Spatio-temporal Characteristics Of Vegetation Cover In China From 2001 To 2017 And Its Response To Moisture Factors

As an important component of terrestrial ecosystems,surface vegetation plays an important role in wind protection and sand fixation,soil and water conservation,and climate regulation.In order to reverse the serious degradation of vegetation since the founding of the People's Republic of China,our country has carried out nationwide vegetation restoration projects such as Natural Forest Protection Projects,Cropland to Forest and Grassland since the late 1990 s.Large-scale vegetation restoration not only improve ecological function,but also causes increasing vegetation water consumption,threatening water supply in water-deficient areas.Therefore,studying the spatio-temporal dynamics of vegetation coverage at the national scale and the temporal and spatial differentiation of its response to moisture factors is of great significance for assessing the water resources carrying capacity of vegetation restoration in various regions and formulating vegetation restoration plans based on the sustainability of water resources.However,the current research on the spatio-temporal characteristics of surface vegetation cover changes and the response to moisture factors are combined with remote sensing technology and traditional geostatistical methods for simple spatio-temporal correlation analysis,lacking effective detection of spatio-temporal differentiation of remote sensing data such as vegetation and moisture factors.It hinders the in-depth study of the spatial heterogeneity of the response of regional vegetation changes to moisture factors,and affects the accuracy of the zoning simulation modeling of the spatial and temporal effects of vegetation-moisture.In order to solve the above problems,this thesis uses Normalized Difference Vegetation Index data,precipitation data,soil moisture data and drought index data,and comprehensively uses maximum value synthesis processing,Mann-Kendall trend analysis,partial correlation analysis,Hurst index analysis and regression analysis to analyze the characteristics of spatio-temporal changes of vegetation coverage and moisture conditions in China from 2001 to 2017,and the response relationship between moisture factors and vegetation coverage changes was discussed.Finally,a concept drift detection method was introduced.The spatial heterogeneity of vegetation coverage and moisture conditions in China and the response relationship between them were tested to explore the spatial differentiation characteristics of vegetation response to moisture factors.The main findings are as follows:(1)On the whole,our country's summer vegetation grows well.From 2001 to 2017,about 33% of the area showed an increasing trend,about 3% of the area showed a decreasing trend,and about 64% of the area showed no obvious trend of change.In terms of time,our country's summer NDVI grew the fastest in 2001-2002,followed by 2010-2017,and slowed in 2002-2009.From a spatial point of view,although the vegetation in most areas of our country shows an increasing trend and the distribution of vegetation growth is relatively concentrated,there are still some areas with reduced vegetation that are scattered.The Northeast temperate mixed forest area shows the largest proportion of NDVI increasing trend,and nearly half of the area(48.46%)shows an increasing trend.The Qinghai-Tibet Plateau's alpine vegetation area has the smallest proportion,which is only 13.85%.(2)The vegetation growth in each vegetation type area is affected by different moisture factors.The precipitation in the cold temperate coniferous forest area and temperate mixed forest area in the northeast is relatively small,but the soil moisture is sufficient.The growth status of these two vegetation types is very good.Temperate zone grassland area,temperate desert area and warm temperate deciduous broad-leaved forest area are drought-prone or half-drought-prone areas in China.Vegetation growth in temperate grassland area and temperate desert area is sparsely mainly affected by less natural precipitation and less soil moisture.The warm temperate deciduous broad-leaved forest area is affected by seasonal precipitation.Northeast cold temperate coniferous forest area and temperate mixed forest area are important wood production bases in China.The vegetation coverage of these two vegetation types is good,but the instability of precipitation is relatively strong.In the process of forestry and forestry production,it is necessary to focus on changes in precipitation.(3)The distribution pattern of soil moisture in China has the characteristics of "two wet and two dry".The two soil dry centers in the country are temperate grassland area and temperate desert area,and the two soil wet centers are northeast cold temperate coniferous forest area and subtropical evergreen broad-leaved forest area in southern China.The change trend distribution of shallow soil moisture and middle soil moisture is consistent with the that of the precipitation,and the area of significant growth and that of the significant reduction of shallow and middle soil are approximately equal.Deep soil moisture in the sparsely-grassland areas shows a decreasing trend as a whole,while the areas showing an increasing trend are mainly in forest or desert areas.(4)The effect of moisture on vegetation is most obvious in the northwest region,that is,vegetation growth in the northwest region is more sensitive to moisture.The response relationship between NDVI and summer average soil moisture has a "two negative and one positive" spatial distribution pattern,that is,the response relationship in the northeast and southwest regions is mostly negatively correlated,while the response relationship between soil moisture and NDVI in the central and western inland areas is mostly positively correlated.The relationship between the cumulative precipitation in summer and the NDVI has a strong positive correlation in most areas.The more precipitation it has in most areas,the better the vegetation growth is.(5)According to the concept drift detection results,the summer vegetation,moisture factor,vegetation-precipitation response relationship,and vegetation-soil moisture response relationship all show significant spatial differentiation.Drift areas of NDVI spatial distribution in summer of China are widely distributed and relatively concentrated in some areas,mainly composed of two areas of northwestern and southern China;there are few areas where summer accumulated rainfall drifts,mainly distributed in a few areas of northwestern and southern China;soil moisture spatial drift mainly occurs in the central and western parts of China,high-altitude areas and parts of tropical rain forest areas of China.The drift regions of shallow soil moisture are widely distributed and relatively concentrated,and due to the increase of soil depth,the drift regions are significantly reduced,but there will be scattered distribution.