Spatio-temporal Variation And Influencing Factors Of Soil Erosion On The Loess Plateau Forestland In Recent 20 Years

Soil erosion leads to loss of soil and water,causes land degradation and poses a great threat to both ecological and food security.It is considered that vegetation restoration is one of the effective ways to prevent soil erosion.Since the implementation of the Grain for Green Program in 1999,which aimed to convert agricultural land into forest and grassland,the vegetation cover on the Loess Plateau,known to be one of the most severe areas affected by soil erosion,has significantly increased,and soil erosion intensity has decreased.However,there is still uncertainty surrounding the impact of vegetation restoration on soil erosion,especially in areas where large-scale afforestation has been carried out,as there are instances of forest degradation and increased soil erosion.Additionally,factors such as vegetation canopy height,horizontal coverage,and growth conditions can directly or indirectly affect soil erosion intensity,and the specific thresholds require further exploration.Therefore,investigating the spatio-temporal distribution patterns and the relationship between vegetation characteristics and soil erosion in forestland post Grain for Green Program implementation can provide reliable evidence for assessing the effectiveness of vegetation restoration programs in preventing soil erosion and support subsequent vegetation restoration management and construction on the Loess Plateau.This study focuses on the impact of afforestation on soil erosion in the Loess Plateau region,using long-term time series remote sensing images,land use data,Aridity Index（AI）,and vegetation character data（forest canopy height,leaf area index,above-ground biomass,gross primary productivity,net primary productivity）.Based on the RUSLE model and on the basis of clarifying the soil erosion status of the Loess Plateau,trend analysis,correlation analysis,and breakpoint regression methods were used to quantify the spatiotemporal characteristics of soil erosion in the region and explore the effects of different forest land changes on soil erosion under different dry and wet areas.Furthermore,the relationship between vegetation characteristics and soil erosion in forest land was further explained.The primary research findings are as follows:（1）Soil erosion in the Loess Plateau exhibits spatial heterogeneity.From 2000 to2020,the overall intensity of soil erosion in the Loess Plateau has shown a decreasing trend,with erosion intensity at all levels gradually decreasing,and the proportion of low erosion intensity areas increasing year by year.The spatial distribution of the rainfall erosivity factor in the RUSLE model shows a decreasing trend from southeast to northwest,while it increases annually over time.In contrast,the spatio-temporal changes in vegetation coverage and management factors are opposite to this trend.The spatial distribution of the soil and water conservation measures factor is determined by the land use type and the slope of the cultivated land,and its trend remains stable during the study period.（2）During the implementation of the Grain for Green Project,the forest area on the Loess Plateau has significantly increased,with a notable improvement in vegetation coverage.Soil erosion in the forested areas is mainly at a mild or non-apparent level.The average soil erosion modulus of different forest types from highest to lowest is artificial forest(216.53 t·km^-2·a^-1),persisting forest(78.41 t·km^-2·a^-1),persisting shrubland(82.57 t·km^-2·a^-1),and artificial shrubland(46.05 t·km^-2·a^-1).Throughout the implementation of the Grain for Green Project,soil erosion in all forest types on the Loess Plateau has shown a decreasing trend,with the highest reduction in soil erosion modulus occurring in artificial forests.（3）There is a negative correlation between canopy height and soil erosion in semi-arid zonesⅢandⅣ（0.3<AI≤0.5）,indicating that increasing canopy height can control soil erosion in forested areas to some extent.However,in arid zoneⅠ（AI≤0.2）,there is a limiting threshold for canopy height and artificial forest soil erosion.When the canopy height exceeds the threshold（18.62 m）,soil erosion in artificial forests increases significantly.In addition,in semi-arid zoneⅡ（0.2<AI≤0.3）,when aboveground biomass exceeds the threshold,soil erosion in global forests,natural forests,and artificial forests intensifies,with thresholds of 98.02 Mg/ha,99.29 Mg/ha,and 101.2 Mg/ha,respectively.Furthermore,leaf area index,total primary productivity of vegetation,net primary productivity of vegetation,and soil erosion in forested areas are negatively correlated,but they do not have a limiting effect on soil erosion.In summary,with the implementation of the vegetation restoration project on the Loess Plateau in the past two decades,the soil erosion situation in the forested areas has significantly improved.Among different vegetation characteristics,leaf area index and vegetation productivity have a significant positive effect on improving soil erosion in forest lands,while canopy height and aboveground biomass exceeding the threshold have a negative impact on soil erosion in arid and semi-arid forest lands.Reasonable selection of tree species and spatial structure are crucial for the prevention and control of forest soil erosion.Therefore,vegetation restoration in the arid and semi-arid regions of the Loess Plateau should focus on the selection of tree species in the early stages and subsequent management methods.This study not only provides a certain data basis for the effectiveness evaluation of regional vegetation restoration projects,but also provides theoretical support and data support for subsequent vegetation restoration and soil and water conservation construction projects.