基于PLS-SEM和SHAP模型的腾格里沙漠土壤水分影响因素分析
Analysis of Factors Affecting Soil Moisture in the Tengger Desert ased on PLS-SEM and SHAP Models
[目的]土壤水分对干旱半干旱生态系统的结构和功能有重要影响，其分布受地形、植被等多种因素影响。为制定有效的土壤水分保持策略，针对沙丘不同微地貌类型和土壤水分水分特征建立固沙植被提供参考。[方法]以腾格里沙漠东南缘半固定沙丘 4 种微地貌类型为研究对象，运用描述统计分析土壤水分在不同微地貌类型上的分布特征。采用偏最小二乘结构方程模型构建地形、灌木植被和草本植被对不同深度土壤水分的影响路径，分析对土壤水分产生直接和间接影响的地形植被变量。利用基于随机森林和XGBoost算法的SHAP模型按照不同地形植被变量对土壤水分的贡献大小进行排序，选择贡献程度大的变量分析对土壤水分的影响机制。[结果]1)背风坡、丘底的土壤水分高于迎风坡和丘顶。表层和中层土壤水分分布的差异较大，变化较为剧烈，而深层土壤水分波动较小，分布较为稳定。2)地形对土壤水分存在显著的负面影响，对草本植被存在显著的正面影响。草本植被和灌木植被直接对土壤水分存在显著的负面影响。且灌木植被对草本植被存在显著的负面影响，从而间接影响土壤水分。3)随机森林模型与基于随机森林和XGBoost的SHAP模型对不同地形植被变量贡献程度的排序存在差异，但 3 种模型均表明草本盖度、坡向和高差对表层土壤水分的贡献程度较大，坡向、草本盖度和生物量对中层土壤水分的贡献程度较大，灌木盖度、坡向和草本盖度对深层土壤水分的贡献程度较大。4)不同深度土壤水分均随草本盖度和坡向增加呈先下降后上升趋势，表层土壤水分随高差增加呈先下降后上升趋势，中层和深层土壤水分分别随生物量和灌木盖度增加呈先上升后下降趋势。[结论]在防风固沙实践中，要针对不同微地貌类型上土壤水分特征合理建设固沙植被。在土壤水分更低的丘顶和迎风坡设置更加密集的草方格网和沙障。在表层和中层土壤水分变化较大的区域，种植耐旱的草本植物和浅根灌木，以快速覆盖地表，减少水分蒸发，在能够扎根到深层土壤的位置，种植深根灌木，利用深层稳定的水分，提高植被的整体抗旱能力。根据坡向和高差合理进行植被布局，在不利于保持土壤水分的高坡和阳坡应选择耐旱性更强的植物种植。在固沙植被管理过程中，需要对植被进行适当的修枝和间伐，减少植被体积和密度过大对土壤水分的消耗。
[Objective]Soil moisture has a significant impact on the structure and function of arid and semi-arid ecosystems,and its distribution is influenced by various factors such as topography and vegetation.To formulate effective soil moisture conservation strategies,it is essential to provide references for stabilizing sand vegetation based on the different microtopographic types of dunes and the characteristics of soil moisture.[Method]Taking four microtopographic types of semi-fixed dunes on the southeastern edge of the Tengger Desert as the research object,descriptive statistics are used to analyze the distribution characteristics of soil moisture in different microtopographic types.A partial least squares structural equation model(PLS-SEM)is used to construct the influence paths of topography,shrub vegetation,and herbaceous vegetation on soil moisture at different depths,analyzing the topographical and vegetation variables that have direct and indirect impacts on soil moisture.The SHAP model,based on random forest and XGBoost algorithms,is used to rank the contributions of different topographical and vegetation variables to soil moisture,and the variables with the highest contributions are selected to analyze the mechanisms affecting soil moisture.[Result]1)The soil moisture on the leeward slope and at the dune bottom is higher than on the windward slope and dune top.There is a significant difference in the distribution of soil moisture between the surface and middle layers,with more dramatic changes,whereas the deep soil moisture fluctuates less and is more stable.2)Topography has a significant negative impact on soil moisture and a significant positive impact on herbaceous vegetation.Herbaceous vegetation and shrub vegetation have a significant direct negative impact on soil moisture.Additionally,shrub vegetation has a significant negative impact on herbaceous vegetation,thereby indirectly affecting soil moisture.3)There are differences in the ranking of the contribution levels of different topographical and vegetation variables to soil moisture between the random forest model and the SHAP model based on random forest and XGBoost algorithms.However,all three models indicate that herbaceous coverage,slope aspect,and height difference have a significant contribution to surface soil moisture;slope aspect,herbaceous coverage,and biomass have a significant contribution to middle layer soil moisture;and shrub coverage,slope aspect,and herbaceous coverage have a significant contribution to deep soil moisture.4)Soil moisture at different depths shows a trend of initially decreasing and then increasing with the increase in herbaceous coverage and slope aspect.Surface soil moisture shows a trend of initially decreasing and then increasing with the increase in height difference.Middle layer and deep soil moisture show a trend of initially increasing and then decreasing with the increase in biomass and shrub coverage,respectively.[Conclusion]In windbreak and sand fixation practices,it is essential to reasonably establish sand-fixing vegetation based on the soil moisture characteristics of different microtopographic types.On dune tops and windward slopes,where soil moisture is lower,denser grass grids and sand barriers should be set up.In areas where surface and middle layer soil moisture vary greatly,drought-resistant herbaceous plants and shallow-rooted shrubs should be planted to quickly cover the surface and reduce moisture evaporation.In locations where roots can reach deep soil,deep-rooted shrubs should be planted to utilize stable deep soil moisture and improve the overall drought resistance of the vegetation.Vegetation should be arranged reasonably according to slope aspect and elevation difference.On high slopes and sunny slopes,which are less favorable for retaining soil moisture,more drought-tolerant plants should be selected for planting.In the management of sand-fixing vegetation,appropriate pruning and thinning are needed to reduce excessive vegetation volume and density,thereby minimizing the consumption of soil moisture.
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