan,it showed the fastest decrease in Hubei.In general,while the climate in most areas of northwestern and southeastern China showed a warming and humid trend,it showed a warming and drying trend in the strips extending from the northeastern to the southwestern.?2?The area of regions where have experienced land use change in China during1980-2015 accounted for about 17%of the total land area.Among the six major land use types,areas of farmland and constructed land increased,while areas of grassland,forest,unused land,and water decreased.From the annual change rate,while the farmland area showed the fastest increase in V1,it the fastest decease in V4;the construction land area showed the fastest increase in V8,followed by V7 and V4.At the provincial scale,while the farmland area showed a rapid increased in Xinjiang,Heilongjiang,and Inner Mongolia,it showed a rapid decrease in Shanghai,Beijing,and Zhejiang;while the constructed land area showed a rapid increase in Chongqing,Guizhou,and Shanghai,it showed the slowest increase in Heilongjiang.In general,Eastern China showed an obvious increase in constructed land,which mainly came from farmland transforming;Northeastern and northwestern China showed an obvious increase in farmland,which mainly came from grassland and forest transforming.All of these implying the strong impacts of human activities on the spatial distribution of vegetation cover.?3?It was suggested that the GIMMS NDVI3g data is suitable for monitoring and analyzing vegetation cover change in China after a systemic comparing the GIMMS NDVI3g dataset with the SPOT-VGT NDVI dataset.Based on the annual mean NDVI trend generated from the GIMMS NDVI3g data,we found that about 70%areas of the country showed an increase in vegetation cover during 1982-2015;while the middle and lower reaches of the Yellow River,the middle reaches of the Yangtze River,and the Huaihe River Basin showed a rapid increase in vegetation cover,some areas in the northeastern and northwestern China showed a decrease in vegetation cover;at provincial scale,all provinces except Heilongjiang and Jilin showed an increasing trend in annual mean NDVI.On average,annual mean NDVI showed an increasing trend in China during 1982-2015with a rate of 0.51×10-3 yr-1?P<0.01?,while it showed rapid,slow,and rapid increasing trends during 1982-1990,1991-2004,and 2005-2015,respectively.NDVI trends of the eight vegetation sub-regions in China varied from-0.42×10-3 yr-1 to 1.55×10-3 yr-1,which were ranked as follows:V3>V4>V5>V6>V7>V8>V1>V2.NDVI trend in each season showed a large spatial heterogeneity.On average,NDVI in spring and autumn showed rapider increasing trends than that in summer and winter.In generally,vegetation in China showed a well inter-annual stability during 1982-2015;in the areas covered by vegetation?except such as bare land and water?,the better the vegetation cover in the regions?or seasons?,the stronger inter-annual stability of vegetation.?4?Regional differences in status and changes of vegetation cover in China were closely related to the spatiotemporal variation in climate and land-use.Climate warming,especially the warming in spring,was the major climatic factor contributed to the increase of vegetation cover in China in the past 34 years.Responses of vegetation cover to the temporal changes in temperature and precipitation showed large difference in different regions.For example,vegetation cover was sensitive to the precipitation change in arid and semi-arid regions.Vegetation cover changes were impacted by both climate change and human activities in most regions of China.The contributions of climate change and human activities to the increase in vegetation cover in China in the past 34 years were estimated to be about 40%and 60%,respectively.The contributions of human activities to the vegetation cover changes for the eight vegetation sub-regions of China were varied from 0to 81%,and ranking from great to weak being as follows V6,V3,V2,V5,V4,V1,V7 and V8.At the provincial scale,the trend rate of growing season NDVI caused by human activities was the largest in Shanxi(1.77×10-3 yr-1),while it was the smallest in Shanghai(-0.32×10-3 yr-1).Moreover,vegetation cover changes in most provinces of China were mainly caused by human activities?contribution>50%?,and the largest contribution of human activities to the vegetation cover change was found in Shanghai.Overall,human activities showed a positive effect on the restoration of vegetation cover in China in the past 34 years,and their contribution was larger than that of climate change.?5?The OMR temperature in the central and eastern parts of the Loess Plateau showed a rapid decline from 1982 to 2015,indicating that surface temperature in these areas was negatively impacted by land surface processes such as vegetation cover change and urbanization.For the non-urbanized areas of the Loess Plateau,both the basic status and long-term change of vegetation cover could affect the surface air temperature,but the effects varied with spatial scale.For the whole Loess Plateau,the better the vegetation cover and the faster the vegetation restoration,the more effective it is to alleviate the rise of surface air temperature;for the eco-geographical sub-regions,the increase in vegetation cover could alleviate surface air temperature warming in semi-arid and semi-humid regions,yet it could promote surface air temperature warming in arid areas.For the non-urbanized areas,the impacts of the basic status and long-term change of vegetation cover on surface air temperature changes in the past 34 years were estimated to be-0.02,0.04,-0.01,and-0.07?/10a for the whole Loess Plateau,arid region,semi-arid region,and semi-humid region,respectively;comparing with the period before implementing Grain for Green Project?GGP?,the increase in vegetation cover in the period implementing GGP led to a decrease in regional average surface air temperature by 0.05?.The results showed that the obvious increase in vegetation cover in the Loess Plateau has alleviated the regional climate warming to some extent in the past 34 years.In summary,vegetation cover in China showed an increasing trend in the past 34 years,but its variation showed obvious spatiotemporal differences,which can be mainly attributed to the impacts of both climate change and human activities.Human activities could positively and negatively affect vegetation cover,such as vegetation increase due to ecological construction?e.g.,GGP?and vegetation degradation due to urbanization.While the impact of precipitation change on vegetation cover was relatively obvious in arid and semi-arid regions,climate warming,especially the dramatic warming in spring was the main climatic contributor to the increase in vegetation cover in China.Overall,the contribution of human activities to the increase in vegetation cover was larger than that of climate change in China;meanwhile,climate warming in the Loess Plateau was alleviated by the increase in vegetation cover to some extent.However,because of the limitations of climate,soil moisture,and land use/land cover types,surface air temperature warming was amplified by vegetation restoration in arid region of the Loess Plateau,which may further worsen the water crisis in this region.In view of this,afforestation efforts perhaps should focus on the humid and semi-humid regions with adequate water supply.The understanding of relationship between vegetation,climate,and human activities in this study could promote a deeper understanding of the key role of human activities in the sustainable development of the ecosystem,and provide associated theoretical support for the ongoing ecological civilization construction in China. |
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