Temporal And Spatial Change Characteristics Of Vegetation Coverage In Northern China And Its Response To Climate Factors And Drought

Under the combined influence of climate change and human activities,global vegetation cover is changing year by year.Vegetation in northern China is more susceptible to climate change than that in southern China where vegetation is well covered.Based on the GIMMS NDVI 3g data from 1982 to 2015,the temperature and precipitation data of 342 meteorological stations in northern China and surrounding areas combining with the standardized precipitation evapotranspiration index SPEI,the response of temporal and spatial variation characteristics of climatic factors and droughts,temporal and spatial variation characteristics of vegetation NDVI values and vegetation cover to climate change in the northern part of China are analyzed through software such as ENVI,Arc GIS and Matlab.The main conclusions of this study are:(1)In the study area,only the Tien Shan area in Xinjiang has experienced an insignificant decrease in temperature during the study period.The average annual temperature in other regions has increased year by year,and the rate of rising annual average temperature is about 0.2 °C-0.6.°C/10 a.The precipitation in the study area shows the scarcity of the northwest region and the abundant spatial distribution of the southeastern region.In the 34 years studied in this paper,the annual precipitation rate in the study area is-0.52mm/10 a,and there are seasonal differences,except for summer precipitation.The precipitation in other seasons showed an increasing trend year by year,but the changes were not significant(P>0.05).Through the analysis of SPEI time variation in the study area,the drought and wetness changes in the study area are mainly divided into two time periods.The first stage is 1982-1996,the average SPEI is 0.32.The rate of change is-0.01/10 a,which gradually shows a transition from wetness to drought.The second stage is from 1997 to 2015,the average SPEI of this stage is-0.23,and the rate of change of SPEI is 0.22/10 a.This stage shows a gradual transition from drought to wet,drought has eased.(2)The NDVI of vegetation in northern China showed a fluctuating trend with a growth rate of 0.004/10 a,and the change was significant(P<0.01).Through the analysis of the MK test results of the annual average NDVI value change,the change of vegetation NDVI value in the study area can be divided into four different stages:1982-1989,the upward trend,NDVI change rate of which is 0.0111/10a(P<0.05).The areas with improved vegetation cover at this stage are mainly distributed in the North China Plain,the Liao River Plain and the northern part of the Greater Khingan Range.The trend of NDVI is increasing from 1989 to 2000,and the NDVI rate of change is 0.0016/10a(P>0.05).Vegetation improvement and degraded areas are interlaced.The degree of vegetation cover improvement and degradation is not obvious.From 2000 to 2006,the trend is downward,and the NDVI change rate is-0.0010/10a(P>0.05).The vegetation in the study area appears in this stage.The degraded trend and the severely degraded areas are mainly distributed in Changbai Mountain area,Lesser Khingan Range,northern Greater Khingan Range and Xinjiang Tien Shan area.The trend of NDVI is 0.0018/10a(P<0.01).During the period,the vegetation cover in the study area showed a gradual recovery.The areas with obvious improvement in vegetation cover were mainly located in the Loess Plateau and Eastern Inner Mongolia Plateau..(3)The NDVI value of vegetation in the study area is synergistically affected by both temperature and precipitation.That the area of vegetation NDVI value positively correlated with temperature and precipitation is much larger than that of negative correlation.Vegetation NDVI values are roughly the same as SPEI-12 and SPEI-24,which means that most of the northern vegetation has a significant response to precipitation in the previous year.The correlation of natural grassland vegetation to SPEI-12 is high,indicating that grassland vegetation activity is driven by precipitation accumulated over the past 12 months.The correlation between natural woodland vegetation and SPEI-24 is high,indicating that long-term precipitation conditions can affect the growth status of forest vegetation.In the vegetation NDVI value and shorter time scale SPEI-3,the response of spring crops to SPEI-3 was more obvious,and the response of summer grassland vegetation to SPEI-3 was more obvious.