| The vegetation cover of the Loess Plateau has improved significantly over the past 20 years.Exploring the spatial and temporal characteristics of vegetation cover in the region and its influencing factors is of great significance to scientifically evaluate the effectiveness of ecological construction in the region and the influence and feedback of regional vegetation and climate.Based on MODIS-NDVI remote sensing data,this paper uses the image element dichotomy method to calculate the growing season vegetation cover of the Loess Plateau from 2000 to 2018,and uses the trend analysis method and transfer matrix method to explore the spatial and temporal distribution of vegetation cover in the study area in the past 20 years and the change trend characteristics.The correlation between climate factors and vegetation cover status was investigated by using partial correlation analysis.Residual analysis was used to isolate climatic factors and to investigate the spatial variation of human activities on the study area.By comparing three different scales: the Loess Plateau,Yan'an City and Wuqi County,the differences in vegetation restoration drivers in the three areas were summarised.The main findings of the paper are as follows.(1)From 2000 to 2018,the mean vegetation cover of the Loess Plateau increased from 37% to 49%during the growing season,showing an overall fluctuating growth trend.The vegetation cover of the Loess Plateau has increased at a rate of 0.0046/a in the last 20 years,and the distribution of vegetation cover has spatial variability,showing a decreasing trend from southeast to northwest.In the past 20 years,the low cover and medium-low cover of the Loess Plateau have turned out significantly,and the medium cover and medium-high cover have turned in significantly.(2)From 2000 to 2018,the average annual precipitation and average annual temperature of the Loess Plateau showed an increasing trend.The vegetation cover of the Loess Plateau was positively correlated with the average annual temperature and precipitation,and the degree of response to annual precipitation was stronger.In the Loess Plateau,30.52% of the growing season vegetation cover showed a significant positive correlation with annual precipitation,concentrated in the semi-arid areas of the western part of the Loess Plateau in the middle and upper part of the Jin-Shaan Valley and west of the Liupan Mountains.The correlation between summer vegetation cover and mean spring seasonal temperature and seasonal precipitation is stronger than that with the same period in summer,and the influence of climate factors on vegetation cover has a certain lag.(3)The rate of vegetation cover recovery in Yan'an City and Wuqi County within the city limits is higher than that of the Loess Plateau as a whole,with vegetation cover in Yan'an City increasing from50% to 74% and that in Wuqi County increasing from 25% to 55%,with a growth rate of 0.0098/a and0.0111/a respectively in the last 20 years,with significant ecological construction effects.The correlation coefficient between vegetation cover and annual precipitation during the growing season in Yan'an and Wuqi counties is comparable to that of the Loess Plateau,while the correlation for temperature is stronger than that of the Loess Plateau,and the lagging effect of climatic factors on seasonal vegetation cover is more obvious.(4)The positive effect of human activities on the restoration of vegetation on the Loess Plateau accounts for 71.90% of the area,and the positive effect of human activities on the restoration of vegetation in Yan'an City and Wuqi County,as typical areas for returning farmland to forest,is 84.63%and 95.61% respectively. |
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