Variation Characteristics Of NDVI And GPP And Their Response To Extreme Climate In The Loess Plateau

Terrestrial vegetation is a key component of the earth’s carbon cycle.Normalized difference vegetation index(NDVI)and gross primary productivity(GPP)can reflect the growth of ecosystem vegetation to varying degrees.Vegetation growth is mainly driven by climate change and human activities.Facing the challenge of global warming,scientific understanding of the response of vegetation dynamics to extreme climate change has become one of the current research hotspots.As an important ecological strategic region in China,the Loess Plateau has a series of environmental problems,such as fragile ecological environment,serious water and soil loss,and scarce vegetation cover,which restrict the development of local economy.Therefore,it is of great significance to study the interaction and feedback mechanism between extreme climate events and NDVI and GPP in this region.Based on the remote sensing normalized vegetation index(NDVI),gross primary productivity(GPP)data and daily temperature and precipitation data of 79 meteorological stations in the Loess Plateau from 1986 to 2019,the temporal and spatial variation characteristics of NDVI and GPP and their response to extreme climate are analyzed by means of mean method,univariate linear regression,Mann Kendall(MK)trend test and Pearson correlation analysis,The results show that:(1)The temporal and spatial variation characteristics and development trend of vegetation NDVI and GPP in the Loess Plateau are proved.(1)NDVI and GPP in the annual and growing seasons of the Loess Plateau showed a significant upward trend from1986 to 2019,especially after 2000.The seasonal variation of NDVI and GPP in the Loess Plateau is obvious,and the NDVI and GPP in summer are greater than those in spring and autumn.(2)Spatially,NDVI and GPP on each time scale show a spatial distribution pattern of high in the southeast and low in the northwest,among which the upward trend is more significant in Loess Hilly and gully areas,earth rock mountainous areas and river valley plain areas.(2)The temporal and spatial distribution law of extreme climate events in the Loess Plateau is clarified.The extreme climate index selects 16 indexes,including 4 extreme precipitation indexes(CDD,R25,RX1 day,RX5day),5 extreme warm indexes(SU25,TN90 p,TX90p,TMAXmean,TMINmean),4 extreme indexes(TNn,TNx,TXn,TXx)and3 extreme cold indexes(FD0,TX10 p,TN10p).These indexes are based on the extreme temperature and extreme precipitation events defined by the world expert group on climate change detection and indicators(ETCCDMI),It is found that:(1)In the extreme climate index,the loess plateau develops towards warm and humid on the whole during the study period,and the extreme precipitation index shows an upward trend,among which R25 and RX1 day show a significant upward trend,the extreme warm index shows a significant upward trend,the extreme cold index shows a significant downward trend,and the extreme value index does not change significantly.(2)Spatially,the spatial distribution of extreme precipitation index is relatively consistent,showing a distribution pattern of high in the southeast and low in the northwest.Among them,the extreme precipitation index of Zhengzhou,Sanmenxia and their surrounding areas in Henan has an obvious upward trend.Compared with the extreme precipitation index,the spatial distribution of extreme temperature is more complex.The spatial distribution patterns of SU25,TMAXmean,TMINmean,TNx and TXx are similar.The high-value areas are distributed in a few areas in the southeast and northwest,while the high-altitude areas in the West are lower.Among them,the warming trend of Loess Plateau and gully areas in the west loess sorghum gully region and the east earth-rocky mountainous region is more significant.(3)The temporal and spatial response characteristics of GPP and NDVI to extreme climate index in the Loess Plateau are expounded.(1)Under different time scales,the Loess Plateau as a whole is positively correlated with extreme warm index and precipitation,and negatively correlated with cold index.Among them,the correlation between NDVI and GPP and FD0 is the strongest on the interannual scale,and the correlation between NDVI and GPP and tminmean and TN90 p is the strongest on the growth season,spring and summer scale,while it is more sensitive to RX5 day in autumn.(2)Spatially,in the interannual and growing season scales,NDVI and GPP in most areas of the Loess Plateau are positively correlated with the extreme precipitation index,while in the seasonal scale,the areas with positive correlation are mostly distributed in the middle and North.For extreme temperature index,under different time scales,NDVI and GPP in most areas are significantly positively correlated with extreme warm index tminmean and tn90 p,while they are negatively correlated with warm index in Western and Eastern high-altitude areas,Yan’an and Xi’an stations.(3)The lag responses of NDVI and GPP on the Loess Plateau to extreme climate events are different.The monthly NDVI is more sensitive to the change of extreme temperature index in the previous month,indicating that the response of vegetation NDVI to extreme temperature is lagging,while the monthly GPP has a greater correlation with the extreme climate index in the current month,that is,the response to extreme climate index is very timely.In conclusion,the change trend of NDVI and GPP on the Loess Plateau determined in this study and its correlation with extreme climate show that the frequent occurrence of extreme high temperature events caused by global warming may have a serious impact on the ecology and environment of the Loess Plateau in the future.