Spatio-temporal Variation Of Vegetation Coverage And Its Response To Climate Change In The Source Region Of The Yellow River From 2000 To 2017

Vegetation is a sensitive indicator in the study of environment and global change,while climate change also affects the growth dynamics and distribution of vegetation.As an important part of the ecosystem of the Tibetan Plateau,monitoring the distribution,change trend and driving mechanism of the vegetation coverage in the source area of the Yellow River is of great significance for maintaining the ecological security and sustainable development of the whole basin.Based on MODIS NDVI,meteorological data,DEM data,vegetation type,and soil type,through the techniques of the dimidiate pixel model,spatial interpolation method,trend line analysis and correlation analysis,the spatial and temporal change of vegetation coverage and its response to climate change is studied in the source region of the Yellow River from 2000 to 2017.Results show the following conclusions:(1)The mean annual FVC of vegetation in the source area of the Yellow River fluctuates greatly,with a range of 0.49 to 0.56,and the growth rate is 0.50%/(10a).The spatial characteristics of the vegetation show a trend of increasing gradually from northwest to southeast.The high value areas of vegetation FVC mainly concentrate in the eastern of the study areas,while the low value areas are mainly distributed in the Qumalai,Maduo and northern Xinghai counties.The annual variation range and fluctuation rate of vegetation FVC varied in different seasons.The maximum change rate of vegetation FVC value in summer was 0.82%/(10a).In addition,with the increase of altitude,the average annual vegetation FVC in the source area of the Yellow River and the vegetation FVC in different seasons take the elevation of 4000 m as the demarcation point,showing a trend of first increase and then decrease.(2)The FVC degraded areas of the source area of the Yellow River(26.65%)are distributed in Qumalai,Maduo,Dari and Maqu counties,while the improved area accounts for 28.92% of the total area,mainly around Longyangxia Reservoir in the north.The vegetation types in the improved area were alpine grassland and alpine meadow.The change trend of vegetation FVC in different seasons is basically the same as that in the whole year.The largest area of the vegetation FVC in autumn,growing season and summer shows a degenerative,unchanged,and increased trend respectively.The relative change rate of the FVC is between-0.32 and 0.32,which has obvious seasonal difference.(3)There is a time-delay effect on response between the FVC and climatic factors in the source area of the Yellow River.The spring vegetation FVC is the most closely related to monthly mean air temperature and monthly accumulated precipitation in the same period;In summer,vegetation FVC lags one month for both air temperature and precipitation;In autumn and growing season,vegetation FVC responds rapidly to air temperature in the same period,but lags one month for precipitation.(4)The vegetation FVC is positively correlated with air temperature,mainly distributed in Zeku,Henan,Maqu,Jiuzhi,Aba and north of the Maduo counties.And it is also positively correlated with precipitation,mainly distributed in Guinan,Xinghai,Tongde,Maduo and Aba counties.The annual average and seasonal vegetation FVC in the source area of the Yellow River shows significant differences with air temperature and precipitation.The pixels of vegetation FVC in spring and autumn are dispersed,and FVC of vegetation in summer and growing season is highly significantly correlated,which is mainly distributed in Xinghai and guinan Counties.With the increase of altitude,the correlation between vegetation FVC and air temperature changes from negative correlation to positive correlation,and the partial correlation coefficient with precipitation shows a significant downward trend.(5)Under the 0.05 significance test,the proportion of vegetation FVC driven by climate factors was 10.22%,in which there are 3.18% and 4.53% area impacted by air temperature and precipitation respectively.The strong driven of air temperature and precipitation accounts for 1.26% and the weak driven of air temperature and precipitation only accounts for 1.25%.The regions affected by climate factors in FVC of vegetation in each season are growing season,summer,spring and autumn from large to small.The vegetation FVC in the source area of the Yellow River is generally driven by non-climatic factors(air temperature and precipitation).